Frontiers in nephrology: heterologous immunity, T cell cross-reactivity, and alloreactivity

Deciphering the True Immunologic Risk in Renal Transplantation in Patients With HIV

Since 1995, rates of end-stage renal disease have risen dramatically in patients living with HIV infection. However, given the concern for higher rates of acute rejection in this patient population, the immunologic threat posed by HIV infection is a specter clinicians must continually confront. Living donor transplantation may negate this risk; this study aims to assess the benefit of living donor transplantation in this population and to ascertain the immunologic risk faced by patients who are HIV-infected. The 2021 UNOS database was queried, and all HIV-infected kidney transplant recipients since 1987 were identified. Recipients were stratified based on deceased (DDKT) vs living (LDKT) donor status. Overall survival, allograft survival, acute rejection, panel reactive antibody (PRA) percentage, and crossmatch positivity were compared between groups. One thousand two hundred twenty-six patients underwent DDKT, and 304 patients underwent LDKT. Living donor kidney transplantation demonstrated greater overall survival (P = .045) and graft survival (P < .001). However, no difference in acute rejection was noted between the cohorts, and no difference in overall or graft survival was evident based on PRA percentage. Crossmatch positive status did not negatively affect graft survival. Patients with HIV undergoing LDKT fared better than those undergoing DDKT. Nevertheless, patients at higher immunologic risk-elevated PRA% and crossmatch positivity-did not experience graft loss at a higher rate than patients at lower immunologic risk. These results were valid in both DDKT and LDKT cohorts. These findings suggest that infection with HIV does not overtly increase patients' immunologic risk, and concerns surrounding transplantation in this population may be overestimated.

Acute and Chronic Changes in Gene Expression After CMV DNAemia in Kidney Transplant Recipients

Cytomegalovirus (CMV) viremia continues to cause significant morbidity and mortality in kidney transplant patients with clinical complications including organ rejection and death. Whole blood gene expression dynamics in CMV viremic patients from onset of DNAemia through convalescence has not been well studied to date in humans. To evaluate how CMV infection impacts whole blood leukocyte gene expression over time, we evaluated a matched cohort of 62 kidney transplant recipients with and without CMV DNAemia using blood samples collected at multiple time points during the 12-month period after transplant. While transcriptomic differences were minimal at baseline between DNAemic and non-DNAemic patients, hundreds of genes were differentially expressed at the long-term timepoint, including genes enriching for pathways important for macrophages, interferon, and IL-8 signaling. Amongst patients with CMV DNAemia, the greatest amount of transcriptomic change occurred between baseline and 1-week post-DNAemia, with increase in pathways for interferon signaling and cytotoxic T cell function. Time-course gene set analysis of these differentially expressed genes revealed that most of the enriched pathways had a significant time-trend. While many pathways that were significantly down- or upregulated at 1 week returned to baseline-like levels, we noted that several pathways important in adaptive and innate cell function remained upregulated at the long-term timepoint after resolution of CMV DNAemia. Differential expression analysis and time-course gene set analysis revealed the dynamics of genes and pathways involved in the immune response to CMV DNAemia in kidney transplant patients. Understanding transcriptional changes caused by CMV DNAemia may identify the mechanism behind patient vulnerability to CMV reactivation and increased risk of rejection in transplant recipients and suggest protective strategies to counter the negative immunologic impact of CMV. These findings provide a framework to identify immune correlates for risk assessment and guiding need for extending antiviral prophylaxis.

Epstein-Barr Virus Epitope-Major Histocompatibility Complex Interaction Combined with Convergent Recombination Drives Selection of Diverse T Cell Receptor α and β Repertoires

Recognition modes of individual T cell receptors (TCRs) are well studied, but factors driving the selection of TCR repertoires from primary through persistent human virus infections are less well understood. Using deep sequencing, we demonstrate a high degree of diversity of Epstein-Barr virus (EBV)-specific clonotypes in acute infectious mononucleosis (AIM). Only 9% of unique clonotypes detected in AIM persisted into convalescence; the majority (91%) of unique clonotypes detected in AIM were not detected in convalescence and were seeming replaced by equally diverse "de novo" clonotypes. The persistent clonotypes had a greater probability of being generated than nonpersistent clonotypes due to convergence recombination of multiple nucleotide sequences to encode the same amino acid sequence, as well as the use of shorter complementarity-determining regions 3 (CDR3s) with fewer nucleotide additions (i.e., sequences closer to germ line). Moreover, the two most immunodominant HLA-A2-restricted EBV epitopes, BRLF1109 and BMLF1280, show highly distinct antigen-specific public (i.e., shared between individuals) features. In fact, TCRα CDR3 motifs played a dominant role, while TCRβ played a minimal role, in the selection of TCR repertoire to an immunodominant EBV epitope, BRLF1. This contrasts with the majority of previously reported repertoires, which appear to be selected either on TCRβ CDR3 interactions with peptide/major histocompatibility complex (MHC) or in combination with TCRα CDR3. Understanding of how TCR-peptide-MHC complex interactions drive repertoire selection can be used to develop optimal strategies for vaccine design or generation of appropriate adoptive immunotherapies for viral infections in transplant settings or for cancer.IMPORTANCE Several lines of evidence suggest that TCRα and TCRβ repertoires play a role in disease outcomes and treatment strategies during viral infections in transplant patients and in cancer and autoimmune disease therapy. Our data suggest that it is essential that we understand the basic principles of how to drive optimum repertoires for both TCR chains, α and β. We address this important issue by characterizing the CD8 TCR repertoire to a common persistent human viral infection (EBV), which is controlled by appropriate CD8 T cell responses. The ultimate goal would be to determine if the individuals who are infected asymptomatically develop a different TCR repertoire than those that develop the immunopathology of AIM. Here, we begin by doing an in-depth characterization of both CD8 T cell TCRα and TCRβ repertoires to two immunodominant EBV epitopes over the course of AIM, identifying potential factors that may be driving their selection.

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.

The impact of vaccines on heterologous adaptive immunity

BACKGROUND

Vaccines induce antigen-specific memory in adaptive immune cells that enables long-lived protection against the target pathogen. In addition to this, several vaccines have beneficial effects greater than protection against their target pathogen. These non-specific effects are proposed to be the result of vaccine-induced immunomodulation. In the case of bacille Calmette-Guérin (BCG) vaccine, this involves induction of innate immune memory, termed 'trained immunity', in monocytes and natural killer cells.

OBJECTIVES

This review discusses current evidence for vaccine-induced immunomodulation of adaptive immune cells and heterologous adaptive immune responses.

CONTENT

The three vaccines that have been associated with changes in all-cause infant mortality: BCG, diphtheria-tetanus-pertussis (DTP) and measles-containing vaccines (MCV) alter T-cell and B-cell immunity. The majority of studies that investigated non-specific effects of these vaccines on the adaptive immune system report changes in numbers or proportions of adaptive immune cell populations. However, there is also evidence for effects of these vaccines on adaptive immune cell function and responses to heterologous stimuli. There is some evidence that, in addition to BCG, DTP and MCV, other vaccines (that have not been associated with changes in all-cause mortality) may alter adaptive immune responses to unrelated stimuli.

IMPLICATIONS

This review concludes that vaccines alter adaptive immune cell populations and heterologous immune responses. The non-specific effects differ between various vaccines and their effects on heterologous adaptive immune responses may also involve bystander activation, cross-reactivity and other as yet undefined mechanisms. This has major implications for future vaccine design and vaccination scheduling.

Old game, new players: Linking classical theories to new trends in transplant immunology

The evolutionary emergence of an efficient immune system has a fundamental role in our survival against pathogenic attacks. Nevertheless, this same protective mechanism may also establish a negative consequence in the setting of disorders such as autoimmunity and transplant rejection. In light of the latter, although research has long uncovered main concepts of allogeneic recognition, immune rejection is still the main obstacle to long-term graft survival. Therefore, in order to define effective therapies that prolong graft viability, it is essential that we understand the underlying mediators and mechanisms that participate in transplant rejection. This multifaceted process is characterized by diverse cellular and humoral participants with innate and adaptive functions that can determine the type of rejection or promote graft acceptance. Although a number of mediators of graft recognition have been described in traditional immunology, recent studies indicate that defining rigid roles for certain immune cells and factors may be more complicated than originally conceived. Current research has also targeted specific cells and drugs that regulate immune activation and induce tolerance. This review will give a broad view of the most recent understanding of the allogeneic inflammatory/tolerogenic response and current insights into cellular and drug therapies that modulate immune activation that may prove to be useful in the induction of tolerance in the clinical setting.

Anti-CD28 Antibody and Belatacept Exert Differential Effects on Mechanisms of Renal Allograft Rejection

Belatacept is a biologic that targets CD80/86 and prevents its interaction with CD28 and its alternative ligand, cytotoxic T lymphocyte antigen 4 (CTLA-4). Clinical experience in kidney transplantation has revealed a high incidence of rejection with belatacept, especially with intensive regimens, suggesting that blocking CTLA-4 is deleterious. We performed a head to head assessment of FR104 (n=5), a selective pegylated Fab' antibody fragment antagonist of CD28 that does not block the CTLA-4 pathway, and belatacept (n=5) in kidney allotransplantation in baboons. The biologics were supplemented with an initial 1-month treatment with low-dose tacrolimus. In cases of acute rejection, animals also received steroids. In the belatacept group, four of five recipients developed severe, steroid-resistant acute cellular rejection, whereas FR104-treated animals did not. Assessment of regulatory T cell-specific demethylated region methylation status in 1-month biopsy samples revealed a nonsignificant trend for higher regulatory T cell frequencies in FR104-treated animals. Transcriptional analysis did not reveal significant differences in Th17 cytokines but did reveal higher levels of IL-21, the main cytokine secreted by CD4 T follicular helper (Tfh) cells, in belatacept-treated animals. In vitro, FR104 controlled the proliferative response of human preexisting Tfh cells more efficiently than belatacept. In mice, selective CD28 blockade also controlled Tfh memory cell responses to KLH stimulation more efficiently than CD80/86 blockade. Our data reveal that selective CD28 blockade and belatacept exert different effects on mechanisms of renal allograft rejection, particularly at the level of Tfh cell stimulation.

Co-Stimulatory Blockade of the CD28/CD80-86/CTLA-4 Balance in Transplantation: Impact on Memory T Cells?

CD28 and CTLA-4 are prototypal co-stimulatory and co-inhibitory cell surface signaling molecules interacting with CD80/86, known to be critical for immune response initiation and regulation, respectively. Initial “bench-to-beside” translation, two decades ago, resulted in the development of CTLA4-Ig, a biologic that targets CD80/86 and prevents T-cell costimulation. In spite of its proven effectiveness in inhibiting allo-immune responses, particularly in murine models, clinical experience in kidney transplantation with belatacept (high-affinity CTLA4-Ig molecule) reveals a high incidence of acute, cell-mediated rejection. Originally, the etiology of belatacept-resistant graft rejection was thought to be heterologous immunity, i.e., the cross-reactivity of the pool of memory T cells from pathogen-specific immune responses with alloantigens. Recently, the standard view that memory T cells arise from effector cells after clonal contraction has been challenged by a “developmental” model, in which less differentiated memory T cells generate effector cells. This review delineates how this shift in paradigm, given the differences in co-stimulatory and co-inhibitory signal depending on the maturation stage, could profoundly affect our understanding of the CD28/CD80-86/CTLA-4 blockade and highlights the potential advantages of selectively targeting CD28, instead of CD80/86, to control post-transplant immune responses.

Viral-induced CD28 loss evokes costimulation independent alloimmunity

BACKGROUND

Belatacept, a B7-specific fusion protein, blocks CD28-B7 costimulation and prevents kidney allograft rejection. However, it is ineffective in a sizable minority of patients. Although T-cell receptor and CD28 engagement are known to initiate T-cell activation, many human antigen-experienced T-cells lose CD28, and can be activated independent of CD28 signals. We posit that these cells are central drivers of costimulation blockade resistant rejection (CoBRR) and propose that CoBRR might relate to an accumulation of CD28(-) T-cells resulting from viral antigen exposure.

MATERIALS AND METHODS

We infected C57BL/6 mice with polyomavirus (a BK virus analog), murine cytomegalovirus (a human cytomegalovirus analog), and gammaherpesvirus (HV68; an Epstein-Barr virus analog) and assessed for CD28 expression relative to mock infection controls. We then used mixed lymphocyte reaction (MLR) assays to assess the alloreactive response of these mice against major histocompatibility complex-mismatched cells.

RESULTS

We demonstrated that infection with polyomavirus, murine CMV, and HV68 can induce CD28 downregulation in mice. We showed that these analogs of clinically relevant human viruses enable lymphocytes from infected mice to launch an anamnestic, costimulation blockade resistant, alloreactive response against major histocompatibility complex-mismatched cells without prior alloantigen exposure. Further analysis revealed that gammherpesvirus-induced oligoclonal T-cell expansion is required for the increased alloreactivity.

CONCLUSIONS

Virus exposure results in reduced T-cell expression of CD28, the target of costimulation blockade therapy. These viruses also contribute to increased alloreactivity. Thus, CD28 downregulation after viral infection may play a seminal role in driving CoBRR.

Taming inflammation by targeting cytokine signaling: new perspectives in the induction of transplantation tolerance

Transplantation tolerance remains an elusive goal, partly due to limitations in our understanding of the interplay between inflammatory mediators and their role in the activation and regulation of T lymphocytes. Although multiple mechanisms acting both centrally and peripherally are responsible for tolerance induction, the signaling pathways leading to activation or regulation of adaptive immunity are often complex, branched, redundant and modulated by the microenvironment's inflammatory milieu. Accumulating evidence clearly indicates that inflammatory cytokines limit the tolerogenic potential of immunomodulatory protocols by supporting priming of the immune system and counteracting regulatory mechanisms, ultimately promoting rejection. In this review, we summarize recent progress in the development of novel therapeutics to manipulate this inflammatory environment and achievements in targeted inhibition of inflammatory cytokine signaling. Ultimately, robust transplant tolerance induction will probably require a multifaceted, holistic approach that integrates the various mechanisms of tolerance induction, incorporates the dynamic alterations in costimulatory requirements of alloreactive T cells, while maintaining endogenous mechanisms of immune regulation.

Impaired CD8(+) T cell immunity after allogeneic bone marrow transplantation leads to persistent and severe respiratory viral infection

RATIONALE

Bone marrow transplant (BMT) recipients experience frequent and severe respiratory viral infections (RVIs). However, the immunological mechanisms predisposing to RVIs are uncertain. Therefore, we hypothesized that antiviral T cell immunity is impaired as a consequence of allogeneic BMT, independent of pharmacologic immunosuppression, and is responsible for increased susceptibility to RVI.

METHODS

Bone marrow and splenocytes from C57BL/6(H2(b)) mice were transplanted into B10.BR(H2(k)) (Allo) or C57BL/6(H2(b)) (Syn) recipients. Five weeks after transplantation, recipient mice were inoculated intranasally with mouse parainfluenza virus type 1 (mPIV-1), commonly known as Sendai virus (SeV), and monitored for relevant immunological and disease endpoints.

MAIN RESULTS

Severe and persistent airway inflammation, epithelial injury, and enhanced mortality are found after viral infection in Allo mice but not in control Syn and non-transplanted mice. In addition, viral clearance is delayed in Allo mice as evidenced by prolonged detection of viral transcripts at Day 15 post-inoculation (p.i.) but not in control mice. In concert with these events, we also detected decreased levels of total and virus-specific CD8(+) T cells, as well as increased T cellexpression of inhibitory receptor programmed death-1 (PD-1), in the lungs of Allo mice at Day 8 p.i. Adoptive transfer of CD8(+) T cells from non-transplanted mice recovered from SeV infection into Allo mice at Day 8 p.i. restored normal levels of viral clearance, epithelial repair, and lung inflammation.

CONCLUSIONS

Taken together these results indicate that allogeneic BMT results in more severe RVI based on the failure to develop an appropriate pulmonary CD8(+) T cell response, providing an important potential mechanism to target in improving outcomes of RVI after BMT.

Adaptation in the innate immune system and heterologous innate immunity

The innate immune system recognizes deviation from homeostasis caused by infectious or non-infectious assaults. The threshold for its activation seems to be established by a calibration process that includes sensing of microbial molecular patterns from commensal bacteria and of endogenous signals. It is becoming increasingly clear that adaptive features, a hallmark of the adaptive immune system, can also be identified in the innate immune system. Such adaptations can result in the manifestation of a primed state of immune and tissue cells with a decreased activation threshold. This keeps the system poised to react quickly. Moreover, the fact that the innate immune system recognizes a wide variety of danger signals via pattern recognition receptors that often activate the same signaling pathways allows for heterologous innate immune stimulation. This implies that, for example, the innate immune response to an infection can be modified by co-infections or other innate stimuli. This "design feature" of the innate immune system has many implications for our understanding of individual susceptibility to diseases or responsiveness to therapies and vaccinations. In this article, adaptive features of the innate immune system as well as heterologous innate immunity and their implications are discussed.

The two faces of heterologous immunity: protection or immunopathology

Immunity to previously encountered viruses can alter responses to unrelated pathogens. This phenomenon, which is known as heterologous immunity, has been well established in animal model systems. Heterologous immunity appears to be relatively common and may be beneficial by boosting protective responses. However, heterologous reactivity can also result in severe immunopathology. The key features that define heterologous immune modulation include alterations in the CD4(+) and CD8(+) T cell compartments and changes in viral dynamics and disease progression. In this review, we discuss recent advances and the current understanding of antiviral immunity in heterologous infections. The difficulties of studying these complex heterologous infections in humans are discussed, with special reference to the variations in HLA haplotypes and uncertainties about individuals' infection history. Despite these limitations, epidemiological analyses in humans and the data from mouse models of coinfection can be applied toward advancing the design of therapeutics and vaccination strategies.

Memory T cells in transplantation - progress and challenges

PURPOSE OF REVIEW

Memory T cells present a different set of challenges to transplant patients; they are needed for protection against invading pathogens, especially under conditions of immunosuppression. But their presence also threatens transplant survival, as some of them are alloreactive. Efforts to resolve this paradox will be critical in the induction of transplant tolerance.

RECENT FINDINGS

There has been significant progress made in the past few years in the areas of population diversity of memory T cells, metabolic control of their induction, and mechanisms and pathways involved in memory cell exhaustion. Multiple targets on memory T cells have been identified, some of which are under vigorous testing in various transplant models.

SUMMARY

Memory T cells are both friends and foes to transplant patients, and tolerance strategies should selectively target alloreactive memory T cells and leave other memory cells unaltered. This situation remains a major challenge in the clinic.

Heterologous immunity triggered by a single, latent virus in Mus musculus: combined costimulation- and adhesion- blockade decrease rejection

The mechanisms underlying latent-virus-mediated heterologous immunity, and subsequent transplant rejection, especially in the setting of T cell costimulation blockade, remain undetermined. To address this, we have utilized MHV68 to develop a rodent model of latent virus-induced heterologous alloimmunity. MHV68 infection was correlated with multimodal immune deviation, which included increased secretion of CXCL9 and CXCL10, and with the expansion of a CD8(dim) T cell population. CD8(dim) T cells exhibited decreased expression of multiple costimulation molecules and increased expression of two adhesion molecules, LFA-1 and VLA-4. In the setting of MHV68 latency, recipients demonstrated accelerated costimulation blockade-resistant rejection of skin allografts compared to non-infected animals (MST 13.5 d in infected animals vs 22 d in non-infected animals, p<.0001). In contrast, the duration of graft acceptance was equivalent between non-infected and infected animals when treated with combined anti-LFA-1/anti-VLA-4 adhesion blockade (MST 24 d for non-infected and 27 d for infected, p = n.s.). The combination of CTLA-4-Ig/anti-CD154-based costimulation blockade+anti-LFA-1/anti-VLA-4-based adhesion blockade led to prolonged graft acceptance in both non-infected and infected cohorts (MST>100 d for both, p<.0001 versus costimulation blockade for either). While in the non-infected cohort, either CTLA-4-Ig or anti-CD154 alone could effectively pair with adhesion blockade to prolong allograft acceptance, in infected animals, the prolonged acceptance of skin grafts could only be recapitulated when anti-LFA-1 and anti-VLA-4 antibodies were combined with anti-CD154 (without CTLA-4-Ig, MST>100 d). Graft acceptance was significantly impaired when CTLA-4-Ig alone (no anti-CD154) was combined with adhesion blockade (MST 41 d). These results suggest that in the setting of MHV68 infection, synergy occurs predominantly between adhesion pathways and CD154-based costimulation, and that combined targeting of both pathways may be required to overcome the increased risk of rejection that occurs in the setting of latent-virus-mediated immune deviation.

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.

Anti-IFN-γ and peptide-tolerization therapies inhibit acute lung injury induced by cross-reactive influenza A-specific memory T cells

Viral infections have variable outcomes, with severe disease occurring in only few individuals. We hypothesized that this variable outcome could correlate with the nature of responses made to previous microbes. To test this, mice were infected initially with influenza A virus (IAV) and in memory phase challenged with lymphocytic choriomeningitis virus (LCMV), which we show in this study to have relatively minor cross-reactivity with IAV. The outcome in genetically identical mice varied from mild pneumonitis to severe acute lung injury with extensive pneumonia and bronchiolization, similar to that observed in patients who died of the 1918 H1N1 pandemic. Lesion expression did not correlate with virus titers. Instead, disease severity directly correlated with and was predicted by the frequency of IAV-PB1703- and IAV-PA224-specific responses, which cross-reacted with LCMV-GP34 and LCMV-GP276, respectively. Eradication or functional ablation of these pathogenic memory T cell populations, using mutant-viral strains, peptide-based tolerization strategies, or short-term anti-IFN-γ treatment, inhibited severe lesions such as bronchiolization from occurring. Heterologous immunity can shape outcome of infections and likely individual responses to vaccination, and can be manipulated to treat or prevent severe pathology.

T-cell activation and transplantation tolerance

Transplantation of allogeneic or "nonself" tissues stimulates a robust immune response leading to graft rejection, and therefore, most recipients of allogeneic organ transplants require the lifelong use of immune suppressive agents. Excellent outcomes notwithstanding, contemporary immunosuppressive medications are toxic, are often not taken by patients, and pose long-term risks of infection and malignancy. The ultimate goal in transplantation is to develop new treatments that will supplant the need for general immunosuppression. Here, we will describe the development and application of costimulation blockade to induce transplantation tolerance and discuss how the diverse array of signals that act on T cells will determine the balance between graft survival and rejection.

Viral infection in renal transplant recipients

Viruses are among the most common causes of opportunistic infection after transplantation. The risk for viral infection is a function of the specific virus encountered, the intensity of immune suppression used to prevent graft rejection, and other host factors governing susceptibility. Although cytomegalovirus is the most common opportunistic pathogen seen in transplant recipients, numerous other viruses have also affected outcomes. In some cases, preventive measures such as pretransplant screening, prophylactic antiviral therapy, or posttransplant viral monitoring may limit the impact of these infections. Recent advances in laboratory monitoring and antiviral therapy have improved outcomes. Studies of viral latency, reactivation, and the cellular effects of viral infection will provide clues for future strategies in prevention and treatment of viral infections. This paper will summarize the major viral infections seen following transplant and discuss strategies for prevention and management of these potential pathogens.

Developing immunologic tolerance for transplantation at the fetal stage

Given the shortage of human organs for transplantation, the waiting lists are increasing annually and consequently so is the time and deaths during the wait. As most immune suppression therapy is not antigen specific and the risk of infection tends to increase, scientists are looking for new options for immunosuppression or immunotolerance. Tolerance induction would avoid the complications caused by immunosupressive drugs. As such, taking into account the experience with autoimmune diseases, one strategy could be immune modulation-induced changes in T-cell cytokine secretion or antigen therapy; however, most clinical trials have failed. Gene transfer of MHC genes across species may be used to induce tolerance to xenogenic solid organs. Other options are induction of central tolerance by the establishment of mixed chimerism through hematopoietic stem cell transplantation and the induction of 'operational tolerance' through immunodeviation involving dendritic or Tregs. I propose that, as the recognition and tolerance of proteins takes place in the thymus, this organ should be the main target for immunotolerance research protocols even as early as during the fetal development.

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.

Computer simulations of heterologous immunity: highlights of an interdisciplinary cooperation

The relationship between biological research and mathematical modeling is complex, critical, and vital. In this review, we summarize the results of the collaboration between two laboratories, exploring the interaction between mathematical modeling and wet-lab immunology. During this collaboration several aspects of the immune defence against viral infections were investigated, focusing primarily on the subject of heterologous immunity. In this manuscript, we emphasize the topics where computational simulations were applied in conjunction with experiments, such as immune attrition, the growing and shrinking of cross-reactive T cell repertoires following repeated infections, the short and long-term effects of cross-reactive immunological memory, and the factors influencing the appearance of new clonal specificities. For each topic, we describe how the mathematical model used was adapted to answer specific biological questions, and we discuss the hypotheses that were generated by simulations. Finally, we propose rules for testing hypotheses that emerge from model experimentation in the wet lab, and vice-versa.

Heterologous immunity: immunopathology, autoimmunity and protection during viral infections

Heterologous immunity is a common phenomenon present in all infections. Most of the time it is beneficial, mediating protective immunity, but in some individuals that have the wrong crossreactive response it leads to a cascade of events that result in severe immunopathology. Infections have been associated with autoimmune diseases such as diabetes, multiple sclerosis and lupus erythematosis, but also with unusual autoimmune like pathologies where the immune system appears dysregulated, such as, sarcoidosis, colitis, panniculitis, bronchiolitis obliterans, infectious mononucleosis and even chronic fatigue syndrome. Here we review the evidence that to better understand these autoreactive pathologies it requires an evaluation of how T cells are regulated and evolve during sequential infections with different pathogens under the influence of heterologous immunity.

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.

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

Accumulating evidence suggests that alloreactive memory T-cells may be generated as a result of viral infection. So far, a suitable tool to define the individual human leukocyte antigen (HLA) cross-reactivity of virus-specific memory T-cells is not available. We therefore aimed to develop a novel system for the detection of cross-reactive alloresponses using single HLA antigen expressing cell lines (SALs) as stimulator. Herein, we generated Epstein-Barr Virus (EBV) EBNA3A specific CD8 memory T-cell clones (HLA-B*0801/FLRGRAYGL peptide restricted) and assayed for alloreactivity against a panel of SALs using interferon-gamma Elispot as readout. Generation of the T-cell clones was performed by single cell sorting based on staining with viral peptide/major histocompatibility complex-specific tetramer. Monoclonality of the T-cell clones was confirmed by T-cell receptor (TCR) polymerase chain reaction analysis. First, we confirmed the previously described alloreactivity of the EBV EBNA3A-specific T-cell clones against SAL-expressing HLA-B*4402. Further screening against the entire panel of SALs also showed additional cross-reactivity against SAL-expressing HLA-B*5501. Functionality of the cross-reactive T-cell clones was confirmed by chromium release assay using phytohemagglutinin blasts as targets. SALs are an effective tool to detect cross-reactivity of viral-specific CD8 memory T-cell clones against individual class I HLA molecules. This technique may have important implications for donor selection and monitoring of transplant recipients.