Chronic Lymphocytic Leukemia
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《新英格兰医药杂志》
When chronic lymphocytic leukemia (CLL) was last reviewed in the Journal,1 it was considered a homogeneous disease of immature, immune-incompetent, minimally self-renewing B cells,2 which accumulate relentlessly because of a faulty apoptotic mechanism.3 In the past decade, these views have been transformed by a wealth of new information about the leukemic cells. CLL is now viewed as two related entities, both originating from antigen-stimulated mature B lymphocytes, which either avoid death through the intercession of external signals or die by apoptosis, only to be replenished by proliferating precursor cells (Table 1).
Table 1. A Comparison of Historical and Current Views of CLL.
Normal B Lymphocytes
B lymphocytes mature in the bone marrow (Figure 1) and in the process rearrange immunoglobulin variable (V) gene segments to create the code for an immunoglobulin molecule that serves as the B-cell receptor for antigen. When an antigen of adequate affinity engages the receptor, the cell enters a germinal center in lymphoid follicles, where, as a centroblast, it rapidly divides and its V genes undergo somatic hypermutation (Figure 2). This process introduces mutations in the rearranged VHDJH and VLJL gene segments that code for the binding site of the receptor. Through these mutations, the receptors of the descendant B cells, called centrocytes, acquire new properties. Cells with receptors that have enhanced antigen-binding affinity proliferate in the presence of the antigen, whereas centrocytes with receptors that no longer bind the antigen or do bind autoantigens are normally eliminated.4
Figure 1. Normal Development of a B Lymphocyte.
Genetic units that will encode the variable (V) regions of the H and L (heavy and light) chains are rearranged in a stepwise fashion. Initially, 1 of the 27 D gene segments in the germ line links up with one of six JH genes in pro-B cells. Next, 1 of the 51 VH segments forms the DJH unit in pre-B cells. At this stage, the monomorphic pseudo-L chain (VpreB + 5) is also synthesized, permitting the surface expression of the VHDJH–μ + VpreB–5 complex (pre-B cell receptor). Similar recombination events occur at the L chain locus in pre-B cells, although they involve only two gene segments (VL and JL, not shown). The recombinase-activating genes RAG1 and RAG2 are essential for these molecular events. In addition, the enzyme terminal deoxytransferase may introduce additional nucleotides at the D–JH and VH–DJH junctions, thereby increasing diversity of the V regions of developing B cells. Each B cell uses a single set of VHDJH and VLJL rearrangements to create its antigen-binding site, thus maintaining the unique B-cell–receptor structure that identifies an individual B-lymphocyte clone. Each of these rearrangements must lead to the formation of recombined genes encoding an intact immunoglobulin molecule to allow the cell to progress to the next stage of development.
Figure 2. A Comparison of Pathways of B-Cell Maturation According to T-Cell Dependency.
In Panel A, the lymph-node follicle, consisting of the germinal center and the follicular mantle, is depicted with the extrafollicular marginal zone (courtesy of Dr. Stefano Pileri, University of Bologna). In Panel B, mature B cells in peripheral lymphoid tissues can follow two maturation pathways. The response of follicular B cells to antigens requires T-cell help, whereas marginal-zone B cells respond to antigens without T-cell participation. T-cell–dependent antigenic stimulation invariably induces V-gene mutations in follicular B cells. T-cell–independent antigenic stimulation may or may not induce V-gene mutations in marginal-zone B cells. V-gene mutations develop in follicular B cells within germinal centers, whereas in marginal-zone B cells, V-gene mutations occur outside of typical germinal-center structures. Both the T-cell–dependent and T-cell–independent pathways lead to either plasma cells or memory cells. Memory cells without V-gene mutations that derive from T-cell–independent stimulation are often referred to as antigen-experienced B cells, to distinguish them from memory cells with somatic mutations.
This stimulation and selection pathway usually requires the help of T lymphocytes and occurs in germinal centers,4 the structure of which ensures the selection of antigen-avid B cells. However, the process can proceed without T cells5,6,7,8 and outside germinal centers, in the marginal zones around lymphoid follicles,9 most often in response to carbohydrates of encapsulated bacteria or viruses (Figure 2). Both processes lead to the development of plasma cells or memory (antigen-experienced) B cells.
Concomitant with B-cell activation, the proteins on the surface of the B cell change. These modifications help activated B cells to interact with other cells and soluble mediators and thereby increase in number or mature into antibody-producing plasma cells. One surface molecule that supports B-cell interactions and differentiation is CD38.10 CD38 has adenosine diphosphate–ribose cyclase activity, and under certain circumstances augments signaling of B-cell receptors11 and delivers signals that regulate the apoptosis of B cells.12
Signals received through B-cell surface receptors are transferred to the nucleus by a cascade of interacting molecules whose structures are temporarily modified during the process.13,14 These modifications frequently involve the attachment of phosphate groups to tyrosines of target proteins by specific enzymes. The enzymes involved in the initial phases of the signaling cascade include Syk and Lyn, members of the Src family of protein tyrosine kinases.13 For T lymphocytes, the zeta-chain–associated protein 70 (ZAP-70) is a crucial player with similar activity.14
CLL and the Biology of Leukemic Lymphocytes
The monoclonal population of B cells in CLL expresses CD19, CD5, and CD23 and has reduced levels of membrane IgM, IgD, and CD79b, a phenotype of mature, activated B lymphocytes.15,16,17 The pathological features of the lymph node are those of a small lymphocytic lymphoma.
Some patients with CLL survive for many years without therapy and eventually succumb to unrelated diseases, whereas others have a rapidly fatal disease despite aggressive therapy. Recognizing this heterogeneity, Rai and colleagues18,19 and Binet and colleagues20 devised staging systems for use in assessing the extent of disease in an individual patient. These systems remain the cornerstones on which decisions regarding medical follow-up and treatment are built, but they fail to predict the course of the disease in patients in whom CLL is diagnosed in early stages.
Within the past decade, CLL has been shown to be a remarkably diverse disorder. Its heterogeneity reflects differences in the mutation status of V genes,21,22,23,24,25,26 expression of CD38 and ZAP-70,27,28,29,30,31,32,33,34,35 and profiling of the expression of genes genome-wide.36,37 When cases of CLL were divided into categories on the basis of these differences, profoundly disparate clinical courses were revealed.25,26,28,29,30,38,39,40 Patients with clones having few or no V-gene mutations or with many CD38+ or ZAP-70+ B cells had an aggressive, usually fatal course, whereas patients with mutated clones or few CD38+ or ZAP-70+ B cells had an indolent course.
Mutations of V genes are detected by comparing DNA sequences of the genes in B cells with corresponding genes in the germ line. A sequence that differs from its germ-line counterpart by 2 percent or more is defined as mutated.21 According to this criterion, CLL cases are divisible into two groups: in the first, leukemic cells have rearranged VH genes with 2 percent or more mutations ("mutated" CLL); in the second, there are few or no mutations (less than 2 percent; "unmutated" CLL).41,42 The presence of V-gene mutations and the presence of few CD38+ cells do not always correlate.26,27,28,43,44
ZAP-70 is an intracellular protein that promulgates activation signals delivered to T lymphocytes and natural killer cells by their surface receptors for antigen14 (Figure 3). It is rarely present in normal B cells but has been found in B cells from patients with CLL.36 When the expression of ZAP-70 is manipulated experimentally in B cells, it can facilitate signal transmission down the pathway initiated by antigen engagement with the B-cell receptor.45 Gene-expression profiles indicate that unmutated CLL cells express more ZAP-70 mRNA than do mutated CLL cells.31,32,33,34,36 The analysis of DNA sequences to determine the status of immunoglobulin V-gene mutations is laborious and not performed routinely in clinical laboratories, whereas testing for ZAP-70, when appropriately standardized, can more readily serve as a clinical test.31,33,34,35
Figure 3. The Promoting Roles of Antigen Stimulation and Accessory Signals from the Microenvironment in CLL.
B cells in patients with CLL who have unfavorable prognostic markers (left side of figure) are stimulated by the binding of self-antigens to the B-cell receptor. The dynamic balance of negative and positive signals delivered by the B-cell receptor and the survival signals transduced by IgD and delivered by other cells, cytokines, and chemokines determine whether the leukemic cell proliferates or dies by apoptosis. B cells from patients with CLL who have favorable prognostic markers (right side of figure) are less capable of triggering apoptosis, survival, or proliferation owing to an inability to bind antigen because of changes in the shape of B-cell receptors, because of V-gene mutations (blue rectangular antigen does not fit B-cell receptors, a condition known as "clonal ignorance"), or because of defective B-cell receptor signaling (despite an adequate fit of green diamond-shaped antigen in B-cell receptors). This lack of receptor stimulation may be a factor associated with a less aggressive form of the disease.
The Biology of Leukemic Lymphocytes and the Clinical Course of CLL
Inducing Factors in CLL
Chromosomal translocations involving oncogenes frequently cause B-cell lymphomas.46 CLL is a special case, however, because chromosomal translocations are rare, and no unifying mutations have been identified. Yet the monoclonal nature of the B lymphocytes that proliferate in this disease imply that inducing lesions must exist in the progenitor clone.
Cytogenetic lesions are rare in the leukemic clone early in the course of the disease and therefore are not likely to be inducing factors. Nevertheless, some appear as the disease progresses. The most common is a deletion at 13q14.3, which occurs in more than 50 percent of cases over time.47 This deleted region contains a nontranscribed gene48 and two micro-RNA genes.49 Micro-RNA is made normally by cells, including B lymphocytes, and regulate the functions of many genes,50 some of which may have relevance to cancer in general and CLL in particular.51 Two micro-RNA genes located at 13q14 are deleted or down-regulated in most cases of CLL.51 The frequency of this deletion implies that it confers a selective advantage, possibly predisposing B-cell clones to undergo additional mutations.
The most ominous alterations are deletions at 11q22–23, 17p13, and 6q21.47 Although the genes that are involved in these lesions are unknown, it is likely that p53 is included in the deletion at 17p13 and that the ataxia–telangiectasia mutated (ATM) gene is involved in the deletion at 11q22–23.47,52 Both genes regulate apoptosis and confer resistance to chemotherapy.52 These deletions are relatively frequent in unmutated CLL cases with a poor outcome.23,52
Interestingly, TCL-1, located at 14q32.1 and involved in the pathogenesis of T-cell prolymphocytic leukemia,53 is expressed in CLL.54 In mice that were genetically manipulated to overexpress Tcl-1 in B cells, a leukemia or lymphoma of CD5+ B cells developed that was reminiscent of CLL.55 Although attractive candidates for an inducing factor, abnormalities of TCL-1 or its regulation have not been identified in patients with CLL.
Promoting Factors in CLL
New evidence suggests that antigenic stimulation, along with interactions with accessory cells and cytokines, is a promoting factor that stimulates proliferation of CLL cells and allows them to avoid apoptosis. These effects may differ in distinct CLL subgroups and thereby lead to the disparity in clinical outcomes among individual cases.
Inferring the Role of Antigenic Stimulation from B-Cell Receptors
The B-cell receptors of CLL cells from various patients are often structurally very similar, suggesting that the antigens these receptors bind are similar and relevant to the pathogenesis of CLL.41,42 The extent of similarity varies among groups of patients. In some cases, there are shared features in the portion of the antigen-binding site contributed by the H chain (VH, D, and JH genes).41,42,56 In these cases, each VH gene exhibits special patterns of mutations and preferential combinations with particular D or JH segments, which generate distinct features in the antigen-binding pocket.24,57 These VHDJH rearrangements and characteristics of antigen-binding pockets differ from the much broader diversity found in B cells from normal persons.24,58,59
In other groups of cases, the structural similarity of the receptors involves the entire antigen-binding site, coded by both the H and L chains (VH, D, and JH, and VL and JL genes). In these instances, the receptors from various patients are very similar or virtually identical.60,61,62,63,64,65 As many as 10 percent of all CLL cases fall into distinct categories branded by receptors with structurally similar antigen-binding pockets; most are of the unmutated, poor-outcome type.63 These findings are very striking since, given the number of possible combinations of V-gene segments that can encode antigen-binding domains, one would not expect to find 2 cases of CLL with such structurally similar B-cell receptors in more than 1 million cases.
These cases suggest that a limited set of antigens promotes division of the leukemic cells, increasing the likelihood of dangerous DNA mutations. What are these promoting antigens? They are unknown, but it is possible that latent viruses or commensal bacteria repetitively activate particular B-cell clones through the B-cell receptor. CLL would result, directly or indirectly, from specific infections and would be perpetuated by them — in a manner similar to the gastric lymphomas that evolve in response to Helicobacter pylori.66
Alternatively, environmental antigens or autoantigens could provoke clonal expansion. CLL cells frequently have polyreactive receptors, which bind multiple antigens, including autoantigens,67,68,69,70 allowing stimulation by both autoantigens and microbial antigens. This mechanism is plausible for unmutated CLL and also for a few cases of mutated CLL, since many unmutated71,72 and some mutated73 immunoglobulin V genes encode such polyreactive receptors; this immune-stimulation mechanism is in keeping with the view that the basis of CLL is autoimmunization.74
As constitutive low-level signaling is delivered through the B-cell receptor in normal B lymphocytes, perhaps to maintain the memory response and the B-cell repertoire,75,76 antigen may not be necessary to continue clonal expansion — antigen-independent triggering might occur through the B-cell–receptor signaling pathway because of another genetic lesion.
Signal Transduction after Antigen Engagement
For antigenic stimulation to underlie clonal expansion, the B-cell receptor must propagate an efficient signal to the cell nucleus (Figure 3). Leukemic cells from different CLL subgroups can differ in this capacity. Cross-linking B-cell receptor molecules with antibodies to IgM in vitro mimics the engagement of antigens with B-cell receptors and transmits signals to the cell nucleus in approximately 50 percent of cases of CLL.77,78,79,80,81 This phenomenon seems to occur mainly in unmutated CLL,27,32,82 but more patients need to be studied for this to be confirmed. CLL cells that do not respond to stimulation from the B-cell receptor may be frozen at a stage at which even normal B lymphocytes would be unresponsive to antigen.41 Alternatively, these cells could be anergic, possibly because of previous antigenic experience.42 Finally, these CLL cells may have become incapable of responding to antigens because of changes in the structure of their B-cell receptors caused by somatic mutations or an inability of the cells to come into contact with relevant antigens in vivo.41 Considering the number of cells that make up the leukemic clone in many patients (1011 to 1012), it is likely that only a fraction of the members of the clone could encounter the antigen, especially if they are restricted to discrete anatomical locations or compartments.
Other, not mutually exclusive, possibilities to explain the lack of B-cell–receptor signaling include reduced numbers of B-cell–receptor molecules,17 uncoupling of the B-cell receptor from accessory molecules necessary for effective signal transduction,83,84,85 and mutations in these accessory structures.86 It is interesting to note that responsiveness to stimuli delivered through surface IgD is frequently maintained in these cases.82,87
Consequences of Signal Transduction through the B-Cell Receptor
Once signal transduction is initiated by the B-cell receptor, B lymphocytes progress into the cell cycle or die. Cross-linking of surface IgM in CLL cells that can transduce a signal can cause81 or prevent88 apoptosis (Figure 3), whereas cross-linking surface IgD invariably prevents apoptosis.12,81 This difference is unexpected, because the two surface isotypes express the same clone-specific antigen-binding site and provide concordant signals in mature B cells. The final outcome of B-cell receptor signaling in an individual CLL cell, therefore, depends on the balance between signals mediated by the two molecules.
Signals from the Microenvironment
Signals that are delivered by direct cell contact or soluble factors, which may or may not occur concomitantly with B-cell–receptor engagement, probably propagate the growth of CLL cells (Figure 3). Interactions with stromal cells89 or nurse-like cells90 or interactions between CD38 and its natural ligand CD3191 rescue CLL cells from apoptosis in vitro and probably do the same in vivo. Activated T cells or other cells expressing CD40 ligand also support the growth of CLL cells.92 Finally, cytokines such as interleukin-4 and vascular endothelial growth factor93,94,95 and chemokines such as SDF-196 (particularly in the presence of stromal cells) support the expansion of CLL clones.
These signals tip the balance between antiapoptotic signals and proapoptotic signals in favor of cell survival. There is up-regulation of the antiapoptotic genes BCL2, survivin, and MCL1 in leukemic cells.92,97 Rescue from apoptosis and facilitation of cell growth may occur preferentially in lymph-node pseudofollicles and bone marrow clusters,89,98 evidenced by expression of the cycling cell marker Ki-67 by the leukemic cells in these sites.92 Because growth of the clone depends on a variety of interactions with the environment, variations in the requirements for these interactions on part of the leukemic cells may be responsible for changes in the clinical course.99,100
Appearance and Evolution of New Genetic Mutations
The emergence of new, aggressive clonal variants, which can worsen the disease, requires proliferation of the leukemic clone. In vivo studies using radioactive and nonradioactive means suggest that CLL cells are more dynamic than is usually appreciated.101,102,103 CLL cells have surprisingly brisk birth rates, ranging from about 0.1 to more than 1.0 percent of the clone per day.103 If the total clonal burden of a typical patient with CLL is approximately 1012 cells, these birth rates point to the daily production of some 109 to 1010 new leukemic cells.
These rates of cell division are sufficient to permit clonal variants to emerge. Indeed, there is an association between brisk birth rates of CLL cells and progressive disease.103 The rate of birth of leukemic cells, therefore, may be more relevant clinically than is either the blood lymphocyte count or the physical examination, since the lymphocyte count reflects the proliferative capacity of the leukemic cells and their potential to promote new DNA lesions, whereas the sizes of the lymph nodes and spleen on physical examination reflect a balance between cell proliferation and cell death. These findings may explain why telomeres, which cap and protect the ends of chromosomes but shorten with each cell division, are smaller in cells from patients in CLL subgroups that have poor outcomes.104,105
A Unifying Hypothesis for the Development, Growth, and Evolution of CLL
Growth and Evolution of CLL Cells
The above considerations suggest a plausible model on which to build future hypotheses and studies. Stimulatory and growth signals from the environment of CLL cells allow them to avoid apoptosis and proliferate. These signals are delivered by the B-cell receptor, receptors for cytokines or chemokines and other ligands, and direct contact with accessory and stromal cells. The major growth effects mediated by the B-cell receptor appear to occur in cases in which the receptor permits binding of autoantigens and maintains the capacity to transmit stimulatory signals to the cell nucleus (i.e., those with unmutated and, to a lesser degree, mutated CLL B-cell receptors).
This model excludes an intrinsic apoptotic defect in all members of the leukemic clone. Indeed, in vitro observations demonstrate the absence of lesions in the major apoptotic pathways.87,106 Whether continued cell division is facilitated by external signals or not, the level of B-cell turnover in vivo can suffice to promote the development and outgrowth of subclones with new genetic lesions and a growth advantage (Figure 4).
Figure 4. Model of the Development and Evolution of CLL Cells.
An initial inducing lesion occurs in a single B lymphocyte (pink cell in upper and lower portions of Panel A) among billions of distinct normal B cells. Interactions between antigens and B-cell receptors of adequate affinity induce clonal amplification (Panel B). The initial inducing lesion provides the marked cell with a growth advantage over other clones stimulated by the same or other antigens. In clones destined to become unmutated CLL cells, repetitive interactions between antigens and polyreactive B-cell receptors of the initially selected clone promote clonal growth, which persists because V-gene mutations do not occur (Panel C, top). In others, destined to become mutated CLL cells (Panel C, bottom), V-gene mutations develop that can abrogate the polyreactivity of the B-cell receptors and thereby alter their ability to bind the original antigen or autoantigen ("clonal ignorance"). Alternatively, these mutated cells become anergic owing to excessive B-cell–receptor stimulation because of the acquisition of more avid receptors. In both instances, the promoting effect of antigenic stimulation is neutralized. Additional DNA mutations cause the cells to cross the boundary from "normality" to "leukemia" (Panel D). Differences in the signals received through the B-cell receptors and other receptors determine the extent of clonal expansion. B cells that can no longer bind antigen may not cross this boundary. Continued cycling leads to other genetic changes (e.g., deletions at 13q, 11q, and 17p or duplication of chromosome 12) that determine the course of the disease (Panel E). These changes appear to occur more frequently in patients with unmutated CLL.
Development of CLL from Normal B Lymphocytes
Many normal B lymphocytes with unmutated V genes produce antibodies capable of binding multiple antigens (e.g., carbohydrates, nucleic acids, and phospholipids)107 and of providing the first line of defense against microorganisms.108 If one of these cells contained or developed a genetic abnormality that allows it to resist restraint on clonal size (e.g., an initial inducing lesion), then this cell would be primed for leukemic transformation (Figure 4A). Foreign antigens and autoantigens then could be important stimuli for the development of CLL.109 B cells with such unmutated polyreactive B-cell receptors could expand and convert to CLL cells with repetitive exposure to microbes and to autoantigens (Figure 4B).110,111,112,113 A similar mechanism may underlie the origin of mutated CLL, because V-gene mutations can occur without T-cell help5,7,8 outside of germinal centers,9 and these mutations can sometimes favor autoreactivity.73 Such expansion would stop if V-gene mutations altered the structure of the B-cell receptor in a way that caused loss of binding to the stimulatory antigen (i.e., the development of "clonal ignorance") (Figure 4C).
This hypothesis implies that such expansions should be detectable in healthy patients. Recent studies suggest that small numbers of clonal B cells with the characteristics of CLL cells exist in the blood of approximately 3.5 percent of disease-free persons114,115 and in an even higher proportion of first-degree relatives of patients with CLL.116 Although studies of the B-cell receptors of these B-lymphocyte expansions are limited, initial information suggests that they are monoclonal and use some of the genes that encode the B-cell receptors of CLL clones.
From Which Subpopulation Do CLL Cells Develop?
Since CLL cells resemble activated B lymphocytes,117 their cellular origin cannot be deduced solely from phenotypic analyses, a fact that makes it difficult to draw a direct parallel with B1 cells described in mice.118 However, certain functional features may help delineate their origin. Normal adult B cells that produce autoantibodies and antibodies against bacterial or viral carbohydrates reside in the marginal zone. It is possible that marginal-zone B cells are the precursors of both unmutated and mutated CLL cells, because B-cell receptors of CLL cells are structurally similar to those of antibodies that react with autoantigens and carbohydrate components of infectious agents41,62,63 (Figure 2B). Alternatively, mutated CLL cells could originate from B cells stimulated in a T-cell–dependent manner that have passed through a germinal center.
Other potential precursors are B1 cells, which share several features with marginal-zone B cells,118 as well as immature pre–B cells119 and transitional B cells120 that can also express self-reactive receptors (Figure 1). A few pre–B cells emerge from the bone marrow into the periphery, and transitional B cells routinely exit the marrow and traverse the circulation to solid lymphoid tissues. A genetic abnormality could allow one of these cells to survive, thereby making it available for autoantigenic drive and leukemic transformation into unmutated CLL cells.
Clinical Implications
Prognosis
The primary role of the Rai and Binet staging systems is to help clinicians decide when patients should be started on therapy. However, since these approaches do not predict the clinical course of a patient with precision, they are less helpful as long-term prognostic indicators. Therefore, physicians have postponed therapeutic decisions until the patients reach advanced Rai or Binet stages.
However, the molecular and cellular features we have discussed can distinguish patients with better or worse clinical courses, regardless of the Rai and Binet risk categories. Determination of V-gene mutation is not routinely available, but measurement of ZAP-70 is becoming widely available; it may be the most reliable indicator of prognosis.35
Several points still require clarification and refinement. For example, can a single marker provide a sufficiently accurate and reliable prognostic assessment to permit early decisions about clinical management? Or should several markers be used to increase the degree of accuracy of prognosis? What are the clinically most useful cutoff points for the percentages of expression of CD3826,44 and ZAP-7031,32,33,34 and the levels of immunoglobulin V-gene mutation26 that most reliably define the clinical subgroups with various outcomes? Should the cases at the borderlines of these arbitrary cutoffs be handled differently?
Management
In the past, physicians told patients with CLL that a "watchful waiting" mode had to be adopted until the disease progressed, whereupon therapy would be initiated. In my opinion, this approach is especially disturbing, given that the novel prognostic markers indicate that some 50 percent of the patients assigned to watchful waiting have one or more features portending a poor outcome. Although this approach is still being followed, it will probably change considerably when the new prognostic markers are more readily available to all clinicians. On the basis of such information, an early start of therapy may be justified in groups with a poor prognosis. However, before any guidelines can be proposed, the results of large, prospectively conducted clinical trials that test the use of early intervention in patients in poor-prognosis groups must become available. Only one such trial has been initiated, and the accrual of required numbers of patients and the analysis of those results will take several years.
New Therapeutic Approaches
Since CLL cells must interact with the stroma in bone marrow or other peripheral lymphoid tissues to survive, these interactions need to be explored as targets of innovative therapies. Furthermore, specific inhibition of the B-cell receptor signaling pathway, in particular ZAP-70 or its signaling partners, may be an option. Targeting the actively proliferating cells that maintain the CLL clone by a cell-cycle–active agent could also be considered. Finally, since as many as 20 percent of patients with the worst prognostic markers have stereotypic antigen receptors, these common structures may be practical and valuable points of attack. When the antigens that engage these receptors are precisely defined, it may become possible to develop another arsenal of specific therapies.
Supported in part by grants from the National Cancer Institute (RO1 CA 81554 and CA 87956) and a General Clinical Research Center Grant (M01 RR018535) from the National Center for Research Resources, the Associazione Italiana per la Ricerca sul Cancro, and the Ministero dell'Istruzione dell'Università e della Ricerca; the Peter J. Sharp Foundation, the Marks Family Foundation, the Jean Walton Fund for Lymphoma and Myeloma Research, the Joseph Eletto Leukemia Research Fund, the Tebil Foundation, the Horace W. Goldsmith Foundation, and the Chemotherapy Foundation. Drs. Rai and Chiorazzi are members of the NCI-sponsored Chronic Lymphocytic Leukemia Research Consortium (PO1 CA 081534).
Dr. Chiorazzi holds a patent on the use of CD38 as a prognostic indicator in chronic lymphocytic leukemia. Dr. Rai reports having received lecture fees from Genentech and Berlex Laboratories and grant support from Berlex. Dr. Ferrarini reports having received lecture fees from Schering AG.
We are indebted to the present and past members of the Laboratory of Experimental Immunology, Institute for Medical Research, North Shore–LIJ Health System, of the North Shore University Hospital, and of the Division of Medical Oncology C, Istituto Nazionale per la Ricerca sul Cancro, for the work that we have discussed in this article.
Source Information
From the Institute for Medical Research, North Shore–LIJ Health System (N.C., K.R.R.), and the Departments of Medicine, North Shore University Hospital, Manhasset, N.Y., and New York University School of Medicine, New York (N.C.); the Departments of Medicine, Long Island Jewish Medical Center, New Hyde Park, N.Y., and Albert Einstein School of Medicine, Bronx, N.Y. (K.R.R.); and the Division of Medical Oncology C, Istituto Nazionale per la Ricerca sul Cancro, and the Dipartimento di Oncologia Clinica e Sperimentale, Universitá di Genova — both in Genoa, Italy (M.F.).
Address reprint requests to Dr. Chiorazzi at the Institute for Medical Research, North Shore–LIJ Health System, 350 Community Dr., Manhasset, NY 11030, or at nchizzi@nshs.edu.
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Table 1. A Comparison of Historical and Current Views of CLL.
Normal B Lymphocytes
B lymphocytes mature in the bone marrow (Figure 1) and in the process rearrange immunoglobulin variable (V) gene segments to create the code for an immunoglobulin molecule that serves as the B-cell receptor for antigen. When an antigen of adequate affinity engages the receptor, the cell enters a germinal center in lymphoid follicles, where, as a centroblast, it rapidly divides and its V genes undergo somatic hypermutation (Figure 2). This process introduces mutations in the rearranged VHDJH and VLJL gene segments that code for the binding site of the receptor. Through these mutations, the receptors of the descendant B cells, called centrocytes, acquire new properties. Cells with receptors that have enhanced antigen-binding affinity proliferate in the presence of the antigen, whereas centrocytes with receptors that no longer bind the antigen or do bind autoantigens are normally eliminated.4
Figure 1. Normal Development of a B Lymphocyte.
Genetic units that will encode the variable (V) regions of the H and L (heavy and light) chains are rearranged in a stepwise fashion. Initially, 1 of the 27 D gene segments in the germ line links up with one of six JH genes in pro-B cells. Next, 1 of the 51 VH segments forms the DJH unit in pre-B cells. At this stage, the monomorphic pseudo-L chain (VpreB + 5) is also synthesized, permitting the surface expression of the VHDJH–μ + VpreB–5 complex (pre-B cell receptor). Similar recombination events occur at the L chain locus in pre-B cells, although they involve only two gene segments (VL and JL, not shown). The recombinase-activating genes RAG1 and RAG2 are essential for these molecular events. In addition, the enzyme terminal deoxytransferase may introduce additional nucleotides at the D–JH and VH–DJH junctions, thereby increasing diversity of the V regions of developing B cells. Each B cell uses a single set of VHDJH and VLJL rearrangements to create its antigen-binding site, thus maintaining the unique B-cell–receptor structure that identifies an individual B-lymphocyte clone. Each of these rearrangements must lead to the formation of recombined genes encoding an intact immunoglobulin molecule to allow the cell to progress to the next stage of development.
Figure 2. A Comparison of Pathways of B-Cell Maturation According to T-Cell Dependency.
In Panel A, the lymph-node follicle, consisting of the germinal center and the follicular mantle, is depicted with the extrafollicular marginal zone (courtesy of Dr. Stefano Pileri, University of Bologna). In Panel B, mature B cells in peripheral lymphoid tissues can follow two maturation pathways. The response of follicular B cells to antigens requires T-cell help, whereas marginal-zone B cells respond to antigens without T-cell participation. T-cell–dependent antigenic stimulation invariably induces V-gene mutations in follicular B cells. T-cell–independent antigenic stimulation may or may not induce V-gene mutations in marginal-zone B cells. V-gene mutations develop in follicular B cells within germinal centers, whereas in marginal-zone B cells, V-gene mutations occur outside of typical germinal-center structures. Both the T-cell–dependent and T-cell–independent pathways lead to either plasma cells or memory cells. Memory cells without V-gene mutations that derive from T-cell–independent stimulation are often referred to as antigen-experienced B cells, to distinguish them from memory cells with somatic mutations.
This stimulation and selection pathway usually requires the help of T lymphocytes and occurs in germinal centers,4 the structure of which ensures the selection of antigen-avid B cells. However, the process can proceed without T cells5,6,7,8 and outside germinal centers, in the marginal zones around lymphoid follicles,9 most often in response to carbohydrates of encapsulated bacteria or viruses (Figure 2). Both processes lead to the development of plasma cells or memory (antigen-experienced) B cells.
Concomitant with B-cell activation, the proteins on the surface of the B cell change. These modifications help activated B cells to interact with other cells and soluble mediators and thereby increase in number or mature into antibody-producing plasma cells. One surface molecule that supports B-cell interactions and differentiation is CD38.10 CD38 has adenosine diphosphate–ribose cyclase activity, and under certain circumstances augments signaling of B-cell receptors11 and delivers signals that regulate the apoptosis of B cells.12
Signals received through B-cell surface receptors are transferred to the nucleus by a cascade of interacting molecules whose structures are temporarily modified during the process.13,14 These modifications frequently involve the attachment of phosphate groups to tyrosines of target proteins by specific enzymes. The enzymes involved in the initial phases of the signaling cascade include Syk and Lyn, members of the Src family of protein tyrosine kinases.13 For T lymphocytes, the zeta-chain–associated protein 70 (ZAP-70) is a crucial player with similar activity.14
CLL and the Biology of Leukemic Lymphocytes
The monoclonal population of B cells in CLL expresses CD19, CD5, and CD23 and has reduced levels of membrane IgM, IgD, and CD79b, a phenotype of mature, activated B lymphocytes.15,16,17 The pathological features of the lymph node are those of a small lymphocytic lymphoma.
Some patients with CLL survive for many years without therapy and eventually succumb to unrelated diseases, whereas others have a rapidly fatal disease despite aggressive therapy. Recognizing this heterogeneity, Rai and colleagues18,19 and Binet and colleagues20 devised staging systems for use in assessing the extent of disease in an individual patient. These systems remain the cornerstones on which decisions regarding medical follow-up and treatment are built, but they fail to predict the course of the disease in patients in whom CLL is diagnosed in early stages.
Within the past decade, CLL has been shown to be a remarkably diverse disorder. Its heterogeneity reflects differences in the mutation status of V genes,21,22,23,24,25,26 expression of CD38 and ZAP-70,27,28,29,30,31,32,33,34,35 and profiling of the expression of genes genome-wide.36,37 When cases of CLL were divided into categories on the basis of these differences, profoundly disparate clinical courses were revealed.25,26,28,29,30,38,39,40 Patients with clones having few or no V-gene mutations or with many CD38+ or ZAP-70+ B cells had an aggressive, usually fatal course, whereas patients with mutated clones or few CD38+ or ZAP-70+ B cells had an indolent course.
Mutations of V genes are detected by comparing DNA sequences of the genes in B cells with corresponding genes in the germ line. A sequence that differs from its germ-line counterpart by 2 percent or more is defined as mutated.21 According to this criterion, CLL cases are divisible into two groups: in the first, leukemic cells have rearranged VH genes with 2 percent or more mutations ("mutated" CLL); in the second, there are few or no mutations (less than 2 percent; "unmutated" CLL).41,42 The presence of V-gene mutations and the presence of few CD38+ cells do not always correlate.26,27,28,43,44
ZAP-70 is an intracellular protein that promulgates activation signals delivered to T lymphocytes and natural killer cells by their surface receptors for antigen14 (Figure 3). It is rarely present in normal B cells but has been found in B cells from patients with CLL.36 When the expression of ZAP-70 is manipulated experimentally in B cells, it can facilitate signal transmission down the pathway initiated by antigen engagement with the B-cell receptor.45 Gene-expression profiles indicate that unmutated CLL cells express more ZAP-70 mRNA than do mutated CLL cells.31,32,33,34,36 The analysis of DNA sequences to determine the status of immunoglobulin V-gene mutations is laborious and not performed routinely in clinical laboratories, whereas testing for ZAP-70, when appropriately standardized, can more readily serve as a clinical test.31,33,34,35
Figure 3. The Promoting Roles of Antigen Stimulation and Accessory Signals from the Microenvironment in CLL.
B cells in patients with CLL who have unfavorable prognostic markers (left side of figure) are stimulated by the binding of self-antigens to the B-cell receptor. The dynamic balance of negative and positive signals delivered by the B-cell receptor and the survival signals transduced by IgD and delivered by other cells, cytokines, and chemokines determine whether the leukemic cell proliferates or dies by apoptosis. B cells from patients with CLL who have favorable prognostic markers (right side of figure) are less capable of triggering apoptosis, survival, or proliferation owing to an inability to bind antigen because of changes in the shape of B-cell receptors, because of V-gene mutations (blue rectangular antigen does not fit B-cell receptors, a condition known as "clonal ignorance"), or because of defective B-cell receptor signaling (despite an adequate fit of green diamond-shaped antigen in B-cell receptors). This lack of receptor stimulation may be a factor associated with a less aggressive form of the disease.
The Biology of Leukemic Lymphocytes and the Clinical Course of CLL
Inducing Factors in CLL
Chromosomal translocations involving oncogenes frequently cause B-cell lymphomas.46 CLL is a special case, however, because chromosomal translocations are rare, and no unifying mutations have been identified. Yet the monoclonal nature of the B lymphocytes that proliferate in this disease imply that inducing lesions must exist in the progenitor clone.
Cytogenetic lesions are rare in the leukemic clone early in the course of the disease and therefore are not likely to be inducing factors. Nevertheless, some appear as the disease progresses. The most common is a deletion at 13q14.3, which occurs in more than 50 percent of cases over time.47 This deleted region contains a nontranscribed gene48 and two micro-RNA genes.49 Micro-RNA is made normally by cells, including B lymphocytes, and regulate the functions of many genes,50 some of which may have relevance to cancer in general and CLL in particular.51 Two micro-RNA genes located at 13q14 are deleted or down-regulated in most cases of CLL.51 The frequency of this deletion implies that it confers a selective advantage, possibly predisposing B-cell clones to undergo additional mutations.
The most ominous alterations are deletions at 11q22–23, 17p13, and 6q21.47 Although the genes that are involved in these lesions are unknown, it is likely that p53 is included in the deletion at 17p13 and that the ataxia–telangiectasia mutated (ATM) gene is involved in the deletion at 11q22–23.47,52 Both genes regulate apoptosis and confer resistance to chemotherapy.52 These deletions are relatively frequent in unmutated CLL cases with a poor outcome.23,52
Interestingly, TCL-1, located at 14q32.1 and involved in the pathogenesis of T-cell prolymphocytic leukemia,53 is expressed in CLL.54 In mice that were genetically manipulated to overexpress Tcl-1 in B cells, a leukemia or lymphoma of CD5+ B cells developed that was reminiscent of CLL.55 Although attractive candidates for an inducing factor, abnormalities of TCL-1 or its regulation have not been identified in patients with CLL.
Promoting Factors in CLL
New evidence suggests that antigenic stimulation, along with interactions with accessory cells and cytokines, is a promoting factor that stimulates proliferation of CLL cells and allows them to avoid apoptosis. These effects may differ in distinct CLL subgroups and thereby lead to the disparity in clinical outcomes among individual cases.
Inferring the Role of Antigenic Stimulation from B-Cell Receptors
The B-cell receptors of CLL cells from various patients are often structurally very similar, suggesting that the antigens these receptors bind are similar and relevant to the pathogenesis of CLL.41,42 The extent of similarity varies among groups of patients. In some cases, there are shared features in the portion of the antigen-binding site contributed by the H chain (VH, D, and JH genes).41,42,56 In these cases, each VH gene exhibits special patterns of mutations and preferential combinations with particular D or JH segments, which generate distinct features in the antigen-binding pocket.24,57 These VHDJH rearrangements and characteristics of antigen-binding pockets differ from the much broader diversity found in B cells from normal persons.24,58,59
In other groups of cases, the structural similarity of the receptors involves the entire antigen-binding site, coded by both the H and L chains (VH, D, and JH, and VL and JL genes). In these instances, the receptors from various patients are very similar or virtually identical.60,61,62,63,64,65 As many as 10 percent of all CLL cases fall into distinct categories branded by receptors with structurally similar antigen-binding pockets; most are of the unmutated, poor-outcome type.63 These findings are very striking since, given the number of possible combinations of V-gene segments that can encode antigen-binding domains, one would not expect to find 2 cases of CLL with such structurally similar B-cell receptors in more than 1 million cases.
These cases suggest that a limited set of antigens promotes division of the leukemic cells, increasing the likelihood of dangerous DNA mutations. What are these promoting antigens? They are unknown, but it is possible that latent viruses or commensal bacteria repetitively activate particular B-cell clones through the B-cell receptor. CLL would result, directly or indirectly, from specific infections and would be perpetuated by them — in a manner similar to the gastric lymphomas that evolve in response to Helicobacter pylori.66
Alternatively, environmental antigens or autoantigens could provoke clonal expansion. CLL cells frequently have polyreactive receptors, which bind multiple antigens, including autoantigens,67,68,69,70 allowing stimulation by both autoantigens and microbial antigens. This mechanism is plausible for unmutated CLL and also for a few cases of mutated CLL, since many unmutated71,72 and some mutated73 immunoglobulin V genes encode such polyreactive receptors; this immune-stimulation mechanism is in keeping with the view that the basis of CLL is autoimmunization.74
As constitutive low-level signaling is delivered through the B-cell receptor in normal B lymphocytes, perhaps to maintain the memory response and the B-cell repertoire,75,76 antigen may not be necessary to continue clonal expansion — antigen-independent triggering might occur through the B-cell–receptor signaling pathway because of another genetic lesion.
Signal Transduction after Antigen Engagement
For antigenic stimulation to underlie clonal expansion, the B-cell receptor must propagate an efficient signal to the cell nucleus (Figure 3). Leukemic cells from different CLL subgroups can differ in this capacity. Cross-linking B-cell receptor molecules with antibodies to IgM in vitro mimics the engagement of antigens with B-cell receptors and transmits signals to the cell nucleus in approximately 50 percent of cases of CLL.77,78,79,80,81 This phenomenon seems to occur mainly in unmutated CLL,27,32,82 but more patients need to be studied for this to be confirmed. CLL cells that do not respond to stimulation from the B-cell receptor may be frozen at a stage at which even normal B lymphocytes would be unresponsive to antigen.41 Alternatively, these cells could be anergic, possibly because of previous antigenic experience.42 Finally, these CLL cells may have become incapable of responding to antigens because of changes in the structure of their B-cell receptors caused by somatic mutations or an inability of the cells to come into contact with relevant antigens in vivo.41 Considering the number of cells that make up the leukemic clone in many patients (1011 to 1012), it is likely that only a fraction of the members of the clone could encounter the antigen, especially if they are restricted to discrete anatomical locations or compartments.
Other, not mutually exclusive, possibilities to explain the lack of B-cell–receptor signaling include reduced numbers of B-cell–receptor molecules,17 uncoupling of the B-cell receptor from accessory molecules necessary for effective signal transduction,83,84,85 and mutations in these accessory structures.86 It is interesting to note that responsiveness to stimuli delivered through surface IgD is frequently maintained in these cases.82,87
Consequences of Signal Transduction through the B-Cell Receptor
Once signal transduction is initiated by the B-cell receptor, B lymphocytes progress into the cell cycle or die. Cross-linking of surface IgM in CLL cells that can transduce a signal can cause81 or prevent88 apoptosis (Figure 3), whereas cross-linking surface IgD invariably prevents apoptosis.12,81 This difference is unexpected, because the two surface isotypes express the same clone-specific antigen-binding site and provide concordant signals in mature B cells. The final outcome of B-cell receptor signaling in an individual CLL cell, therefore, depends on the balance between signals mediated by the two molecules.
Signals from the Microenvironment
Signals that are delivered by direct cell contact or soluble factors, which may or may not occur concomitantly with B-cell–receptor engagement, probably propagate the growth of CLL cells (Figure 3). Interactions with stromal cells89 or nurse-like cells90 or interactions between CD38 and its natural ligand CD3191 rescue CLL cells from apoptosis in vitro and probably do the same in vivo. Activated T cells or other cells expressing CD40 ligand also support the growth of CLL cells.92 Finally, cytokines such as interleukin-4 and vascular endothelial growth factor93,94,95 and chemokines such as SDF-196 (particularly in the presence of stromal cells) support the expansion of CLL clones.
These signals tip the balance between antiapoptotic signals and proapoptotic signals in favor of cell survival. There is up-regulation of the antiapoptotic genes BCL2, survivin, and MCL1 in leukemic cells.92,97 Rescue from apoptosis and facilitation of cell growth may occur preferentially in lymph-node pseudofollicles and bone marrow clusters,89,98 evidenced by expression of the cycling cell marker Ki-67 by the leukemic cells in these sites.92 Because growth of the clone depends on a variety of interactions with the environment, variations in the requirements for these interactions on part of the leukemic cells may be responsible for changes in the clinical course.99,100
Appearance and Evolution of New Genetic Mutations
The emergence of new, aggressive clonal variants, which can worsen the disease, requires proliferation of the leukemic clone. In vivo studies using radioactive and nonradioactive means suggest that CLL cells are more dynamic than is usually appreciated.101,102,103 CLL cells have surprisingly brisk birth rates, ranging from about 0.1 to more than 1.0 percent of the clone per day.103 If the total clonal burden of a typical patient with CLL is approximately 1012 cells, these birth rates point to the daily production of some 109 to 1010 new leukemic cells.
These rates of cell division are sufficient to permit clonal variants to emerge. Indeed, there is an association between brisk birth rates of CLL cells and progressive disease.103 The rate of birth of leukemic cells, therefore, may be more relevant clinically than is either the blood lymphocyte count or the physical examination, since the lymphocyte count reflects the proliferative capacity of the leukemic cells and their potential to promote new DNA lesions, whereas the sizes of the lymph nodes and spleen on physical examination reflect a balance between cell proliferation and cell death. These findings may explain why telomeres, which cap and protect the ends of chromosomes but shorten with each cell division, are smaller in cells from patients in CLL subgroups that have poor outcomes.104,105
A Unifying Hypothesis for the Development, Growth, and Evolution of CLL
Growth and Evolution of CLL Cells
The above considerations suggest a plausible model on which to build future hypotheses and studies. Stimulatory and growth signals from the environment of CLL cells allow them to avoid apoptosis and proliferate. These signals are delivered by the B-cell receptor, receptors for cytokines or chemokines and other ligands, and direct contact with accessory and stromal cells. The major growth effects mediated by the B-cell receptor appear to occur in cases in which the receptor permits binding of autoantigens and maintains the capacity to transmit stimulatory signals to the cell nucleus (i.e., those with unmutated and, to a lesser degree, mutated CLL B-cell receptors).
This model excludes an intrinsic apoptotic defect in all members of the leukemic clone. Indeed, in vitro observations demonstrate the absence of lesions in the major apoptotic pathways.87,106 Whether continued cell division is facilitated by external signals or not, the level of B-cell turnover in vivo can suffice to promote the development and outgrowth of subclones with new genetic lesions and a growth advantage (Figure 4).
Figure 4. Model of the Development and Evolution of CLL Cells.
An initial inducing lesion occurs in a single B lymphocyte (pink cell in upper and lower portions of Panel A) among billions of distinct normal B cells. Interactions between antigens and B-cell receptors of adequate affinity induce clonal amplification (Panel B). The initial inducing lesion provides the marked cell with a growth advantage over other clones stimulated by the same or other antigens. In clones destined to become unmutated CLL cells, repetitive interactions between antigens and polyreactive B-cell receptors of the initially selected clone promote clonal growth, which persists because V-gene mutations do not occur (Panel C, top). In others, destined to become mutated CLL cells (Panel C, bottom), V-gene mutations develop that can abrogate the polyreactivity of the B-cell receptors and thereby alter their ability to bind the original antigen or autoantigen ("clonal ignorance"). Alternatively, these mutated cells become anergic owing to excessive B-cell–receptor stimulation because of the acquisition of more avid receptors. In both instances, the promoting effect of antigenic stimulation is neutralized. Additional DNA mutations cause the cells to cross the boundary from "normality" to "leukemia" (Panel D). Differences in the signals received through the B-cell receptors and other receptors determine the extent of clonal expansion. B cells that can no longer bind antigen may not cross this boundary. Continued cycling leads to other genetic changes (e.g., deletions at 13q, 11q, and 17p or duplication of chromosome 12) that determine the course of the disease (Panel E). These changes appear to occur more frequently in patients with unmutated CLL.
Development of CLL from Normal B Lymphocytes
Many normal B lymphocytes with unmutated V genes produce antibodies capable of binding multiple antigens (e.g., carbohydrates, nucleic acids, and phospholipids)107 and of providing the first line of defense against microorganisms.108 If one of these cells contained or developed a genetic abnormality that allows it to resist restraint on clonal size (e.g., an initial inducing lesion), then this cell would be primed for leukemic transformation (Figure 4A). Foreign antigens and autoantigens then could be important stimuli for the development of CLL.109 B cells with such unmutated polyreactive B-cell receptors could expand and convert to CLL cells with repetitive exposure to microbes and to autoantigens (Figure 4B).110,111,112,113 A similar mechanism may underlie the origin of mutated CLL, because V-gene mutations can occur without T-cell help5,7,8 outside of germinal centers,9 and these mutations can sometimes favor autoreactivity.73 Such expansion would stop if V-gene mutations altered the structure of the B-cell receptor in a way that caused loss of binding to the stimulatory antigen (i.e., the development of "clonal ignorance") (Figure 4C).
This hypothesis implies that such expansions should be detectable in healthy patients. Recent studies suggest that small numbers of clonal B cells with the characteristics of CLL cells exist in the blood of approximately 3.5 percent of disease-free persons114,115 and in an even higher proportion of first-degree relatives of patients with CLL.116 Although studies of the B-cell receptors of these B-lymphocyte expansions are limited, initial information suggests that they are monoclonal and use some of the genes that encode the B-cell receptors of CLL clones.
From Which Subpopulation Do CLL Cells Develop?
Since CLL cells resemble activated B lymphocytes,117 their cellular origin cannot be deduced solely from phenotypic analyses, a fact that makes it difficult to draw a direct parallel with B1 cells described in mice.118 However, certain functional features may help delineate their origin. Normal adult B cells that produce autoantibodies and antibodies against bacterial or viral carbohydrates reside in the marginal zone. It is possible that marginal-zone B cells are the precursors of both unmutated and mutated CLL cells, because B-cell receptors of CLL cells are structurally similar to those of antibodies that react with autoantigens and carbohydrate components of infectious agents41,62,63 (Figure 2B). Alternatively, mutated CLL cells could originate from B cells stimulated in a T-cell–dependent manner that have passed through a germinal center.
Other potential precursors are B1 cells, which share several features with marginal-zone B cells,118 as well as immature pre–B cells119 and transitional B cells120 that can also express self-reactive receptors (Figure 1). A few pre–B cells emerge from the bone marrow into the periphery, and transitional B cells routinely exit the marrow and traverse the circulation to solid lymphoid tissues. A genetic abnormality could allow one of these cells to survive, thereby making it available for autoantigenic drive and leukemic transformation into unmutated CLL cells.
Clinical Implications
Prognosis
The primary role of the Rai and Binet staging systems is to help clinicians decide when patients should be started on therapy. However, since these approaches do not predict the clinical course of a patient with precision, they are less helpful as long-term prognostic indicators. Therefore, physicians have postponed therapeutic decisions until the patients reach advanced Rai or Binet stages.
However, the molecular and cellular features we have discussed can distinguish patients with better or worse clinical courses, regardless of the Rai and Binet risk categories. Determination of V-gene mutation is not routinely available, but measurement of ZAP-70 is becoming widely available; it may be the most reliable indicator of prognosis.35
Several points still require clarification and refinement. For example, can a single marker provide a sufficiently accurate and reliable prognostic assessment to permit early decisions about clinical management? Or should several markers be used to increase the degree of accuracy of prognosis? What are the clinically most useful cutoff points for the percentages of expression of CD3826,44 and ZAP-7031,32,33,34 and the levels of immunoglobulin V-gene mutation26 that most reliably define the clinical subgroups with various outcomes? Should the cases at the borderlines of these arbitrary cutoffs be handled differently?
Management
In the past, physicians told patients with CLL that a "watchful waiting" mode had to be adopted until the disease progressed, whereupon therapy would be initiated. In my opinion, this approach is especially disturbing, given that the novel prognostic markers indicate that some 50 percent of the patients assigned to watchful waiting have one or more features portending a poor outcome. Although this approach is still being followed, it will probably change considerably when the new prognostic markers are more readily available to all clinicians. On the basis of such information, an early start of therapy may be justified in groups with a poor prognosis. However, before any guidelines can be proposed, the results of large, prospectively conducted clinical trials that test the use of early intervention in patients in poor-prognosis groups must become available. Only one such trial has been initiated, and the accrual of required numbers of patients and the analysis of those results will take several years.
New Therapeutic Approaches
Since CLL cells must interact with the stroma in bone marrow or other peripheral lymphoid tissues to survive, these interactions need to be explored as targets of innovative therapies. Furthermore, specific inhibition of the B-cell receptor signaling pathway, in particular ZAP-70 or its signaling partners, may be an option. Targeting the actively proliferating cells that maintain the CLL clone by a cell-cycle–active agent could also be considered. Finally, since as many as 20 percent of patients with the worst prognostic markers have stereotypic antigen receptors, these common structures may be practical and valuable points of attack. When the antigens that engage these receptors are precisely defined, it may become possible to develop another arsenal of specific therapies.
Supported in part by grants from the National Cancer Institute (RO1 CA 81554 and CA 87956) and a General Clinical Research Center Grant (M01 RR018535) from the National Center for Research Resources, the Associazione Italiana per la Ricerca sul Cancro, and the Ministero dell'Istruzione dell'Università e della Ricerca; the Peter J. Sharp Foundation, the Marks Family Foundation, the Jean Walton Fund for Lymphoma and Myeloma Research, the Joseph Eletto Leukemia Research Fund, the Tebil Foundation, the Horace W. Goldsmith Foundation, and the Chemotherapy Foundation. Drs. Rai and Chiorazzi are members of the NCI-sponsored Chronic Lymphocytic Leukemia Research Consortium (PO1 CA 081534).
Dr. Chiorazzi holds a patent on the use of CD38 as a prognostic indicator in chronic lymphocytic leukemia. Dr. Rai reports having received lecture fees from Genentech and Berlex Laboratories and grant support from Berlex. Dr. Ferrarini reports having received lecture fees from Schering AG.
We are indebted to the present and past members of the Laboratory of Experimental Immunology, Institute for Medical Research, North Shore–LIJ Health System, of the North Shore University Hospital, and of the Division of Medical Oncology C, Istituto Nazionale per la Ricerca sul Cancro, for the work that we have discussed in this article.
Source Information
From the Institute for Medical Research, North Shore–LIJ Health System (N.C., K.R.R.), and the Departments of Medicine, North Shore University Hospital, Manhasset, N.Y., and New York University School of Medicine, New York (N.C.); the Departments of Medicine, Long Island Jewish Medical Center, New Hyde Park, N.Y., and Albert Einstein School of Medicine, Bronx, N.Y. (K.R.R.); and the Division of Medical Oncology C, Istituto Nazionale per la Ricerca sul Cancro, and the Dipartimento di Oncologia Clinica e Sperimentale, Universitá di Genova — both in Genoa, Italy (M.F.).
Address reprint requests to Dr. Chiorazzi at the Institute for Medical Research, North Shore–LIJ Health System, 350 Community Dr., Manhasset, NY 11030, or at nchizzi@nshs.edu.
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