Cross-reactivities in memory cytotoxic T lymphocyte recognition of heterologous viruses

Anti-HLA antibodies may be a subset of polyreactive immunoglobulins generated after viral superinfection

Chronic rejection remains an obstacle to long-term allograft survival. Donor-specific anti-HLA antibodies (DSA) play a significant role in causing chronic antibody-mediated allograft rejection. Exposure to mismatched HLA antigens via transfusion, pregnancy, or transplanted tissue has been described in the literature as an immunogenic stimulus of anti-HLA antibodies. Yet anti-HLA antibodies also develop in the absence of traditional sensitization events and molecular mimicry has been postulated as a stimulus for these naturally occurring alloantibodies. While heterologous reactivity has been documented between virus components and allogeneic T cells, there is insufficient evidence to support the development of anti-HLA antibodies from viral components. We hypothesized that anti-HLA antibodies may develop following viral coinfection or superinfection. The objectives of this investigation included: 1) developing an in-silico algorithm to identify viral peptide components that exhibit HLA-specific homology, and 2) identifying cellular changes that take place during ischemia/reperfusion injury which could facilitate the generation of novel anti-HLA antibodies from viral sources. We developed the neoepitope transplant rejection and autoimmune disease (NETRAD) algorithm to identify amino acid sequence homology between viral envelope proteins and HLA. The algorithm integrates post-translational protein modifications that are consistent with ischemia/reperfusion injury. Seventy-two HLA-specific epitopes were demarcated as examples using this approach. In conclusion, we present in-silico evidence which supports the identification of anti-HLA antibodies as a subset of polyreactive antibodies generated from stress-modified viral envelope proteins. Remarkably, each targeted HLA epitope associated with a distinct anti-HLA antibody could be consistently attributed to a major envelope glycoprotein component of Epstein Barr virus. Transplant Immunology manuscript # TRIM-D-24-00351. Dryad data repository:https://doi.org/10.5061/dryad.qjq2bvqpq.

BCG immunization induces CX3CR1hi effector memory T cells to provide cross-protection via IFN-γ-mediated trained immunity

After a century of using the Bacillus Calmette-Guérin (BCG) vaccine, our understanding of its ability to provide protection against homologous (Mycobacterium tuberculosis) or heterologous (for example, influenza virus) infections remains limited. Here we show that systemic (intravenous) BCG vaccination provides significant protection against subsequent influenza A virus infection in mice. We further demonstrate that the BCG-mediated cross-protection against influenza A virus is largely due to the enrichment of conventional CD4+ effector CX3CR1hi memory αβ T cells in the circulation and lung parenchyma. Importantly, pulmonary CX3CR1hi T cells limit early viral infection in an antigen-independent manner via potent interferon-γ production, which subsequently enhances long-term antimicrobial activity of alveolar macrophages. These results offer insight into the unknown mechanism by which BCG has persistently displayed broad protection against non-tuberculosis infections via cross-talk between adaptive and innate memory responses.

p40 homodimers bridge ischemic tissue inflammation and heterologous alloimmunity in mice via IL-15 transpresentation

Virus-induced memory T cells often express functional cross-reactivity, or heterologous immunity, to other viruses and to allogeneic MHC molecules that is an important component of pathogenic responses to allogeneic transplants. During immune responses, antigen-reactive naive and central memory T cells proliferate in secondary lymphoid organs to achieve sufficient cell numbers to effectively respond, whereas effector memory T cell proliferation occurs directly within the peripheral inflammatory microenvironment. Mechanisms driving heterologous memory T cell proliferation and effector function expression within peripheral tissues remain poorly understood. Here, we dissected proliferation of heterologous donor-reactive memory CD8+ T cells and their effector functions following infiltration into heart allografts with low or high intensities of ischemic inflammation. Proliferation within both ischemic conditions required p40 homodimer-induced IL-15 transpresentation by graft DCs, but expression of effector functions mediating acute allograft injury occurred only in high-ischemic allografts. Transcriptional responses of heterologous donor-reactive memory CD8+ T cells were distinct from donor antigen-primed memory CD8+ T cells during early activation in allografts and at graft rejection. Overall, the results provide insights into mechanisms driving heterologous effector memory CD8+ T cell proliferation and the separation between proliferation and effector function that is dependent on the intensity of inflammation within the tissue microenvironment.

Pre-existing T Cell Memory to Novel Pathogens

Immunological experiences lead to the development of specific T and B cell memory, which readies the host for a later pathogen rechallenge. Currently, immunological memory is best understood as a linear process whereby memory responses are generated by and directed against the same pathogen. However, numerous studies have identified memory cells that target pathogens in unexposed individuals. How "pre-existing memory" forms and impacts the outcome of infection remains unclear. In this review, we discuss differences in the composition of baseline T cell repertoire in mice and humans, factors that influence pre-existing immune states, and recent literature on their functional significance. We summarize current knowledge on the roles of pre-existing T cells in homeostasis and perturbation and their impacts on health and disease.

Autoimmunity: Are we asking the right question?

For decades, the main question immunologists have asked about autoimmunity is "what causes a break in self-tolerance?" We have not found good answers to that question, and I believe we are still so ignorant because it's the wrong question. Rather than a break in self-tolerance, I suggest that many autoimmune diseases might be due to defects in normal tissue physiology.

The Ugly Duckling Turned to Swan: A Change in Perception of Bystander-Activated Memory CD8 T Cells

Memory T cells (T_mem) rapidly mount Ag-specific responses during pathogen reencounter. However, T_mem also respond to inflammatory cues in the absence of an activating TCR signal, a phenomenon termed bystander activation. Although bystander activation was first described over 20 years ago, the physiological relevance and the consequences of T cell bystander activation have only become more evident in recent years. In this review, we discuss the scenarios that trigger CD8 T_mem bystander activation including acute and chronic infections that are either systemic or localized, as well as evidence for bystander CD8 T_mem within tumors and following vaccination. We summarize the possible consequences of bystander activation for the T cell itself, the subsequent immune response, and the host. We highlight when T cell bystander activation appears to benefit or harm the host and briefly discuss our current knowledge gaps regarding regulatory signals that can control bystander activation.

Endogenous memory T cells with donor-reactivity: early post-transplant mediators of acute graft injury in unsensitized recipients

The pretransplant presence of endogenous donor-reactive memory T cells is an established risk factor for acute rejection and poorer transplant outcomes. A major source of these memory T cells in unsensitized recipients is heterologously generated memory T cells expressing reactivity to donor allogeneic MHC molecules. Multiple clinical studies have shown that the pretransplant presence of high numbers of circulating endogenous donor-reactive memory T cells correlates with higher incidence of acute rejection and decreased graft function during the first-year post-transplant. These findings have spurred investigation in preclinical models to better understand mechanisms underlying endogenous donor-reactive memory T-cell-mediated allograft injury in unsensitized graft recipients. These studies have led to the identification of unique mechanisms underlying the activation of these memory T cells within allografts at early times after transplant. In particular, optimal activation to mediate acute allograft injury is dependent on the intensity of ischaemia-reperfusion injury. Therapeutic strategies directed at the recruitment and activation of endogenous donor-reactive memory T cells are effective in attenuating acute injury in allografts experiencing increased ischaemia-reperfusion injury in preclinical models and should be translatable to clinical transplantation.

Heterologous boosting of nonrelated toxoid immunity during acute Puumala hantavirus infection

Persistence of immune memory in humans is a crucial yet poorly understood aspect of immunology. Here we have studied the effect of Puumala hantavirus infection on unrelated, pre-existing immune memory by studying T cell- and antibody responses against toxoid vaccine antigens of diphtheria, tetanus and pertussis in a cohort of 45 patients. We found that tetanus- and pertussis -specific IgG concentrations elevate during acute Puumala virus infection. Increase in vaccine IgG was associated with proliferation of heterologous T cells. Interestingly, increases in tetanus-specific IgG persisted a year after the infection while pertussis-specific IgG declined rapidly; a difference in IgG kinetics resembling the difference seen after vaccination against tetanus and pertussis. These results suggest that persistence of immune memory is facilitated by heterologous boosting of old memory during memory formation against newly encountered antigens. They also show that different toxoid antigens may be treated differently. Our study gives new insight into how immune memory formation may alter pre-existing immune memory, and also shows that heterologous immunity may have an impact on vaccination outcomes.

T Cells: Warriors of SARS-CoV-2 Infection

Severe infection with severe acute respiratory syndrome coronavirus (SARS-CoV)-2 is characterized by massive cytokine release and T cell loss. The exaggerated host immune response, incapable of viral clearance, instead aggravates respiratory distress, as well as cardiac, and/or damage to other organs. The mortality pattern of SARS-CoV-2 infection, higher in older versus younger adults and almost absent in children, is possibly caused by the effects of age and pre-existing comorbidities on innate and adaptive immunity. Here, we speculate that the abnormal and excessive immune response to SARS-CoV-2 infection partly depends on T cell immunological memory, which is more pronounced in adults compared with children, and may significantly contribute to immunopathology and massive collateral damage in coronavirus disease 2019 (COVID-19) patients.

T cells in the brain enhance neonatal mortality during peripheral LCMV infection

In adult mice the severity of disease from viral infections is determined by the balance between the efficiency of the immune response and the magnitude of viral load. Here, the impact of this dynamic is examined in neonates. Newborns are highly susceptible to infections due to poor innate responses, lower numbers of T cells and Th2-prone immune responses. Eighty-percent of 7-day old mice, immunologically equivalent to human neonates, succumbed to extremely low doses (5 PFU) of the essentially non-lethal lymphocytic choriomeningitis virus (LCMV-Armstrong) given intraperitoneally. This increased lethality was determined to be dependent upon poor early viral control, as well as, T cells and perforin as assessed in knockout mice. By day 3, these neonatal mice had 400-fold higher viral loads as compared to adults receiving a 10,000-fold (5X104 PFU) higher dose of LCMV. The high viral load in combination with the subsequent immunological defect of partial CD8 T cell clonal exhaustion in the periphery led to viral entry and replication in the brain. Within the brain, CD8 T cells were protected from exhaustion, and thus were able to mediate lethal immunopathology. To further delineate the role of early viral control, neonatal mice were infected with Pichinde virus, a less virulent arenavirus, or LCMV was given to pups of LCMV-immune mothers. In both cases, peak viral load was at least 29-fold lower, leading to functional CD8 T cell responses and 100% survival.

Anti-fungal T cell responses in the lung and modulation by the gut-lung axis

The lung is a central organ for immune-environmental interactions ranging from tolerance against harmless substances to protection against pathogens, which are particularly sensitive to regulation by the intestinal microbiota. Airborne fungi, can cause variety of diseases, including allergies and inflammatory disorders, as well as life-threatening invasive infections. Remarkable differences exist between ubiquitous fungal species with regard to protective immune mechanisms. Recent data have surprisingly identified Aspergillus-specific regulatory T cells as an essential tolerance checkpoint and provided mechanistic insight for the loss of tolerance in the course of immune pathologies. Furthermore, pathogenic Th17 cells in Aspergillus-associated inflammatory disease seem to be induced by cross-reactivity to the intestinal commensal Candida albicans. Here we review and discuss what is known about fungus-specific T cell responses in the lung how they are modulated by the gut-lung axis and in particular discussing the modulation of adaptive immune responses by cross-reactivity to the microbiota.

Infection History Determines Susceptibility to Unrelated Diseases

Epidemiological data suggest that previous infections can alter an individual's susceptibility to unrelated diseases. Nevertheless, the underlying mechanisms are not completely understood. Substantial research efforts have expanded the classical concept of immune memory to also include long-lasting changes in innate immunity and antigen-independent reactivation of adaptive immunity. Collectively, these processes provide possible explanations on how acute infections might induce long-term changes that also affect immunity to unrelated diseases. Here, we review lasting changes the immune compartment undergoes upon infection and how infection experience alters the responsiveness of immune cells towards universal signals. This heightened state of alert enhances the ability of the immune system to combat even unrelated infections but may also increase susceptibility to autoimmunity. At the same time, infection-induced changes in the regulatory compartment may dampen subsequent immune responses and promote pathogen persistence. The concepts presented here outline how infection-induced changes in the immune system may affect human health.

Loss of Resistance to Mousepox during Chronic Lymphocytic Choriomeningitis Virus Infection Is Associated with Impaired T-Cell Responses and Can Be Rescued by Immunization

It is well established that chronic viral infections can cause immune suppression, resulting in increased susceptibility to other infectious diseases. However, the effects of chronic viral infection on T-cell responses and vaccination against highly pathogenic viruses are not well understood. We have recently shown that C57BL/6 (B6) mice lose their natural resistance to wild-type (WT) ectromelia virus (ECTV) when chronically infected with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13). Here we compared the T-cell response to ECTV in previously immunologically naive mice that were chronically infected with CL13 or that were convalescent from acute infection with the Armstrong (Arm) strain of LCMV. Our results show that mice that were chronically infected with CL13 but not those that had recovered from Arm infection have highly defective ECTV-specific CD8⁺ and CD4⁺ T-cell responses to WT ECTV. These defects are at least partly due to the chronic infection environment. In contrast to mice infected with WT ECTV, mice chronically infected with CL13 survived without signs of disease when infected with ECTV-Δ036, a mutant ECTV strain that is highly attenuated. Strikingly, mice chronically infected with CL13 mounted a strong CD8⁺ T-cell response to ECTV-Δ036 and survived without signs of disease after a subsequent challenge with WT ECTV. Our work suggests that enhanced susceptibility to acute viral infections in chronically infected individuals can be partly due to poor T-cell responses but that sufficient T-cell function can be recovered and resistance to acute infection can be restored by immunization with highly attenuated vaccines.IMPORTANCE Chronic viral infections may result in immunosuppression and enhanced susceptibility to infections with other pathogens. For example, we have recently shown that mice chronically infected with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13) are highly susceptible to mousepox, a disease that is caused by ectromelia virus and that is the mouse homolog of human smallpox. Here we show chronic CL13 infection severely disrupts the expansion, proliferation, activation, and cytotoxicity of T cells in response due at least in part to the suppressive effects of the chronic infection milieu. Notably, despite this profound immunodeficiency, mice chronically infected with CL13 could be protected by vaccination with a highly attenuated variant of ECTV. These results demonstrate that protective vaccination of immunosuppressed individuals is possible, provided that proper immunization tools are used.

Association of HLA Antigen Mismatch With Risk of Developing Skin Cancer After Solid-Organ Transplant

Importance

Risk factors for the development of skin cancer after solid-organ transplant can inform clinical care, but data on these risk factors are limited.

Objective

To study the association between HLA antigen mismatch and skin cancer incidence after solid-organ transplant.

Design, Setting, and Participants

This retrospective cohort study is a secondary analysis of the multicenter Transplant Skin Cancer Network study of 10 649 adults who underwent a primary solid-organ transplant between January 1, 2003, and December 31, 2003, or between January 1, 2008, and December 31, 2008. These participants were identified through the Scientific Registry of Transplant Recipients standard analysis files, which contain data collected mostly by the Organ Procurement and Transplantation Network. Participants were matched to skin cancer outcomes by medical record review. This study was conducted from August 1, 2016, to July 31, 2017.

Main Outcomes and Measures

The primary outcome was time to diagnosis of posttransplant skin cancer, including squamous cell carcinoma, melanoma, and Merkel cell carcinoma. The HLA antigen mismatch was calculated based on the 2016 Organ Procurement and Transplantation Network guidelines. Risk of skin cancer was analyzed using a multivariate Cox proportional hazards regression model.

Results

In total, 10 649 organ transplant recipients (6776 men [63.6%], with a mean [SD] age of 51 [12] years) contributed 59 923 years of follow-up. For each additional mismatched allele, a 7% to 8% reduction in skin cancer risk was found (adjusted hazard ratio [HR], 0.93; 95% CI, 0.87-0.99; P = .01). Subgroup analysis found the protective effect of HLA antigen mismatch to be statistically significant in lung (adjusted HR, 0.70; 95% CI, 0.56-0.87; P = .001) and heart (adjusted HR, 0.75; 95% CI, 0.60-0.93; P = .008) transplant recipients but not for recipients of liver, kidney, or pancreas. The degree of HLA-DR mismatch, but not HLA-A or HLA-B mismatch, was the most statistically significant for skin cancer risk (adjusted HR, 0.85; 95% CI, 0.74-0.97; P = .01).

Conclusions and Relevance

The HLA antigen mismatch appears to be associated with reductions in the risk of skin cancer after solid-organ transplant among heart and lung transplant recipients; this finding suggests that HLA antigen mismatch activates the tumor surveillance mechanisms that protect against skin cancer in transplant recipients and that skin cancer risk may be higher in patients who received a well-matched organ.

Viral Infections and Autoimmune Disease: Roles of LCMV in Delineating Mechanisms of Immune Tolerance

Viral infections are a natural part of our existence. They can affect us in many ways that are the result of the interaction between the viral pathogen and our immune system. Most times, the resulting immune response is beneficial for the host. The pathogen is cleared, thus protecting our vital organs with no other consequences. Conversely, the reaction of our immune system against the pathogen can cause organ damage (immunopathology) or lead to autoimmune disease. To date, there are several mechanisms for virus-induced autoimmune disease, including molecular mimicry and bystander activation, in support of the “fertile field” hypothesis (terms defined in our review). In contrast, viral infections have been associated with protection from autoimmunity through mechanisms that include Treg invigoration and immune deviation, in support of the “hygiene hypothesis”, also defined here. Infection with lymphocytic choriomeningitis virus (LCMV) is one of the prototypes showing that the interaction of our immune system with viruses can either accelerate or prevent autoimmunity. Studies using mouse models of LCMV have helped conceive and establish several concepts that we now know and use to explain how viruses can lead to autoimmune activation or induce tolerance. Some of the most important mechanisms established during the course of LCMV infection are described in this short review.

Severity of Acute Infectious Mononucleosis Correlates with Cross-Reactive Influenza CD8 T-Cell Receptor Repertoires

Fifty years after the discovery of Epstein-Barr virus (EBV), it remains unclear how primary infection with this virus leads to massive CD8 T-cell expansion and acute infectious mononucleosis (AIM) in young adults. AIM can vary greatly in severity, from a mild transient influenza-like illness to a prolonged severe syndrome. We questioned whether expansion of a unique HLA-A2.01-restricted, cross-reactive CD8 T-cell response between influenza virus A-M1₅₈ (IAV-M1) and EBV BMLF1₂₈₀ (EBV-BM) could modulate the immune response to EBV and play a role in determining the severity of AIM in 32 college students. Only ex vivo total IAV-M1 and IAV-M1+EBV-BM cross-reactive tetramer⁺ frequencies directly correlated with AIM severity and were predictive of severe disease. Expansion of specific cross-reactive memory IAV-M1 T-cell receptor (TCR) Vβ repertoires correlated with levels of disease severity. There were unique profiles of qualitatively different functional responses in the cross-reactive and EBV-specific CD8 T-cell responses in each of the three groups studied, severe-AIM patients, mild-AIM patients, and seropositive persistently EBV-infected healthy donors, that may result from differences in TCR repertoire use. IAV-M1 tetramer⁺ cells were functionally cross-reactive in short-term cultures, were associated with the highest disease severity in AIM, and displayed enhanced production of gamma interferon, a cytokine that greatly amplifies immune responses, thus frequently contributing to induction of immunopathology. Altogether, these data link heterologous immunity via CD8 T-cell cross-reactivity to CD8 T-cell repertoire selection, function, and resultant disease severity in a common and important human infection. In particular, it highlights for the first time a direct link between the TCR repertoire with pathogenesis and the diversity of outcomes upon pathogen encounter.

The pathogenic impact of immune responses that by chance cross-react to unrelated viruses has not been established in human infections. Here, we demonstrate that the severity of acute infectious mononucleosis (AIM), an Epstein-Barr virus (EBV)-induced disease prevalent in young adults but not children, is associated with increased frequencies of T cells cross-reactive to EBV and the commonly acquired influenza A virus (IAV). The T-cell receptor (TCR) repertoire and functions of these cross-reactive T cells differed between mild- and severe-AIM patients, most likely because these two groups of patients had selected different memory TCR repertoires in response to IAV infections encountered earlier. This heterologous immunity may explain variability in disease outcome and why young adults with more-developed IAV-specific memory T-cell pools have more-severe disease than children, who have less-developed memory pools. This study provides a new framework for understanding the role of heterologous immunity in human health and disease and highlights an important developing field examining the role of T-cell repertoires in the mediation of immunopathology.

Reconstitution of CD8 T Cells Protective against Cytomegalovirus in a Mouse Model of Hematopoietic Cell Transplantation: Dynamics and Inessentiality of Epitope Immunodominance

Successful reconstitution of cytomegalovirus (CMV)-specific CD8⁺ T cells by hematopoietic cell transplantation (HCT) gives a favorable prognosis for the control of CMV reactivation and prevention of CMV disease after hematoablative therapy of hematopoietic malignancies. In the transient immunocompromised state after HCT, pre-emptive cytoimmunotherapy with viral epitope-specific effector or memory CD8⁺ T cells is a promising option to speed up antiviral control. Despite high-coding capacity of CMVs and a broad CD8⁺ T-cell response on the population level, which reflects polymorphism in major histocompatibility complex class-I (MHC-I) glycoproteins, the response in terms of quantity of CD8⁺ T cells in any individual is directed against a limited set of CMV-encoded epitopes selected for presentation by the private repertoire of MHC-I molecules. Such epitopes are known as “immunodominant” epitopes (IDEs). Besides host immunogenetics, genetic variance in CMV strains harbored as latent viruses by an individual HCT recipient can also determine the set of IDEs, which complicates a “personalized immunotherapy.” It is, therefore, an important question if IDE-specific CD8⁺ T-cell reconstitution after HCT is critical or dispensable for antiviral control. As viruses with targeted mutations of IDEs cannot be experimentally tested in HCT patients, we employed the well-established mouse model of HCT. Notably, control of murine CMV (mCMV) after HCT was comparably efficient for IDE-deletion mutant mCMV-Δ4IDE and the corresponding IDE-expressing revertant virus mCMV-Δ4IDE-rev. Thus, antigenicity-loss mutations in IDEs do not result in loss-of-function of a polyclonal CD8⁺ T-cell population. Although IDE deletion was not associated with global changes in the response to non-IDE epitopes, the collective of non-IDE-specific CD8⁺ T-cells infiltrates infected tissue and confines infection within nodular inflammatory foci. We conclude from the model, and predict also for human CMV, that there is no need to exclusively aim for IDE-specific immunoreconstitution.

iVAX: An integrated toolkit for the selection and optimization of antigens and the design of epitope-driven vaccines

Computational vaccine design, also known as computational vaccinology, encompasses epitope mapping, antigen selection and immunogen design using computational tools. The iVAX toolkit is an integrated set of tools that has been in development since 1998 by De Groot and Martin. It comprises a suite of immunoinformatics algorithms for triaging candidate antigens, selecting immunogenic and conserved T cell epitopes, eliminating regulatory T cell epitopes, and optimizing antigens for immunogenicity and protection against disease. iVAX has been applied to vaccine development programs for emerging infectious diseases, cancer antigens and biodefense targets. Several iVAX vaccine design projects have had success in pre-clinical studies in animal models and are progressing toward clinical studies. The toolkit now incorporates a range of immunoinformatics tools for infectious disease and cancer immunotherapy vaccine design. This article will provide a guide to the iVAX approach to computational vaccinology.

In situ expansion of T cells that recognize distinct self-antigens sustains autoimmunity in the CNS

Polyspecific T cells recognizing multiple distinct self-antigens have been identified in multiple sclerosis and other organ-specific autoimmune diseases, but their pathophysiological relevance remains undetermined. Using a mouse model of multiple sclerosis, we show that autoimmune encephalomyelitis induction is strictly dependent on reactivation of pathogenic T cells by a peptide (35-55) derived from myelin oligodendrocyte glycoprotein (MOG). This disease-inducing response wanes after onset. Strikingly, the progression of disease is driven by the in situ activation and expansion of a minority of MOG35-55-specific T cells that also recognize neurofilament-medium (NF-M)15-35, an intermediate filament protein expressed in neurons. This mobilization of bispecific T cells is critical for disease progression as adoptive transfer of NF-M15-35/MOG35-55 bispecific T cell lines caused full-blown disease in wild-type but not NF-M-deficient recipients. Moreover, specific tolerance through injection of NF-M15-35 peptide at the peak of disease halted experimental autoimmune encephalomyelitis progression. Our findings highlight the importance of polyspecific autoreactive T cells in the aggravation and perpetuation of central nervous system autoimmunity.

The Unspecific Side of Acquired Immunity Against Infectious Disease: Causes and Consequences

Acquired immunity against infectious disease (AIID) has long been considered as strictly dependent on the B and T lymphocytes of the adaptive immune system. Consequently, AIID has been viewed as highly specific to the antigens expressed by pathogens. However, a growing body of data motivates revision of this central paradigm of immunology. Unrelated past infection, vaccination, and chronic infection have been found to induce cross-protection against numerous pathogens. These observations can be partially explained by the poly-specificity of antigenic T and B receptors, the Mackaness effect and trained immunity. In addition, numerous studies highlight the importance of microbiota composition on resistance to infectious disease via direct competition or modulation of the immune response. All of these data support the idea that a non-negligible part of AIID in nature can be nonspecific to the pathogens encountered and even of the antigens expressed by pathogens. As this protection may be dependent on the private T and B repertoires produced by the random rearrangement of genes, past immune history, chronic infection, and microbiota composition, it is largely unpredictable at the individual level. However, we can reasonably expect that a better understanding of the underlying mechanisms will allow us to statistically predict cross-protection at the population level. From an evolutionary perspective, selection of immune mechanisms allowing for partially nonspecific AIID would appear to be advantageous against highly polymorphic and rapidly evolving pathogens. This new emerging paradigm may have several important consequences on our understanding of individual infectious disease susceptibility and our conception of tolerance, vaccination and therapeutic strategies against infection and cancer. It also underscores the importance of viewing the microbiota and persisting infectious agents as integral parts of the immune system.

Heterologous Immunity between Adenoviruses and Hepatitis C Virus: A New Paradigm in HCV Immunity and Vaccines

Adenoviruses (Ad) are commonly used as vectors for gene therapy and/or vaccine delivery. Recombinant Ad vectors are being tested as vaccines for many pathogens. We have made a surprising observation that peptides derived from various hepatitis C virus (HCV) antigens contain extensive regions of homology with multiple adenovirus proteins, and conclusively demonstrate that adenovirus vector can induce robust, heterologous cellular and humoral immune responses against multiple HCV antigens. Intriguingly, the induction of this cross-reactive immunity leads to significant reduction of viral loads in a recombinant vaccinia-HCV virus infected mouse model, supporting their role in antiviral immunity against HCV. Healthy human subjects with Ad-specific pre-existing immunity demonstrated cross-reactive cellular and humoral immune responses against multiple HCV antigens. These findings reveal the potential of a previously uncharacterized property of natural human adenovirus infection to dictate, modulate and/or alter the course of HCV infection upon exposure. This intrinsic property of adenovirus vectors to cross-prime HCV immunity can also be exploited to develop a prophylactic and/or therapeutic vaccine against HCV.

Non-cognate bystander cytolysis by clonal epitope-specific CTL lines through CD28-CD80 interaction inhibits antibody production: A potential caveat to CD8 T-cell immunotherapy

Adoptive transfer of virus epitope-specific CD8 T cells is an immunotherapy option to control cytomegalovirus (CMV) infection and prevent CMV organ disease in immunocompromised solid organ transplantation (SOT) and hematopoietic cell transplantation (HCT) recipients. The therapy aims at an early, selective recognition and cytolysis of infected cells for preventing viral spread in tissues with no adverse immunopathogenic side-effects by attack of uninfected bystander cells. Here we describe that virus epitope-specific, cloned T-cell lines lyse target cells that present the cognate antigenic peptide to the TCR, but simultaneously have the potential to lyse uninfected cells expressing the CD28 ligand CD80 (B7-1). While TCR-mediated cytolysis requires co-receptor CD8 and depends on perforin, the TCR-independent and viral epitope-independent cytolysis through CD28-CD80 signaling does not require CD8 on the effector cells and is perforin-independent. Importantly, this non-cognate cytolysis pathway leads to bystander cytolysis of CD80-expressing B-cell blasts and thereby inhibits pan-specific antibody production.

Evolution of the immune system in humans from infancy to old age

This article reviews the development of the immune response through neonatal, infant and adult life, including pregnancy, ending with the decline in old age. A picture emerges of a child born with an immature, innate and adaptive immune system, which matures and acquires memory as he or she grows. It then goes into decline in old age. These changes are considered alongside the risks of different types of infection, autoimmune disease and malignancy.

Increased Immune Response Variability during Simultaneous Viral Coinfection Leads to Unpredictability in CD8 T Cell Immunity and Pathogenesis

T cell memory is usually studied in the context of infection with a single pathogen in naive mice, but how memory develops during a coinfection with two pathogens, as frequently occurs in nature or after vaccination, is far less studied. Here, we questioned how the competition between immune responses to two viruses in the same naive host would influence the development of CD8 T cell memory and subsequent disease outcome upon challenge. Using two different models of coinfection, including the well-studied lymphocytic choriomeningitis (LCMV) and Pichinde (PICV) viruses, several differences were observed within the CD8 T cell responses to either virus. Compared to single-virus infection, coinfection resulted in substantial variation among mice in the size of epitope-specific T cell responses to each virus. Some mice had an overall reduced number of virus-specific cells to either one of the viruses, and other mice developed an immunodominant response to a normally subdominant, cross-reactive epitope (nucleoprotein residues 205 to 212, or NP205). These changes led to decreased protective immunity and enhanced pathology in some mice upon challenge with either of the original coinfecting viruses. In mice with PICV-dominant responses, during a high-dose challenge with LCMV clone 13, increased immunopathology was associated with a reduced number of LCMV-specific effector memory CD8 T cells. In mice with dominant cross-reactive memory responses, during challenge with PICV increased immunopathology was directly associated with these cross-reactive NP205-specific CD8 memory cells. In conclusion, the inherent competition between two simultaneous immune responses results in significant alterations in T cell immunity and subsequent disease outcome upon reexposure.

IMPORTANCE

Combination vaccines and simultaneous administration of vaccines are necessary to accommodate required immunizations and maintain vaccination rates. Antibody responses generally correlate with protection and vaccine efficacy. However, live attenuated vaccines also induce strong CD8 T cell responses, and the impact of these cells on subsequent immunity, whether beneficial or detrimental, has seldom been studied, in part due to the lack of known T cell epitopes to vaccine viruses. We questioned if the inherent increased competition and stochasticity between two immune responses during a simultaneous coinfection would significantly alter CD8 T cell memory in a mouse model where CD8 T cell epitopes are clearly defined. We show that some of the coinfected mice have sufficiently altered memory T cell responses that they have decreased protection and enhanced immunopathology when reexposed to one of the two viruses. These data suggest that a better understanding of human T cell responses to vaccines is needed to optimize immunization strategies.

Cross-Reactivity Between Influenza Matrix- and HIV-1 P17-Specific CTL-A Large Cohort Study

BACKGROUND

It has been reported that HIV-1-specific cytotoxic T cells (CTL) recognizing the HLA-A2-restricted p17 epitope SLYNTVATL (SL9) can cross-react with the HLA-A2-restricted influenza matrix epitope GILGFVFTL (GL9). So far, the prevalence of GL9-cross-reacting HIV-1-specific CTL in larger cohorts of HIV-1-infected patients is unknown, and there are no data yet on whether SL9/GL9-cross-reactive CTL may influence the course of HIV-1 infection.

METHODS

We analyzed the presence of SL9/GL9-cross-reacting CTL in a cohort of 175 HLA-A2-positive HIV-1-infected patients. Peripheral blood mononuclear cells were stimulated in vitro with SL9 and GL9 peptides, and outgrowing cell lines regarding cross-reactivity and recognition of viral variants in γ-interferon enzyme-linked immunospot assays were analyzed.

RESULTS

SL9- and GL9-specific CTL could be generated in 52.6% and 53.7% of 175 patients, respectively. Both SL9- and GL9-specific CTL were more frequently observed in patients on antiretroviral therapy (ART). Of the 92 SL9-specific CTL and the 94 GL9-specific CTL, 65.2% and 66%, respectively, showed at least partial SL9/GL9 cross-reactivity. SL9/GL9-cross-reactive CTL could be detected in 42.9% of the 175 patients. Recognition of SL9 was associated with lower viral loads and higher CD4 cell counts in patients on ART. Patients with GL9/SL9 cross-reactivity displayed similar CD4 cell counts than patients without GL9/SL9-cross-reactive cells. GL9/SL9-cross-reactive cells were associated with higher viral loads in patients on ART.

CONCLUSIONS

Partially SL9/GL9-cross-reactive CTL are frequently observed in HIV-1-infected patients. So far, we could not detect a significant influence of the presence of SL9/GL9-cross-reacting CTL on the course of HIV-1 infection.

Vaccination and heterologous immunity: educating the immune system

This review discusses three inter-related topics: (1) the immaturity of the neonatal and infant immune response; (2) heterologous immunity, where prior infection history with unrelated pathogens alters disease outcome resulting in either enhanced protective immunity or increased immunopathology to new infections, and (3) epidemiological human vaccine studies that demonstrate vaccines can have beneficial or detrimental effects on subsequent unrelated infections. The results from the epidemiological and heterologous immunity studies suggest that the immune system has tremendous plasticity and that each new infection or vaccine that an individual is exposed to during a lifetime will potentially alter the dynamics of their immune system. It also suggests that each new infection or vaccine that an infant receives is not only perturbing the immune system but is educating the immune system and laying down the foundation for all subsequent responses. This leads to the question, is there an optimum way to educate the immune system? Should this be taken into consideration in our vaccination protocols?

Improved structural method for T-cell cross-reactivity prediction

Cytotoxic T-lymphocytes (CTLs) are the key players of adaptive cellular immunity, being able to identify and eliminate infected cells through the interaction with peptide-loaded major histocompatibility complexes class I (pMHC-I). Despite the high specificity of this interaction, a given lymphocyte is actually able to recognize more than just one pMHC-I complex, a phenomenon referred as cross-reactivity. In the present work we describe the use of pMHC-I structural features as input for multivariate statistical methods, to perform standardized structure-based predictions of cross-reactivity among viral epitopes. Our improved approach was able to successfully identify cross-reactive targets among 28 naturally occurring hepatitis C virus (HCV) variants and among eight epitopes from the four dengue virus serotypes. In both cases, our results were supported by multiscale bootstrap resampling and by data from previously published in vitro experiments. The combined use of data from charges and accessible surface area (ASA) of selected residues over the pMHC-I surface provided a powerful way of assessing the structural features involved in triggering cross-reactive responses. Moreover, the use of an R package (pvclust) for assessing the uncertainty in the hierarchical cluster analysis provided a statistical support for the interpretation of results. Taken together, these methods can be applied to vaccine design, both for the selection of candidates capable of inducing immunity against different targets, or to identify epitopes that could trigger undesired immunological responses.

Generation of cellular immune memory and B-cell immunity is impaired by natural killer cells

The goal of most vaccines is the induction of long-lived memory T and B cells capable of protecting the host from infection by cytotoxic mechanisms, cytokines and high-affinity antibodies. However, efforts to develop vaccines against major human pathogens like HIV and HCV have not been successful, thereby highlighting the need for novel approaches to circumvent immunoregulatory mechanisms that limit induction of protective immunity. Here we show that mouse natural killer (NK) cells inhibit generation of long-lived virus-specific memory T- and B-cells as well as virus-specific antibody production after acute infection. Mechanistically, NK cells suppressed CD4 T cells and follicular helper T cells (T_FH) in a perforin-dependent manner during the first few days of infection, resulting in a weaker germinal center (GC) response and diminished immune memory. We anticipate that innovative strategies to relieve NK cell-mediated suppression of immunity should facilitate development of efficacious new vaccines targeting difficult-to-prevent infections.

Antiviral memory phenotype T cells in unexposed adults

αβ T cells are an integral part of protective immunity against pathogens. After precursor cells arise in the adult bone marrow or fetal liver, they migrate to the thymus where they rearrange their T-cell receptor genes (TCR) and undergo selection on the basis of their interactions with ligands expressed by thymic stroma and other cells. Those that survive then exit the thymus to populate the peripheral immune compartment, where they patrol the blood and lymphoid systems. The composition of this pre-immune peripheral repertoire is critically important in determining the robustness of an immune response. In both mice and humans, the magnitude and diversity of a response are directly correlated with the frequency of precursor T cells. Equally relevant are the functional characteristics of these lymphocytes. Engagement of a specific antigen to the TCR activates signaling pathways in the naive T cell that result in cellular proliferation and the acquisition of particular effector functions. A portion of these persist following the resolution of infection and become memory cells. These memory cells can mount a faster and stronger response when they encounter the same antigen at a later time. As the molecular basis for TCR ligand interaction has become better defined, it is clear that some T cells can recognize multiple distinct ligands and therefore T-cell memory developed by exposure to one ligand may play a significant role in the response to a different antigen. Thus, there is an increasing focus on understanding how exposure to related or unrelated antigens influences the T-cell repertoire and impacts subsequent immunity. In this review, we discuss the issue of TCR cross-reactivity in the development of memory phenotype CD4(+) T cells and the implications for pathogen-specific responses. We review both the human and mouse data and discuss the therapeutic implications of these findings in the contexts of infection and vaccination.

The promised land of human immunology

Advances in technology and data analysis have made it possible to take a new look at human immunology. These advances run the gamut from systems biology approaches, which are likely in the vanguard of how we can start "to put the pieces together" of immune function, to a deeper understanding of specific diseases and vaccines and the immune repertoire. In our own experience, we have also found that asking simple questions about human immunity has often given us very surprising answers, causing a rethink of established dogma. Thus, we have developed a new perspective on the nature of the αβ TCR repertoire and also the likely role of T-cell repertoire (TCR) cross-reactivity in generating T memory independent of specific antigen interactions. These findings show that human immunology is not just a necessary step for "translating" basic immunology to treat diseases or develop better vaccines, but is also an important complement to the inbred mouse model.

Control of influenza virus infection by immunity to conserved viral features

Influenza has circulated among humans for centuries and kills more people than many newly emerging diseases. The present methods for control of influenza are not adequate, especially for dealing with a pandemic. In the face of a rapidly spreading outbreak, a race to isolate the virus and prepare a vaccine would probably not succeed in time to avoid great losses. Thus, additional anti-infection strategies are needed. Broad cross-protection against widely divergent influenza A subtypes is readily achieved in animals by several means of immunization. How does cross-protection work in animals, and can we apply what we have learned about it to induce broad cross-protection in humans?

Estimating the diversity, completeness, and cross-reactivity of the T cell repertoire

In order to recognize and combat a diverse array of pathogens the immune system has a large repertoire of T cells having unique T cell receptors (TCRs) with only a few clones specific for any given antigen. We discuss how the number of different possible TCRs encoded in the genome (the potential repertoire) and the number of different TCRs present in an individual (the realized repertoire) can be measured. One puzzle is that the potential repertoire greatly exceeds the realized diversity of naïve T cells within any individual. We show that the existing hypotheses fail to explain why the immune system has the potential to generate far more diversity than is used in an individual, and propose an alternative hypothesis of “evolutionary sloppiness.” Another immunological puzzle is why mice and humans have similar repertoires even though humans have over 1000-fold more T cells. We discuss how the idea of the “protecton,” the smallest unit of protection, might explain this discrepancy and estimate the size of “protecton” based on available precursor frequencies data. We then consider T cell cross-reactivity – the ability of a T cell clone to respond to more than one epitope. We extend existing calculations to estimate the extent of expected cross-reactivity between the responses to different pathogens. Our results are consistent with two observations: a low probability of observing cross-reactivity between the immune responses to two randomly chosen pathogens; and the ensemble of memory cells being sufficiently diverse to generate cross-reactive responses to new pathogens.

PC61 (anti-CD25) treatment inhibits influenza A virus-expanded regulatory T cells and severe lung pathology during a subsequent heterologous lymphocytic choriomeningitis virus infection

Prior immunity to influenza A virus (IAV) in mice changes the outcome to a subsequent lymphocytic choriomeningitis virus (LCMV) infection and can result in severe lung pathology, similar to that observed in patients that died of the 1918 H1N1 pandemic. This pathology is induced by IAV-specific memory CD8(+) T cells cross-reactive with LCMV. Here, we discovered that IAV-immune mice have enhanced CD4(+) Foxp3(+) T-regulatory (Treg) cells in their lungs, leading us to question whether a modulation in the normal balance of Treg and effector T-cell responses also contributes to enhancing lung pathology upon LCMV infection of IAV-immune mice. Treg cell and interleukin-10 (IL-10) levels remained elevated in the lungs and mediastinal lymph nodes (mLNs) throughout the acute LCMV response of IAV-immune mice. PC61 treatment, used to decrease Treg cell levels, did not change LCMV titers but resulted in a surprising decrease in lung pathology upon LCMV infection in IAV-immune but not in naive mice. Associated with this decrease in pathology was a retention of Treg in the mLN and an unexpected partial clonal exhaustion of LCMV-specific CD8(+) T-cell responses only in IAV-immune mice. PC61 treatment did not affect cross-reactive memory CD8(+) T-cell proliferation. These results suggest that in the absence of IAV-expanded Treg cells and in the presence of cross-reactive memory, the LCMV-specific response was overstimulated and became partially exhausted, resulting in a decreased effector response. These studies suggest that Treg cells generated during past infections can influence the characteristics of effector T-cell responses and immunopathology during subsequent heterologous infections. Thus, in humans with complex infection histories, PC61 treatment may lead to unexpected results.

Disparate epitopes mediating protective heterologous immunity to unrelated viruses share peptide-MHC structural features recognized by cross-reactive T cells

Closely related peptide epitopes can be recognized by the same T cells and contribute to the immune response against pathogens encoding those epitopes, but sometimes cross-reactive epitopes share little homology. The degree of structural homology required for such disparate ligands to be recognized by cross-reactive TCRs remains unclear. In this study, we examined the mechanistic basis for cross-reactive T cell responses between epitopes from unrelated and pathogenic viruses, lymphocytic choriomeningitis virus (LCMV) and vaccinia virus. Our results show that the LCMV cross-reactive T cell response toward vaccinia virus is dominated by a shared asparagine residue, together with other shared structural elements conserved in the crystal structures of K(b)-VV-A11R and K(b)-LCMV-gp34. Based on analysis of the crystal structures and the specificity determinants for the cross-reactive T cell response, we were able to manipulate the degree of cross-reactivity of the T cell response, and to predict and generate a LCMV cross-reactive response toward a variant of a null OVA-derived peptide. These results indicate that protective heterologous immune responses can occur for disparate epitopes from unrelated viruses.

Opportunistic infections--coming to the limits of immunosuppression?

Possible etiologies of infection in the solid organ recipient are diverse, ranging from common bacterial and viral pathogens to opportunistic pathogens that cause invasive disease only in immunocompromised hosts. The recognition of infectious syndromes in this population is limited by alterations in the clinical manifestations by immunosuppression. The risk of serious infections in the organ transplant patient is determined by the interaction between the patients' recent and distant epidemiological exposures and all factors that contribute to the patient's net state of immune suppression. This risk is altered by antimicrobial prophylaxis and changes in immunosuppressive therapies. In addition to the direct effects of infection, opportunistic infections, and the microbiome may adversely shape the host immune responses with diminished graft and patient survivals. Antimicrobial therapies are more complex than in the normal host with a significant incidence of drug toxicity and a propensity for drug interactions with the immunosuppressive agents used to maintain graft function. Rapid and specific microbiologic diagnosis is essential. Newer microbiologic assays have improved the diagnosis and management of opportunistic infections. These tools coupled with assays that assess immune responses to infection and to graft antigens may allow optimization of management for graft recipients in the future.

A small jab - a big effect: nonspecific immunomodulation by vaccines

Recent epidemiological studies have shown that, in addition to disease-specific effects, vaccines against infectious diseases have nonspecific effects on the ability of the immune system to handle other pathogens. For instance, in randomized trials tuberculosis and measles vaccines are associated with a substantial reduction in overall child mortality, which cannot be explained by prevention of the target disease. New research suggests that the nonspecific effects of vaccines are related to cross-reactivity of the adaptive immune system with unrelated pathogens, and to training of the innate immune system through epigenetic reprogramming. Hence, epidemiological findings are backed by immunological data. This generates a new understanding of the immune system and about how it can be modulated by vaccines to impact the general resistance to disease.

Assessment of CD4(+) and CD8 (+) T cell responses using MHC class I and II tetramers

The low frequency of T cells specific for given antigens makes the study of antigen-specific T cell responses difficult. The development of MHC class I and II tetramer staining techniques allows precise quantification and tracking of antigen-specific CD8(+) and CD4(+) T cell responses. Here, we describe a protocol for MHC class I and II tetramer staining of mouse T cells isolated from various tissues of mice infected with lymphocytic choriomeningitis virus (LCMV) or with murine cytomegalovirus (MCMV).

Role of lymphocytic choriomeningitis virus (LCMV) in understanding viral immunology: past, present and future

Lymphocytic choriomeningitis virus (LCMV) is a common infection of rodents first identified over eighty years ago in St. Louis, MO, U.S.A. It is best known for its application in immunological studies. The history of LCMV closely correlates with the development of modern immunology. With the use of LCMV as a model pathogen several key concepts have emerged: Major Histocompatibility Complex (MHC) restriction, T cell memory, persistent infections, T cell exhaustion and the key role of immune pathology in disease. Given the phenomenal infrastructure within this field (e.g., defined immunodominant and subdominant epitopes to all T cell receptor specificities as well as the cognate tetramers for enumeration in vivo) the study of LCMV remains an active and productive platform for biological research across the globe to this day. Here we present a historical primer that highlights several breakthroughs since the discovery of LCMV. Next, we highlight current research in the field and conclude with our predictions for future directions in the remarkable field of LCMV research.

Reovirus type-2 infection in newborn DBA/1J mice reduces the development of late allergic asthma

The aim of the present study was to determine whether or not the development of a helper T (Th) 1 response induced by Reovirus type-2 (Reo-2) infection would protect against the development of Th2-mediated late allergic asthma. This hypothesis was examined by infecting one day old neonatal DB A/1J mice with Reo-2 in an ovalbumin (OVA)-induced late asthma model. Compared with the controls (either infected or uninfected mice with or without OVA sensitization and/or OVA challenge), Reo-2 infection lessened the magnitude of the subsequent allergic Th2-mediated late asthma. In infected mice with allergic late asthma, there was decreased infiltration of interleukin (IL)-4(+), IL-5(+), IL-13(+) and very late antigen (VLA)-4(+) lymphocytes, and eotaxin-2(+) and VLA-4(+) eosinophils, in both bronchial and bronchiolar lesions. Also the expression of vascular cell adhesion molecule (VCAM)-1 and eotaxin-2 on vascular endothelial cells was reduced. Moreover, the systemic production of IL-4, IL-5, tumour necrosis factor-α and OVA-specific IgE was reduced, whereas systemic IFN-γ production was increased. In addition, there was no increase in IFN-α production. Thus the present study suggests that systemic Reo-2 infection at birth may reduce the development of subsequent late allergic asthma by the induction of a Th1 response. Therefore the potential suppressive mechanism(s) that might be induced by Reo-2 infection in newborn mice and their effects on the development of late allergic asthma are discussed.

Memory CD4+ T cells: fate determination, positive feedback and plasticity

Naïve CD4(+) T cells undergo massive cell proliferation upon encountering their cognate ligand. This proliferation depends upon appropriate cues from the antigen-presenting cells that have processed the antigen and present the peptide to the T cells, and requires the establishment of a cytokine environment that can support such proliferation. Expansion of antigen-specific CD4(+) T cells needs to be coupled with differentiation into one of several effector/regulatory phenotypes if the priming event is to result in cells that can initially act to control the particular pathogen that elicited the response, and later to serve as memory cells to insure an appropriate response upon reintroduction of the pathogen. Here, we discuss the initiation of T helper lineage commitment, the positive feedback regulation by the cytokine environment to enhance and stabilize the differentiation into distinct T helper subsets, and the biological significance of CD4(+) T cell plasticity and long-term CD4(+) T cell memory.

Overcoming memory T-cell responses for induction of delayed tolerance in nonhuman primates

The presence of alloreactive memory T cells is a major barrier for induction of tolerance in primates. In theory, delaying conditioning for tolerance induction until after organ transplantation could further decrease the efficacy of the regimen, since preexisting alloreactive memory T cells might be stimulated by the transplanted organ. Here, we show that such "delayed tolerance" can be induced in nonhuman primates through the mixed chimerism approach, if specific modifications to overcome/avoid donor-specific memory T-cell responses are provided. These modifications include adequate depletion of CD8+ memory T cells and timing of donor bone marrow administration to minimize levels of proinflammatory cytokines. Using this modified approach, mixed chimerism was induced successfully in 11 of 13 recipients of previously placed renal allografts and long-term survival without immunosuppression could be achieved in at least 6 of these 11 animals.

Preclinical and clinical studies on the induction of renal allograft tolerance through transient mixed chimerism

PURPOSE OF REVIEW

The present review updates the current status of research for induction of tolerance through a mixed chimerism approach in nonhuman primates and humans.

RECENT FINDINGS

Allograft tolerance has been successfully achieved with a nonmyeloablative conditioning regimen and donor bone marrow transplantation in human leukocyte antigen (HLA)-matched and mismatched kidney transplantation. In HLA-matched kidney transplantation, persistent mixed chimerism and renal allograft tolerance has been achieved in some patients. In HLA-mismatched combinations, induction of persistent mixed chimerism has not been achieved using a nonmyeloablative preparative regimen. Nevertheless, the transient mixed chimerism that has been achieved has resulted in long-term renal allograft tolerance in the majority of patients. Recent preclinical studies have demonstrated that the presence of heterologous memory T-cell responses observed in primates, but not in rodents, may be a major barrier for induction of durable chimerism and tolerance in primates. Strategies to overcome such memory T-cell responses may, therefore, be of great value in the development of reliable protocols for clinical tolerance induction.

SUMMARY

Induction of tolerance in clinical kidney transplantation has been achieved via mixed chimerism approaches. Improvements in the consistency and safety of tolerance induction and extension of successful protocols to other organs and to organs from deceased donors will all be among the next steps in bringing tolerance to a wider range of clinical applications.

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.

Characterization of cross-reactive CD8+ T-cell recognition of HLA-A2-restricted HIV-Gag (SLYNTVATL) and HCV-NS5b (ALYDVVSKL) epitopes in individuals infected with human immunodeficiency and hepatitis C viruses

The immunologic mechanisms underlying the faster progression of hepatitis C virus (HCV) disease in the presence of human immunodeficiency virus (HIV) coinfection are not clearly understood. T-cell cross-reactivity between HCV and influenza virus-specific epitopes has been associated with rapid progression of HCV disease (S. Urbani, B. Amadei, P. Fisicaro, M. Pilli, G. Missale, A. Bertoletti, and C. Ferrari, J. Exp. Med. 201:675-680, 2005). We asked whether T-cell cross-reactivity between HCV and HIV could exist during HCV/HIV coinfection and affect pathogenesis. Our search for amino acid sequence homology between the HCV and HIV proteomes revealed two similar HLA-A2-restricted epitopes, HIV-Gag (SLYNTVATL [HIV-SL9]) and HCV-NS5b (ALYDVVSKL [HCV-AL9]). We found that 4 out of 20 HLA-A2-positive (HLA-A2(+)) HIV-infected individuals had CD8(+) T cells that recognized both the HIV-SL9 and HCV-AL9 epitopes. However, the AL9 epitope was generally shown to be a weak agonist. Although HCV-monoinfected individuals in our study did not show AL9-specific responses, we found that about half of HCV/HIV-coinfected individuals had dual responses to both epitopes. High dual T-cell recognition among coinfected subjects was usually due to separate T-cell populations targeting each epitope, as determined by pentamer staining. The one individual demonstrating cross-reactive T cells to both epitopes showed the most advanced degree of liver disease. In coinfected individuals, we observed a positive correlation between the magnitudes of T-cell responses to both the SL9 and the AL9 epitopes, which was also positively associated with the clinical parameter of liver damage. Thus, we find that HIV infection induces T cells that can cross-react to heterologous viruses or prime for T cells that are closely related in sequence. However, the induction of cross-reactive T cells may not be associated with control of disease caused by the heterologous virus. This demonstrates that degeneracy of HIV-specific T cells may play a role in the immunopathology of HCV/HIV coinfection.

Bi-specific MHC heterodimers for characterization of cross-reactive T cells

T cell cross-reactivity describes the phenomenon whereby a single T cell can recognize two or more different peptide antigens presented in complex with MHC proteins. Cross-reactive T cells have previously been characterized at the population level by cytokine secretion and MHC tetramer staining assays, but single-cell analysis is difficult or impossible using these methods. In this study, we describe development of a novel peptide-MHC heterodimer specific for cross-reactive T cells. MHC-peptide monomers were independently conjugated to hydrazide or aldehyde-containing cross-linkers using thiol-maleimide coupling at cysteine residues introduced into recombinant MHC heavy chain proteins. Hydrazone formation provided bi-specific MHC heterodimers carrying two different peptides. Using this approach we prepared heterodimers of the murine class I MHC protein H-2K(b) carrying peptides from lymphocytic choriomeningitis virus and vaccinia virus, and used these to identify cross-reactive CD8+ T cells recognizing both lymphocytic choriomeningitis virus and vaccinia virus antigens. A similar strategy could be used to develop reagents to analyze cross-reactive T cell responses in humans.