New tools to monitor the impact of viral infection on the alloreactive T-cell repertoire

Abacavir inhibits but does not cause self-reactivity to HLA-B*57:01-restricted EBV specific T cell receptors

Pre-existing pathogen-specific memory T cell responses can contribute to multiple adverse outcomes including autoimmunity and drug hypersensitivity. How the specificity of the T cell receptor (TCR) is subverted or seconded in many of these diseases remains unclear. Here, we apply abacavir hypersensitivity (AHS) as a model to address this question because the disease is linked to memory T cell responses and the HLA risk allele, HLA-B*57:01, and the initiating insult, abacavir, are known. To investigate the role of pathogen-specific TCR specificity in mediating AHS we performed a genome-wide screen for HLA-B*57:01 restricted T cell responses to Epstein-Barr virus (EBV), one of the most prevalent human pathogens. T cell epitope mapping revealed HLA-B*57:01 restricted responses to 17 EBV open reading frames and identified an epitope encoded by EBNA3C. Using these data, we cloned the dominant TCR for EBNA3C and a previously defined epitope within EBNA3B. TCR specificity to each epitope was confirmed, however, cloned TCRs did not cross-react with abacavir plus self-peptide. Nevertheless, abacavir inhibited TCR interactions with their cognate ligands, demonstrating that TCR specificity may be subverted by a drug molecule. These results provide an experimental road map for future studies addressing the heterologous immune responses of TCRs including T cell mediated adverse drug reactions.

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.

Could COVID-19 induce remission of acute leukemia?

BACKGROUND

COVID-19 viral pandemic caused many mortalities in cancer patients especially those with hematological malignancies. The immunological response to COVID-19 infection is responsible for the outcome of cases whether mild, severe or critical.

CASE PRESENTATION

Two cases presented with moderate COVID-19 viral infection, concomitant with acute myeloid leukemia and T acute lymphoblastic leukemia, respectively. Surprisingly, after the administration of COVID-19 supportive therapy, the cases showed disease remission after a follow-up period of 12 and 5 months, respectively. Additionally, the blast cells dropped to only 3% and 0% in the bone marrow aspirates of those two cases, respectively, after it was 30% in both cases at diagnosis.

CONCLUSION

The immune response that emerged against COVID-19 infection could potentially produce anti-tumor immunity in some patients, or the virus may act as an oncolytic virus. However, further investigations are required to explain this phenomenon, which may help in finding a possible new targeted therapy for these cases.

Drug-induced alloreactivity: A new paradigm for allorecognition

Abacavir administration is associated with drug-induced hypersensitivity reactions in HIV+ individuals expressing the HLA-B*57:01 allele. However, the immunological effects of abacavir administration in an HLA-B57 mismatched transplantation setting have not been studied. We hypothesized that abacavir exposure could induce de novo HLA-B57-specific allorecognition. HIV-specific CD8 T cell clones were generated from HIV+ individuals, using single cell sorting based on HIV peptide/HLA tetramer staining. The T cell clones were assayed for alloreactivity against a panel of single HLA-expressing cell lines, in the presence or absence of abacavir. Cytokine assay, CD137 upregulation, and cytotoxicity were used as readout. Abacavir exposure can induce de novo HLA-B57 allorecognition by HIV-specific T cells. A HIV Gag RK9/HLA-A3-specific T cell did exhibit interferon-γ production, CD137 upregulation, and cytolytic effector function against allogeneic HLA-B57, but only in the presence of abacavir. Allorecognition was specific to the virus specificity, HLA restriction, and T cell receptor TRBV use of the T cell. We provide proof-of-principle evidence that administration of a drug could induce specific allorecognition of mismatched HLA molecules in the transplant setting. We suggest that HIV-seropositive recipients of an HLA-B57 mismatched graft should not receive abacavir until further studies are completed.

Cross-Reactivity of Virus-Specific CD8+ T Cells Against Allogeneic HLA-C: Possible Implications for Pregnancy Outcome

Heterologous immunity of virus-specific T cells poses a potential barrier to transplantation tolerance. Cross-reactivity to HLA-A and -B molecules has broadly been described, whereas responses to allo-HLA-C have remained ill defined. In contrast to the transplant setting, HLA-C is the only polymorphic HLA molecule expressed by extravillous trophoblasts at the maternal-fetal interface during pregnancy. Uncontrolled placental viral infections, accompanied by a pro-inflammatory milieu, can alter the activation status and stability of effector T cells. Potential cross-reactivity of maternal decidual virus-specific T cells to fetal allo-HLA-C may thereby have detrimental consequences for the success of pregnancy. To explore the presence of cross-reactivity to HLA-C and the other non-classical HLA antigens expressed by trophoblasts, HLA-A and -B-restricted CD8+ T cells specific for Epstein-Barr virus, Cytomegalovirus, Varicella-Zoster virus, and Influenza virus were tested against target cells expressing HLA-C, -E, and -G molecules. An HLA-B^*08:01-restricted EBV-specific T cell clone displayed cross-reactivity against HLA-C^*01:02. Furthermore, cross-reactivity of HLA-C-restricted virus-specific CD8+ T cells was observed for HCMV HLA-C^*06:02/TRA CD8+ T cell lines and clones against HLA-C^*03:02. Collectively, these results demonstrate that cross-reactivity against HLA-C can occur and thereby may affect pregnancy outcome.

Mycobacterium tuberculosis peptide E7/HLA-DRB1 tetramers with different HLA-DR alleles bound CD4+ T cells might share identical CDR3 region

Human CD4+ T cells play an important role in the immune response to Mycobacterium tuberculosis (MTB). However, little is known about the spectratyping characteristics of the CD4+ T-cell receptor (TCR) α- and β-chains CDR3 region in tuberculosis (TB) patients. We sorted MTB peptide E7-bound CD4+ T cells by using E7/HLA-DR tetramers constructed with different HLA-DRB1 alleles and extracted the CDR3 amino-acid sequences of TCR α- and β-chains. The results showed that the CDR3 sequences of E7-bound CD4+ T cells were completely or partially identical in a single patient. The sequences of MTB peptide C5-bound CD4+ T cells shared another, and non-peptide bound CD4+ T cells, as well as unbound CD4+ T cells with tetramers were different from each other. Specifically, diverse CDR3 sequences of E7-bound CD4+ T cells displayed similar protein tertiary structure in one TB patient. In summary, the TCR α- and β-chains of CDR3 lineage of CD4+ T cells in TB patients apparently drifted, and the predominant CDR3 sequences of TCR α- and β-chains that recognized the MTB antigen exhibited peptide specificity, and certain HLA-DR restriction was also established. This study elucidates the possible causes and mechanisms of peptide-specific CD4+ T-cell-related presentation against MTB.

Allo-HLA Cross-Reactivities of Cytomegalovirus-, Influenza-, and Varicella Zoster Virus-Specific Memory T Cells Are Shared by Different Healthy Individuals

Virus-specific T cells can recognize allogeneic HLA (allo-HLA) through TCR cross-reactivity. The allospecificity often differs by individual (private cross-reactivity) but also can be shared by multiple individuals (public cross-reactivity); however, only a few examples of the latter have been described. Because these could facilitate alloreactivity prediction in transplantation, we aimed to identify novel public cross-reactivities of human virus-specific CD8⁺ T cells directed against allo-HLA by assessing their reactivity in mixed-lymphocyte reactions. Further characterization was done by studying TCR usage with primer-based DNA sequencing, cytokine production with ELISAs, and cytotoxicity with ⁵¹ chromium-release assays. We identified three novel public allo-HLA cross-reactivities of human virus-specific CD8⁺ T cells. CMV B35/IPS CD8⁺ T cells cross-reacted with HLA-B51 and/or HLA-B58/B57 (23% of tetramer-positive individuals), FLU A2/GIL (influenza IMP[58-66] HLA-A*02:01/GILGFVFTL) CD8⁺ T cells with HLA-B38 (90% of tetramer-positive individuals), and VZV A2/ALW (varicella zoster virus IE62[593-601] HLA-A*02:01/ALWALPHAA) CD8⁺ T cells with HLA-B55 (two unrelated individuals). Cross-reactivity was tested against different cell types including endothelial and epithelial cells. All cross-reactive T cells expressed a memory phenotype, emphasizing the importance for transplantation. We conclude that public allo-HLA cross-reactivity of virus-specific memory T cells is not uncommon and may create novel opportunities for alloreactivity prediction and risk estimation in transplantation.

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.

Stimulation of HIV-specific T cell clonotypes using allogeneic HLA

We hypothesized that HIV-specific CD8 T cell clonotypes can be stimulated by allogeneic HLA molecules. Multiple HIV-specific CD8 T cell clones were derived from 12 individuals with chronic HIV infection, specific for 13 different HIV Gag antigens and restricted to 7 different HLA molecules. The generated T cell clones were assayed for alloreactivity against a panel of single HLA class I expressing cell lines (SALs). HIV-specific T cells recognising at least one allogeneic HLA molecule could be identified from 7 of 12 patients tested. Allorecognition was associated with IFNγ cytokine production, CD137 upregulation and cytotoxicity, suggesting high avidity allo-stimulation. Allo-HLA recognition by HIV-specific T cells was specific to the HIV target peptide/HLA restriction and TCR TRBV usage of the T cells. HIV-specific T cells do crossreact against allogeneic HLA molecules in an epitope and TRBV specific manner. Therefore allo-HLA stimulation could be exploited to induce or augment HIV-specific T cell responses.

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.

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.

The possible role of virus-specific CD8(+) memory T cells in decidual tissue

The most abundant lymphocyte present in decidual tissue is the CD8(+) T cell. It has been shown that most decidual CD8(+) T cells have an effector-memory phenotype, but expressed reduced levels of perforin and granzyme B compared with the peripheral CD8(+) effector-memory T cells. The specificity of these CD8(+) memory T cells has yet to be determined. One hypothesis is that the decidual memory T cells are virus-specific T cells that should protect the fetus against incoming pathogens. As virus-specific CD8(+) memory T cells can cross-react with human leukocyte alloantigens, an alternative, but not mutually exclusive, hypothesis is that these CD8(+) T cells are fetus-specific. Using virus-specific tetramers, we found increased percentages of virus-specific CD8(+) T cells in decidual tissue compared with peripheral blood after uncomplicated pregnancy. So far, no evidence has been obtained for a cross-reactive response of these virus-specific T cells to fetal human leukocyte antigens. These results suggest that the virus-specific memory T cells accumulate in the placenta to protect the fetus from a harmful infection.

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.

CMV driven CD8(+) T-cell activation is associated with acute rejection in lung transplantation

Lung transplantation is the definitive treatment for terminal respiratory disease, but the associated mortality rate is high. Acute rejection of the transplanted lung is a key determinant of adverse prognosis. Furthermore, an epidemiological relationship has been established between the occurrence of acute lung rejection and cytomegalovirus infection. However, the reasons for this association remain unclear. Here, we performed a longitudinal characterization of CMV-specific T-cell responses and immune activation status in the peripheral blood and bronchoalveolar lavage fluid of forty-four lung transplant patients. Acute rejection was associated with high levels of cellular activation in the periphery, reflecting strong CMV-specific CD8(+) T-cell activity post-transplant. Peripheral and lung CMV-specific CD8(+) T-cell responses were very similar, and related to the presence of CMV in the transplanted organ. These findings support that activated CMV-specific CD8(+) T-cells in the lung may play a role in promoting acute rejection.

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.

Relatively low level of antigen-specific monocytes detected in blood from untreated tuberculosis patients using CD4+ T-cell receptor tetramers

The in vivo kinetics of antigen-presenting cells (APCs) in patients with advanced and convalescent tuberculosis (TB) is not well characterized. In order to target Mycobacterium tuberculosis (MTB) peptides- and HLA-DR-holding monocytes and macrophages, 2 MTB peptide-specific CD4⁺ T-cell receptor (TCR) tetramers eu and hu were successfully constructed. Peripheral blood (PBL) samples from inpatients with advanced pulmonary TB (PTB) were analyzed using flow cytometry, and the percentages of tetramer-bound CD14⁺ monocytes ranged from 0.26–1.44% and 0.21–0.95%, respectively; significantly higher than those measured in PBL samples obtained from non-TB patients, healthy donors, and umbilical cords. These tetramers were also able to specifically detect macrophages in situ via immunofluorescent staining. The results of the continuous time-point tracking of the tetramer-positive rates in PBL samples from active PTB outpatients undergoing treatment show that the median percentages were at first low before treatment, increased to their highest levels during the first month, and then began to decrease during the second month until finally reaching and maintaining a relatively low level after 3–6 months. These results suggest that there is a relatively low level of MTB-specific monocytes in advanced and untreated patients. Further experiments show that MTB induces apoptosis in CD14⁺ cells, and the percentage of apoptotic monocytes dramatically decreases after treatment. Therefore, the relatively low level of MTB-specific monocytes is probably related to the apoptosis or necrosis of APCs due to live bacteria and their growth. The bactericidal effects of anti-TB drugs, as well as other unknown factors, would induce a peak value during the first month of treatment, and a relatively low level would be subsequently reached and maintained until all of the involved factors reached equilibrium. These tetramers have diagnostic potential and can provide valuable insights into the mechanisms of antigen presentation and its relationship with TB infection and latent TB infection.

Mycobacterium tuberculosis (MTB) is one of the most dangerous pathogens in the world. It is estimated that one-third of the world population contracts the bacteria during their lives. Approximately 5–10% of infected individuals will eventually develop an active form of the disease. Cellular immunity plays an important role in immunity against tuberculosis (TB); however, the host's defense mechanisms are not completely understood. Here, we developed a novel tool: MTB antigen-specific tetrameric CD4⁺ T-cell receptor (TCR) complexes that can detect MTB peptide-specific antigen presenting cells (APCs) in blood and local tissues. We found that a relatively low level of antigen-specific monocytes (i.e., APCs) was detected in peripheral blood (PBL) samples from untreated TB patients, and then increased to their peak levels during the first month after treatment, which probably had something to do with the decrease in APC apoptosis. Our research provides a new method for tracking dynamic changes in APCs that are associated with TB infection and latent TB infection, and an additional tool for the studies of TB immunity and its pathogenesis.

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.

Detection of allo-HLA cross-reactivity by virus-specific memory T-cell clones using single HLA-transfected K562 cells

The ability to directly measure virus-specific lymphocytes using fluorochrome-labeled tetrameric complexes has proven a great advancement for the transplantation field. Viral peptide/HLA tetrameric complexes allow the rapid generation of virus-specific clones using single cell sorting apparatus, permitting the determination of alloreactivity from a single TCR with known specificity. When combined with new target "detector" cells called single HLA antigen-transfected K562 cells (SALs), the human alloresponse can for the first time be examined specifically and reliably. Here we describe a method for detection of "heterologous immunity" from virus-specific memory T-cells using single HLA expressing cell lines as allogeneic targets.

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.

Vaccine-induced allo-HLA-reactive memory T cells in a kidney transplantation candidate

BACKGROUND

Allo-human leukocyte antigen (HLA) reactivity by naturally acquired viral-specific memory T cells is common. However, the effect of successful vaccination on the alloreactive memory T-cell repertoire is unclear. We hypothesized that vaccination could specifically induce allo-HLA-reactive memory T cells.

METHODS

A varicella-zoster virus (VZV) immediate early 62 (IE62)-specific CD8 memory T-cell clone was single cell sorted from a VZV seronegative renal transplant candidate after response to live attenuated varicella vaccination. To analyze the allo-HLA reactivity, the VZV IE62-specific T-cell clone was tested against HLA-typed target cells and target cells transfected with HLA molecules, in both cytokine production and cytotoxicity assays.

RESULTS

The varicella vaccine-induced VZV IE62-specific T-cell clone specifically produced interferon-γ when stimulated with HLA-B*55:01-expressing Epstein-Barr virus-transformed B cells and HLA-B*55:01-transfected K562 cells (single HLA antigen expressing cell line [SALs]) only. The clone also demonstrated specific cytolytic effector function against HLA-B*55:01 SALs and phytohemagglutinin blasts. Cytotoxicity assays using proximal tubular epithelial cell and human umbilical vein endothelial cell targets confirmed the kidney tissue specificity of the allo-HLA-B*55:01 reactivity, and the relevance of the cross-reactivity to clinical kidney transplantation. The results also suggest that molecular mimicry, and not bystander proliferation, is the mechanism underlying vaccine-induced alloreactivity.

CONCLUSIONS

Varicella vaccination generated a de novo alloreactive kidney cell-specific cytolytic effector memory T cell in a patient awaiting renal transplantation. Vaccination-induced alloreactivity may have important clinical implications, especially for vaccine timing and recipient monitoring.

Tissue specificity of cross-reactive allogeneic responses by EBV EBNA3A-specific memory T cells

BACKGROUND

The crossreactivity of Epstein-Barr virus (EBV Epstein-Barr virus nuclear antigen 3A [EBNA3A])-specific CD8 T cells against allogeneic human leukocyte antigen (HLA)-B*44:02 has been shown to be dependent on presentation of self-peptide EEYLQAFTY by the target antigen. In this study, we report that allogeneic HLA-B*44:02 proximal tubular epithelial cells (PTECs) and human umbilical vein endothelial cells (HUVECs) are poor targets for EBV EBNA3A-specific T cells.

METHODS

The EEY peptide was exogenously loaded onto HLA-B*44:02 and HLA-B*44:03-expressing PTECs and HUVECs. EEY-peptide-loaded, and unloaded, PTECs and HUVECs were then incubated with serial dilutions of our EBNA3A T-cell clone, in a cytotoxicity assay.

RESULTS

Although HLA-B*44:02-expressing PTECs were specifically lysed in proportion to the effector/target ratio by the EBNA3A T-cell clone, without peptide loading, lysis was greatly increased by exogenous EEY peptide loading (15% vs. 75%; P<0.0001). HLA-B*44:02-expressing HUVECs were only lysed when loaded with exogenous EEY peptide (0% vs. 64%; P<0.0001). Lack of HLA expression and lack of ABCD3 gene expression were excluded as a cause for these results. PTECs and HUVECs were specifically targeted by another alloreactive T-cell clone without exogenous peptide loading, suggesting that the lack of recognition of HLA-B*44:02 epithelial and endothelial cells by the EBV EBNA3A T-cell clone was due to lack of EEYLQAFTY peptide presentation.

CONCLUSIONS

Tissue-specific (peptide dependent) alloreactivity may have important implications for transplantation monitoring and rejection.

Alloreactivity from human viral specific memory T-cells

The mechanisms by which alloreactive memory T-cells are generated in non-sensitized individuals have begun to be elucidated. It is generally accepted that a very high level of crossreactivity is an essential feature of the T-cell receptor. Indeed it has recently been shown that alloreactivity from viral specific memory T-cells is far more common than predicted, 45% of viral specific T-cell clones were found to be allo-HLA crossreactive. In this overview the evidence for crossreactive alloresponses from human viral specific memory T-cells is discussed with special emphasis on the unexpected high frequency of these crossreactive responses, the peptide and tissue specificity of the responses, and the mechanistic insights gleaned from the elucidation of the crystal structure of an allo-HLA crossreactive viral specific TCR. The possible implications for clinical solid organ and bone marrow transplantation and tolerance induction will be discussed.

Allo-HLA reactivity of virus-specific memory T cells is common

Graft-versus-host disease and graft rejection are major complications of allogeneic HLA-mismatched stem cell transplantation or organ transplantation that are caused by alloreactive T cells. Because a range of acute viral infections have been linked to initiating these complications, we hypothesized that the cross-reactive potential of virus-specific memory T cells to allogeneic (allo) HLA molecules may be able to mediate these complications. To analyze the allo-HLA reactivity, T cells specific for Epstein-Barr virus, cytomegalovirus, varicella zoster virus, and influenza virus were tested against a panel of HLA-typed target cells, and target cells transduced with single HLA molecules. Eighty percent of T-cell lines and 45% of virus-specific T-cell clones were shown to cross-react against allo-HLA molecules. The cross-reactivity of the CD8 and CD4 T-cell clones was directed primarily against HLA class I and II, respectively. However, a restricted number of CD8 T cells exhibited cross-reactivity to HLA class II. T-cell receptor (TCR) gene transfer confirmed that allo-HLA reactivity and virus specificity were mediated via the same TCR. These results demonstrate that a substantial proportion of virus-specific T cells exert allo-HLA reactivity, which may have important clinical implications in transplantation settings as well as adoptive transfer of third-party virus-specific T cells.