Cross-recognition of HLA DR4 alloantigen by virus-specific CD8+ T cells: a new paradigm for self-/nonself-recognition

Important denominator between autoimmune comorbidities: a review of class II HLA, autoimmune disease, and the gut

Human leukocyte antigen (HLA) genes are associated with more diseases than any other region of the genome. Highly polymorphic HLA genes produce variable haplotypes that are specifically correlated with pathogenically different autoimmunities. Despite differing etiologies, however, many autoimmune disorders share the same risk-associated HLA haplotypes often resulting in comorbidity. This shared risk remains an unanswered question in the field. Yet, several groups have revealed links between gut microbial community composition and autoimmune diseases. Autoimmunity is frequently associated with dysbiosis, resulting in loss of barrier function and permeability of tight junctions, which increases HLA class II expression levels and thus further influences the composition of the gut microbiome. However, autoimmune-risk-associated HLA haplotypes are connected to gut dysbiosis long before autoimmunity even begins. This review evaluates current research on the HLA-microbiome-autoimmunity triplex and proposes that pre-autoimmune bacterial dysbiosis in the gut is an important determinant between autoimmune comorbidities with systemic inflammation as a common denominator.

A class-mismatched TCR bypasses MHC restriction via an unorthodox but fully functional binding geometry

MHC restriction, which describes the binding of TCRs from CD4⁺ T cells to class II MHC proteins and TCRs from CD8⁺ T cells to class I MHC proteins, is a hallmark of immunology. Seemingly rare TCRs that break this paradigm exist, but mechanistic insight into their behavior is lacking. TIL1383I is a prototypical class-mismatched TCR, cloned from a CD4⁺ T cell but recognizing the tyrosinase tumor antigen presented by the class I MHC HLA-A2 in a fully functional manner. Here we find that TIL1383I binds this class I target with a highly atypical geometry. Despite unorthodox binding, TCR signaling, antigen specificity, and the ability to use CD8 are maintained. Structurally, a key feature of TIL1383I is an exceptionally long CDR3β loop that mediates functions that are traditionally performed separately by hypervariable and germline loops in canonical TCR structures. Our findings thus expand the range of known TCR binding geometries compatible with normal function and specificity, provide insight into the determinants of MHC restriction, and may help guide TCR selection and engineering for immunotherapy.

CD8 coreceptor engagement of MR1 enhances antigen responsiveness by human MAIT and other MR1-reactive T cells

Mucosal-associated invariant T (MAIT) cells detect microbial infection via recognition of riboflavin-based antigens presented by the major histocompatibility complex class I (MHC-I)-related protein 1 (MR1). Most MAIT cells in human peripheral blood express CD8αα or CD8αβ coreceptors, and the binding site for CD8 on MHC-I molecules is relatively conserved in MR1. Yet, there is no direct evidence of CD8 interacting with MR1 or the functional consequences thereof. Similarly, the role of CD8αα in lymphocyte function remains ill-defined. Here, using newly developed MR1 tetramers, mutated at the CD8 binding site, and by determining the crystal structure of MR1-CD8αα, we show that CD8 engaged MR1, analogous to how it engages MHC-I molecules. CD8αα and CD8αβ enhanced MR1 binding and cytokine production by MAIT cells. Moreover, the CD8-MR1 interaction was critical for the recognition of folate-derived antigens by other MR1-reactive T cells. Together, our findings suggest that both CD8αα and CD8αβ act as functional coreceptors for MAIT and other MR1-reactive T cells.

Construction of the prognostic enhancer RNA regulatory network in osteosarcoma

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Our enhancer RNAs-based prognostic model showed good predictive ability in osteosarcoma.

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CCAAT enhancer binding protein alpha (CEBPA) may regulate CD8A molecule (CD8A).

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CD8A activation may promote CD3E molecule (CD3E) expression and activate allograft rejection in CD8+ T cells.

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Above signal axis provided new insights in the mechanism of osteosarcoma tumorigenesis.

Background

Osteosarcoma (OS) is a common malignant tumor in osteoarticular system, the 5-year overall survival of which is poor. Enhancer RNAs (eRNAs) have been implicated in the tumorigenesis of various cancer types, whereas their roles in OS tumorigenesis remains largely unclear.

Methods

Differentially expressed eRNAs (DEEs), transcription factors (DETFs), target genes (DETGs) were identified using limma (Linear Models for Microarray Analysis) package. Prognosis-related DEEs were accessed by univariate Cox regression analysis. A multivariate model was constructed to evaluate the prognosis of OS samples. Prognosis-related DEEs, DETFs, DETGs, immune cells, and hallmark gene sets were co-analyzed to construct an regulatory network. Specific inhibitors were also filtered by connectivity Map analysis. External validation and scRNA-seq analysis were performed to verify our key findings.

Results

3,981 DETGs, 468 DEEs, 51 DETFs, and 27 differentially expressed hallmark gene sets were identified. A total of Multivariate risk predicting model based on 18 prognosis-related DEEs showed a high accuracy (area under curve (AUC) = 0.896). GW-8510 was the candidate inhibitor targeting prognosis-related DEEs (mean = 0.670, p < 0.001). Based on the OS tumorigenesis-related regulation network, we identified that CCAAT enhancer binding protein alpha (CEBPA, DETF) may regulate CD8A molecule (CD8A, DEE), thereby promoting the transcription of CD3E molecule (CD3E, DETG), which may affect allograft rejection based on CD8+ T cells.

Conclusion

We constructed an eRNA-based prognostic model for predicting the OS patients’ prognosis and explored the potential regulation network for OS tumorigenesis by an integrated bioinformatics analysis, providing promising therapeutic targets for OS patients.

Promiscuity of Peptides Presented in HLA-DP Molecules from Different Immunogenicity Groups Is Associated With T-Cell Cross-Reactivity

In the context of HLA-DP-mismatched allogeneic stem cell transplantation, mismatched HLA-DP alleles can provoke profound allo-HLA-DP-specific immune responses from the donor T-cell repertoire leading to graft-versus-leukemia effect and/or graft-versus-host disease in the patient. The magnitude of allo-HLA-DP-specific immune responses has been shown to depend on the specific HLA-DP disparity between donor and patient and the immunogenicity of the mismatched HLA-DP allele(s). HLA-DP peptidome clustering (DPC) was developed to classify the HLA-DP molecules based on similarities and differences in their peptide-binding motifs. To investigate a possible categorization of HLA-DP molecules based on overlap of presented peptides, we identified and compared the peptidomes of the thirteen most frequently expressed HLA-DP molecules. Our categorization based on shared peptides was in line with the DPC classification. We found that the HLA-DP molecules within the previously defined groups DPC-1 or DPC-3 shared the largest numbers of presented peptides. However, the HLA-DP molecules in DPC-2 segregated into two subgroups based on the overlap in presented peptides. Besides overlap in presented peptides within the DPC groups, a substantial number of peptides was also found to be shared between HLA-DP molecules from different DPC groups, especially for groups DPC-1 and -2. The functional relevance of these findings was illustrated by demonstration of cross-reactivity of allo-HLA-DP-reactive T-cell clones not only against HLA-DP molecules within one DPC group, but also across different DPC groups. The promiscuity of peptides presented in various HLA-DP molecules and the cross-reactivity against different HLA-DP molecules demonstrate that these molecules cannot be strictly categorized in immunogenicity groups.

Heterologous Immunity of Virus-Specific T Cells Leading to Alloreactivity: Possible Implications for Solid Organ Transplantation

Exposure of the adaptive immune system to a pathogen can result in the activation and expansion of T cells capable of recognizing not only the specific antigen but also different unrelated antigens, a process which is commonly referred to as heterologous immunity. While such cross-reactivity is favourable in amplifying protective immune responses to pathogens, induction of T cell-mediated heterologous immune responses to allo-antigens in the setting of solid organ transplantation can potentially lead to allograft rejection. In this review, we provide an overview of murine and human studies investigating the incidence and functional properties of virus-specific memory T cells cross-reacting with allo-antigens and discuss their potential relevance in the context of solid organ transplantation.

HLA Class-II‒Restricted CD8+ T Cells Contribute to the Promiscuous Immune Response in Dapsone-Hypersensitive Patients

HLA-B∗13:01 is associated with dapsone (DDS)-induced hypersensitivity, and it has been shown that CD4+ and CD8+ T cells are activated by DDS and its nitroso metabolite (nitroso dapsone [DDS-NO]). However, there is a need to define the importance of the HLA association in the disease pathogenesis. Thus, DDS- and DDS-NO‒specific CD8+ T-cell clones (TCCs) were generated from hypersensitive patients expressing HLA-B∗13:01 and were assessed for phenotype and function, HLA allele restriction, and killing of target cells. CD8+ TCCs were stimulated to proliferate and secrete effector molecules when exposed to DDS and/or DDS-NO. DDS-responsive and several DDS-NO‒responsive TCCs expressing a variety of TCR sequences displayed HLA class-I restriction, with the drug (metabolite) interacting with multiple HLA-B alleles. However, activation of certain DDS-NO‒responsive CD8+ TCCs was inhibited with HLA class-II block, with DDS-NO binding to HLA-DQB1∗05:01. These TCCs were of different origin but expressed TCRs displaying the same amino acid sequences. They were activated through a hapten pathway; displayed CD45RO, CD28, PD-1, and CTLA-4 surface molecules; secreted the same panel of effector molecules as HLA class-I‒restricted TCCs; but displayed a lower capacity to lyse target cells. To conclude, DDS and DDS-NO interact with a number of HLA molecules to activate CD8+ TCCs, with HLA class-II‒restricted CD8+ TCCs that display hybrid CD4‒CD8 features also contributing to the promiscuous immune response that develops in patients.

Genetic Bias, Diversity Indices, Physiochemical Properties and CDR3 Motifs Divide Auto-Reactive from Allo-Reactive T-Cell Repertoires

The distinct properties of allo-reactive T-cell repertoires are not well understood. To investigate whether auto-reactive and allo-reactive T-cell repertoires encoded distinct properties, we used dextramer enumeration, enrichment, single-cell T-cell receptor (TCR) sequencing and multiparameter analysis. We found auto-reactive and allo-reactive T-cells differed in mean ex vivo frequency which was antigen dependent. Allo-reactive T-cells showed clear differences in TCR architecture, with enriched usage of specific T-cell receptor variable (TRBJ) genes and broader use of T-cell receptor variable joining (TRBJ) genes. Auto-reactive T-cell repertoires exhibited complementary determining regions three (CDR3) lengths using a Gaussian distribution whereas allo-reactive T-cell repertoires exhibited distorted patterns in CDR3 length. CDR3 loops from allo-reactive T-cells showed distinct physical-chemical properties, tending to encode loops that were more acidic in charge. Allo-reactive T-cell repertoires differed in diversity metrics, tending to show increased overall diversity and increased homogeneity between repertoires. Motif analysis of CDR3 loops showed allo-reactive T-cell repertoires differed in motif preference which included broader motif use. Collectively, these data conclude that allo-reactive T-cell repertoires are indeed different to auto-reactive repertoires and provide tangible metrics for further investigations and validation. Given that the antigens studied here are overexpressed on multiple cancers and that allo-reactive TCRs often show increased ligand affinity, this new TCR bank also has translational potential for adoptive cell therapy, soluble TCR-based therapy and rational TCR design.

The human T-cell receptor repertoire in health and disease and potential for omics integration

The adaptive immune system arose 600 million years ago in a cold-blooded fish. Over countless generations, our antecedents tuned the function of the T-cell receptor (TCR). The TCR system is arguably the most complex known to science. The TCR evolved hypervariability to fight the hypervariability of pathogens and cancers that look to consume our resources. This review describes the genetics and architecture of the human TCR and highlights surprising new discoveries over the past years that have disproved very old dogmas. The standardization of TCR sequencing data is discussed in preparation for big data bioinformatics and predictive analysis. We next catalogue new signatures and phenomenon discovered by TCR next generation sequencing (NGS) in health and disease and work that remain to be done in this space. Finally, we discuss how TCR NGS can add to immunodiagnostics and integrate with other omics platforms for both a deeper understanding of TCR biology and its use in the clinical setting.

Interrogating the recognition landscape of a conserved HIV-specific TCR reveals distinct bacterial peptide cross-reactivity

T cell cross-reactivity ensures that diverse pathogen-derived epitopes encountered during a lifetime are recognized by the available TCR repertoire. A feature of cross-reactivity where previous exposure to one microbe can alter immunity to subsequent, non-related pathogens has been mainly explored for viruses. Yet cross-reactivity to additional microbes is important to consider, especially in HIV infection where gut-intestinal barrier dysfunction could facilitate T cell exposure to commensal/pathogenic microbes. Here we evaluated the cross-reactivity of a ‘public’, HIV-specific, CD8 T cell-derived TCR (AGA1 TCR) using MHC class I yeast display technology. Via screening of MHC-restricted libraries comprising ~2×10⁸ sequence-diverse peptides, AGA1 TCR specificity was mapped to a central peptide di-motif. Using the top TCR-enriched library peptides to probe the non-redundant protein database, bacterial peptides that elicited functional responses by AGA1-expressing T cells were identified. The possibility that in context-specific settings, MHC class I proteins presenting microbial peptides influence virus-specific T cell populations in vivo is discussed.

Priming of Allo-HLA-DP-Specific Reactivity from the Naïve T Cell Compartment Is Not Exclusively Mediated by Professional Antigen-Presenting Cells

Allogeneic (allo) stem cell transplantation is applied to patients suffering from hematologic malignancies to replace the diseased hematopoietic system with cells derived from a donor stem cell graft. The majority of 10/10-matched unrelated donors are HLA-DP-mismatched, and this may result in varying degrees of the graft-versus-leukemia (GVL) effect with or without the occurrence of graft-versus-host disease (GVHD). Allo-HLA-reactive T cells are commonly present in the donor T cell repertoire, and thus a very profound alloreactive immune response can be provoked in the HLA-DP-mismatched setting. The magnitude and the diversity of the allo-HLA-DP-specific immune response likely dictates the balance between the occurrence of GVL and/or GVHD after transplantation. To understand the nature of the allo-HLA-DP-specific immune response provoked under different stimulatory conditions, immune responses were induced from both the naïve and memory T cell compartments using either HLA-DP-mismatched professional antigen-presenting cells (APCs) (monocyte-derived dendritic cells [allo-DCs]) or HLA-DP-mismatched nonprofessional APCs (skin-derived fibroblasts [allo-fibroblasts]) as stimulator cells. In this study, we observed that allo-HLA-DP-reactive T cells could be provoked from both the naïve and memory compartments by both types of APCs. However, the magnitude of the allo-HLA-DP-specific immune response was greater when stimulation was performed with allo-DCs. Moreover, we found that the frequency of allo-HLA-DP-reactive T cells was greater in the naïve T cell compartment compared with the memory T cell compartment, but we observed a comparable lineage specificity of these allo-HLA-DP-specific reactivities. Overall, the data from this study illustrate that the presence of professional APCs of recipient origin will mostly dictate the magnitude of the allo-HLA-DP-specific immune response derived from both the naïve and memory T cell compartments, but does not exclusively mediate the induction of these immune responses.

Regulation of NK-Cell Function by HLA Class II

Natural Killer (NK) cells were initially described as part of the innate immune system and characterized by their ability to lyse malignant and virus-infected cells. The cytolytic function of NK cells is tightly controlled by activating and inhibitory receptors expressed on the cell surface. Ligands that interact with a variety of NK-cell receptors include the human leukocyte antigen (HLA) molecules, and the regulation of NK-cell function by HLA class I molecules is well-established. Earlier studies also suggested a role of HLA class II molecules in regulating NK cell activity; yet, interactions between HLA class II molecules and NK cell receptors have not been well-characterized. We recently identified a subset of HLA-DP molecules that can serve as ligands for the natural cytotoxicity receptor NKp44 and activate NK cells. This novel receptor-ligand interaction provides a potential mechanism to explain the strong associations of HLA-DP molecules with HBV infection outcomes, graft-vs.-host disease and inflammatory bowel disease. Furthermore, it adds a new mechanism for NK-cell crosstalk with immune cells expressing HLA class II molecules. In this perspective article, we discuss the potential implications of NK cell receptor interactions with HLA class II molecules for the regulation of immune responses.

HLA class II-Restricted CD8+ T cells in HIV-1 Virus Controllers

A paradigm shifting study demonstrated that induction of MHC class E and II-restricted CD8+ T cells was associated with the clearance of SIV infection in rhesus macaques. Another recent study highlighted the presence of HIV-1-specific class II-restricted CD8+ T cells in HIV-1 patients who naturally control infection (virus controllers; VCs). However, questions regarding class II-restricted CD8+ T cells ontogeny, distribution across different HIV-1 disease states and their role in viral control remain unclear. In this study, we investigated the distribution and anti-viral properties of HLA-DRB1*0701 and DQB1*0501 class II-restricted CD8+ T cells in different HIV-1 patient cohorts; and whether class II-restricted CD8+ T cells represent a unique T cell subset. We show that memory class II-restricted CD8+ T cell responses were more often detectable in VCs than in chronically infected patients, but not in healthy seronegative donors. We also demonstrate that VC CD8+ T cells inhibit virus replication in both a class I- and class II-dependent manner, and that in two VC patients the class II-restricted CD8+ T cells with an anti-viral gene signature expressed both CD4+ and CD8+ T cell lineage-specific genes. These data demonstrated that anti-viral memory class II-restricted CD8+ T cells with hybrid CD4+ and CD8+ features are present during natural HIV-1 infection.

Immunodominant cytomegalovirus-specific CD8+ T-cell responses in sub-Saharan African populations

More than 90% of children in Africa are infected with cytomegalovirus (CMV) by the age of 12 months. However, the high-frequency, immunodominant CD8+ T-cell responses that control CMV infection have not been well studied in African populations. We therefore sought to define the immunodominant CMV-specific CD8+ T-cell responses within sub-Saharan African study subjects. Among 257 subjects, we determined the CD8+ T-cell responses to overlapping peptides spanning three of the most immunogenic CMV proteins, pp65, IE-1 and IE-2, using IFN-γ ELISpot assays. A bioinformatics tool was used to predict optimal epitopes within overlapping peptides whose recognition was statistically associated with expression of particular HLA class I molecules. Using this approach, we identified 16 predicted novel CMV-specific epitopes within CMV-pp65, IE-1 and IE-2. The immunodominant pp65-specific, IE-1, IE-2 responses were all either previously well characterised or were confirmed using peptide-MHC tetramers. The novel epitopes identified included an IE-2-specific epitope restricted by HLA*B*44:03 that induced high-frequency CD8+ T-cell responses (mean 3.4% of CD8+ T-cells) in 95% of HLA-B*44:03-positive subjects tested, in one individual accounting for 18.8% of all CD8+ T-cells. These predicted novel CMV-specific CD8+ T-cell epitopes identified in an African cohort will facilitate future analyses of immune responses in African populations where CMV infection is almost universal during infancy.

Antiviral CD8+ T Cells Restricted by Human Leukocyte Antigen Class II Exist during Natural HIV Infection and Exhibit Clonal Expansion

CD8⁺ T cell recognition of virus-infected cells is characteristically restricted by major histocompatibility complex (MHC) class I, although rare examples of MHC class II restriction have been reported in Cd4-deficient mice and a macaque SIV vaccine trial using a recombinant cytomegalovirus vector. Here, we demonstrate the presence of human leukocyte antigen (HLA) class II-restricted CD8⁺ T cell responses with antiviral properties in a small subset of HIV-infected individuals. In these individuals, T cell receptor β (TCRβ) analysis revealed that class II-restricted CD8⁺ T cells underwent clonal expansion and mediated killing of HIV-infected cells. In one case, these cells comprised 12% of circulating CD8⁺ T cells, and TCRα analysis revealed two distinct co-expressed TCRα chains, with only one contributing to binding of the class II HLA-peptide complex. These data indicate that class II-restricted CD8⁺ T cell responses can exist in a chronic human viral infection, and may contribute to immune control.

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CD8⁺ T cells restricted by HLA-DRB1 exist in a small number of HIV-infected persons

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These CD8⁺ T cells exhibit potent antiviral functions against HIV-infected cells

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TCRβ usage patterns indicate clonal expansion of class II-restricted CD8⁺ T cells

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CD8⁺ T cells that violate immunologic paradigms may contribute to viral control

Class II-Restricted CD8s: New Lessons Violate Old Paradigms

CD8⁺ T cells that recognize peptides presented by MHC class II molecules have been observed in a macaque SIV vaccine model. A new study by Ranasinghe et al. (2016) shows that virus-specific class-II-restricted CD8⁺ T cells can be found in some HIV-infected patients.

Infectious pathogens may trigger specific allo-HLA reactivity via multiple mechanisms

Transplant recipients can be sensitized against allo-HLA antigens by previous transplantation, blood transfusion, or pregnancy. While there is growing awareness that multiple components of the immune system can act as effectors of the alloresponse, the role of infectious pathogen exposure in triggering sensitization and allograft rejection has remained a matter of much debate. Here, we describe that exposure to pathogens may enhance the immune response to allogeneic HLA antigens via different pathways. The potential role of allo-HLA cross-reactivity of virus-specific memory T cells, activation of innate immunity leading to a more efficient induction of the adaptive alloimmune response by antigen-presenting cells, and bystander activation of existing memory B cell activation will be discussed in this review.

IRG1 increases MHC class I level in macrophages through STAT-TAP1 axis depending on NADPH oxidase mediated reactive oxygen species

The major histocompatibility complex (MHC) is the connection between innate immunity and acquired immune system. Recently, many studies reported that the immunoresponsive gene 1 (IRG1) play an important role on innate immunity including reactive oxygen species (ROS), antiviral effect and expression of inflammatory factors. However, the function of IRG1 in antigen presenting remains unclear. In this study, we found that overexpressed-IRG1 promoted MHC I level instead of MHC II in macrophages membrane. Besides, IRG1 increased expression of some transporter proteins associated with antigen processing involving TAP1, PSMB9 depending on ROS. By detecting the activation of glucose-6-phosphate dehydrogenase (G6PD), we confirmed that IRG1 could increase ROS level by promoting pentose phosphate pathway (PPP). DPI, an inhibitor of NADPH oxidase (NOX), also significant attenuated TAP1 and MHC I level in IRG1-overexpressed macrophages. Finally, results showed that phosphorylation of STAT1/3 involved in IRG1-mediated TAP1 and MHC I expression. In conclusion, IRG1 increased MHC class I level in macrophages through STAT1/3-TAP1 axis depending on PPP and NOX mediated ROS.

Deciphering the clinical relevance of allo-human leukocyte antigen cross-reactivity in mediating alloimmunity following transplantation

PURPOSE OF REVIEW

Despite a growing awareness regarding the potential of cross-reactive virus-specific memory T cells to mediate alloimmunity, there has been limited clinical evaluation on allograft immunopathology. This review will explore published models of human T-cell cross-reactivity and discuss criteria required to drive this mechanism as a contributing cause of allograft dysfunction in transplantation.

RECENT FINDINGS

Published models of human allogeneic (allo)-human leukocyte antigen (HLA) cross-reactivity have enabled dissection of the cross-reactive T cell receptor/peptide/major histocompatibility complex (TCR/peptide/MHC) interaction. In many of the models, the cross-reactive T cells express a unique TCR, although the relevance of a public cross-reactive TCR repertoire has yet to be determined. Equally, allopeptide identity, a vital component driving cross-recognition, remains unknown in the majority of models thereby prompting further characterization utilizing novel technologies. Although clinical studies examining the presence and impact of specific cross-reactive virus-specific T cells have been minimally explored, the existing data suggest that there may be a marginal set of requirements that need to be satisfied before the potentially damaging effects of allo-HLA cross-reactivity can be realized.

SUMMARY

Our understanding of allo-HLA cross-reactivity continues to evolve as improved technology and novel strategies allow us to better question the contribution of allo-HLA cross-reactivity in clinically relevant allograft dysfunction.

Recent advances in viral evasion of the MHC Class I processing pathway

T-cell mediated adaptive immunity against viruses relies on recognition of virus-derived peptides by CD4(+) and CD8(+) T cells. Detection of pathogen-derived peptide-MHC-I complexes triggers CD8(+) T cells to eliminate the infected cells. Viruses have evolved several mechanisms to avoid recognition, many of which target the MHC-I antigen-processing pathway. While many immune evasion strategies have been described in the context of herpesvirus infections, it is becoming clear that this 'disguise' ability is more widespread. Here, we address recent findings in viral evasion of the MHC-I antigen presentation pathway and the impact on CD8(+) T cell responses.

Cross-Reactivity of TCR Repertoire: Current Concepts, Challenges, and Implication for Allotransplantation

Being able to track donor reactive T cells during the course of organ transplantation is a key to improve the graft survival, to prevent graft dysfunction, and to adapt the immunosuppressive regimen. The attempts of transplant immunologists have been for long hampered by the large size of the alloreactive T cell repertoire. Understanding how self-TCR can interact with allogeneic MHC is a key to critically appraise the different assays available to analyze the TCR Vβ repertoire usage. In this report, we will review conceptually and experimentally the process of cross-reactivity. We will then highlight what can be learned from allotransplantation, a situation of artificial cross-reactivity. Finally, the low- and high-resolution techniques to characterize the TCR Vβ repertoire usage in transplantation will be critically discussed.

Detection of Virus-Specific CD8+ T Cells With Cross-Reactivity Against Alloantigens: Potency and Flaws of Present Experimental Methods

Supplemental digital content is available in the text.

Virus-specific T cells have the intrinsic capacity to cross-react against allogeneic HLA antigens, a phenomenon known as heterologous immunity. In transplantation, these cells may contribute to the alloimmune response and negatively impact graft outcome. This study describes the various techniques that can be used to detect heterologous immune responses of virus-specific CD8⁺ T cells against allogeneic HLA antigens. The strengths and weaknesses of the different approaches are discussed and illustrated by experimental data.

Allo-reactive T cells for the treatment of hematological malignancies

Several mechanisms can be responsible for control of hematological tumors by allo-reactive T cells. Following allogeneic stem cell transplantation (alloSCT) donor T cells recognizing genetic disparities presented on recipient cells and not on donor cells are main effectors of tumor control, but also of the detrimental graft versus host disease (GVHD). Since after transplantation normal hematopoiesis is of donor origin, any T cell response directed against polymorphic antigens expressed on hematopoietic recipient cells but not on donor cells will result in an anti-tumor response not affecting normal hematopoiesis. After fully HLA-matched alloSCT, T cells recognizing polymorphic peptides derived from proteins encoded by genes selectively expressed in hematopoietic lineages may result in anti-tumor responses without GVHD. Due to the high susceptibility of hematopoietic cells for T cell recognition, a low amplitude of the overall T cell response may also be in favor of the anti-tumor reactivity in hematological malignancies. A mismatch between donor and patient for specific HLA-alleles can also be exploited to induce a selective T cell response against patient (malignant) hematopoietic cells. If restricting HLA class II molecules are selectively expressed on hematopoietic cells under non-inflammatory circumstances, allo HLA class-II responses may control the tumor with limited risk of GVHD. Alternatively, T cells recognizing hematopoiesis-restricted antigens presented in the context of mismatched HLA alleles may be used to treat patients with hematological cancers. This review discusses various ways to manipulate the allo-immune response aiming to exploit the powerful ability of allo-reactive T-cells to control the malignancies without causing severe damage to non-hematopoietic tissues.

Optimization of T-cell Reactivity by Exploiting TCR Chain Centricity for the Purpose of Safe and Effective Antitumor TCR Gene Therapy

Adoptive transfer of T cells redirected by a high-affinity antitumor T-cell receptor (TCR) is a promising treatment modality for cancer patients. Safety and efficacy depend on the selection of a TCR that induces minimal toxicity and elicits sufficient antitumor reactivity. Many, if not all, TCRs possess cross-reactivity to unrelated MHC molecules in addition to reactivity to target self-MHC/peptide complexes. Some TCRs display chain centricity, in which recognition of MHC/peptide complexes is dominated by one of the TCR hemi-chains. In this study, we comprehensively studied how TCR chain centricity affects reactivity to target self-MHC/peptide complexes and alloreactivity using the TCR, clone TAK1, which is specific for human leukocyte antigen-A*24:02/Wilms tumor 1(235-243) (A24/WT1(235)) and cross-reactive with B*57:01 (B57). The TAK1β, but not the TAK1α, hemi-chain possessed chain centricity. When paired with multiple clonotypic TCRα counter-chains encoding TRAV12-2, 20, 36, or 38-2, the de novo TAK1β-containing TCRs showed enhanced, weakened, or absent reactivity to A24/WT1(235) and/or to B57. T cells reconstituted with these TCRα genes along with TAK1β possessed a very broad range (>3 log orders) of functional and structural avidities. These results suggest that TCR chain centricity can be exploited to enhance desired antitumor TCR reactivity and eliminate unwanted TCR cross-reactivity. TCR reactivity to target MHC/peptide complexes and cross-reactivity to unrelated MHC molecules are not inextricably linked and are separable at the TCR sequence level. However, it is still mandatory to carefully monitor for possible harmful toxicities caused by adoptive transfer of T cells redirected by thymically unselected TCRs.

Identification of novel CD8+ T cell epitopes in human herpesvirus 6B U11 and U90

Human herpesvirus 6B (HHV6B) infects over 90% of the population, and normally establishes a latent infection, where episodes of reactivation are asymptomatic. However, in immunocompromised patients HHV6B reactivation is associated with high morbidity and mortality. Cellular immunotherapy has been utilised against other herpesvirus in immunocompromised settings. However, limited information on the immune response against HHV6B has hampered the development of immunotherapy for HHV6B-driven disease. In this study, we have analysed the cellular immune response against four HHV6B antigens in a panel of 30 healthy donors. We show that the base-line level of T cell reactivity in peripheral blood is very low to undetectable. A short-term reactivation step enabled expansion of T cell responses, and all donors responded to at least 1 antigen, but more commonly 3 or 4. A hierarchy of immunogenicity was determined with antigens U90 and U54 being co-dominant, followed by U11 and U39. Putative CD8+ T cell epitopes were mapped to U90 and U11, predicted to be presented in the context of HLA-A1, A29, B39 and C6. T cells reactive against these novel epitopes were able to recognise virus-infected cells. Our data is supportive of the application and on-going development of T cell immunotherapy against HHVB-driven disease in the immunocompromised host.

Understanding the complexity and malleability of T-cell recognition

T cells are the master regulators of immune system function, continually walking the biological tightrope between adequate host defence and accidental host pathology. Tolerance is maintained or broken through an intricate structural interplay between the T-cell receptor (TCR) and major histocompatibility complex (MHC) molecule cradling peptide antigens (p). Recent advances in structural biology have shown that the TCR/pMHC interface is surprising precise and extraordinarily malleable. We have seen that seemingly minor changes in the TCR/pMHC interface can abrogate function, as well as substantial conformational changes before and after TCR docking. Our understanding of T-cell biology has also been altered with the knowledge that MHC molecules can bind not only peptides, but also an array of natural and synthetic compounds. Here, we review some examples of the precision and flexibility intrinsic to the TCR/p/MHCI axis.

Novel CD8 T cell alloreactivities in CCR5-deficient recipients of class II MHC disparate kidney grafts

Recipient CD4 T regulatory cells inhibit the acute T cell-mediated rejection of renal allografts in wild-type mice. The survival of single class II MHC-disparate H-2(bm12) renal allografts was tested in B6.CCR5(-/-) recipients, which have defects in T regulatory cell activities that constrain alloimmune responses. In contrast to wild-type C57BL/6 recipients, B6.CCR5(-/-) recipients rejected the bm12 renal allografts. However, donor-reactive CD8 T cells rather than CD4 T cells were the primary effector T cells mediating rejection. The CD8 T cells induced to bm12 allografts in CCR5-deficient recipients were reactive to peptides spanning the 3 aa difference in the I-A(bm12) versus I-A(b) β-chains presented by K(b) and D(b) class I MHC molecules. Allograft-primed CD8 T cells from CCR5-deficient allograft recipients were activated during culture either with proinflammatory cytokine-stimulated wild-type endothelial cells pulsed with the I-A(bm12) peptides or with proinflammatory cytokine-simulated bm12 endothelial cells, indicating their presentation of the I-A(bm12) β-chain peptide/class I MHC complexes. In addition to induction by bm12 renal allografts, the I-A(bm12) β-chain-reactive CD8 T cells were induced in CCR5-deficient, but not wild-type C57BL/6, mice by immunization with the peptides. These results reveal novel alloreactive CD8 T cell specificities in CCR5-deficient recipients of single class II MHC renal allografts that mediate rejection of the allografts.

The dynamics of herpesvirus reactivations during and after severe drug eruptions: their relation to the clinical phenotype and therapeutic outcome

Background

Drug‐induced hypersensitivity syndrome/drug rash with eosinophilia and systemic symptoms (DIHS/DRESS) and Stevens–Johnson syndrome (SJS)/toxic epidermal necrolysis (TEN) represent contrasting poles of severe drug eruptions, and sequential reactivations of several herpesviruses have exclusively been demonstrated in the former. No previous studies, however, were extended beyond the acute stage. We sought to investigate whether herpesvirus reactivations could also be observed in SJS/TEN and beyond the acute stage of both diseases.

Methods

Patients with SJS (n = 16), SJS/TEN overlap (n = 2), TEN (n = 10), and DIHS/DRESS (n = 34) were enrolled. We performed a retrospective analysis of Epstein–Barr virus (EBV), human herpesvirus 6 (HHV‐6), and cytomegalovirus (CMV) DNA loads sequentially determined by real‐time polymerase chain reaction during a 2‐year period after onset.

Results

Persistently increased EBV loads were detected in SJS during the acute stage and long after resolution, but not in others. In contrast, high HHV‐6 loads were exclusively detected in DIHS/DRESS during the acute stage. The dynamics of herpesvirus reactivation varied in DIHS/DRESS according to the use of systemic corticosteroids: While EBV loads were higher in patients not receiving systemic corticosteroids, CMV and HHV‐6 loads were higher in those receiving them.

Conclusions

Distinct patterns of herpesvirus reactivation according to the pathological phenotype and to the use of systemic corticosteroids were observed during the acute stage and follow‐up period, which may contribute, at least in part, to the difference in the clinical manifestations and long‐term outcomes. Systemic corticosteroids during the acute stage may improve the outcomes in DIHS/DRESS.

Human cytomegalovirus and autoimmune disease

Human cytomegalovirus (HCMV) represents a prototypic pathogenic member of the β-subgroup of the herpesvirus family. A range of HCMV features like its lytic replication in multiple tissues, the lifelong persistence through periods of latency and intermitting reactivation, the extraordinary large proteome, and extensive manipulation of adaptive and innate immunity make HCMV a high profile candidate for involvement in autoimmune disorders. We surveyed the available literature for reports on HCMV association with onset or exacerbation of autoimmune disease. A causative linkage between HCMV and systemic lupus erythematosus (SLE), systemic sclerosis (SSc), diabetes mellitus type 1, and rheumatoid arthritis (RA) is suggested by the literature. However, a clear association of HCMV seroprevalence and disease could not be established, leaving the question open whether HCMV could play a coresponsible role for onset of disease. For convincing conclusions population-based prospective studies must be performed in the future. Specific immunopathogenic mechanisms by which HCMV could contribute to the course of autoimmune disease have been suggested, for example, molecular mimicry by UL94 in SSc and UL83/pp65 in SLE patients, as well as aggravation of joint inflammation by induction and expansion of CD4+/CD28− T-cells in RA patients. Further studies are needed to validate these findings and to lay the grounds for targeted therapeutic intervention.

Recognition of distinct cross-reactive virus-specific CD8+ T cells reveals a unique TCR signature in a clinical setting

Human CMV still remains problematic in immunocompromised patients, particularly after solid organ transplantation. CMV primary disease and reactivation greatly increase the risks associated with incidences of chronic allograft rejection and decreased survival in transplant recipients. But whether this is due to direct viral effects, indirect viral effects including cross-reactive antiviral T cell immunopathology, or a combination of both remains undetermined. In this article, we report the novel TCR signature of cross-reactive HLA-A*02:01 (A2) CMV (NLVPMVATV [NLV])-specific CD8(+) T cells recognizing a specific array of HLA-B27 alleles using technical advancements that combine both IFN-γ secretion and multiplex nested RT-PCR for determining paired CDR3α/β sequences from a single cell. This study represents the first evidence, to our knowledge, of the same A2-restricted cross-reactive NLV-specific TCR-α/β signature (TRAV3TRAJ31_TRBV12-4TRBJ1-1) in two genetically distinct individuals. Longitudinal posttransplant monitoring of a lung transplant recipient (A2, CMV seropositive) who received a HLA-B27 bilateral lung allograft showed a dynamic expansion of the cross-reactive NLV-specific TCR repertoire before CMV reactivation. After resolution of the active viral infection, the frequency of cross-reactive NLV-specific CD8(+) T cells reduced to previremia levels, thereby demonstrating immune modulation of the T cell repertoire due to antigenic pressure. The dynamic changes in TCR repertoire, at a time when CMV reactivation was subclinical, illustrates that prospective monitoring in susceptible patients can reveal nuances in immune profiles that may be clinically relevant.

Endogenous-peptide-dependent alloreactivity: new scientific insights and clinical implications

T-cell alloreactivity is generated via immune responsiveness directed against allogeneic (allo) human leucocyte antigen (HLA) molecules. Whilst the alloresponse is of extraordinary potency and frequency, it has often been assumed to be less peptide-specific than conventional T-cell reactivity. Recently, several human studies have shown that both alloreactive CD8(+) and CD4(+) T cells exhibit exquisite allo-HLA and endogenous peptide specificity that has also underpinned tissue-specific allorecognition. In this review, we summarize former and recent scientific evidence in support of endogenous peptide (self-peptide)-dependence of T-cell alloreactivity. The clinical implications of these findings will be discussed in the context of both solid organ transplantation and haematopoietic stem cell transplantation (HSCT). Insights into the understanding of the molecular basis of T-cell allorecognition will probably translate into improved allograft survival outcomes, lower frequencies of graft vs host disease and could potentially be exploited for selective graft vs leukaemia effect to improve clinical outcomes following HSCT.

Alloreactivity of virus-specific T cells: possible implication of graft-versus-host disease and graft-versus-leukemia effects

Immune reconstitution of functional virus-specific T cells after allogeneic hematopoietic stem cell transplantation (HSCT) has been intensively investigated. However, the possible role of crossreactivity of these virus-specific T cells against allogeneic targets is still unclear. Theoretically, as in the field of organ transplantation, virus-specific T cells possess crossreactivity potential after allogeneic HSCT. Such crossreactivity is assumed to play a role in graft-versus-host disease and graft-versus-leukemia effects. In this article, we aim to give a comprehensive overview of current understanding about crossreactivity of virus-specific T cells.

Immune parameters to consider when choosing T-cell receptors for therapy

T-cell receptor (TCR) therapy has arrived as a realistic treatment option for many human diseases. TCR gene therapy allows for the mass redirection of T-cells against a defined antigen while high affinity TCR engineering allows for the creation of a new class of soluble drugs. However, deciding which TCR blueprint to take forward for gene therapy or engineering is difficult. More than one quintillion TCR combinations can be generated by somatic recombination and we are only now beginning to appreciate that not all are functionally equal. TCRs can exhibit high or low degrees of HLA-restricted cross-reactivity and alloreact against one or a combination of HLA alleles. Identifying TCR candidates with high specificity and minimal cross-reactivity/alloreactivity footprints before engineering is obviously highly desirable. Here we will summarize what we currently know about TCR biology with regard to immunoengineering.

Safety and clinical efficacy of rapidly-generated trivirus-directed T cells as treatment for adenovirus, EBV, and CMV infections after allogeneic hematopoietic stem cell transplant

Adoptive transfer of virus-specific T cells can prevent and treat serious infections with Epstein-Barr virus (EBV), cytomegalovirus (CMV), and adenovirus (Adv) after allogeneic hematopoietic stem cell transplant. It has, however, proved difficult to make this approach widely available since infectious virus and viral vectors are required for T cell activation, followed by an intensive and prolonged culture period extending over several months. We now show that T cells targeting a range of viral antigens derived from EBV, CMV, and Adv can be reproducibly generated in a single culture over a 2-3-week period, using methods that exclude all viral components and employ a much-simplified culture technology. When administered to recipients of haploidentical (n = 5), matched unrelated (n = 3), mismatched unrelated (n = 1) or matched related (n = 1) transplants with active CMV (n = 3), Adv (n = 1), EBV (n = 2), EBV+Adv (n = 2) or CMV+Adv (n = 2) infections, the cells produced complete virological responses in 80%, including all patients with dual infections. In each case, a decrease in viral load correlated with an increase in the frequency of T cells directed against the infecting virus(es); both immediate and delayed toxicities were absent. This approach should increase both the feasibility and applicability of T cell therapy. The trial was registered at www.clinicaltrials.gov as NCT01070797.

Alloreactivity

The alloimmune response between individuals genetically disparate for antigens encoded within the major histocompatibility complex (MHC) remains a substantial barrier to transplantation of solid organs, tissues, and hematopoietic stem cells. Alloreactivity has been an immunological paradox because of its apparent contradiction to the requirement of MHC restriction for the induction of normal T lymphocyte mediated immune responses. Through crystallographic analyses and experimental systems utilizing murine CD8(+) cytolytic T cell clones, major advances have been achieved in understanding the molecular and structural basis of T cell receptor recognition of MHC-peptide complexes and the basis of T cell mediated alloreactivity. These studies have further provided an explanation for the relatively high frequencies of alloreactive T cells compared to the frequencies of T cells for microbial derived antigens.

Cytomegalovirus vectors violate CD8+ T cell epitope recognition paradigms

CD8(+) T cell responses focus on a small fraction of pathogen- or vaccine-encoded peptides, and for some pathogens, these restricted recognition hierarchies limit the effectiveness of antipathogen immunity. We found that simian immunodeficiency virus (SIV) protein-expressing rhesus cytomegalovirus (RhCMV) vectors elicit SIV-specific CD8(+) T cells that recognize unusual, diverse, and highly promiscuous epitopes, including dominant responses to epitopes restricted by class II major histocompatibility complex (MHC) molecules. Induction of canonical SIV epitope-specific CD8(+) T cell responses is suppressed by the RhCMV-encoded Rh189 gene (corresponding to human CMV US11), and the promiscuous MHC class I- and class II-restricted CD8(+) T cell responses occur only in the absence of the Rh157.5, Rh157.4, and Rh157.6 (human CMV UL128, UL130, and UL131) genes. Thus, CMV vectors can be genetically programmed to achieve distinct patterns of CD8(+) T cell epitope recognition.

The two-faced T cell epitope: examining the host-microbe interface with JanusMatrix

Advances in the field of T cell immunology have contributed to the understanding that cross-reactivity is an intrinsic characteristic of the T cell receptor (TCR), and that each TCR can potentially interact with many different T cell epitopes. To better define the potential for TCR cross-reactivity between epitopes derived from the human genome, the human microbiome, and human pathogens, we developed a new immunoinformatics tool, JanusMatrix, that represents an extension of the validated T cell epitope mapping tool, EpiMatrix. Initial explorations, summarized in this synopsis, have uncovered what appear to be important differences in the TCR cross-reactivity of selected regulatory and effector T cell epitopes with other epitopes in the human genome, human microbiome, and selected human pathogens. In addition to exploring the T cell epitope relationships between human self, commensal and pathogen, JanusMatrix may also be useful to explore some aspects of heterologous immunity and to examine T cell epitope relatedness between pathogens to which humans are exposed (Dengue serotypes, or HCV and Influenza, for example). In Hand-Foot-Mouth disease (HFMD) for example, extensive enterovirus and human microbiome cross-reactivity (and limited cross-reactivity with the human genome) seemingly predicts immunodominance. In contrast, more extensive cross-reactivity with proteins contained in the human genome as compared to the human microbiome was observed for selected Treg epitopes. While it may be impossible to predict all immune response influences, the availability of sequence data from the human genome, the human microbiome, and an array of human pathogens and vaccines has made computationally–driven exploration of the effects of T cell epitope cross-reactivity now possible. This is the first description of JanusMatrix, an algorithm that assesses TCR cross-reactivity that may contribute to a means of predicting the phenotype of T cells responding to selected T cell epitopes. Whether used for explorations of T cell phenotype or for evaluating cross-conservation between related viral strains at the TCR face of viral epitopes, further JanusMatrix studies may contribute to developing safer, more effective vaccines.

Cross-reactive anti-viral T cells increase prior to an episode of viral reactivation post human lung transplantation

Human Cytomegalovirus (CMV) reactivation continues to influence lung transplant outcomes. Cross-reactivity of anti-viral memory T cells against donor human leukocyte antigens (HLA) may be a contributing factor. We identified cross-reactive HLA-A*02:01-restricted CMV-specific cytotoxic T lymphocytes (CTL) co-recognizing the NLVPMVATV (NLV) epitope and HLA-B27. NLV-specific CD8+ T cells were expanded for 13 days from 14 HLA-A*02:01/CMV seropositive healthy donors and 11 lung transplant recipients (LTR) then assessed for the production of IFN-γ and CD107a expression in response to 19 cell lines expressing either single HLA-A or -B class I molecules. In one healthy individual, we observed functional and proliferative cross-reactivity in response to B*27:05 alloantigen, representing approximately 5% of the NLV-specific CTL population. Similar patterns were also observed in one LTR receiving a B27 allograft, revealing that the cross-reactive NLV-specific CTL gradually increased (days 13-193 post-transplant) before a CMV reactivation event (day 270) and reduced to basal levels following viral clearance (day 909). Lung function remained stable with no acute rejection episodes being reported up to 3 years post-transplant. Individualized immunological monitoring of cross-reactive anti-viral T cells will provide further insights into their effects on the allograft and an opportunity to predict sub-clinical CMV reactivation events and immunopathological complications.

Peptide length determines the outcome of TCR/peptide-MHCI engagement

αβ-TCRs expressed at the CD8(+) T-cell surface interact with short peptide fragments (p) bound to MHC class I molecules (pMHCI). The TCR/pMHCI interaction is pivotal in all aspects of CD8(+) T-cell immunity. However, the rules that govern the outcome of TCR/pMHCI engagement are not entirely understood, and this is a major barrier to understanding the requirements for both effective immunity and vaccination. In the present study, we discovered an unexpected feature of the TCR/pMHCI interaction by showing that any given TCR exhibits an explicit preference for a single MHCI-peptide length. Agonists of nonpreferred length were extremely rare, suboptimal, and often entirely distinct in sequence. Structural analysis indicated that alterations in peptide length have a major impact on antigenic complexity, to which individual TCRs are unable to adapt. This novel finding demonstrates that the outcome of TCR/pMHCI engagement is determined by peptide length in addition to the sequence identity of the MHCI-bound peptide. Accordingly, the effective recognition of pMHCI Ag, which is a prerequisite for successful CD8(+) T-cell immunity and protective vaccination, can only be achieved by length-matched Ag-specific CD8(+) T-cell clonotypes.

Stimulation of human EBV- and CMV-specific cytolytic effector function using allogeneic HLA molecules

Viral infection is a major cause of morbidity and mortality, and there are few therapeutic options available to augment a virus-specific T cell response. Although allo-HLA cross-reactivity from virus-specific memory T cells is common, it is unclear whether priming with specific allogeneic cells could conversely elicit a viral peptide/self-HLA restricted cytotoxic T cell response in humans. First, we used the previously described allo-HLA-B*44:02 cross-reactivity of EBV peptide/HLA-B8 restricted T cells, to determine whether allogeneic HLA stimulation can elicit a cytolytic immune response against EBV. HLA-B8(+) HLA-B44(-) EBV-seropositive PBMCs were stimulated with either HLA-B*44:02(+) or HLA-B*44:03(+) mismatched irradiated PBMCs in a 7-10 d MLR. The allo-HLA stimulated responder cells were then evaluated for cytotoxicity using EBV peptide loaded autologous target cells and unloaded HLA-B8(+) EBV LCL target cells. PBMCs from EBV-seropositive donors gained EBV-specific cytolytic effector function following specific allo-HLA stimulation. Finally, we also elicited cytolytic CMV-specific responses using specific allogeneic cell stimulation, to confirm that this technique can be used to elicit viral peptide/self-HLA restricted responses even from nonpublic TCR responses. Allogeneic cell stimulation used as a cell therapy may be a potential tool to augment an antiviral T cell response in patients with EBV or CMV infection.

Advances in direct T-cell alloreactivity: function, avidity, biophysics and structure

Although T-cell-based adaptive immunity plays a crucial role in protection against infectious pathogens and uncontrolled outgrowth of malignant cells, a large portion of these T cells are also capable of responding to allogeneic HLA molecules, violating the paradigm of self-major histocompatibility complex (MHC) restriction. Recent studies have provided insights into the mechanisms by which these T cells recognize allogeneic targets. The role of antiviral T cells in direct alloreactivity through peptide-dependent molecular mimicry and alternate peptide-MHC docking modes has emerged as major models for the human alloresponse. Here, we review in depth recent advances in this field and discuss how molecular interactions between T cells and HLA molecules drive the activation of these effector cells and its potential implications for alloreactivity in human transplantation.

TCR cross-reactivity and allorecognition: new insights into the immunogenetics of allorecognition

Alloreactive T cells are core mediators of graft rejection and are a potent barrier to transplantation tolerance. It was previously unclear how T cells educated in the recipient thymus could recognize allogeneic HLA molecules. Recently it was shown that both naïve and memory CD4+ and CD8+ T cells are frequently cross-reactive against allogeneic HLA molecules and that this allorecognition exhibits exquisite peptide and HLA specificity and is dependent on both public and private specificities of the T cell receptor. In this review we highlight new insights gained into the immunogenetics of allorecognition, with particular emphasis on how viral infection and vaccination may specifically activate allo-HLA reactive T cells. We also briefly discuss the potential for virus-specific T cell infusions to produce GvHD. The progress made in understanding the molecular basis of allograft rejection will hopefully be translated into improved allograft function and/or survival, and eventually tolerance induction.