Much ado about minor histocompatibility antigens

MSCs Become Collagen-Type I Producing Cells with Different Phenotype in Allogeneic and Syngeneic Bone Marrow Transplantation

Mesenchymal stem cells (MSCs) have been widely used in therapeutic applications for many decades. However, more and more evidence suggests that factors such as the site of origin and pre-implantation treatment have a crucial impact on the result. This study investigates the role of freshly isolated MSCs in the lacrimal gland after allogeneic transplantation. For this purpose, MSCs from transgenic GFP mice were isolated and transplanted into allogeneic and syngeneic recipients. While the syngeneic MSCs maintained a spherical shape, allogeneic MSCs engrafted into the tissue as spindle-shaped cells in the interstitial stroma. Furthermore, the MSCs produced collagen type I in more than 85% to 95% of the detected GFP+ MSCs in the recipients of both models, supposedly contributing to pathogenic fibrosis in allogeneic recipients compared to syngeneic models. These findings indicate that allogeneic MSCs act completely differently from syngeneic MSCs, highlighting the importance of understanding the exact mechanisms behind MSCs.

Impact of genomic polymorphisms on the repertoire of human MHC class I-associated peptides

For decades, the global impact of genomic polymorphisms on the repertoire of peptides presented by major histocompatibility complex (MHC) has remained a matter of speculation. Here we present a novel approach that enables high-throughput discovery of polymorphic MHC class I-associated peptides (MIPs), which play a major role in allorecognition. On the basis of comprehensive analyses of the genomic landscape of MIPs eluted from B lymphoblasts of two MHC-identical siblings, we show that 0.5% of non-synonymous single nucleotide variations are represented in the MIP repertoire. The 34 polymorphic MIPs found in our subjects are encoded by bi-allelic loci with dominant and recessive alleles. Our analyses show that, at the population level, 12% of the MIP-coding exome is polymorphic. Our method provides fundamental insights into the relationship between the genomic self and the immune self and accelerates the discovery of polymorphic MIPs (also known as minor histocompatibility antigens).

Mass spectrometry (MS) has furthered our understanding of MHC class I-associated peptides (MIPs), but the technique is inadequate for studying MIP-associated polymorphisms. Here, the authors combine high-throughput MS with exome and transcriptome sequencing to identify polymorphic MIPs from two female siblings.

Occurrence and Impact of Minor Histocompatibility Antigens' Disparities on Outcomes of Hematopoietic Stem Cell Transplantation from HLA-Matched Sibling Donors

We have examined the alleles of eleven minor histocompatibility antigens (MiHAs) and investigated the occurrence of immunogenic MiHA disparities in 62 recipients of allogeneic hematopoietic cell transplantation (allo-HCT) with myeloablative conditioning performed between 2000 and 2008 and in their HLA-matched sibling donors. Immunogenic MiHA mismatches were detected in 42 donor-recipient pairs: in 29% MiHA was mismatched in HVG direction, in another 29% in GVH direction; bidirectional MiHA disparity was detected in 10% and no MiHA mismatches in 32%. Patients with GVH-directed HY mismatches had lower both overall survival and disease-free survival at 3 years than patients with compatible HY; also higher incidence of both severe acute GvHD and extensive chronic GVHD was observed in patients with GVH-directed HY mismatch. On contrary, GVH-directed mismatches of autosomally encoded MiHAs had no negative effect on overall survival. Results of our study help to understand why posttransplant courses of allo-HCT from siblings may vary despite the complete high-resolution HLA matching of a donor and a recipient.

Immunological barriers to stem-cell based cardiac repair

Repair of damaged myocardium with pluripotent stem cell derived cardiomyocytes is becoming increasingly more feasible. Developments in stem cell research emphasize the need to address the foreseeable problem of immune rejection following transplantation. Pluripotent stem cell (PSC) derived cardiomyocytes have unique immune characteristics, some of which are not advantageous for transplantation. Here we review the possible mechanisms of PSC-derived cardiomyocytes rejection, summarize the current knowledge pertaining to immunogenicity of such cells and describe the existing controversies. Myocardial graft rejection can be reduced by modifying PSCs prior to their differentiation into cardiomyocytes. Overall, this approach facilitates the development of universal donor stem cells suitable for the regeneration of many different tissue types.

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.

Concurrent allorecognition has a limited impact on posttransplant vaccination

Transplantation of allogeneic hematopoietic stem cells with or without immunocompetent lymphocytes has proved a successful strategy in the treatment of hematological malignancies. We have recently shown that this approach can also cure mouse prostate cancer, provided that it is combined with tumor-specific vaccination. Whether the response to alloantigens acts by providing helper function to enhance vaccine-specific responses or in other ways impinges on vaccine immunogenicity remains to be clarified, and this question is of clinical relevance. In this study, we have addressed this issue by comparing the immunogenicity of dendritic cells pulsed with a peptide derived from a tumor/viral model Ag in recipients of donor cells either syngeneic to the host or differing for either Y-encoded or multiple minor H antigens. We report that vaccination elicits comparable proliferation and differentiation of peptide-specific CD8(+) T cells despite concurrent expansion and differentiation of minor H antigen-specific IFN-γ effector T cells. Depletion of alloreactive CD4(+) T cells reduced alloreactivity but not vaccine-induced CD8(+) T cell priming, suggesting that alloresponses do not provide helper functions in peripheral lymphoid tissues. Vaccine-mediated T cell priming was also preserved in the case of multiple minor H antigen disparities, prone to graft-versus-host disease. Thus, in the context of nonmyeloablative allotransplantation aimed at restoring an effective tumor-specific T cell repertoire, minor H antigen-specific T cells do not interfere with vaccine-induced T cell priming, supporting the notion that posttransplant vaccination is a valuable strategy to boost tumor and pathogen-specific protective immunity.

Effects of mismatching for minor histocompatibility antigens on clinical outcomes in HLA-matched, unrelated hematopoietic stem cell transplants

Several studies in HLA-matched sibling hematopoietic stem cell transplantation (HSCT) have reported an association between mismatches in minor histocompatibility antigens (mHAg) and outcomes. We assessed whether single and multiple minor mHAg mismatches are associated with outcomes in 730 unrelated donor, HLA-A, B, C, DRB1, and DQB1 allele-matched hematopoietic stem cell transplants (HSCT) facilitated by the National Marrow Donor Program (NMDP) between 1996 and 2003. Patients had acute and chronic leukemia or myelodysplastic syndrome (MDS), received myeloablative conditioning regimens and calcineurin inhibitor-based graft-versus-host-disease (GVHD) prophylaxis, and most received bone marrow (BM; 85%). Donor and recipient DNA samples were genotyped for mHAg including: HA-1, HA-2, HA-3, HA-8, HB-1 and CD31(125/563). Primary outcomes included grades III-IV acute GVHD (aGVHD) and survival; secondary outcomes included chronic GVHD (cGVHD), engraftment, and relapse. Single disparities at HA-1, HA-2, HA-3, HA-8, and HB-1 were not significantly associated with any of the outcomes analyzed. In HLA-A2-positive individuals, single CD31(563) or multiple mHAg mismatches in the HVG vector were associated with lower risk of grades III-IV aGVHD. Based on these data, we conclude that mHAg incompatibility at HA-1, HA-2, HA-3, HA-8, HB-1, and CD31 has no detectable effect on the outcome of HLA matched unrelated donor HSCT.

Immunological behavior of enhanced green fluorescent protein (EGFP) as a minor histocompatibility antigen with a special reference to skin isograft and specific regulation of local graft-versus-host reaction (GvHR)

Although enhanced green fluorescent protein (EGFP) is widely used as a molecular tag in cell biology, it has become evident that immunogenicity of transgenic or transduced EGFP is important when it applies to transplantation model. Indeed, it appears that applications of EGFP-expressing cells, tissues and organ transplantation are limited in vivo due to the ultimate rejection of the graft. Nevertheless, the immunological behavior of transduced EGFP, in particular, as a minor histocompatibility antigen is not fully understood. Thus employing two strains of EGFP transgenic (Tg) rats generated by the same vector construct, e.g., EGFP-F344 Tg (RT11) and EGFP-DA Tg (RT1a), and its F(1) hybrid with a non-transgenic rat, behavior of EGFP-transgenic antigen(s) was examined by in vivo assays, such as EGFP-transgenic test skin grafts or regulation of EGFP-transgenic lymphocytes. In the latter system, EGFP-specific, T-cell-mediated immune regulation of local graft-versus-host reaction (GvHR) was further investigated with a special reference of in vivo cytotoxic assay, i.e., elimination of colored lymphocytes with either EGFP-incompatible or CFSE-labeled sex-mismatched lymphocytes. We provide evidence that differential immunological behavior of EGFP-transgenic minor histocompatibility antigen was observed in vivo. Thus, immune responses to EGFP-minor histocompatibility antigen(s) were not always accompanied with the rejection of test skin isograft. It only becomes apparent for EGFP-specific elimination and suppression of both systemic and local GvHR induced by EGFP-transgenic T lymphocytes after EGFP-specific sensitization. However, this was not the case where test skin isografting was applied even under extensive sensitization protocols. These findings demonstrate that minor histocompatibility antigen specific immune elimination of EGFP-transgenic T lymphocytes or regulation of local GvHR provides more sensitive and better immune assay systems in vivo than classical test skin isograft systems.

Phenotype frequencies of autosomal minor histocompatibility antigens display significant differences among populations

Minor histocompatibility (H) antigens are allogeneic target molecules having significant roles in alloimmune responses after human leukocyte antigen–matched solid organ and stem cell transplantation (SCT). Minor H antigens are instrumental in the processes of transplant rejection, graft-versus-host disease, and in the curative graft-versus-tumor effect of SCT. The latter characteristic enabled the current application of selected minor H antigens in clinical immunotherapeutic SCT protocols. No information exists on the global phenotypic distribution of the currently identified minor H antigens. Therefore, an estimation of their overall impact in human leukocyte antigen–matched solid organ and SCT in the major ethnic populations is still lacking. For the first time, a worldwide phenotype frequency analysis of ten autosomal minor H antigens was executed by 31 laboratories and comprised 2,685 randomly selected individuals from six major ethnic populations. Significant differences in minor H antigen frequencies were observed between the ethnic populations, some of which appeared to be geographically correlated.

Peptides from cellular proteins evoking alloimmune responses in human leukocyte antigen–identical transplantation are called minor histocompatibility (H) antigens. Upon hematopoietic stem cell transplantation (HSCT) for hematological malignancies or for solid tumors, responses to minor H antigens may have detrimental effects, e.g., graft-versus-host-disease and graft rejection, but can also significantly contribute to the eradication of the tumor cells. We designated the latter antigens as “tumor” minor H antigens. Current clinical trials aim at using these tumor minor H antigens to boost the graft-versus-tumor response. So far, it is unclear how frequently the HSCT recipient and donor differ in their minor H antigens and thus how many cancer patients are eligible for minor H antigen-based treatment. Therefore, worldwide 31 laboratories joined forces to determine the genotype and phenotype frequencies of ten autosomally encoded minor H antigens in six ethnic populations. The frequencies vary depending upon ethnic background and geographic location of the population, implying that the potential applicability of the tumor minor H antigens differs from one population to another. Depending on the population, the current theoretical percentages of clinical application of the eight tumor minor H antigens in HLA-matched combinations is estimated at 12% to 34%.

Sensitization to enhanced green fluorescence protein minor histocompatibility antigen by gene transduction into dendritic cells and peritoneal exudate macrophages

Enhanced green fluorescence protein (EGFP) has been widely applied to gene transduction in cellular and molecular biology as a reporter element. When applied to cell transplantation, it raises fundamental issues concerning cell-associated antigens, in particular, a model of minor histocompatibility antigen(s). Although it is well known that immunological behavior of minor histocompatibility antigens mimic tumor associated antigens (TAA), identified genes coding minor histocompatibility antigens are few and far between. Inasmuch as immunity and tolerance to TAA are provided by immunological behavior of minor histocompatibility antigen such as histocompatibility antigen of the Y chromosome, H-Y, it occurs to us that transgenic as well as transduced EGFP provides a useful model system to be applied to tumor immunology. In this respect, genetic modification of specialized antigen-presenting cells (APC), i.e., dendritic cells (DC), such as gene transduction of EGFP into DC, would provide one of the most important strategies in transplantation as well as tumor immunology inasmuch as DC play a key role in initiating primary immune responses, As far as gene transduction into DC is concerned, others have reported that successful gene transduction occurs in DC by adenoviral vector systems. However, our previous studies concerning EGFP transduction into DC suggested that this view should be carefully examined and interpreted. Employing adenoviral and lentiviral vector systems as well as specialized APC of rat DC and peritoneal exudate macrophages (PEM), EGFP-transduced APC were examined to determine whether and to what extent the EGFP-transduced APC were able to sensitize non-transgenic littermates against transgenic EGFP as antigen(s). Thus EGFP-transgenic cardiac isografts were transplanted to non-transgenic littermates and examined to determine if sensitization of non-transgenic littermate recipients with the EGFP-transduced APC was able to reject the test grafts in an accelerated manner. In this study, we examined this and provide further evidence that widely used viral vector systems are unable to transfer the reporter gene EGFP into mature rat DC generated from bone marrow cells (BMC), driven by Flt3/Flk2 ligand and IL-6. Nevertheless, successful gene transduction was obtained by either applying a lentiviral vector system to the developing DC progenitor cells during a long-term culture of rat BMC or by applying an adenoviral vector system to PEM. Thus, successful gene transduction into specialized APC was verified by in vivo priming of non-transgenic littermates with the EGFP-transduced APC, followed by accelerated rejection of EGFP-transgenic cardiac isografts.

Allorecognition and the alloresponse: clinical implications

The artificial transfer of tissues or cells between genetically diverse individuals elicits an immune response that is adaptive and specific. This response is orchestrated by T lymphocytes that are recognizing, amongst others, major histocompatibility complex (MHC) molecules expressed on the surface of the transferred cells. Three pathways of recognition are described: direct, indirect and semi-direct. The sets of antigens that are recognized in this setting are also discussed, namely, MHC protein products, the MHC class I-related chain (MIC) system, minor histocompatibility antigens and natural killer cell receptor ligands. The end product of the effector responses are hyperacute, acute and chronic rejection. Special circumstances surround the situation of pregnancy and bone marrow transplantation because in the latter, the transferred cells are the ones originating the immune response, not the host. As the understanding of these processes improves, the ability to generate clinically viable immunotherapies will increase.

Autoantibodies to vimentin cause accelerated rejection of cardiac allografts

Autoimmune responses to vimentin occur after solid organ transplantation, but their pathogenic effects are unclear. The aim of these studies was to investigate the effects of vimentin preimmunization on allogeneic and isografted hearts in a murine transplant model. Immunization of C57BL/6 mice with murine vimentin in complete Freund's adjuvant resulted in anti-vimentin antibodies and vimentin-reactive Th-1 cells. Transplantation of 129/sv hearts into vimentin-immunized C57BL/6 recipients resulted in accelerated rejection (8.4 +/- 1.5 days; n = 18), compared with hen egg lysozyme-immunized C57BL/6 (13.3 +/- 2.2 days; n = 10; P < 0.0001, log-rank test). In contrast, isografts continued to beat beyond 90 days. Immunohistochemical analysis of allografts from vimentin/complete Freund's adjuvant mice demonstrated increased numbers of T cells and enhanced microvascular deposition of C3d, CD41, and P-selectin compared with controls. Antibodies were necessary for accelerated rejection, shown by the fact that vimentin-immunized B-cell-deficient IgH6 mice did not show accelerated rejection of 129/sv allografts, but rejection was restored by adoptive transfer of serum containing anti-vimentin antibodies. Eluates from donor hearts placed in vimentin/complete Freund's adjuvant recipients contained anti-vimentin antibodies, shown by Western blotting. Confocal imaging of rejected hearts de-monstrated presence of vimentin and C3d on apoptosed leukocytes, endothelial cells, and platelet/leukocyte conjugates. These results demonstrate that autoantibodies to vimentin, in conjunction with the alloimmune response, have a pathogenic role in allograft rejection.

A novel approach to detect donor/recipent immune responses between HLA-identical pairs

Renal transplant rejection and graft versus host reactions between HLA genetically-identical sibling (HLAgi) donor/recipient (D/R) pairs are thought to result from minor histocompatibility antigen (mHAg) disparities. We have compared two methods of measuring HLAgi D/R T lymphocyte responses to "matured" dendritic cells: 1.) a modified Cylex assay of CD4(+) ATP levels (MLDC-ATP) versus 2.) (3)H-thymidine uptake (MLDC-(3)H). The MLDC-ATP kinetics peaked at 48 hours versus the MLDC-(3)H at 7 days, and appeared more sensitive. We tested HLAgi (normal) volunteer siblings (NLs), and D/R sibling pairs before and after renal transplantation (pre-Tx and post-Tx). The overall frequencies of positive responses in the MLDC-ATP for HLAgi NLs, pre-Tx, and post-Tx D/R pairs were 63%, 50%, and 42%, respectively. The percentage with reciprocal responses was 37.5%, 20%, and 22.22%, respectively. In one set of three HLAgi (NLs) siblings (two males and one female), there was a nongender-associated differential response. There was no MLDC correlation with class I MHC-associated mHAg (SSP) incompatibility, nor could some MLDC positive reactive pairs theoretically process the necessary HLA-class I restriction molecules for presentation of known (nanomeric) mHAg peptides. Speculatively, the MLDC reflects class II MHC-restricted mHAg reactions (not yet definable), with possible effects of other polymorphic (nonhistocompatibility) immune response genes, and thereby may be a useful measurement of CD4(+) T-cell HLAgi transplantation immunity.

Multiplex Genotyping of Human Minor Histocompatibility Antigens

Minor histocompatibility antigens (mHAg) induce major histocompatibility complex-restricted, T cell-mediated immune responses that may contribute to increased risk of graft-versus-host disease and graft-versus-leukemia effects. Unlike human leukocyte antigen genes, mHAg are encoded by genetically and functionally unrelated genes located throughout the chromosome. The role of mHAg in stem cell transplantation and the population frequencies of mHAg alleles remain unknown due in part to the lack of suitable high throughput methods for genotyping these diverse genes. Here we describe the development and utility of a multiplexed Luminex assay for genotyping human mHAg, including HA-1, HA-2, HA-3, HA-8, HB-1, CD31(125), and CD31(563). The assay uses a multiplexed, allele-independent, gated amplification of mHAg genes followed by differential detection of allele-specific primer extension products using the MultiCode PLx system (EraGen Biosciences, Madison, WI). The alleles are interrogated using a multiplex allele-specific primer extension reaction using primers tagged with EraCodes. The products are hybridized to Luminex beads and the hybridization duplexes are detected using streptavidin-phycoerythrin. The assay resolved the mHAg genotypes of 259 Caucasian donors and provided population estimates of mHAg gene and phenotypic frequencies. All mHAg alleles evaluated in this study exhibited Hardy-Weinberg equilibrium, although some mHAg phenotypes were present in large majority of individuals tested (HA-2, HB-1). This assay will provide a valuable tool for determining mHAg frequencies in other ethnic populations, as well as for establishing the clinical importance of mHAg disparities in stem cell transplantation.

Differential immunogenicity of HLA mismatches in clinical transplantation

Although HLA matching is beneficial in clinical transplantation, it is not feasible to select a completely HLA matched donor for every potential recipient because of the enormous polymorphism of the HLA system. As a consequence, the majority of the recipients will be transplanted with a mismatched donor organ or hematopoietic stem cell transplant. For this large group of patients it is important to take advantage of the differential immunogenicity of HLA mismatches and to select for them a donor with HLA mismatches of low immunogenicity, the so-called acceptable mismatches. The differential immunogenicity of HLA mismatches can be determined by either retrospective analysis of graft survival data or by in vitro assays measuring T-cell and B-cell alloreactivity. A recently developed computer algorithm (HLAMatchmaker) can be instrumental in selecting donors with HLA mismatches, which do not lead to alloantibody formation. The theoretical background and practical implications of this acceptable mismatch approach are discussed.

Modulation of tissue-specific immune response to cardiac myosin can prolong survival of allogeneic heart transplants

The role of immune response to tissue-specific Ags in transplant rejection is poorly defined. We have previously reported that transplantation of cardiac allografts triggers a CD4(+) Th1 cell response to cardiac myosin (CM), a major contractile protein of the heart, and that pretransplant activation of proinflammatory CM-specific T cells accelerates rejection. In this study, we show that administration of CM together with IFA (CM/IFA) can prevent acute rejection of an allogeneic heart transplant. Prolongation of cardiac graft survival is associated with activation of CM- and allo-specific T cells secreting type 2 cytokines (IL-4, IL-5) and reduction of the frequency of proinflammatory IFN-gamma-secreting (type 1) alloreactive T cells. Blocking of IL-4 cytokine with Abs abrogates the prolongation. CM/IFA treatment prevents acute rejection of MHC class I-mismatched, but not fully mismatched grafts. However, if donor heart is devoid of MHC class II expression, CM-IFA administration delays rejection of fully allogeneic cardiac transplants. This finding suggests that the effect of CM modulation depends on the type (direct vs indirect) and strength of recipient's CD4(+) T cell alloresponse. Our results underscore the important role of host immunity to tissue-specific Ags in the rejection of an allograft. This study demonstrates that modulation of the immune response to a tissue-specific Ag can significantly prolong cardiac allograft survival, an observation that may have important implications for the development of novel selective immune therapies in transplantation.

Improving the outcome of unrelated donor stem cell transplantation by molecular matching

Volunteer unrelated donor (VUD) stem cell transplantation is now a well-established procedure in the treatment for many haematological and other disorders. The improved success of this modality of treatment is related, in part, to the existence of large volunteer donor registries (with well characterized tissue typing), as well as to the improved understanding of the molecular factors that have an influence on transplantation outcome. It is clear that close attention to human leukocyte antigen (HLA) matching is essential in ensuring a satisfactory transplant outcome, however the extent to which donor-recipient pairs need to be matched is not yet clear. There is also an increased understanding that factors other than HLA do affect clinical outcome. The ability to perform high resolution molecular typing techniques has allowed researchers to begin assessing the significance of mismatches at particular loci against an otherwise matched background, and in this way highlight the effects of individual genetic factors on transplantation outcome.

TCR usage in naive and committed alloreactive cells: implications for the understanding of TCR biases in transplantation

The direct pathway of allorecognition is involved in acute allograft rejection and is characterised by TCR-mediated recognition of the MHC framework; this is thought to occur in a peptide-dependent but not peptide-specific manner. In contrast, the indirect pathway is restricted to the recipient's own MHC molecules and prevails in chronic rejection. In this pathway, the peptide has a major influence on the TCR recognition and selects alloreactive T cells with altered TCR Vbeta usage. However, qualitative analysis of Vbeta usage alone might limit our understanding of alloreactivity. The advantages of a combined quantitative assessment of Vbeta mRNA usage are discussed.

Immature dendritic cells acquire CD8(+) cytotoxic T lymphocyte priming capacity upon activation by T helper cell-independent or -dependent stimuli

The well defined, immature murine dendritic cell (DC) line D1 was used to study the role of DC maturation in CTL induction in vitro and in vivo. Maturation of D1 cells, characterized by markedly increased expression of MHC and costimulatory molecules, was induced by incubation with lipopolysaccharide, agonistic CD40 antibody, or specific CD4(+) T helper (Th) cells. Activated, but not immature, D1 cells efficiently primed alloreactive T cell responses in vitro. Similarly, priming of CTL immunity in vivo in CD4-depleted mice was only observed if these mice were immunized with activated D1 cells. This study provides formal evidence that activation of DCs, induced by Th-independent as well as Th-dependent stimuli, is essential for efficient induction of CTL responses.