Cellular Immunotherapy with Dendritic Cells in Cancer: Current Status
http://www.100md.com
《干细胞学杂志》
a Massachusetts Institute of Technology, Center for Cancer Research, Cambridge, Massachusetts, USA;
b Department of Internal Medicine, University of Genova, Genova, Italy;
c Department of Hematology, Oncology and Immunology, University of Tübingen, Tübingen, Germany
Key Words. Dendritic cells ? Tumor antigens ? Vaccinations
Correspondence: Peter Brossart, M.D., Department of Hematology, Oncology and Immunology, University of Tübingen, Otfried-Müller Str. 10, D-72076 Tübingen, Germany. Telephone: 49-7071-298-2726; Fax: 49-7071-295709; e-mail: peter.brossart@med.uni-tuebingen.de
ABSTRACT
The first demonstration that tumor rejection antigens exist goes back to the late 1980s when tumor-infiltrating lymphocytes from melanoma patients were shown to lyse HLA-matched melanoma cell lines, suggesting the existence of shared melanoma antigens . In the subsequent years, the first genes encoding tumor antigens (such as tyrosinase, gp-100, the MART and MAGE genes) were cloned, and the immunogenic epitopes were identified. These and subsequent studies pointed out that tumors often upregulate the expression of molecules that are normally suppressed or expressed at much lower levels in adult tissues. T lymphocytes capable of recognizing these antigens usually exist in the periphery, possibly due to the lack of presentation of these antigens during thymic selection or lower avidity of the T-cell receptor (TCR) . However, in most cases, the immune system fails to recognize and destroy tumor cells that may give rise to clinically relevant malignancies. The tumor escape mechanisms include the inefficiency of tumor cells as antigen-presenting cells (APCs) and the lack of efficient contact between immune system and tumor cells .
There is also evidence that antitumor immune responses can extinguish established tumors, especially in patients affected by melanoma or renal cell carcinoma. Infiltration of the primary tumor with lymphocytes has been associated with a better prognosis in different types of malignancies . Similarly, immune-mediated paraneoplastic syndromes characterized by an immune response directed against antigens shared by tumor and normal tissues (such as the central nervous system) have been associated with a better clinical outcome and even with spontaneous tumor regressions . However, such spontaneous immune responses are rare and still remain largely elusive. Thus, the goal of modern tumor immunotherapy is to trigger the immune system in order to mimic such rejection events and improve clinical outcome.
Particularly, the induction CD8+ cytotoxic T lymphocytes (CTLs) directed against tumor epitopes in vivo is the desirable effect of a specific immunotherapy approach, given that these immune effectors are mainly responsible for tumor rejection . These lymphocytes recognize via the TCR 8–11 amino acids–long peptide epitopes in the context of the HLA class I molecules. Upon encounter with cells that express the target antigen, the CTLs activate their lytic machinery and kill the cells. The induction of CD4+ helper T cells also plays a major role in antitumor immunity, and immunization strategies should probably take into account providing immunogenic epitopes for these lymphocytes.
The first immunization strategies for cancer patients often involved the administration of tumor lysates or irradiated tumor cells together with immunological adjuvants such as bacillus Calmette-Guérin (BCG) . This vaccination method has recently been reported by Vermorken et al. to be possibly associated with a protection from relapses in patients with stage-II colorectal cancer, but this study needs further confirmation by other groups. Such an approach is limited by the requirement of sufficient amount of tumor material and by potential concerns related to the administration of autologous tumor cells, though irradiated, to patients in clinical remission of disease.
Recent advances in the knowledge of the immune system have opened new perspectives for the development of antitumor immunization strategies. In particular, the administration of immunogenic APCs such as dendritic cells (DCs) loaded with tumor antigens is now considered one of the most promising approaches to the specific cancer immunotherapy and is being evaluated in many cancer centers for different malignancies and in different clinical settings.
DCs are leukocytes that are highly specialized in the capture and presentation of antigens to T cells . They are presently believed to control the induction (and, possibly, the suppression) of antigen-specific immune responses in vivo . DCs for clinical use can be generated in sufficient numbers from circulating precursors, including peripheral blood CD14+ monocytes and CD34+ stem cells . Injection of DCs loaded with tumor-associated antigens (TAAs) into patients was shown to break tolerance and to induce antitumor cytotoxic immune responses in vivo . The DC-based clinical trials performed so far have demonstrated that this form of immunotherapy is feasible and safe . Moreover, some studies reported cases of tumor regression or growth arrest following DC administration.
DEFINITION OF DCS AND METHODS FOR DC GENERATION
Anticancer vaccinations attempt to elicit tumor-directed CD8+ CTLs that lyse tumor cells presenting MHC class I–associated peptides derived from tumor-associated proteins. Several different strategies are currently available to deliver antigens into DCs during the ex vivo manipulation for further presentation to T cells in the recipient. DCs can be pulsed with synthetic peptide epitopes derived from known TAAs such as MUC1, Her-2/neu, survivin, tyrosinase, telomerase, CEA, p53, MAGE, or Melan-A/MART . Although most of these peptides are designed to bind HLA-A2, the most common HLA class I molecule among Caucasians, several peptide epitopes have been identified that bind to other HLA class I alleles. Moreover, HLA class II binding peptides have also been reported that either unspecifically trigger CD4+ lymphocyte activation or are derived from tumor antigens and induce antigen-specific CD4+ helper T cells . Many of these peptides are now commercially available and ready to use under GMP conditions. Hence, peptide-based vaccinations are potentially applicable to most patients. In some immunization studies, peptides are injected directly into the patients without previous incubation with DCs ex vivo. For this kind of approach, the peptides are usually coinjected with immune cytokines such as GM-CSF, which favors DC migration and activation in situ, or with incomplete Freund adjuvant . Major drawbacks related to the use of peptides are (a) the restriction to some HLA class I alleles, (b) the need to determine the expression of the target antigen by a tumor, and (c) the likelihood that targeting single or few tumor epitopes may impede the detection of tumor cells that downregulate those antigens. However, to some extent, tumor escape may be prevented by the expansion of lymphocytes directed against epitopes other than those used for immunization, a phenomenon named "epitope spreading," which has already been observed in some clinical studies .
Another approach is to use recombinant proteins as antigens. These are captured by DCs, then processed and presented in the form of immunogenic peptides in the context of HLA molecules. This approach bypasses the HLA restriction of the peptides and was successfully applied to the treatment of patients with follicular lymphoma . In this case, DCs were pulsed ex vivo with tumor-specific idiotype protein to induce antitumor immunity, with encouraging clinical results. The effectiveness of the same approach for the treatment of myeloma is still under investigation . Similarly, a recombinant prostatic acid phosphatase (PAP) has been used to load autologous DCs by Fong et al. . An alternative strategy is gene-based delivery of TAAs into DCs. DCs can be transduced with recombinant viruses (retroviral or adenoviral vectors, vaccinia virus) or transfected with RNA encoding for a specific tumor antigen .
Several other approaches also exist that, instead of using single or few antigens, make use of whole tumor material as an antigenic source. These approaches use tumor lysates, dead tumor cells (apoptotic bodies, necrotic cells), DCs fused with tumor cells, or total tumor RNA . All of these methods were shown to induce immunity against the parental tumor and are being evaluated in the clinical setting. Importantly, whole tumor–derived materials represent the entire antigenic repertoire of a tumor; thus the resulting immune response simultaneously targets many tumor antigens. In fact, it is likely that preferential expansion of CTLs directed against immunodominant epitopes will happen in some cases; this may be related to the higher frequency of some epitope-specific effectors or to the strong immunogenicity of some tumor-derived peptides (or both) . One potential advantage of the use of RNA compared with the other tumor-derived materials cited above is that methods exist for the unspecific amplification of messenger RNA . This translates into the applicability of this method also in those cases when small tumor specimens such as needle biopsies would not be sufficient to obtain lysates or apoptotic tumor cells for DC pulsing. Moreover, the antigens encoded by the transfected RNA may be processed and presented on both HLA class I and class II molecules, thus inducing CD8+ as well as CD4+ antitumor lymphocytes . The elicited immune response, at least according to the in vitro experiments, seems to be restricted to immunodominant tumor epitopes while saving nonmalignant autologous cells . Thus, RNA transfection of DCs appears as a very attractive approach for the induction of antitumor CTLs in a variety of malignancies.
In vivo DC loading has also been evaluated in preclinical models. In particular, immunization with DNA vaccines by gene gun represents an attractive approach; here gold particles coated with expression plasmid DNA encoding target genes are "bombarded" into the skin . This procedure transfects plasmid DNA directly into the DCs present in the skin. Transfected DCs express the encoded antigen and present the processed peptides to the antigen-specific T cells to initiate an immune response in the afferent lymph nodes. In light of the results reported by Sudowe et al. and Garg et al. in the animal model, this approach may reveal as an effective method for antitumor immunity induction.
ROUTES OF DC DELIVERY
The detection of the immune response to tumor antigens following vaccination represents one of the major endpoints of the clinical vaccination studies. The DTH assay represents a possible approach to this goal. It is usually performed by intradermal injection of tumor-derived material or DCs loaded with tumor antigen(s) before and after the vaccination course . In the case of tumor regression after vaccine administration, the detection of either infiltrating lymphocytes or inflammatory cells (or both) in tumor specimens, whenever these are easily reachable, should be performed in order to correlate the clinical outcome with the elicited immune response . In some cases, the tumor-infiltrating lymphocytes can be isolated and further characterized . However, in most studies the lymphocytes reacting to the tumor antigens have been detected in the peripheral blood mononuclear cells. The T cells specific for a defined tumor-derived epitope can be tracked via different approaches, which typically include ELISPOT, cytokine secretion assay from Miltenyi, intracellular staining for IFN-, tetramers, proliferation assays, ELISAs, cytotoxicity assays, and real-time polymerase chain reaction for IFN- . The results obtained with different methods are often, though not necessarily, consistent, and further refining of these techniques is still required .
When whole tumor–derived material (tumor lysates, total tumor RNA, fusions, tumor-derived peptides) is used as an antigen source for vaccine preparation, the autologous tumor cells or tumor material, when available, can be used to determine immunoreactivity before and after vaccine administration . The same DCs loaded with tumor antigens may work as a suitable target in immunological monitoring . In this kind of approach, the antigens involved in the immune response are often not known. However, in selected patients who express defined HLA alleles and tumor antigens, the immunization against known tumor epitopes could also be evaluated .
Some studies found no correlation between the immune response to the antigen used for immunization and the clinical outcome, since some tumor regressions were observed in patients who showed little response to vaccination . Besides, different studies have already reported the expansion of lymphocytes specific for different tumor epitopes following vaccination , and in the study by Butterfield et al. the only complete clinical remission was induced in a patient showing epitope spreading. These data indicate that immunity versus an array of different tumor antigens, including molecules not present in the vaccine preparation, should possibly be monitored.
An improved characterization (phenotypic and functional) of the antitumor T lymphocytes will also be necessary for a better understanding of the lymphocyte subsets involved in tumor rejections. This goal can be pursued by combining tetramer staining with antibodies for surface markers such as CD45RA, CD45RO, CD27, CCR7, CD28, and CD25 or with intracellular staining for cytokines such as IFN-, IL-4, IL-10 . The antigen-specific lymphocytes can be isolated by fluorescence-activated cell sorter (FACS) or magnetic cell sorting (MACS) technology, expanded, and further characterized.
Finally, pulsing DCs with immunogenic epitopes (such as influenza peptides or CD4 epitopes) or antigens (such as keyhole limpet hemocyanin or HBsAg) has been performed with the double intent to exploit them as an immune adjuvant and to use them as an immunological tracer to evaluate DC priming efficacy in vivo and responsiveness of the immune system to vaccine administration . The use of these immunogens may be particularly useful in order to compare different vaccine administration routes and schedules or DCs generated according to different protocols.
CLINICAL STUDIES WITH DCS
The phase I and II clinical studies with DCs are hardly comparable, given that different methods for DC culture, antigen loading, and administration have been used. Altogether, the data reported so far indicate that these ex vivo–generated APCs are immunogenic in vivo and that DC injection was associated with a clinical response in some patients. Phase III studies are necessary to evaluate the potential clinical advantages of DC vaccination and are already ongoing for some diseases, such as melanoma and prostate cancer . It is a general conviction that, if any, the clinical benefits associated with this immunotherapeutic approach are more likely to be recorded among patients in remission of disease or with small tumor burden. Meanwhile, it seems probable that the efficacy of DC vaccinations will be improved by the novel methods of antigen loading and by the concomitant administration of cytokines or immunogenic factors such as IL-2, IL-12, or CpG dinucleotides, which should amplify the immune response in vivo.
Particularly appealing is the application of DCs to allogeneic bone marrow and PBPC transplantations. In this context, the recently developed protocols for reduced-intensity conditioning (the so-called mini-allo) have increased the safety of this kind of treatment and extended its applicability in leukemia (also in older patients), Hodgkin and non-Hodgkin lymphoma, myeloma, and nonhematological malignancies such as renal cell carcinoma and breast cancer . In this context, DCs could be used for the expansion and adoptive transfer of lymphocytes against TAAs or minor histocompatibility antigens . This may help to selectively target the graft-versus-tumor reaction, while possibly minimizing the graft-versus-host effect.
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b Department of Internal Medicine, University of Genova, Genova, Italy;
c Department of Hematology, Oncology and Immunology, University of Tübingen, Tübingen, Germany
Key Words. Dendritic cells ? Tumor antigens ? Vaccinations
Correspondence: Peter Brossart, M.D., Department of Hematology, Oncology and Immunology, University of Tübingen, Otfried-Müller Str. 10, D-72076 Tübingen, Germany. Telephone: 49-7071-298-2726; Fax: 49-7071-295709; e-mail: peter.brossart@med.uni-tuebingen.de
ABSTRACT
The first demonstration that tumor rejection antigens exist goes back to the late 1980s when tumor-infiltrating lymphocytes from melanoma patients were shown to lyse HLA-matched melanoma cell lines, suggesting the existence of shared melanoma antigens . In the subsequent years, the first genes encoding tumor antigens (such as tyrosinase, gp-100, the MART and MAGE genes) were cloned, and the immunogenic epitopes were identified. These and subsequent studies pointed out that tumors often upregulate the expression of molecules that are normally suppressed or expressed at much lower levels in adult tissues. T lymphocytes capable of recognizing these antigens usually exist in the periphery, possibly due to the lack of presentation of these antigens during thymic selection or lower avidity of the T-cell receptor (TCR) . However, in most cases, the immune system fails to recognize and destroy tumor cells that may give rise to clinically relevant malignancies. The tumor escape mechanisms include the inefficiency of tumor cells as antigen-presenting cells (APCs) and the lack of efficient contact between immune system and tumor cells .
There is also evidence that antitumor immune responses can extinguish established tumors, especially in patients affected by melanoma or renal cell carcinoma. Infiltration of the primary tumor with lymphocytes has been associated with a better prognosis in different types of malignancies . Similarly, immune-mediated paraneoplastic syndromes characterized by an immune response directed against antigens shared by tumor and normal tissues (such as the central nervous system) have been associated with a better clinical outcome and even with spontaneous tumor regressions . However, such spontaneous immune responses are rare and still remain largely elusive. Thus, the goal of modern tumor immunotherapy is to trigger the immune system in order to mimic such rejection events and improve clinical outcome.
Particularly, the induction CD8+ cytotoxic T lymphocytes (CTLs) directed against tumor epitopes in vivo is the desirable effect of a specific immunotherapy approach, given that these immune effectors are mainly responsible for tumor rejection . These lymphocytes recognize via the TCR 8–11 amino acids–long peptide epitopes in the context of the HLA class I molecules. Upon encounter with cells that express the target antigen, the CTLs activate their lytic machinery and kill the cells. The induction of CD4+ helper T cells also plays a major role in antitumor immunity, and immunization strategies should probably take into account providing immunogenic epitopes for these lymphocytes.
The first immunization strategies for cancer patients often involved the administration of tumor lysates or irradiated tumor cells together with immunological adjuvants such as bacillus Calmette-Guérin (BCG) . This vaccination method has recently been reported by Vermorken et al. to be possibly associated with a protection from relapses in patients with stage-II colorectal cancer, but this study needs further confirmation by other groups. Such an approach is limited by the requirement of sufficient amount of tumor material and by potential concerns related to the administration of autologous tumor cells, though irradiated, to patients in clinical remission of disease.
Recent advances in the knowledge of the immune system have opened new perspectives for the development of antitumor immunization strategies. In particular, the administration of immunogenic APCs such as dendritic cells (DCs) loaded with tumor antigens is now considered one of the most promising approaches to the specific cancer immunotherapy and is being evaluated in many cancer centers for different malignancies and in different clinical settings.
DCs are leukocytes that are highly specialized in the capture and presentation of antigens to T cells . They are presently believed to control the induction (and, possibly, the suppression) of antigen-specific immune responses in vivo . DCs for clinical use can be generated in sufficient numbers from circulating precursors, including peripheral blood CD14+ monocytes and CD34+ stem cells . Injection of DCs loaded with tumor-associated antigens (TAAs) into patients was shown to break tolerance and to induce antitumor cytotoxic immune responses in vivo . The DC-based clinical trials performed so far have demonstrated that this form of immunotherapy is feasible and safe . Moreover, some studies reported cases of tumor regression or growth arrest following DC administration.
DEFINITION OF DCS AND METHODS FOR DC GENERATION
Anticancer vaccinations attempt to elicit tumor-directed CD8+ CTLs that lyse tumor cells presenting MHC class I–associated peptides derived from tumor-associated proteins. Several different strategies are currently available to deliver antigens into DCs during the ex vivo manipulation for further presentation to T cells in the recipient. DCs can be pulsed with synthetic peptide epitopes derived from known TAAs such as MUC1, Her-2/neu, survivin, tyrosinase, telomerase, CEA, p53, MAGE, or Melan-A/MART . Although most of these peptides are designed to bind HLA-A2, the most common HLA class I molecule among Caucasians, several peptide epitopes have been identified that bind to other HLA class I alleles. Moreover, HLA class II binding peptides have also been reported that either unspecifically trigger CD4+ lymphocyte activation or are derived from tumor antigens and induce antigen-specific CD4+ helper T cells . Many of these peptides are now commercially available and ready to use under GMP conditions. Hence, peptide-based vaccinations are potentially applicable to most patients. In some immunization studies, peptides are injected directly into the patients without previous incubation with DCs ex vivo. For this kind of approach, the peptides are usually coinjected with immune cytokines such as GM-CSF, which favors DC migration and activation in situ, or with incomplete Freund adjuvant . Major drawbacks related to the use of peptides are (a) the restriction to some HLA class I alleles, (b) the need to determine the expression of the target antigen by a tumor, and (c) the likelihood that targeting single or few tumor epitopes may impede the detection of tumor cells that downregulate those antigens. However, to some extent, tumor escape may be prevented by the expansion of lymphocytes directed against epitopes other than those used for immunization, a phenomenon named "epitope spreading," which has already been observed in some clinical studies .
Another approach is to use recombinant proteins as antigens. These are captured by DCs, then processed and presented in the form of immunogenic peptides in the context of HLA molecules. This approach bypasses the HLA restriction of the peptides and was successfully applied to the treatment of patients with follicular lymphoma . In this case, DCs were pulsed ex vivo with tumor-specific idiotype protein to induce antitumor immunity, with encouraging clinical results. The effectiveness of the same approach for the treatment of myeloma is still under investigation . Similarly, a recombinant prostatic acid phosphatase (PAP) has been used to load autologous DCs by Fong et al. . An alternative strategy is gene-based delivery of TAAs into DCs. DCs can be transduced with recombinant viruses (retroviral or adenoviral vectors, vaccinia virus) or transfected with RNA encoding for a specific tumor antigen .
Several other approaches also exist that, instead of using single or few antigens, make use of whole tumor material as an antigenic source. These approaches use tumor lysates, dead tumor cells (apoptotic bodies, necrotic cells), DCs fused with tumor cells, or total tumor RNA . All of these methods were shown to induce immunity against the parental tumor and are being evaluated in the clinical setting. Importantly, whole tumor–derived materials represent the entire antigenic repertoire of a tumor; thus the resulting immune response simultaneously targets many tumor antigens. In fact, it is likely that preferential expansion of CTLs directed against immunodominant epitopes will happen in some cases; this may be related to the higher frequency of some epitope-specific effectors or to the strong immunogenicity of some tumor-derived peptides (or both) . One potential advantage of the use of RNA compared with the other tumor-derived materials cited above is that methods exist for the unspecific amplification of messenger RNA . This translates into the applicability of this method also in those cases when small tumor specimens such as needle biopsies would not be sufficient to obtain lysates or apoptotic tumor cells for DC pulsing. Moreover, the antigens encoded by the transfected RNA may be processed and presented on both HLA class I and class II molecules, thus inducing CD8+ as well as CD4+ antitumor lymphocytes . The elicited immune response, at least according to the in vitro experiments, seems to be restricted to immunodominant tumor epitopes while saving nonmalignant autologous cells . Thus, RNA transfection of DCs appears as a very attractive approach for the induction of antitumor CTLs in a variety of malignancies.
In vivo DC loading has also been evaluated in preclinical models. In particular, immunization with DNA vaccines by gene gun represents an attractive approach; here gold particles coated with expression plasmid DNA encoding target genes are "bombarded" into the skin . This procedure transfects plasmid DNA directly into the DCs present in the skin. Transfected DCs express the encoded antigen and present the processed peptides to the antigen-specific T cells to initiate an immune response in the afferent lymph nodes. In light of the results reported by Sudowe et al. and Garg et al. in the animal model, this approach may reveal as an effective method for antitumor immunity induction.
ROUTES OF DC DELIVERY
The detection of the immune response to tumor antigens following vaccination represents one of the major endpoints of the clinical vaccination studies. The DTH assay represents a possible approach to this goal. It is usually performed by intradermal injection of tumor-derived material or DCs loaded with tumor antigen(s) before and after the vaccination course . In the case of tumor regression after vaccine administration, the detection of either infiltrating lymphocytes or inflammatory cells (or both) in tumor specimens, whenever these are easily reachable, should be performed in order to correlate the clinical outcome with the elicited immune response . In some cases, the tumor-infiltrating lymphocytes can be isolated and further characterized . However, in most studies the lymphocytes reacting to the tumor antigens have been detected in the peripheral blood mononuclear cells. The T cells specific for a defined tumor-derived epitope can be tracked via different approaches, which typically include ELISPOT, cytokine secretion assay from Miltenyi, intracellular staining for IFN-, tetramers, proliferation assays, ELISAs, cytotoxicity assays, and real-time polymerase chain reaction for IFN- . The results obtained with different methods are often, though not necessarily, consistent, and further refining of these techniques is still required .
When whole tumor–derived material (tumor lysates, total tumor RNA, fusions, tumor-derived peptides) is used as an antigen source for vaccine preparation, the autologous tumor cells or tumor material, when available, can be used to determine immunoreactivity before and after vaccine administration . The same DCs loaded with tumor antigens may work as a suitable target in immunological monitoring . In this kind of approach, the antigens involved in the immune response are often not known. However, in selected patients who express defined HLA alleles and tumor antigens, the immunization against known tumor epitopes could also be evaluated .
Some studies found no correlation between the immune response to the antigen used for immunization and the clinical outcome, since some tumor regressions were observed in patients who showed little response to vaccination . Besides, different studies have already reported the expansion of lymphocytes specific for different tumor epitopes following vaccination , and in the study by Butterfield et al. the only complete clinical remission was induced in a patient showing epitope spreading. These data indicate that immunity versus an array of different tumor antigens, including molecules not present in the vaccine preparation, should possibly be monitored.
An improved characterization (phenotypic and functional) of the antitumor T lymphocytes will also be necessary for a better understanding of the lymphocyte subsets involved in tumor rejections. This goal can be pursued by combining tetramer staining with antibodies for surface markers such as CD45RA, CD45RO, CD27, CCR7, CD28, and CD25 or with intracellular staining for cytokines such as IFN-, IL-4, IL-10 . The antigen-specific lymphocytes can be isolated by fluorescence-activated cell sorter (FACS) or magnetic cell sorting (MACS) technology, expanded, and further characterized.
Finally, pulsing DCs with immunogenic epitopes (such as influenza peptides or CD4 epitopes) or antigens (such as keyhole limpet hemocyanin or HBsAg) has been performed with the double intent to exploit them as an immune adjuvant and to use them as an immunological tracer to evaluate DC priming efficacy in vivo and responsiveness of the immune system to vaccine administration . The use of these immunogens may be particularly useful in order to compare different vaccine administration routes and schedules or DCs generated according to different protocols.
CLINICAL STUDIES WITH DCS
The phase I and II clinical studies with DCs are hardly comparable, given that different methods for DC culture, antigen loading, and administration have been used. Altogether, the data reported so far indicate that these ex vivo–generated APCs are immunogenic in vivo and that DC injection was associated with a clinical response in some patients. Phase III studies are necessary to evaluate the potential clinical advantages of DC vaccination and are already ongoing for some diseases, such as melanoma and prostate cancer . It is a general conviction that, if any, the clinical benefits associated with this immunotherapeutic approach are more likely to be recorded among patients in remission of disease or with small tumor burden. Meanwhile, it seems probable that the efficacy of DC vaccinations will be improved by the novel methods of antigen loading and by the concomitant administration of cytokines or immunogenic factors such as IL-2, IL-12, or CpG dinucleotides, which should amplify the immune response in vivo.
Particularly appealing is the application of DCs to allogeneic bone marrow and PBPC transplantations. In this context, the recently developed protocols for reduced-intensity conditioning (the so-called mini-allo) have increased the safety of this kind of treatment and extended its applicability in leukemia (also in older patients), Hodgkin and non-Hodgkin lymphoma, myeloma, and nonhematological malignancies such as renal cell carcinoma and breast cancer . In this context, DCs could be used for the expansion and adoptive transfer of lymphocytes against TAAs or minor histocompatibility antigens . This may help to selectively target the graft-versus-tumor reaction, while possibly minimizing the graft-versus-host effect.
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