Immunodominant minor histocompatibility antigens: the major ones

H60: A Unique Murine Hematopoietic Cell-Restricted Minor Histocompatibility Antigen for Graft-versus-Leukemia Effect

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an important treatment for many types of hematological malignancies. Matching of donor and recipient for the major histocompatibility complex (MHC) improves the HSCT reconstitution, but donor-derived T cells reactive to non-MHC encoded minor histocompatibility antigens (MiHAs) can induce graft-versus-host disease (GVHD) while also being needed for graft-versus-leukemia (GVL) effects. MiHAs are allelically variant self-peptides presented conventionally on MHC molecules, but are alloantigenic in transplantation settings. Immunodominant MiHAs are most strongly associated with GVHD and GVL. There is need for mouse paradigms to understand these contradictory effects. H60 is a highly immunodominant mouse MiHA with hematopoietic cell-restricted expression. Immunodominance of H60 is tightly associated with its allelic nature (presence vs. absence of the transcripts), and the qualitative (TCR diversity) and quantitative (frequency) traits of the reactive T cells. The identity as a hematopoietic cell-restricted antigen (HRA) of H60 assists the appearance of the immunodominace in allo-HSCT circumstances, and generation of GVL effects without induction of serious GVHD after adoptive T cell transfer. Also it allows the low avidity T cells to escape thymic negative selection and exert GVL effect in the periphery, which is a previously unevaluated finding related to HRAs. In this review, we describe the molecular features and immunobiology in detail through which H60 selectively exerts its potent GVL effect. We further describe how lessons learned can be extrapolated to human allo-HCST.

The Genomic Landscape of Antigenic Targets for T Cell-Based Leukemia Immunotherapy

Intensive fundamental and clinical research in cancer immunotherapy has led to the emergence and evolution of two parallel universes with surprisingly little interactions: the realm of hematologic malignancies and that of solid tumors. Treatment of hematologic cancers using allogeneic hematopoietic cell transplantation (AHCT) serendipitously led to the discovery that T cells specific for minor histocompatibility antigens (MiHAs) could cure hematopoietic cancers. Besides, studies based on treatment of solid tumor with ex vivo-expanded tumor infiltrating lymphocytes or immune checkpoint therapy demonstrated that anti-tumor responses could be achieved by targeting tumor-specific antigens (TSAs). It is our contention that much insight can be gained by sharing the tremendous amount of data generated in the two-abovementioned universes. Our perspective article has two specific goals. First, to discuss the value of methods currently used for MiHA and TSA discovery and to explain the key role of mass spectrometry analyses in this process. Second, to demonstrate the importance of broadening the scope of TSA discovery efforts beyond classic annotated protein-coding genomic sequences.

Allodepleted T-cell immunotherapy after haploidentical haematopoietic stem cell transplantation without severe acute graft-versus-host disease (GVHD) in the absence of GVHD prophylaxis

Graft‐versus‐host disease (GVHD) is a major cause of transplant‐related mortality (TRM) after allogeneic haematopoietic stem cell transplantation (HSCT) and presents a challenge in haploidentical HSCT. GVHD may be prevented by ex vivo graft T‐cell depletion or in vivo depletion of proliferating lymphocytes. However, both approaches pose significant risks, particularly infections and relapse, compromising survival. A photodepletion strategy to eliminate alloreactive T cells from mismatched donor lymphocyte infusions (enabling administration without immunosuppression), was used to develop ATIR101, an adjunctive therapy for use after haploidentical HSCT. In this phase I dose‐finding study, 19 adults (median age: 54 years) with high‐risk haematological malignancies were treated with T‐cell‐depleted human leucocyte antigen‐haploidentical myeloablative HSCT followed by ATIR101 at doses of 1 × 10⁴–5 × 10⁶ CD3⁺ cells/kg (median 31 days post‐transplant). No patient received post‐transplant immunosuppression or developed grade III/IV acute GVHD, demonstrating the feasibility of ATIR101 infusion for evaluation in two subsequent phase 2 studies. Additionally, we report long‐term follow ‐up of patients treated with ATIR101 in this study. At 1 year, all 9 patients receiving doses of 0·3–2 × 10⁶ CD3⁺ cells/kg ATIR101 remained free of serious infections and after more than 8 years, TRM was 0%, relapse‐related mortality was 33% and overall survival was 67% in these patients.

Proteogenomic-based discovery of minor histocompatibility antigens with suitable features for immunotherapy of hematologic cancers

Pre-clinical studies have shown that injection of allogeneic T cells primed against a single minor histocompatibility antigen (MiHA) could cure hematologic cancers (HC) without causing any toxicity to the host. However, translation of this approach in humans has been hampered by the paucity of molecularly defined human MiHAs. Using a novel proteogenomic approach, we have analyzed cells from 13 volunteers and discovered a vast repertoire of MiHAs presented by the most common HLA haplotype in European Americans: HLA-A*02:01;B*44:03. Notably, out of >6000 MiHAs, we have identified a set of 39 MiHAs that share optimal features for immunotherapy of HCs. These 'optimal MiHAs' are coded by common alleles of genes that are preferentially expressed in hematopoietic cells. Bioinformatic modeling based on MiHA allelic frequencies showed that the 39 optimal MiHAs would enable MiHA-targeted immunotherapy of practically all HLA-A*02:01;B*44:03 patients. Further extension of this strategy to a few additional HLA haplotypes would allow treatment of almost all patients.

Subdominant H60 antigen-specific CD8 T-cell response precedes dominant H4 antigen-specific response during the initial phase of allogenic skin graft rejection

In allogeneic transplantation, including the B6 anti-BALB.B settings, H60 and H4 are two representative dominant minor histocompatibility antigens that induce strong CD8 T-cell responses. With different distribution patterns, H60 expression is restricted to hematopoietic cells, whereas H4 is ubiquitously expressed. H60-specific CD8 T-cell response has been known to be dominant in most cases of B6 anti-BALB.B allo-responses, except in the case of skin transplantation. To understand the mechanism underlying the subdominance of H60 during allogeneic skin transplantation, we investigated the dynamics of the H60-specific CD8 T cells in B6 mice transplanted with allogeneic BALB.B tail skin. Unexpectedly, longitudinal bioluminescence imaging and flow cytometric analyses revealed that H60-specific CD8 T cells were not always subdominant to H4-specific cells but instead showed a brief dominance before the H4 response became predominant. H60-specific CD8 T cells could expand in the draining lymph node and migrate to the BALB.B allografts, indicating their active participation in the anti-BALB.B allo-response. Enhancing the frequencies of H60-reactive CD8 T cells prior to skin transplantation reversed the immune hierarchy between H60 and H4. Additionally, H60 became predominant when antigen presentation was limited to the direct pathway. However, when antigen presentation was restricted to the indirect pathway, the expansion of H60-specific CD8 T cells was limited, whereas H4-specific CD8 T cells expanded significantly, suggesting that the temporary immunodominance and eventual subdominance of H60 could be due to their reliance on the direct antigen presentation pathway. These results enhance our understanding of the immunodominance phenomenon following allogeneic tissue transplantation.

Rejection of leukemic cells requires antigen-specific T cells with high functional avidity

In a context where injection of antigen (Ag)-specific T cells probably represents the future of leukemia immunotherapy, identification of optimal target Ags is crucial. We therefore sought to discover a reliable marker for selection of the most potent Ags. To this end, (1) we immunized mice against 8 individual Ags: 4 minor histocompatibility Ags (miHAs) and 4 leukemia-associated Ags (LAAs) that were overexpressed on leukemic relative to normal thymocytes; (2) we assessed their ability to reject EL4 leukemic cells; and (3) we correlated the properties of our Ags (and their cognate T cells) with their ability to induce protective antileukemic responses. Overall, individual miHAs instigated more potent antileukemic responses than LAAs. Three features had no influence on the ability of primed T cells to reject leukemic cells: (1) MHC-peptide affinity; (2) the stability of MHC-peptide complexes; and (3) epitope density at the surface of leukemic cells, as assessed using mass spectrometry. The cardinal feature of successful Ags is that they were recognized by high-avidity CD8 T cells that proliferated extensively in vivo. Our work suggests that in vitro evaluation of functional avidity represents the best criterion for selection of Ags, which should be prioritized in clinical trials of leukemia immunotherapy.

Primary vascularization of the graft determines the immunodominance of murine minor H antigens during organ transplantation

Grafts can be rejected even when matched for MHC because of differences in the minor histocompatibility Ags (mH-Ags). H4- and H60-derived epitopes are known as immunodominant mH-Ags in H2(b)-compatible BALB.B to C57BL/6 transplantation settings. Although multiple explanations have been provided to explain immunodominance of Ags, the role of vascularization of the graft is yet to be determined. In this study, we used heart (vascularized) and skin (nonvascularized) transplantations to determine the role of primary vascularization of the graft. A higher IFN-γ response toward H60 peptide occurs in heart recipients. In contrast, a higher IFN-γ response was generated against H4 peptide in skin transplant recipients. Peptide-loaded tetramer staining revealed a distinct antigenic hierarchy between heart and skin transplantation: H60-specific CD8(+) T cells were the most abundant after heart transplantation, whereas H4-specific CD8(+) T cells were more abundant after skin graft. Neither the tissue-specific distribution of mH-Ags nor the draining lymph node-derived dendritic cells correlated with the observed immunodominance. Interestingly, non-primarily vascularized cardiac allografts mimicked skin grafts in the observed immunodominance, and H60 immunodominance was observed in primarily vascularized skin grafts. However, T cell depletion from the BALB.B donor prior to cardiac allograft induces H4 immunodominance in vascularized cardiac allograft. Collectively, our data suggest that immediate transmigration of donor T cells via primary vascularization is responsible for the immunodominance of H60 mH-Ag in organ and tissue transplantation.

Hematopoietic stem cell recipients do not develop post-transplantation immune tolerance to antigens present on minimal residual disease

The immune environment present after allogeneic hematopoietic stem cell transplantation (HSCT) contributes to the control of leukemia. Our laboratory has demonstrated in a murine model that vaccination of recipients after transplantation with recipient tumor vaccines does not exacerbate graft-versus-host disease but does induce meaningful graft-versus-tumor effects. We previously demonstrated that part of the reason for the lack of graft-versus-host disease from post-transplantation vaccination is due to gradual acquisition of tolerance or unresponsiveness to recipient immunodominant minor histocompatibility antigens that are ubiquitously expressed in the recipient. However, our prior studies have not critically addressed the question of whether a similar process of acquisition of unresponsiveness to or tolerance of antigens present on minimal residual disease also occurs. The present study tested the hypothesis that unresponsiveness to antigens present on minimal residual disease present at the time of HSCT would also occur. The answer to this question would have a significant effect on the potential efficacy of post-transplantation tumor vaccines. In a murine model of major histocompatibility complex matched, minor histocompatibility antigen mismatched HSCT (C3.SW female donors and C57BL/6 female recipients), we tested whether transplant recipients would acquire unresponsiveness to antigens present on small numbers of residual leukemia/lymphoma cells. We employed a male C57BL/6 lymphoid malignancy with an immunoglobulin/c-myc oncogene in these studies using as a model of tumor-restricted antigen the well-characterized male (HY) antigen system present only on the tumor but not present as ubiquitous minor antigens in the recipient. After HSCT, recipients did not mount immune responses to the ubiquitously distributed immunodominant recipient strain H7 minor histocompatibility antigen, but did retain the capacity to mount significant T cell responses to HY antigens present on small numbers of HY+ tumor cells present at transplantation. Additional studies using small numbers of nonmalignant recipient male B cells or dendritic cells as models of minimal residual disease also demonstrated that the transplant recipients retained their capacity to mount anti-HY T cell responses. After HSCT, recipients may retain the capacity to mount effective T cell responses to antigens present on minimal residual disease and still acquire relative tolerance to ubiquitously distributed immunodominant minor antigens that are related to graft-versus-host disease.

Asynchronous differentiation of CD8 T cells that recognize dominant and cryptic antigens

Restriction of T cell responses to a few epitopes (immunodominance) is a central feature of immune responses. We analyzed the entire transcriptome of effector CD8 T cells specific for a dominant (H7(a)) and a cryptic (HY) mouse Ag and performed a longitudinal analysis of selected T cell differentiation markers. We found that Ag specificity had a relatively modest influence on the repertoire of genes that are transcriptionally modulated by the CD8 T cell differentiation program. Although the differentiation programs of anti-H7(a) and anti-HY T cells were similar, they did not progress simultaneously. The expansion peak of anti-H7(a) T cells was reached on day 10 while that of anti-HY T cells was attained on days 15-20. Between days 10 and 20, anti-H7(a) T cells were in the contraction phase and anti-HY T cells in the expansion phase. Furthermore, expansion and development of effector function were well-synchronized in anti-H7(a) T cells but were disconnected in anti-HY T cells. We propose that, by leading to selective expansion of the fittest CD8 T cells, immunodominance may be beneficial to the host. Inhibition of the T cell response to cryptic Ag would ensure that host resources (APC, cytokines) for which T cells compete are devoted to T cells with the best effector potential. One implication is that favoring expansion of the fittest effector T cells in general may be more important than increasing the diversity of the T cell repertoire.

Prediction of graft-versus-host disease in humans by donor gene-expression profiling

BACKGROUND

Graft-versus-host disease (GVHD) results from recognition of host antigens by donor T cells following allogeneic hematopoietic cell transplantation (AHCT). Notably, histoincompatibility between donor and recipient is necessary but not sufficient to elicit GVHD. Therefore, we tested the hypothesis that some donors may be "stronger alloresponders" than others, and consequently more likely to elicit GVHD.

METHODS AND FINDINGS

To this end, we measured the gene-expression profiles of CD4(+) and CD8(+) T cells from 50 AHCT donors with microarrays. We report that pre-AHCT gene-expression profiling segregates donors whose recipient suffered from GVHD or not. Using quantitative PCR, established statistical tests, and analysis of multiple independent training-test datasets, we found that for chronic GVHD the "dangerous donor" trait (occurrence of GVHD in the recipient) is under polygenic control and is shaped by the activity of genes that regulate transforming growth factor-beta signaling and cell proliferation.

CONCLUSIONS

These findings strongly suggest that the donor gene-expression profile has a dominant influence on the occurrence of GVHD in the recipient. The ability to discriminate strong and weak alloresponders using gene-expression profiling could pave the way to personalized transplantation medicine.

Sampling of major histocompatibility complex class I-associated peptidome suggests relatively looser global association of HLA-B*5101 with peptides

We have analyzed peptides associated with six human major histocompatibility complex (MHC) class I allomorphs expressed by the U937 cell line. Peptides were isolated by mild acid elution or by MHC class I immunoprecipitation by using W6/32 monoclonal antibody. Eighty-five peptides were sequenced by mass spectrometry, and their putative binding alleles were assigned using bioinformatic tools. Only three peptides isolated by the two approaches were identical, suggesting that the approaches may yield distinct partially overlapping peptide populations. Mild acid treatment-derived peptides manifested overall characteristics suggestive of relatively lower affinity of binding for MHC class I. Interestingly, a large proportion of putative HLA-B*5101-binding peptides was evident among the mild acid treatment-eluted peptides, and to a lesser degree in the affinity-purified peptide pool. These results suggest that HLA-B*5101 may bind a potentially large pool of peptides with relatively lower affinity. We suggest that lower affinity of peptide binding may be the basis for inefficient tolerance to HLA-B*5101-binding self-peptides, a predisposing factor for the development of Behçet disease.

Alloreactive T cell clonotype recruitment in a mixed lymphocyte reaction: Implications for graft engineering

OBJECTIVE

The selective elimination of alloreactive T cells from donor stem cell grafts prior to hematopoietic stem cell transplantation (HSCT) is an important goal in the prevention of graft-vs-host disease (GVHD). However, in HLA-identical donor-recipient pairs, it has proven difficult to identify alloreactive T cells using in vitro systems pretransplant due, in part, to their low frequency and a lack of methodological standardization. To better understand the alloresponse between HLA-identical related pairs, we characterized the alloreactive T cells generated in a mixed lymphocyte reaction (MLR) assay system.

METHODS

HSCT donor peripheral blood mononuclear cells (responder) were labeled with carboxyfluorescein diacetate, succinimidyl ester (CFSE) dye and cocultured with irradiated HSCT recipient cells (stimulator) in a one-way MLR. Alloreactive T cells were sorted by upregulation of activation markers (CD25 in most cases) and the responding clonotypes were defined by sequencing the complementarity region 3 (CDR3) of the T cell receptor beta-chain.

RESULTS

We show that the recruitment of alloreactive CD4(+) T cells is highly variable. Oligoclonal CD4(+) T-cell expansions in repeated MLRs performed in the same donor-recipient pair showed inconsistent recruitment of clonotypes. The recruitment of alloreactive CD8(+) T cells was more consistent in repeated assays, with the same clonotypes identified in the same donor-recipient pair performed under different conditions.

CONCLUSION

Taken together, our data show that even in culture conditions constrained to eliminate background proliferation, stochastic events and low precursor frequencies preclude reproducible elicitation of immunodominant T cell clonotypes with the potential to cause GVHD.

Possible involvement of allogeneic antigens recognised by donor-derived CD4 cytotoxic T cells in selective GVL effects after stem cell transplantation of patients with haematological malignancy

Cytotoxic T lymphocyte (CTL) lines specific for allogeneic antigens were generated by in vitro stimulation of donor-derived peripheral blood mononuclear cells obtained from patients who received human leucocyte antigen (HLA)-matched allogeneic haematopoietic stem cell transplantation (HSCT). One of the allogeneic antigen-specific CD4+ CTL lines, CTL-A, generated from a patient with T cell acute lymphoblastic leukaemia, recognised HLA-DPB1*0501-positive Epstein-Barr virus-immortalised human B cell line (EBV-B cells), phytohaemagglutinin blasts and leukaemia cells, but not interferon-gamma (IFN-gamma) treated HLA-DPB1*0501-positive fibroblasts, indicating that this CD4+ T-cell line recognised a minor histocompatibility antigen (mHa) that is preferentially expressed in haematopoietic cells in an HLA-DPB1*0501-restricted manner. The other CD4+ CTL line, CTL-B, generated from a patient with chronic myeloid leukaemia, recognised mismatched HLA-DQB1*0303 on EBV-B cells and phytohaemagglutinin (PHA) blasts. Interestingly, this CTL line did not recognise IFN-gamma-treated recipient's skin fibroblasts, as HLA-DQ was merely upregulated even after IFN-gamma stimulation in non-haematopoietic cells including fibroblasts, endothelial cells and hepatocytes. These results suggest that these CD4 positive CTLs, specific for mismatch HLA-DQ and mHa that are preferentially expressed on haematopoietic cells, may play an important role in induction of selective graft-versus-leukaemia effect without development of graft-versus-host disease after allogeneic HSCT.

Immunodominance and Immunodomination: Critical Factors in Developing Effective CD8+ T‐Cell–Based Cancer Vaccines

The focusing of cellular immunity toward one, or just a few, antigenic determinant, even during immune responses to complex microorganisms or antigens, is known as immunodominance. Although described in many systems, the mechanisms of determinant immunodominance are only just beginning to be appreciated, especially in relation to the interplay between T cells of differing specificities and the interactions between T cells and the antigen-presenting cells (APCs). The outcome of these cellular interactions can lead to a form of immune suppression of one specificity by another-described as "immunodomination". The specific and detailed mechanisms involved in this process are now partly defined. A full understanding of all the factors that control immunodominance and influence immunodomination will help us to develop better viral and cancer vaccines.

Selective depletion strategies in allogeneic stem cell transplantation

Despite improved prophylaxis and treatment, GvHD remains a major limitation to optimal allogeneic stem cell transplantation. Ex vivo selective depletion (SD) is a strategy to prevent GvHD, in which host-reactive donor lymphocytes are selectively eliminated from a PBSC allograft while useful donor immune function is preserved. The elimination of alloreactive and thereby GvHD-mediating T cells has been shown to be feasible in both pre-clinical and more recently clinical studies. However, SD techniques and the translational research needed for clinical application are still under development. Here we summarize and discuss the following aspects of the SD approach: selection of an appropriate allogeneic stimulator; the responder population; the alloresponse; methods for removal of alloreacting T cells; product testing; clinical considerations. Our review highlights the diversity of possible approaches and the need to develop different techniques for specific clinical applications.

CD8 T-cell ability to exert immunodomination correlates with T-cell receptor: Epitope association rate

When presented alone, H7 a and HY antigens elicit CD8 T-cell responses of similar amplitude, but H7 a totally abrogates the response to HY when both antigens are presented on the same antigen-presenting cell. We found that H7a- and HY-specific T-cell precursors had similar frequencies in nonimmune mice and expressed similar levels of CD5. The H7a -specific CD8 T-cell repertoire harvested at the time of primary response showed highly restricted T-cell receptor (TCR) diversity. Furthermore, T cells specific for H7a and HY expressed equivalent levels of CD8 and TCR and displayed similar tetramer decay rates. The key difference was that anti-H7a T cells exhibited a much more rapid TCR:epitope on-rate than anti-HY T cells. Coupled with evidence that primed CD8 T cells limit the duration of antigen presentation by killing or inactivating antigen-presenting cells, our data support a novel and simple model for immunodomination: the main feature of T cells that exert immunodomination is that, compared with other T cells, they are functionally primed after a shorter duration of antigen presentation.

Minor histocompatibility antigen HA-1 and HPA-5 polymorphisms in HLA-identical related bone marrow transplantation

The minor histocompatibility antigens (mHags), HA-1 and HPA-5, are immunogenic alloantigens shown to be responsible for graft-versus-host disease (GVHD) in HLA-identical bone marrow transplantation. Both antigens have two known alleles each, resulting in a single amino acid polymorphism. The HA-1H allele encodes histidine, whereas the HA-1R allele encodes arginine. The HPA-5b (Br(a)) allele encodes lysine, whereas the HPA-5a (Br(b)) encodes glutamic acid. In this study, 49 bone marrow transplant recipients and their genetically related HLA-identical donors were evaluated for the presence of HA-1, whereas 39 recipients, different from the abovementioned ones, and their HLA-identical siblings were analyzed for the presence of HPA-5. The frequencies of the two alleles of HA-1 in the recipient population were HA-1R = 0.663 and HA-1H = 0.336. In the donor population, the respective frequencies were 0.704 and 0.296. Seven donors (14.5%) were mismatched with the recipients for HA-1H. In contrast, the frequencies of the two alleles of HPA-5 in the recipient population were HPA-5a = 0.859 and HPA-5b = 0.141; whereas, among donors, they were 0.820 and 0.180, respectively. Five donors (12.8%) were found to be mismatched with their recipients for HPA-5. These results provide insight into the polymorphism of mH antigens based on the study of their frequencies in bone marrow transplant recipients and their genetically HLA-identical siblings, an endeavor that is essential to investigate the presence of HA-1 and HPA-5 mHags.

Avian thymic hormone treatment of peripheral blood mononuclear cells from young chicks stimulates acute graft-versus-host reaction in chicken embryos

Avian thymic hormone (ATH) is a parvalbumin produced by epithelial cells in the thymic cortex of chickens and circulates in the blood on a 5-day cycle. It stimulates precocious development of cell-mediated immunity. The effect of partially purified extracts of thymus (TE) and purified ATH were tested for their effect on the acute graft-versus-host reaction (GVHR). Treatment of chicks for their first 3-days of life did not enhance the acute GVHR produced by their PBMC in 14-day-old embryos. PBMC from 3-day-old chicks were treated in vitro with TE, ATH, thymosin fraction 5 or thymosin alpha1 for 2 h and injected into 14-day-old embryos. Bone marrow cells and thymic lymphocytes were treated with TE. Only PBMC treated with TE or ATH produced an enhanced acute GVHR. Because ATH targets gammadelta T cells, the data implicate participation of donor gammadelta T cells in the acute GVHR.

Minor histocompatibility antigen-specific T cells with multiple distinct specificities can be isolated by direct cloning of IFNγ-secreting T cells from patients with relapsed leukemia responding to donor lymphocyte infusion

Graft-vs-leukemia reactivity after donor lymphocyte infusion (DLI) can be mediated by donor T cells recognizing minor histocompatibility antigens (mHags) on recipient hematopoietic cells. To study the diversity of cells involved in this immune response, hematopoietic cell reactive T cells were directly clonally isolated from peripheral blood of patients entering complete remission after DLI. T cells were briefly stimulated with bone marrow cells from patients pretransplant, and IFNgamma-secreting T cells were directly clonally isolated, and expanded. Cytotoxic T-lymphocyte (CTL) clones from individual patients used multiple distinct HLA-restricting molecules and varied in reactivity against patient-derived normal and/or malignant hematopoietic cells. For each patient, CTL clones specific for known immunodominant mHags as well as distinct unknown mHags were found. Within individual patients, CTL clones using the same HLA-restricting element could show differential recognition patterns, indicating further diversity in mHag reactivity. CTL clones from individual patients exhibiting identical specificities could show oligoclonal origin. In conclusion, the direct cloning technique shows that the response to hematopoietic cells after DLI is directed against multiple distinct mHags, including but not limited to known immunodominant mHags, implying that immunotherapy with T cells against multiple mHag specificities may be more effective in eradicating malignant cells.

Target Antigens Determine Graft-versus-Host Disease Phenotype

Chronic graft-vs-host disease (cGVHD) is an increasingly frequent complication of allogeneic stem cell transplantation. Phenotypically, cGVHD differs from patient to patient; in particular, a subset of patients develops extensive cutaneous fibrosis. Similarly, graft-vs-host disease (GVHD) is distinct in inbred murine donor:recipient pairings, indicating a genetic component to disease phenotype. The B10.D2 -->BALB/c (H-2d) strain pairing uniquely recapitulates key pathologic features of fibrotic human cutaneous cGVHD. To distinguish whether this genetic component is due to differences in genes that modulate immune responses or to the specific Ags targeted, we asked whether skin-dominant cGVHD also develops in the B10 -->BALB.B (H-2b) and B10.BR -->BALB.K (H-2k) MHC-congenic pairings. Because each MHC haplotype presents different peptides and selects different T cell repertoires, GVHD in each donor:recipient pair undoubtedly targets different Ags. We found that, in contrast to BALB/c recipients, BALB.B mice never manifested skin disease while BALB.K mice developed a modified form of skin disease. Instead, BALB.B and BALB.K recipients developed systemic GVHD which was absent in BALB/c mice. Moreover, in (B10 x B10.D2)F1 -->(BALB.B x BALB/c)F1 H-2b/d transplants, recipients developed both cutaneous and systemic disease. Thus, the selection of immunodominant Ags determines the target and character of GVHD, providing insight into the genetic basis for different forms of GVHD.

Application of discordant sib-pair linkage analysis for mapping minor histocompatibility antigen loci in a novel graft-vs-host-disease model

Graft-vs-host disease (GVHD) is an adverse effect of allogenic bone marrow transplantation. Although a major cause of GVHD following bone marrow transplantation is incompatibility of major histocompatibility antigen (human leukocyte antigen, HLA) in donor-recipient pairs, the incompatibility of minor histocompatibility antigen (mHa) is known as another cause, especially in HLA-matched donor-recipient pairs. In 1998, Lunetta and Rogus proposed the use of discordant sib-pair (DSP) linkage analysis for detecting mHa and calculated the statistical power using the GVHD model, assuming single mHa locus with multiple alleles. Recently, we proposed a different GVHD model, assuming multiple mHa loci with two alleles (biallelic), considering the single-nucleotide polymorphisms. When the effect of each mHa locus on the occurrence of GVHD is independent, the possible triangle for DSP proposed by Lunetta and Rogus is not optimum, but a new possible triangle, named here as GVHD region, is needed. We evaluated, based on Monte Carlo simulation, the test criteria [log of odds (lod) score cutoffs] and power of DSP using the GVHD region for various parameter sets. The GVHD region showed a higher power than the DSP and entire regions in plausible situations. Our results suggest that the application of GVHD region to DSP is effective for the screening of mHa loci.

Significance of selectively targeted apoptotic rete cells in graft-versus-host disease

Considerable data exist regarding the mechanisms of allostimulation and homing (the effector phases) in graft-versus-host disease (GVHD). Current dogma suggests that target specificity involves preferential injury to epithelial surfaces of the skin and squamous mucosae, liver, and gut. Little attention has been devoted, however, to mechanisms of cellular targeting or to whether heterogeneity exists in target tissues with regard to a threshold for cellular injury. A recent breakthrough in understanding the target stage of GVHD indicates that the predominant pathway of injury to squamous epithelial cells involves apoptosis. Moreover, apoptotic injury may be associated or unassociated with local T-cell infiltration and involves phenotypically and antigenically distinctive epithelial cells within the basal layer of the skin and squamous mucosa. These cells are confined to rete ridges in the skin and retelike prominences in the dorsal tongue and are designated as selectively targeted apoptotic rete (STAR) cells. The discovery of STAR cells in GVHD paves the way for speculation and experimentation to determine why these subpopulations are selectively vulnerable and how soluble and cellular effectors of apoptosis contribute to their ultimate demise. Novel approaches to GVHD treatment derived from understanding mechanisms of selective epithelial injury are likely to use strategies to render target cells less susceptible to the apoptosis that is ultimately responsible for organ dysfunction and failure.

Significance of chimerism in hematopoietic stem cell transplantation: new variations on an old theme

The main goal of post-transplantation monitoring in hematopoietic stem cell transplantation (HSCT) is to predict negative events, such as disease relapse, graft rejection and graft-versus-host disease, in order to intervene with appropriate therapy. In this context, chimerism analysis is an important method in monitoring post HSCT outcome. Mixed chimerism (MC) is mainly evaluated to define engraftment and relapse. Detection of MC is a prerequisite in both myeloablative and nonmyeloablative HSCT, in order to assess the graft status and decide later therapeutic strategies such as donor lymphocyte infusion. In this review, we discuss various techniques including erythrocyte phenotyping, cytogenetic analysis, fluorescent in situ hybridization, restriction fragment length polymorphism, STR/VNTR analysis and real-time quantitative PCR, along with the various methods used to detect minimal residual disease (MRD) in different diseases such as chronic myeloid leukemia, acute myelomonocytic leukemia or acute lymphoblastic leukemia. The review mainly highlights the optimal methodological approach, which needs to be informative, sensitive and quantitatively accurate for MC detection. Future of post HSCT graft monitoring lies in the selection of the most accurate and sensitive technique to determine both MC and MRD. Such an approach would be helpful in not only determining relapse or rejection, but also in ascertaining various responses to different treatment modalities.

Specific donor Vβ-associated CD4+ T-cell responses correlate with severe acute graft-versus-host disease directed to multiple minor histocompatibility antigens

CXB-2/By (CXB-2) recombinant inbred mice express a subset of the minor histocompatibility antigen (miHA) repertoire expressed by C.B10-H2(b)/LiMcdJ (BALB.B) mice. On lethal irradiation and the transplantation of H2(b)-matched C57BL/6 (B6) T cell-depleted bone marrow cells, along with naive unfractionated T cells, both strains succumb to acute graft-versus-host disease (GVHD). Although alloreactive B6 CD4(+) T cells are a necessary source of T-cell help for the B6 CD8(+) component of the GVHD response in both recipient strains, they are capable of mediating severe GVHD by themselves only in BALB.B mice. Previous CD4(+) T-cell receptor repertoire analysis demonstrated overlapping oligoclonal Vbeta use between the CD4(+) B6 anti-BALB.B and B6 anti-CXB-2 responses, with indications of additional BALB.B unique T-cell responses (Vbeta2 and Vbeta11). We report here that the more severe B6 anti-BALB.B response is not due to a quantitative difference in the responding cells, because the frequency of alloreactive donor CD4(+) T cells over time was equivalent in the spleens of BALB.B versus CXB-2 recipients. The responses were also similar in the number of infiltrating B6 CD4(+) T cells in the lingual epithelium of the 2 recipients. In contrast, a significantly greater degree of infiltration and injury of BALB.B intestinal epithelium correlated with the increased level of clinical GVHD severity. Of most significance, despite the involvement of at least 11 Vbeta-associated CD4(+) T-cell families in the overall B6 anti-BALB.B response, the development of severe GVHD correlated with the presence of Vbeta2- and Vbeta11-positive donor T cells. Transplantation of donor CD4(+) T cells from Vbeta-associated families that were shared between the B6 anti-BALB.B and anti-CXB-2 responses resulted in minimal GVHD potential. These data suggest that severe GVHD across miHA barriers depends on the involvement of a restricted number of potent T-cell specificities and implies that there are only a limited number of corresponding responsible miHAs.

Importance of minor histocompatibility antigen expression by nonhematopoietic tissues in a CD4+ T cell-mediated graft-versus-host disease model

Minor histocompatibility antigens with expression restricted to the recipient hematopoietic compartment represent prospective immunological targets for graft-versus-leukemia therapy. It remains unclear, however, whether donor T cell recognition of these hematopoietically derived minor histocompatibility antigens will induce significant graft-versus-host disease (GVHD). Using established bone marrow irradiation chimeras across the multiple minor histocompatibility antigen-disparate, C57BL/6-->BALB.B combination, we studied the occurrence of lethal GVHD mediated by CD4+ T cells in recipient mice expressing only hematopoietically derived alloantigens. Even substantial dosages of donor C57BL/6 CD4+ T cells were unable to elicit lethal GVHD when transplanted into [BALB.B-->C57BL/6] chimeras. Instead, chimeric mice displayed transient cachexia with reduced target-tissue injury over time, reflecting an early, limited, graft-versus-host response. On the other hand, the importance of minor histocompatibility antigens derived from nonhematopoietic tissues was demonstrated by the finding that [C57BL/6-->BALB.B] chimeric mice succumbed to C57BL/6 CD4+ T cell-mediated GVHD. These data suggest that severe acute CD4+ T cell-mediated GVHD across this minor histocompatibility antigen barrier depends on the expression of nonhematopoietically rather than hematopoietically derived alloantigens for maximal target-tissue infiltration and injury.

Development of immune response that protects mice from viral pneumonitis after a single intranasal immunization with influenza A virus and nanoemulsion

Nanoemulsion, a water-in-oil formulation stabilized by small amounts of surfactant, is non-toxic to mucous membranes and produces biocidal activity against enveloped viruses. We evaluated nanoemulsion as an adjuvant for mucosal influenza vaccines. Mice (C3H/HeNHsd strain) were vaccinated intranasally with 5 x 10(5) plaque forming units (pfu) of influenza A virus (Ann Arbor/6/60 strain) and a nanoemulsion mixture. The mice were challenged on day 21 after immunization with an intranasal lethal dose of 2 x 10(5) pfu of virus. Animals vaccinated with the influenza A/nanoemulsion mixture were completely protected against infection, while animals vaccinated with either formaldehyde-killed virus or nanoemulsion alone developed viral pneumonitis and died by day 6 after the challenge. Mice vaccinated with virus/nanoemulsion mixture had rapid cytokine responses followed by high levels of specific anti-influenza immunoglobulin G (IgG) and immunoglobulin A (IgA) antibodies. Specificity of the immune response was confirmed by assessment of the proliferation and cytokine production in splenocytes. This paper demonstrates that nanoemulsion can be employed as a non-toxic mucosal adjuvant for influenza virus vaccine.

Post-hematopoietic cell transplantation control of graft-versus-host disease by donor CD425 T cells to allow an effective graft-versus-leukemia response

After allogeneic hematopoietic cell transplantation (HCT), the high inverse correlation between graft-versus-host disease (GVHD) and leukemic relapse requires that calculated measures be taken to reduce GVHD pathology while retaining the graft-versus-leukemia (GVL) effect. We sought to determine whether donor CD4(+)CD25(+) regulatory T cells could control ongoing GVHD, thereby providing an initial window of time in which the alloreactive anti-host response is permitted to begin, with the intent of most effectively eliminating residual leukemia cells. Prevention of lethal GVHD by infusion of donor CD4(+)CD25(+) cells early after HCT (day 2) was achieved across a major histocompatibility complex barrier in the haploidentical C3H-->(B6xC3H)F(1) model. However, in vitro expansion of donor CD4(+)CD25(+) T cells, stimulated by recipient cells in the presence of high-dose interleukin-2, was required for successful regulation. In contrast, in the major histocompatibility complex-matched, minor histocompatibility antigen-disparate, CD8-mediated B10.BR-->CBA GVHD model, lethal disease could be completely prevented by a single infusion of freshly isolated donor CD4(+)CD25(+) cells administered as late as 10 days after HCT. Of importance, this late regulatory effect required only a 3:1 ratio of effector CD8:CD4(+)CD25(+) T cells, indicating a strong potential for the delayed infusion of CD4(+)CD25(+) cells to control GVHD across minor histocompatibility antigen barriers. Furthermore, this regulation did not interfere with complete and lasting donor engraftment of the hematopoietic compartment. Of most significance, the day 10 infusion of donor CD4(+)CD25(+) cells into CBA HCT recipients that had been challenged with the MMCBA6 myeloid leukemia cell line did not block an effective GVL response, despite reducing lethal GVHD. These results suggest that donor CD4(+)CD25(+) T cells infused soon after transplantation can ameliorate the development of GVHD without sacrificing a sufficient GVL effect.

Tissue distribution of target antigen has a decisive influence on the outcome of adoptive cancer immunotherapy

Adoptive transfer of allogeneic T cells has unmatched efficacy to eradicate leukemic cells. We therefore sought to evaluate in kinetic terms interactions between T cells and allogeneic leukemic cells. T cells primed against the model B6(dom1) minor histocompatibility antigen were adoptively transferred in irradiated B10 (B6(dom1)-positive) and congenic B10.H7(b) (B6(dom1)-negative) recipients, some of which were also injected with EL4 leukemia/lymphoma cells (B6(dom1)-positive). A key finding was that the tissue distribution of the target epitope dramatically influenced the outcome of adoptive cancer immunotherapy. Widespread expression of B6(dom1) in B10 recipients induced apoptosis and dysfunction of antigen-specific T cells. Furthermore, in leukemic B10 and B10.H7(b) hosts, a massive accumulation of effector/memory B6(dom1)-specific T cells was detected in the bone marrow, the main site of EL4 cell growth. The accumulation of effector/memory cells in recipient bone marrow was EL4 dependent, and its kinetics was different from that observed in recipient spleen. We conclude that strategies must be devised to prevent apoptosis of adoptively transferred T cells confronted with a high antigen load and that local monitoring of the immune response at the site of tumor growth may be mandatory for a meaningful assessment of the efficacy of adoptive immunotherapy.

New Developments in Allotransplant Immunology

After allogeneic stem cell transplantation, the establishment of the donor’s immune system in an antigenically distinct recipient confers a therapeutic graft-versus-malignancy effect, but also causes graft-versus-host disease (GVHD) and protracted immune dysfunction. In the last decade, a molecular-level description of alloimmune interactions and the process of immune recovery leading to tolerance has emerged. Here, new developments in understanding alloresponses, genetic factors that modify them, and strategies to control immune reconstitution are described.

In Section I, Dr. John Barrett and colleagues describe the cellular and molecular basis of the alloresponse and the mechanisms underlying the three major outcomes of engraftment, GVHD and the graft-versus-leukemia (GVL) effect. Increasing knowledge of leukemia-restricted antigens suggests ways to separate GVHD and GVL. Recent findings highlight a central role of hematopoietic-derived antigen-presenting cells in the initiation of GVHD and distinct properties of natural killer (NK) cell alloreactivity in engraftment and GVL that are of therapeutic importance. Finally, a detailed map of cellular immune recovery post-transplant is emerging which highlights the importance of post-thymic lymphocytes in determining outcome in the critical first few months following stem cell transplantation. Factors that modify immune reconstitution include immunosuppression, GVHD, the cytokine milieu and poorly-defined homeostatic mechanisms which encourage irregular T cell expansions driven by immunodominant T cell–antigen interactions.

In Section II, Prof. Anne Dickinson and colleagues describe genetic polymorphisms outside the human leukocyte antigen (HLA) system that determine the nature of immune reconstitution after allogeneic stem cell transplantation (SCT) and thereby affect transplant outcomethrough GVHD, GVL, and transplant-related mortality. Polymorphisms in cytokine gene promotors and other less characterized genes affect the cytokine milieu of the recipient and the immune reactivity of the donor. Some cytokine gene polymorphisms are significantly associated with transplant outcome. Other non-HLA genes strongly affecting alloresponses code for minor histocompatibility antigens (mHA). Differences between donor and recipient mHA cause GVHD or GVL reactions or graft rejection. Both cytokine gene polymorphisms (CGP) and mHA differences resulting on donor-recipient incompatibilities can be jointly assessed in the skin explant assay as a functional way to select the most suitable donor or the best transplant approach for the recipient.

In Section III, Dr. Nelson Chao describes non-pharmaceutical techniques to control immune reconstitution post-transplant. T cells stimulated by host alloantigens can be distinguished from resting T cells by the expression of a variety of activation markers (IL-2 receptor, FAS, CD69, CD71) and by an increased photosensitivity to rhodamine dyes. These differences form the basis for eliminating GVHD-reactive T cells in vitro while conserving GVL and anti-viral immunity. Other attempts to control immune reactions post-transplant include the insertion of suicide genes into the transplanted T cells for effective termination of GVHD reactions, the removal of CD62 ligand expressing cells, and the modulation of T cell reactivity by favoring Th2, Tc2 lymphocyte subset expansion. These technologies could eliminate GVHD while preserving T cell responses to leukemia and reactivating viruses.

The immunogenomics of minor histocompatibility antigens

Minor histocompatibility (H) antigens are a diverse assemblage of major histocompatibility complex (MHC)-bound peptides with the unifying property of acting as alloantigens that induce allogeneic tissue rejection. They are a consequence of any form of accumulated genetic variation that translates to differential MHC-presented peptide epitopes, the most common form of which is simple sequence polymorphisms. The universe of potential minor H antigens is large when transplantation is performed between genetically unrelated, MHC-matched individuals, especially considering the remarkable discriminative sensitivity of T cells. However, the phenomenon of immunodominance greatly simplifies immune responses that ensue. One mouse minor H antigen, H60, stands out in that the preponderance of the CD8 T cell response elicited in a complex alloantigenic setting is directed against this single minor H antigen epitope. Its immunodominance is because mice lacking H60 develop an unusually robust T cell repertoire dedicated to this single minor H antigen. The now well-characterized mouse minor H antigen system should provide a vehicle to assess the degree to which immunodominant alloantigens contribute to transplant rejection.

Immunodominance of H60 is caused by an abnormally high precursor T cell pool directed against its unique minor histocompatibility antigen peptide

The H60 minor histocompatibility (H) antigen peptide is derived from a glycoprotein that serves as a ligand for the stimulatory NKG2D receptor. We show that this peptide is remarkably immunodominant in that it competes effectively with MHC alloantigens, is efficiently crosspresented by host antigen-presenting cells (APCs), and readily elicits naive CD8 T cell responses in vitro. H60 immunodominance is neither a consequence of NKG2D engagement nor competition among minor H antigens on APCs. Instead, H60 immunodominance is a consequence of an abnormally high naive precursor frequency of H60 peptide reactive CD8 T cells. Understanding why the H60 peptide is so immunogenic has important implications in tissue transplantation and vaccine design.

P-glycoprotein targeting: a unique strategy to selectively eliminate immunoreactive T cells

T lymphocytes have been found to harbor P-glycoprotein (Pgp) and to demonstrate modulation of its ion channel transporter function according to the state of activation of T lymphocytes. We hypothesized that cytotoxic chemicals that are extruded by Pgp could be used to specifically eliminate immunoreactive T-cell populations. In this study, we evaluated the capacity of 4,5-dibromorhodamine methyl ester (TH9402), a photosensitizer structurally similar to rhodamine, a dye transported by Pgp, and which becomes highly cytotoxic on activation with visible light to selectively deplete alloreactive T lymphocytes. Stimulation of T cells with mitogens or allogeneic major histocompatibility complex-mismatched cells resulted in the preferential retention of the TH9402 rhodamine-derivative in activated T cells, both CD4+ and CD8+. Photodynamic cell therapy of TH9402-exposed T cells led to the selective elimination of immunoreactive T-cell populations. In addition, this treatment preserved resting T cells and their capacity to respond to third-party cells. Inhibition of Pgp enhanced cellular trapping of the dye in nonactivated T cells and resulted in their depletion after exposure to light. Targeting of Pgp-deficient cells may therefore represent an appealing strategy for the prevention and treatment of graft-versus-host disease and other alloimmune or autoimmune disorders.

Allogeneic transplantation for multiple myeloma: further evidence for a GVHD-associated graft-versus-myeloma effect

We report a series of 37 consecutive patients with multiple myeloma (MM) who received an allograft between 1990 and 2000 at our institution. Median age was 47 years, and nearly 70% of patients were Durie-Salmon stage III. A median of five cycles of chemotherapy were given before transplant, with a median interval between diagnosis and transplant of 9.3 months. We report a nonrelapse mortality rate of 22% with a median follow-up period of 40 months, whereas complete remission (CR) rate at 12 months is estimated at 57%. Treatment failure rate and overall survival at 40 months are estimated at 52% and 32%, respectively. The number of chemotherapy cycles prior to allotransplantation achieved borderline statistical significance as a poor prognosis factor for overall survival (P = 0.05), while the presence of chronic graft-versus-host disease (cGVHD) was significantly correlated with CR achievement (P = 0.036). Our study confirms that early allografting in MM can yield toxicity rates significantly lower than those associated with historical cohorts, and supports the hypothesis that cumulative chemotoxicity has a negative influence on mortality and survival rates. More importantly, our study clearly demonstrates an association between cGVHD and CR and brings further evidence in favor of a graft-versus-myeloma effect.

Immunodomination results from functional differences between competing CTL

The presence of dominant epitopes suppresses generation of CTL activity toward other non-dominant epitopes found on the same antigen-presenting cell (APC). This phenomenon, termed immunodomination, drastically restricts the diversity of the repertoire of CTL responses. Under various experimental conditions we assessed the in vivo expansion by tetramer staining and function by expression of O-glycans and intracellular perforin of CTL specific for a dominant (B6(dom1)) and a non-dominant (HY) H2D(b)-restricted epitope. Immunodomination abrogated expansion rather than differentiation of HY-specific CTL. When immunodomination was precluded because HY was presented alone or because high numbers of antigen-bearing APC were present, the numbers of HY-specific T cells detected after antigen priming were similar to those of B6(dom1)-specific T cells. The main difference between T cells that recognized B6(dom1) versus HY was functional rather than quantitative. The key feature of T cells specific for B6(dom1) is that they show striking up-regulation of molecules involved in CTL effector activity rather than accumulating to particularly high levels, as assessed by tetramer staining. These results support the emerging concept that following antigen priming, CTL populations of similar size can display important differences in effector function, and suggest that these functional differences are instrumental in shaping the repertoire of CTL responses.

Adoptive transfer of minor histocompatibility antigen-specific T lymphocytes eradicates leukemia cells without causing graft-versus-host disease

Adoptive transfer of T cells reactive to minor histocompatibility antigens has the unmatched ability to eradicate malignant hematopoietic cells. Unfortunately, its use is hampered by the associated graft-versus-host disease. The critical issue of a possible dissociation of the antileukemic effect and graft-versus-host disease by targeting specific minor histocompatibility antigens remains unresolved because of the unknown nature and number of minor histocompatibility antigens necessary or sufficient to elicit anti-leukemic activity and graft-versus-host disease. We found that injection of T lymphocytes primed against a single major histocompatibility complex class I-restricted immunodominant minor histocompatibility antigen (B6dom1) caused no graft-versus-host disease but produced a curative anti-leukemic response. Avoidance of graft-versus-host disease required that no other host-reactive T cells be co-injected with T cells primed with B6dom1. Here we show that effective and non-toxic immunotherapy of hematologic malignancies can be achieved by targeting a single immunodominant minor histocompatibility antigen.

Generation of helper and cytotoxic CD4+T cell clones specific for the minor histocompatibility antigen H-Y, after in vitro priming of human T cells by HLA-identical monocyte-derived dendritic cells

BACKGROUND

There is now convincing evidence that minor histocompatibility antigens (mHag) may play a significant role in the pathogenesis of graft-versus-host disease after HLA-identical bone marrow transplantation. Indeed, in this clinical situation, T cells specific for mHag have been isolated. Here, we addressed whether one can generate mHag-specific T cells in vitro, without any in vivo immunization, among healthy blood donors.

METHODS

We used monocyte-derived dendritic cells (Mo-DCs) as antigen presenting cells to induce primary responses between healthy HLA-identical siblings, in mixed lymphocyte dendritic cell reactions (MLDCRs).

RESULTS

We show that CD4+ T-cell clones, specific for the mHag H-Y, can be generated in vitro. These clones were derived from a gender-mismatched positive MLDCR pair of HLA-identical siblings and were restricted by the HLA DQB1*0502 molecule. In addition, these CD4+ T clones were also able to lyse allogeneic targets with the same pattern of restriction and specificity than helper function. Finally, acute myeloid leukemia (AML) blast cells were susceptible to lysis by these clones.

CONCLUSIONS

Altogether, these results predict that Mo-DCs could help to generate class II-associated, mHag-specific, T-cell lines or clones in vitro, between healthy blood donors, without any need of transplantation-mediated immunization.

Genetic factors influencing the development of chronic graft-versus-host disease in a murine model

Graft-versus-host disease (GVHD) is a major complication of bone marrow transplantation that can occur in either acute or chronic forms. Much of the long-term pathology seen in chronic GVHD is a result of autoantibody production. In the DBA/2-->B6D2F1 murine model of chronic GVHD, anti-ssDNA autoantibodies can be detected by 14 days post cell transfer. These autoantibodies are not observed in B6D2F1 recipients of cells from C57BL/6 or B10.D2 donors, which develop acute rather than chronic GVHD. Therefore, in this model, donor genetic factors predispose to the development of chronic GVHD in recipients. We performed a genetic analysis aimed at mapping donor loci that influence the magnitude of early autoantibody production in B6D2F1 recipients of cells from DBA/2 donor mice. Linkage analysis suggested an influence of two loci: a locus on chromosome 11 linked to D11Mit278 and a locus on chromosome 4 linked to D4Mit226. The locus on chromosome 11 also appeared to influence the development of renal pathology associated with chronic GVHD.

Detection of a potent humoral response associated with immune-induced remission of chronic myelogenous leukemia

The effectiveness of donor-lymphocyte infusion (DLI) for treatment of relapsed chronic myelogenous leukemia (CML) after allogeneic bone marrow transplantation is a clear demonstration of the graft-versus-leukemia (GVL) effect. T cells are critical mediators of GVL, but the antigenic targets of this response are unknown. To determine whether patients who respond to DLI also develop B-cell immunity to CML-associated antigens, we analyzed sera from three patients with relapsed CML who achieved a complete molecular remission after infusion of donor T cells. Sera from these individuals recognized 13 distinct gene products represented in a CML-derived cDNA library. Two proteins, Jkappa-recombination signal-binding protein (RBP-Jkappa) and related adhesion focal tyrosine kinase (RAFTK), were recognized by sera from three of 19 DLI responders. None of these antigens were recognized by sera from healthy donors or patients with chronic graft-versus-host disease. Four gene products were recognized by sera from CML patients treated with hydroxyurea and nine were detected by sera from CML patients who responded to IFN-alpha. Antibody titers specific for RAFTK, but not for RBP-Jkappa, were found to be temporally associated with the response to DLI. These results demonstrate that patients who respond to DLI generate potent antibody responses to CML-associated antigens, suggesting the development of coordinated T- and B-cell immunity. The characterization of B cell-defined antigens may help identify clinically relevant targets of the GVL response in vivo.

Relapse after bone marrow transplantation: evidence for distinct immunological mechanisms between adult and paediatric populations

Donor lymphocyte infusions are particularly effective for remission induction in malignant cells in patients who relapse after allogeneic progenitor cell transplantation (PCT) and who remain sensitive to the administration of unprimed donor T and/or natural killer (NK) cells present in donor lymphocyte infusions. To determine whether relapse after unmanipulated PCT could be ascribed to donor T and/or NK cell loss or tolerization, we evaluated the chimeric status of 81 patients with haematological malignancies who were receiving allogeneic unmanipulated PCT. The incidence of mixed chimaerism (MC) in unfractionated mononuclear leucocyte samples decreased rapidly after transplant, and was not detectable 4 months after PCT, even in patients who subsequently relapsed. The chimeric status of immune effector cell subsets was then evaluated in 15 patients at the time of relapse. All adults demonstrated complete donor haematopoiesis (CDH) for all cell lineages, whereas T- and NK-cell MC was only found in patients younger than age 13 years (P = 0.004). MC was not found in T nor NK cells of a control group consisting of age-matched paediatric patients in remission after allogeneic PCT. Thus, in adults, T and NK cell MC disappears early after unmanipulated allogeneic PCT and is absent at the time of relapse. However, the identification of donor T and NK cell loss in the paediatric relapsed but not remission patients suggests a distinct mechanism of relapse.

The In Vivo Fate of APCs Displaying Minor H Antigen and/or MHC Differences Is Regulated by CTLs Specific for Immunodominant Class I-Associated Epitopes

The goal of this work was to evaluate the fate of APCs following interactions with T cells in unprimed mice with a normal T cell repertoire. We elaborated a model in which male adherent peritoneal mononuclear cells were injected into the foreleg footpads of naive female recipients mismatched for either minor or major histocompatibility Ags. At various times after injection, APC numbers in the draining (axillary and brachial) lymph nodes were assessed using a Ube1y gene-specific PCR assay. Our experimental model was designed so that the number of APCs expressing the priming epitope was similar to what is observed under real life conditions. Thus, early after injection, the frequency of afferent lymph-derived APCs expressing the priming epitope was in the range of 101–102/106 lymph node cells. We found that APCs presenting some, but not all, nonself epitopes were killed rapidly after entrance into the lymph nodes. Rapid elimination of APCs occurred following interactions with MHC class I-restricted, but not class II-restricted, T cells and was observed when APCs presented an immunodominant (B6dom1/H7a), but not a nondominant (HY), epitope. Killing of APCs was mediated partly, but not exclusively, by perforin-dependent process. We propose that killing of APCs by CTLs specific for immunodominant MHC class I-restricted epitopes may be instrumental in regulating the intensity, duration, and diversity of T cell responses.

Invertebrates versus vertebrates innate immunity: In the light of evolution

Invertebrates use a nonadaptive, innate immunity, the expression of germline encoded receptors, to identify the allogeneic and xenogeneic attributes. Vertebrates also have the capacity to express ontogenically related adaptive immunity which is a somatically selected gene rearrangement process. Several commonly accepted generalizations are utilized to explain the enigmatic lack of the adaptive immunity in invertebrates. All point to the primitive nature of the innate immunity and the primitive organization of the body plan and the life history patterns of invertebrates. Seven of the most common generalizations are reviewed and confuted by virtue of a biased literature presentation. Subsequently, three evolutionary puzzles are raised and the accepted paradigm that the vertebrate immunity is pathogenically directed is further challenged. This leads to an alternative idea suggesting that preserving the individuality against the threat of invading conspecific cells might have been the original function of the immune system. This ancient system has been co-opted later on to serve as a defence mechanism against pathogens. The secondary role arose in the form of a multiplicity of newly developed phenomena, one of them being the vertebrate adaptive immunity. This proposal is supported by the fact that vertebrates still exhibit two distinct but common types of naturally occurring transplantation events (natural chimerism) and by a variety of recent studies, providing evidence for the crucial role of the vertebrate's innate immunity in signalling and triggering the acquired effector mechanisms.

Correlation Between Disparity for the Minor Histocompatibility Antigen HA-1 and the Development of Acute Graft-Versus-Host Disease After Allogeneic Marrow Transplantation

Results of a previous study suggested that recipient mismatching for the minor histocompatibility antigen HA-1 is associated with acute graft-versus-host disease (GVHD) after allogeneic marrow transplantation. In that study, most patients received either cyclosporine or methotrexate for GVHD prophylaxis, and a cytotoxic T-cell clone was used to test for HA-1 disparity. To facilitate large-scale testing, we developed a method that uses genomic DNA to identify HA-1 alleles. A retrospective study was conducted to correlate HA-1 disparity and the occurrence of acute GVHD in 237 HLA-A2–positive white patients who had received a marrow or peripheral blood stem cell transplant from an HLA-identical sibling. All patients received both methotrexate and cyclosporine for GVHD prophylaxis. The presence of HLA-A*0201 was confirmed in 34 of the 36 HA-1 disparate pairs by sequencing the HLA-A locus. Grades II-IV GVHD occurred in 22 (64.7%) of these 34 patients, compared with 86 (42.8%) of the 201 patients without HA-1 disparity (odds ratio, 2.45; 95% confidence interval [CI], 1.15 to 5.23; P = .02). Recipient HA-1 disparity showed a trend for association with acute GVHD (odds ratio, 2.1; 95% CI, 0.91 to 4.68; P = .08) when a multivariable logistic regression model was used to include additional risk factors. These data are consistent with results of the previous study, suggesting an association between HA-1 disparity and risk of acute GVHD, but the strength of this association may be lower in patients who received both methotrexate and cyclosporine than in those who received methotrexate or cyclosporine alone.

Characterization and manipulation of T cell immunity to skin grafts expressing a transgenic minor antigen

BACKGROUND

Minor histocompatibility antigens play a significant role in allograft rejection when donor and recipient are matched at MHC loci. An improved understanding of T cell immunity directed toward a model minor antigen may provide new approaches for preventing graft rejection.

METHODS

C57BL/6 (B6) recipient mice were engrafted with skin from B6 beta-galactosidase transgenic (beta-gal tg) donors and the induced T cell immune responses were characterized by cytokine ELISA spot assay. beta-gal-specific immunity was manipulated prior to transplant through preinjection with beta-gal in complete Freund's adjuvant (CFA) or through preinjection with soluble beta-gal i.v.

RESULTS

B6 mice rejected beta-gal tg skin by day 25. Rejection was associated with a low frequency of predominantly CD8+, interferon-gamma-producing T cells capable of directly recognizing both beta-gal tg cells and an immunodominant major histocompatibility complex I-restricted peptide derived from the beta-gal protein. Rejection of multiple minor antigen disparate skin and major histocompatibility complex-disparate skin occurred significantly faster, and was associated with a 10- to 30-fold higher frequency of alloreactive T cells, than rejection of beta-gal tg skin. Prepriming of recipients with beta-gal in complete Freund's adjuvant resulted in an increased frequency of beta-gal-specific T cells and accelerated rejection of beta-gal tg skin. Intravenous injection of soluble beta-gal-induced graft tolerance and a lack of detectable beta-gal-specific immunity.

CONCLUSIONS

The findings reveal that transgenically expressed beta-gal behaves as a minor transplantation antigen and that manipulation of the beta-gal-specific T cell repertoire can dramatically affect rejection of beta-gal tg skin grafts. The work provides the foundation for mechanistic studies of tolerogenesis to minor antigenic determinants.

The role of intestinal bacterial flora in the tuning of the T cell repertoire

The role of the intestinal bacterial flora on the Vbeta repertoire was examined using the gnotobiotic murine model. The ratio of Vbeta6-positive T cells in the periphery of DBA/2 mice under SPF conditions was only 2.2% (mean, n = 4), since the cells were eliminated by the endogenous superantigen Mls(a). However, the ratio in germ-free (GF) mice was 31.7%. Similarly, the contamination of the GF Mice with the intestinal flora from SPF mice reduced the ratio of Vbeta6 in GF mice from 22.9% to 13.7%. In contrast, in BALB/c mice (Mls(b)) in which Vbeta6 cells do not react with this endogenous superantigen, the ratio of Vbeta6 cells do not react with this endogenous superantigen, the ratio of Vbeta6 of SPF mice (15.4%, mean, n = 3) was found to be comparable to that of GF mice (15.6%, n = 3). These data suggested that the absence of intestinal flora deteriorated a part of the Mls(a) determinant, which reacted with the Vbeta6 T cells and thereby eliminated them, thus resulting in an increase of these cells in GF mice. Moreover, the alloantigenicity of minor histocompatible alloantigen(s) (mHAg) in SPF mice, which was detected in H-2 identical MLR experiments and a murine graft-versus-host (GVH) model, was reduced in GF and decontaminated SPF mice, thus indicating that the intestinal flora upregulated the mHAg including a part of Mls determinant. These results therefore suggest that the intestinal flora plays a role in the upregulation of mHAg including a part of endogenous superantigen and the consequent tuning of the Vbeta repertoire.

Biochemical and Immunogenetic Analysis of an Immunodominant Peptide (B6dom1) Encoded by the Classical H7 Minor Histocompatibility Locus

Of the many minor histocompatibility (H) Ags that have been detected in mice, the ability to induce graft vs host disease (GVHD) after bone marrow transplantation is restricted to a limited number of immunodominant Ags. One such murine Ag, B6dom1, is presented by the H2-Db MHC class I molecule. We present biochemical evidence that the natural B6dom1 peptide is indistinguishable from AAPDNRETF, and we show that this peptide can be isolated from a wide array of tissues, with highest levels from the lymphoid organs and lung. Moreover, we employ a novel, somatic cell selection technique involving CTL-mediated immunoselection coupled with classical genetics, to show that B6dom1 is encoded by the H7 minor H locus originally discovered ∼40 years ago. These studies provide a molecular genetic framework for understanding B6dom1, and exemplify the fact that mouse minor H loci that encode immunodominant CTL epitopes can correspond to classical H loci originally identified by their ability to confer strong resistance to tumor transplantation. Additionally, these studies demonstrate the utility of somatic cell selection approaches toward resolving H Ag immunogenetics.