Nonmyeloablative hematopoietic cell transplantation can cure patients with hematologic malignancies but has reported limited success against solid tumors. This is possibly because of profound peripheral tolerance mechanisms and/or suboptimal tumor recognition by effector T lymphocytes. We report that in mice developing spontaneous prostate cancer, nonmyeloablative minor histocompatibility mismatched hematopoietic stem cell transplantation, and donor lymphocyte infusion of unmanipulated lymphocytes combined with posttransplant tumor-specific vaccination circumvents tumor-specific tolerance, allowing acute tumor rejection and the establishment of protective immunosurveillance. Although donor-derived tumor-specific T cells readily differentiated into effector cells and infiltrated the tumor soon after infusion, they were alone insufficient for tumor eradication, which instead required the concomitance of minor histocompatibiltiy antigen–specific CD8+ T-cell responses. The establishment of protective immunosurveillance was best induced by posttransplant tumor-specific vaccination. Hence, these results provide the proof of principle that tumor-specific T-cell responses have to be harnessed together with minor histocompatibility responses and sustained by posttransplant tumor-specific vaccination to improve the efficacy of allotransplantion for the cure of solid tumors. Cancer Res; 70(9); 3505–14. ©2010 AACR.
Nonmyeloablative allogeneic hematopoietic stem cell transplantation (HSCT) with or without donor lymphocyte infusion (DLI) may restore immune competence in patients with hematologic malignancies (1), whereas in patients with solid tumors, this treatment has met with limited success (2). This is possibly due to the existence in the latter of more profound mechanisms of peripheral tolerance hindering tumor-specific protective immune responses (3) and/or diverse susceptibility of different malignant tumors to HSCT (4).
The therapeutic benefit of allogeneic HSCT is largely related to graft-versus-tumor (GVT) effects mediated by donor-derived immunocompetent T cells present within the graft or infused to patients following HSCT, i.e., DLI (5). In recipients of genetically different but MHC-matched transplants, donor-derived T cells recognize tumor-associated antigens (TAA) favoring tumor eradication (6) and also host-specific histocompatibility (H) antigens (7). These are the MHC-restricted peptide products of polymorphic proteins encoded by H loci, usually biallelic, outside the MHC, the so-called minor H antigens (7), which typically differ between donor and recipient. As recognition of minor H antigens may favor tumor eradication, but also cause life-threatening graft versus host disease (GVHD; ref. 8), the relative contribution of minor H– and TAA-specific T lymphocytes to tumor aggression is currently needed to define whether they should be further exploited in patients with solid cancer. Furthermore, the possibility to implement graft-versus-tumor without favoring GVHD, by posttransplant tumor-specific vaccination (9, 10), remains largely to be explored.
To address these issues, we have analyzed, at the single cell level, tumor- and minor H antigen–specific immunity following nonmyeloablative allogeneic HSCT in the transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse model (11), one of the few in which the fate of TAA-specific T cells can be followed during autochthonous tumor growth. TRAMP mice express the SV40 early genes (small and large T antigens; Tag) under the control of the androgen-driven rat probasin regulatory element (11). Consequently, at puberty, male mice invariably develop spontaneous prostate intraepithelial neoplasia (week 6–12), adenocarcinoma (week 12–18), with lymph node and visceral metastasis (week 18–30), closely mimicking the human pathology (12). Because of thymic Tag expression, high-affinity Tag-specific thymocytes in TRAMP mice are deleted (13). After puberty, overexpression of Tag within prostate epithelial cells, quantitatively similar to other prostate-associated antigens (e.g., refs. 14, 15), also causes the loss of responsiveness of low-avidity Tag-specific T cells (16). Thus, in TRAMP mice, prostate cancer development and the Tag-specific immune response well recapitulate the tolerant status found in patients with advanced prostate cancer (17).
To independently but simultaneously evaluate tumor and minor H antigen–specific T-cell responses, we adopted a female into the male transplant setting, as MHC class II– and class I–restricted epitopes derived from Y chromosome–encoded minor H antigens (HY) have been described (18, 19) and HY-directed T cells are known to mediate male graft rejection (20). GVHD is not expected due to the single minor H disparity, likely insufficient to sustain GVHD in mice (21), and due to the fact that host-reactive T cells might accumulate within inflamed sites (such as the prostate in TRAMP mice), but not in noninflamed GVHD target sites (22). We deliberately initially chose to avoid GVHD, as this issue might generate potentially confounding clinical and immunologic effects.
We report here that in the context of nonmyeloablative allogeneic HSCT/DLI and posttransplantation tumor-specific vaccination, the concomitance of tumor and minor H antigen–specific T-cell responses is required to obtain acute tumor rejection and disease-free, long-term survival.
Materials and Methods
Mice, cell lines, and reagents
Heterozygous CD45.2+ C57BL/6 TRAMP mice, wild-type (WT) CD45.1+ congenic mice, and CD45.1+ CD45.2+ heterozygous F1 WT offspring were housed, bred, and genotyped (16) in a specific pathogen-free animal facility in accordance with the European Union guidelines and with the approval of the Institutional Ethical Committee (Institutional Animal Care and Use Committee # 311). MataHari and Marilyn Rag2-deficient Thy 1.1+ splenocytes were kindly provided by J-G Chai (Imperial College, London, United Kingdom). CD8+ T cells from MataHari mice express a transgenic TCR specific for the Uty peptide (23), whereas CD4+ T cells of Marilyn origin express a TCR specific for the Dby peptide (24). RMA is a thymoma cell line (25). B6/K-0 is a kidney cell line expressing Tag (26). Unless specified, all chemical reagents were from Sigma-Aldrich and monoclonal antibodies were from BD Pharmingen.
Hematopoietic cell transplantation and tumor-specific vaccination
Mice were sublethally irradiated (600 rad) and, the day after, were transplanted (i.v.) with 1 × 107 viable bone marrow cells. A DLI consisting of 6 × 107 splenocytes was provided 2 wk later. When indicated, female donors were presensitized against HY by i.v. injection of 5 × 106 syngeneic male bone marrow cells (27). Lipopolysaccharide-matured dendritic cells (28) were pulsed with 2 μmol/L Tag-IV peptide (VVYDFLKC; Research Genetics) and injected intradermally (4 × 105 dendritic cell/mouse).
Flow cytometry and cytotoxicity analyses
Cells were stained with phycoerythrin-labeled Kb/OVA or Kb/Tag-IV pentamers (ProImmune) and/or the appropriate fluorochrome-labeled monoclonal antibody, or they were stimulated in vitro with RMA cells pulsed with 2 μmol/L of Tag-IV or 5 μmol/L of Uty or Dby peptides, and intracellular cytokine measurement was performed (16). When indicated, mice were perfused and the prostate glands were treated with collagenase to obtain single-cell suspensions, which were stained either directly or after a Fycoll gradient separation and analyzed by flow cytometry (29). Cytotoxicity was measured in standard 4-h 51Cr release assays (16).
Histology and immunohistochemistry
Urogenital apparata (UGA) were processed for histology and immunohistochemistry, and scored on coded samples by a pathologist as previously described (16, 30) with minor modifications: the score of 0 was given to prostates showing complete tumor regression (CR) and the score of 5 was given to invasive adenocarcinoma (12) or metastases. Prostates with areas of CR scattered among acini affected by adenocarcinoma were defined as partial regression (PR). Mice bearing highly aggressive poorly differentiated neuroendocrine tumors were excluded from the study with the exception of the survival experiments. CD3 (Serotec) and Cleaved Caspase-3 (Cell Signaling) immunodetection was performed according to the manufacturer's instructions. CD3 sections were digitally scanned (ScanScope, Aperio) and then analyzed with the Spectrum Plus software (Aperio).
Statistical analyses were performed using the log-rank test, the χ2 test, or the two-tailed Student's t test. Statistical significance was P < 0.05.
Allotransplantation followed by tumor-specific vaccination circumvents tumor-specific T-cell tolerance, instructs cancer rejection, and allows long-term, disease-free survival
We performed nonmyeloablative transplantation in 17-wk-old TRAMP male mice (CD45.2+), as at this age, the mice reveal developed prostate cancer and are fully tolerant to Tag (the experimental scheme is depicted in Fig. 1A; ref. 16). CD45.2+ mice were subjected to nonmyeloablative total body irradiation (TBI; 600 Rad) and a day later transplanted with 1 × 107 bone marrow cells derived from CD45.1+ or CD45.1+ CD45.2+ F1 congenic female donors (fHSCT). Two weeks after, the HSCT mice received a DLI of 6 × 107 splenocytes derived from congenic CD45.1 or CD45.1+ CD45.2+ F1 congenic females (fDLI), previously sensitized against male antigen to allow the detection of Uty- and Dby-specific T cells (27). This time for DLI was chosen to allow a mixed chimerism (Supplementary Fig. S1), avoid the inflammatory environment induced shortly after conditioning, and favor antigen-driven responses because of decreased competition for survival or proliferation signals (9, 31). To support tumor-specific immunity, a day after the DLI, mice were vaccinated with female bone marrow–derived dendritic cell pulsed with Tag-IV (vax; ref. 16). Mice undergoing such treatment will be hereafter defined as fDLI/vax.
In a first set of experiments, fDLI cells were labeled with the fluorescent dye carboxyfluorescein diacetate succinimidyl ester (CFSE). Following infusion in male (Fig. 1B, top) but not in female (Fig. 1B, bottom) recipients, a fraction of fDLI-derived cells completed several cell division cycles, indicative of in vivo Ag recognition. Fast-dividing cells (identified by the low CFSE content; Fig. 1B) comprised minor H antigen–specific CD4+ and CD8+ T cells and tumor-specific CD8+ T cells, respectively, identified by Dby-, Uty-, and Tag-specific IFN-γ intracellular expression. CFSEdim IFN-γ–competent cells were also found in peripheral blood (data not shown) and selectively accumulated within the prostate (Fig. 1C). Thus, soon after infusion and vaccination, a fraction of DLI-derived T cells is activated, undergo cell division, differentiate into IFN-γ–secreting cells, and relocate to the prostate of TRAMP mice.
To determine whether the observed T-cell responses could be maintained over time and have an effect on tumor progression, fDLI/vax mice were boosted with the vaccine 3 wk after the initial vaccination and either sacrificed 1 wk later (day 42) or boosted monthly thereafter and observed for survival. Age-matched TRAMP mice only receiving vax (TRAMP/vax) and WT mice receiving fDLI/vax (WT/fDLI/vax) were analyzed for comparison. As expected (16), Tag-specific CD8+ T-cell responses remained undetectable in TRAMP/vax mice (Supplementary Fig. S2A; Fig. 1D, left). In contrast, at day 42, tumor (Tag)–specific, IFN-γ–secreting CD8+ T cells, also capable of Ag-specific cytotoxic activity (Supplementary Fig. S3), were found in spleen cultures derived from TRAMP/fDLI/vax mice, with frequencies comparable with those found in WT/fDLI/vax mice (Fig. 1D, left). In the case of TRAMP mice, most of the effector lymphocytes were of donor origin and DLI derived (as determined by relative expression of the CD45.1 congenic marker, Supplementary Fig. S2A), whereas in WT recipients, Tag-specific effectors were of both host and donor origin (Supplementary Fig. S2A; Fig. 1D, left). Transplanted TRAMP and WT mice also revealed sizable and comparable frequencies of IFN-γ–secreting minor H antigen (Uty)–specific CD8+ T cells, which were exclusively of donor origin (Supplementary Fig. S2B; Fig. 1D, right).
The presence of tumor and minor H antigen–specific T-cell responses in the lymphoid organs of transplanted and vaccinated TRAMP mice at day 42 was paralleled by the tumor infiltrates. The prostates of TRAMP/fDLI/vax mice were enriched with CD8+ T cells in comparison with peripheral blood (data not shown) and revealed higher numbers of tumor-infiltrating CD3+, CD4+, and CD8+ T cells ex vivo, when compared with those of WT/fDLI/vax mice (Fig. 2A) and TRAMP/vax mice (Supplementary Fig. S4). Tumor-draining lymph node and prostate tumors contained both tumor and minor H antigen–specific T cells enumerable by fluorescent Kb/Tag-IV pentamer staining (Fig. 2B) and by Ag (Tag-IV and Uty)–specific IFN-γ production, which were mostly of DLI origin (CD45.1 bright cells; Fig. 2C). Immunohistochemical analyses showed that CD3+ cells were in close proximity to prostate epithelial cells immunoreactive to cleaved Caspase-3 in TRAMP/fDLI/vax mice and not in TRAMP/vax controls (Supplementary Fig. S4; Fig. 2D), thus supporting direct killing of transformed cells.
The UGA was next analyzed. Although TRAMP/vax mice revealed an enlarged UGA and engorged seminal vesicles when compared with that recovered from age-matched WT mice, up to 90% of TRAMP/fDLI/vax mice revealed an UGA with almost normal size and weight (Fig. 3). This however was also found in the case in which fDLI was omitted and is thus possibly due to TBI (Fig. 3B). Following staining with H&E (Fig. 4A), prostate tubules of TRAMP/vax mice were expanded by the presence of an adenocarcinoma (disease score, 4.05 ± 0.49; ref. 16) and epithelial cells were all Tag+ (Fig. 4A). In sharp contrast, the histologic appearance of the UGA of the majority of TRAMP/fDLI/vax mice resembled that of transplanted WT/fDLI/vax controls (Fig. 4B and C). The UGA of 15 of 20 TRAMP/fDLI/vax mice showed evidence of CR throughout the organ, which was indicated by the presence of well-lined normal Tag− epithelial cells and the absence of Tag+ cells in the surrounding stroma (Fig. 4B). The persistence of expanded thick wall tubules having hyalinosis of the fibromuscular wall was taken as indicative of past disease occurrence. Some of the mice showed signs of PR revealed by areas of CR scattered among acini affected by adenocarcinoma, and a few of them showed no tumor regression (NR; data not shown). TRAMP/fDLI/vax mice were blindly assigned a disease score reflecting the area of residual transformation. The mean disease score for this group was 0.85 ± 1.57 (Fig. 4D). In most cases, the disease status correlated with the relative representation of minor and tumor-specific IFN-γ–producing cells in TRAMP mice (Fig. 1D, compare white, gray, and black symbols). CR was not observed when fDLI was omitted (i.e., in TBI, HSCT, and vax mice; data not shown). Hence, transplantation followed by tumor-specific vaccination circumvents mechanisms of central and peripheral tolerance normally hindering protective immunity and favors prostate tumor infiltration and rejection of established prostate cancer.
Among the groups of mice maintained beyond day 42, we compared the long-term survival of TRAMP/fDLI/vax mice (n = 9) with control TRAMP/vax (n = 7) and TRAMP mice receiving only PBS (TRAMP/PBS; n = 8). TRAMP/PBS and TRAMP/vax mice had comparable median survival times of 300 and 281 days, respectively (Fig. 5A). This was expected, because although dendritic cell–based vaccination delays the incidence of prostate cancer if administered to young mice (16), it is ineffective in older mice (29). In contrast, TRAMP/fDLI/vax mice revealed prolonged disease-free survival (Fig. 5A). Up to 80% of the mice in this treatment group survived until day 472 when the experiment was concluded to allow histologic and immunologic analyses of the UGA and secondary lymphoid tissues (16, 30). Despite the advanced age (15 months), the UGA of three of seven TRAMP/fDLI/vax mice that survived until day 472 showed evidence of CR on H&E staining (Supplementary Fig. S5A) and Tag expression was mostly undetectable by immunohistochemistry (Supplementary Fig. S5B). Of note, the absence of UGA disease reflected the presence of systemic Tag-specific IFN-γ–producing T cells of donor origin (Fig. 5B, tumor free). Remarkably, the frequency of donor-derived IFN-γ–secreting lymphocytes in the spleen of cured TRAMP was comparable with that found in similarly treated WT mice (Fig. 5B, WT/fDLI/vax), indicating that the combined strategy circumvents the establishment of tumor-specific T-cell tolerance normally hindering endogenous T-cell responses in TRAMP mice. Four TRAMP/fDLI/vax mice had signs of disease (Supplementary Fig. S5A, tumor bearing). In one mouse, this was shown by the enlargement of prostate tubules due to the presence of widely diffused prostate intraepithelial neoplasia (disease score, 3) and in two mice by the presence of microinvasive adenocarcinoma (disease score, 4). One mouse presented a neuroendocrine tumor. In these cases, most of the epithelial cells were Tag+ (Supplementary Fig. S5B) and tumor-specific immune responses were barely detectable (Fig. 5B). Hence, the immune response induced by allotransplantation and tumor-specific vaccination may have sustained rejection of the primary tumor and avoided recurrence for several months in some of the mice, whereas in the remainder, the tumor was kept in a state of equilibrium (32), although eventually tolerance developed (29).
Tumor-specific immunity requires the concomitance of minor H antigen–specific responses for the eradication of spontaneous autochthonous tumors
We asked whether tumor-specific T-cell responses per se were sufficient to cause tumor eradication in TRAMP mice. TRAMP mice received HSCT/DLI from male-derived congeneic WT donors, which contain high-affinity Tag-specific T-cell precursors, but lack minor HY antigen–specific T cells. TRAMP mice undergoing male into male HSCT and DLI transplantation followed by tumor-specific vaccination (mDLI/vax), despite having a sizable Tag-specific effector T-cell population (Fig. 6A), and an almost normal UGA weight and size (1.04 ± 0.24; n = 6) at day 42, revealed in most cases a disease score comparable (3.17 ± 0.6) to that found in TRAMP/vax controls (4.0 ± 0.0) without substantial signs of disease regression (Fig. 6B). Hence, high-affinity tumor-specific T-cell responses are not sufficient for rejection of established autochthonous tumors. Likewise, tumor rejection was not observed when mice were transplanted with fDLI of TRAMP origin (Fig. 6B), which have normal frequencies of minor H antigen–specific T cells but lack high-affinity tumor-specific T cells.7
7R. Hess Michelini and colleagues, in preparation.
The finding that Tag-specific IFN-γ–producing CD8+ T cells were found in comparable frequencies in fDLI and mDLI recipients (respectively harboring and lacking minor H antigen–specific immunity; Fig. 6A), and also in recipients of CD4+-depleted fDLI (data not shown), suggested that minor H antigen–specific T cells might not provide cognate help at the time of tumor-specific T-cell priming in peripheral lymphoid tissues. We thus investigated their role at the tumor site. To this aim, Uty- and Dby-specific Matahari and Marilyn T cells derived from TCR transgenic mice were infused into fHSCT-TRAMP mice and analyzed by flow cytometry. We found that these cells underwent rapid clonal expansion and differentiation (data not shown) and preferentially infiltrated the prostate of TRAMP mice (Fig. 6D, left), in which they preserved the ability to produce IFN-γ after antigen challenge (Fig. 6D, right), likely favoring further tumor infiltration. Accordingly, CD3+ tumor–infiltrating cells were enriched for in fDLI mice when compared with mDLI recipients (Supplementary Fig. S6; Fig. 6C). Furthermore, although depletion of helper CD4+ cells from fDLI did not did not have an effect on the therapeutic potential of fDLI (Fig. 7A, fCD8), depletion of CD8+ cells reduced the DLI therapeutic potential allowing only PR (Fig. 7A, fCD4). Thus, together, these data support the possibility that minor H antigen–specific T cells contribute to tumor eradication by exerting a direct effect at the tumor site.
Posttransplant tumor-specific vaccination protects against disease recurrence
We finally investigated the contribution of posttransplant tumor-specific vaccination to tumor eradication by comparing disease state and long-term survival of fDLI/vax and fDLI mice. Posttransplant tumor-specific vaccination seemed dispensable for short-term tumor control (Fig. 7A, compare fDLI/vax and fDLI). This is possibly explained by the fact that both tumor-specific (Fig. 7B) and minor H antigen–specific (Fig. 7C) IFN-γ+ effectors were both sizable in fDLI/vax and fDLI recipients. Posttransplant vaccination revealed, however, to be critical when long-term survival was compared. Although fDLI mice survived significantly longer than control PBS-treated mice, all the former eventually succumbed to disease by day 579 (Fig. 7D). At difference, all TRAMP/fDLI/vax mice boosted monthly remained viable to date (Fig. 7D). These data further indicate that minor H antigen–specific immunity is not sufficient for long-term survival that requires the concomitance and persistence of tumor-specific immunity best supported by posttransplant vaccination.
We provide evidence that the combination of minor H antigen–mismatched allogeneic transplantation and tumor-specific vaccination circumvents tumor-specific tolerance and results in the concomitant generation of tumor- and alloreactive-specific effector lymphocytes, favoring the eradication of established spontaneous solid tumors and long-term, disease-free survival. To date, this is the first efficacious treatment for TRAMP mice at an advanced stage of disease (29, 30, 33–35) and the only report of the cure of spontaneous tumors with unmanipulated, unselected donor T lymphocytes. These findings are of particular relevance in the TRAMP model in which the genetic pressure for cancer recurrence is particularly strong (11) and able to induce unresponsiveness of high-affinity tumor-specific T cells (36, 37).
The combined strategy might have provided several advantages over individual treatments. Irradiation may have directly damaged tumor cells and their surrounding stroma, and indirectly favored TAA/minor H antigen processing and presentation (38). We found that the UGA weight and size were reduced when DLI was omitted and that this did not reflect disease amelioration. Thus, although these data support an effect of irradiation on the organ mass and are reminiscent of our previous finding showing that chemotherapy causes a drop in UGA weight without tumor remission (30), they warn against the use of UGA weight as the only therapeutic index. The finding that Tag-specific T cells were primed in the absence of vaccination also supports the idea that transplantation favors cross-presentation of TAA. Irradiation might also have favored the upregulation of MHC class I and costimulatory molecules (39) and, by that, the lymphocyte persistence within the tumor. Furthermore, TBI-induced lymphopenia might have favored antigen presenting cell (APC) activation (40) and reduced the number of regulatory T cells and cytokine sinks (41) and, by that, residual host and adoptively transferred lymphocytes expansion and differentiation into memory cells (42). Finally, we found that the infusion of mature donor-derived T cells (DLI) provided a fresh reservoir of T cells capable of responding to TAA, whereas posttransplant TAA-specific vaccination supported their activation and sustained a long-lasting graft-versus-tumor response.
Although the concept of posttransplant tumor-specific vaccination was previously tested against transplantable primary (9, 10) and metastatic tumors (43), it was never evaluated in mice with autochthonous tumors, which might differ in many regards, such as lack of peripheral tolerance to the tumor, frequently observed in patients with advanced cancer (44). For instance Luznik (43) reported a critical role for host immune responses, whereas in our experimental setting, which takes into account preexisting tolerance, protection entirely relied on donor-derived T cells. Tumor-specific immunity, however, was insufficient for disease eradication. When transplantation was performed from male donors (containing tumor-specific and not minor H antigen–specific T-cell precursors), disease remission was not observed. This was in spite of the fact that immune responses to Tag, and likely to other prostate cancer–associated antigens (45), were readily primed. Minor H antigen–specific T cells were also insufficient for long-term, disease-free survival. Indeed, fDLI recipients required posttransplant tumor-specific vaccination for optimal survival, and fDLI of TRAMP female origin (bearing minor H antigen–specific but not high-affinity Tag-specific T-cell precursors), failed to confer protection. Thus, tumor- and minor H antigen–specific T-cell responses are necessary but individually inefficacious in the eradication of autochthonous tumors. These data differ from those reported by Meunier and colleagues (46) who showed the potency of single minor H antigen–specific T cells against transplantable melanoma. As we found that transplanted TRAMP-C1 cells (a prostate cancer cell line derived from a TRAMP tumor; ref. 47) were rejected when injected into female, but not male recipients,8
Which is the mechanism underlying antitumor protection conferred by tumor and minor H antigen–specific T-cell responses in transplanted mice? Our data seem to support a concomitant effect of the cells at the tumor site. Indeed, fDLI and minor H antigen–specific transgenic T cells readily infiltrated the prostate of TRAMP mice shortly after infusion, possibly in response to inflammatory signals produced by the ongoing pathologic process. We speculate that recognition of minor H antigen expressed by the transformed cells and possibly by tumor-associated stromal components elicits a local graft-versus-host response and tissue damage that can directly hinder tumor growth and support a tumor-specific immune response in a nontolerogenic context. Tumor-specific effector T cells also infiltrated the prostate likely increasing inflammation and upregulation of MHC class I molecules following the release of IFN-γ (46) and other soluble factors able to sustain recruitment and in situ reactivation of minor H antigen–specific effector T cells (22). Whether the anti-minor and anti-Tag responses are synergistic or are simply independent and both contributing to survival remains to be determined. Ongoing experiments are also addressing whether the concomitance of minor H antigen and Tag-specific T cells within the tumor might favor epitope spreading contributing to protective immunity.
Although we deliberately avoided the possible confounding effect of concomitant GVHD, this is the major complication of allotransplantation, as minor H antigens likely expressed by tumors might also be expressed by their normal counterparts. Although our data strongly support the idea that tumor and minor H antigen–specific immunity might have to be harnessed together for therapeutic efficacy of allotransplantation against solid tumor, further studies are currently needed to address the efficacy and safety of the strategy in the context of multiple minor H antigen disparity. Nevertheless, the identification of the minimal requirements to obtain protective antitumor immunity following allotransplantation together with the finding that T cells targeted to a single minor H antigen can induce tissue damage (8), opens the way to the design of safer and more effective transplantation strategies.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
We thank Dr. A. Bondanza, M. Bregni, and P. Dellabona (San Raffaele Institute, Milan) for the invaluable discussion and Drs. J. Dyson and J.-G. Chai (Imperial College, London) for the helpful hints and provision of reagents.
Grant Support: Associazione Italiana per la Ricerca sul Cancro and Ministero della Salute (A. Mondino and M. Bellone) and the European Community contract LSHC-CT-2005-018914 (A. Mondino).