Viral infection abrogates CD8(+) T-cell deletion induced by costimulation blockade

Heterologous immune responses in health and disease

Immunological memory and tolerance represent major achievements and advantages of adaptive immunity. Organisms bearing adaptive immunity display prominent competitive advantages in the fight against infections. Memory immune cells are preserved for decades and are able to repel a second attack of an infectious agent. However, studies performed in the XXI century have shown that even unrelated pathogens may be quickly and effectively destroyed by memory cells. This type of response is called heterologous so that heterologous immune response is mainly typical to viral infections and other intracellular infections, where T-cells play a lead role in protection. This review will discuss various mechanisms involved in implementing T-cell cross-reactivity, describe molecular prerequisites for heterologous T-cell responses. Experimental evidence of memory T-cell potential to heterologous immune response in mouse models and in human infections are also discussed. Heterologous immune response is an important immune arm in adults and the elderly when the yield of naive cells to the periphery declines due to thymus involution. Along with obvious advantages, heterologous immune response leads to imbalanced memory T-cell repertoire, replacement of immunodominant epitopes with minor ones allowing viruses to evade immune response that results in virus persistence, or, conversely, fulminant infection course. Another threat of heterologous immune response due to switch in dominant repertoire of recognizable epitopes is presented by random self-epitope recognition, which can lead to development of autoimmune pathology. Heterologous immunity can also disrupt drug-induced tolerance in organ and tissue transplants and lead to graft rejection. Heterologous immune response should be taken into consideration while developing and using new vaccines, especially in adults and the elderly.

CD8+ effector memory T cells induce acute rejection of allogeneic heart retransplants in mice possibly through activating expression of inflammatory cytokines

BACKGROUND

To investigate the effects of CD8⁺ memory T (Tm) cells and CD8⁺ effector memory T (Tem) cells on the results of allogeneic heart retransplantations performed in mice.

METHODS

A skin transplantation model was used to generate sensitized splenic CD8⁺ Tem cells for infusion into BALB/c mice. One week after infusion, the BALB/c mice underwent allogeneic heart transplantation in the abdominal cavity. Cyclosporin A was administered via intraperitoneal injection starting one day prior to transplantation to arrest immunological rejection of the transplanted heart. The effects of sensitized CD8⁺ T_em cells on allogeneic heart graft rejection were examined by monitoring survival of the transplanted hearts, the infiltration of effector memory CD8⁺ T cells into myocardium, and expressions of inflammatory cytokines in blood serum.

RESULTS

Adoptive transfer of sensitized CD8⁺ Tem cells prior to transplantation induced an acute rejection response which decreased the survival of transplanted hearts. The rejection response was accompanied by an infiltration of CD8⁺ Tem cells into the transplanted myocardial tissue. Additionally, infusion of sensitized CD8⁺ Tem cells induced markedly increased expressions of IL-2 and IFN-γ, and decreased expression of TGF-β in the transplanted hearts, as well as higher levels of IFN-γ and CXCL-9 in blood serum.

CONCLUSIONS

The infusion of sensitized CD8⁺ Tem cells induced an acute graft rejection response and decreased the survival of grafted hearts by regulating the expressions of inflammatory cytokines including CXCL-9, IL-2, and INF-γ. Cyclosporin A had no therapeutic effect on the graft rejection response induced by sensitized CD8⁺ Tem cells.

Alloreactive CD8 T cells rescued from apoptosis during co-stimulation blockade by Toll-like receptor stimulation remain susceptible to Fas-induced cell death

Blockade of co-stimulatory signals to T cells is extremely effective for the induction of transplantation tolerance in immunologically naive rodents. However, infections and inflammation compromise the efficacy of co-stimulation blockade regimens for the induction of tolerance, thereby stimulating the rejection of allografts. Previous studies have shown that stimulation of innate immunity abrogates tolerance induction by preventing the deletion of alloreactive CD8(+) T cells that normally occurs during co-stimulation blockade. Although inflammation prevents the deletion of alloreactive T cells during co-stimulation blockade, it is not known if this resistance to cell death is the result of a mechanism intrinsic to the T cell. Here, we used syngeneic bone marrow chimeric mice that contain a trace population of T-cell receptor transgenic alloreactive CD8(+) T cells to investigate the early apoptotic signature and activation status of alloreactive T cells following exposure to inflammatory signals during co-stimulation blockade with an antibody specific for CD154. Our findings revealed that the presence of bacterial lipopolysaccharide during co-stimulation blockade enhanced the early activation of alloreactive CD8(+) T cells, as indicated by the up-regulation of CD25 and CD69, suppressed Fas ligand expression, and prevented apoptotic cell death. However, alloreactive CD8(+) T cells from lipopolysaccharide-treated mice remained sensitive to Fas-mediated apoptosis in vitro. These findings suggest that alloreactive T cells rescued from deletion during co-stimulation blockade by inflammation are still sensitive to pro-apoptotic signals and that stimulating these apoptotic pathways during co-stimulation blockade may augment the induction of tolerance.

CD28 family and chronic rejection: "to belatacept...And beyond!"

Kidneys are one of the most frequently transplanted human organs. Immunosuppressive agents may prevent or reverse most acute rejection episodes; however, the graft may still succumb to chronic rejection. The immunological response involved in the chronic rejection process depends on both innate and adaptive immune response. T lymphocytes have a pivotal role in chronic rejection in adaptive immune response. Meanwhile, we aim to present a general overview on the state-of-the-art knowledge of the strategies used for manipulating the lymphocyte activation mechanisms involved in allografts, with emphasis on T-lymphocyte costimulatory and coinhibitory molecules of the B7-CD28 superfamily. A deeper understanding of the structure and function of these molecules improves both the knowledge of the immune system itself and their potential action as rejection inducers or tolerance promoters. In this context, the central role played by CD28 family, especially the relationship between CD28 and CTLA-4, becomes an interesting target for the development of immune-based therapies aiming to increase the survival rate of allografts and to decrease autoimmune phenomena. Good results obtained by the recent development of abatacept and belatacept with potential clinical use aroused better expectations concerning the outcome of transplanted patients.

Innate pathways of immune activation in transplantation

Studies of the immune mechanisms of allograft rejection have predominantly focused on the adaptive immune system that includes T cells and B cells. Recent investigations into the innate immune system, which recognizes foreign antigens through more evolutionarily primitive pathways, have demonstrated a critical role of the innate immune system in the regulation of the adaptive immune system. Innate immunity has been extensively studied in its role as the host's first-line defense against microbial pathogens; however, it is becoming increasingly recognized for its ability to also recognize host-derived molecules that result from tissue damage. The capacity of endogenous damage signals acting through the innate immune system to lower immune thresholds and promote immune recognition and rejection of transplant grafts is only beginning to be appreciated. An improved understanding of these pathways may reveal novel therapeutic targets to decrease graft alloreactivity and increase graft longevity.

Recent progress and new perspectives in studying T cell responses to allografts

Studies in the past decade advanced our understanding of the development, execution and regulation of T-cell-mediated allograft rejection. This review outlines recent progress and focuses on three major areas of investigation that are likely to guide the development of graft-prolonging therapies in the future. The discussed topics include the contribution of recently discovered molecules to the activation and functions of alloreactive T cells, the emerging problem of alloreactive memory T cells and recently gained insights into the old question of transplantation tolerance.

The early course of kidney allograft rejection: defining the time when rejection begins

We studied the early events in mouse kidney allografts and isografts to define when allorecognition begins and when alloimmune tissue injury begins. Allografts but not isografts showed T-cell infiltration in perivascular areas from day 1, but tubulitis and arteritis did not develop until day 7. Flow cytometry confirmed the early allospecific CD3(+)CD8(+) T-cell infiltrate. At day 1, both allografts and isografts showed extensive transcriptome changes, reflecting the response to surgery, but only allografts showed expression of interferon-gamma (IFN-gamma)-inducible transcripts and T-cell-associated transcripts. Although the number of CD68(+) myeloid cell numbers did not increase in day 1 isografts or allografts, mRNA expression for myeloid markers was increased in isografts and allografts, suggesting activation of resident cells of the macrophage-dendritic cell series (MMDCs) in response to injury, followed by increased CD68(+) cell numbers from day 2. By day 3, an interstitial T-cell and MMDC infiltrate was established in allografts, corresponding with the emergence of allospecific tissue injury, as reflected by decreased parenchymal transcripts. Thus, in renal allografts, allorecognition by T cells occurs in perivascular sites by day 1, but alloimmune parenchymal damage begins at day 3, coinciding with the emergence of the interstitial T-cell-MMDC infiltrate.

Viral infection: a potent barrier to transplantation tolerance

Transplantation of allogeneic organs has proven to be an effective therapeutic for a large variety of disease states, but the chronic immunosuppression that is required for organ allograft survival increases the risk for infection and neoplasia and has direct organ toxicity. The establishment of transplantation tolerance, which obviates the need for chronic immunosuppression, is the ultimate goal in the field of transplantation. Many experimental approaches have been developed in animal models that permit long-term allograft survival in the absence of chronic immunosuppression. These approaches function by inducing peripheral or central tolerance to the allograft. Emerging as some of the most promising approaches for the induction of tolerance are protocols based on costimulation blockade. However, as these protocols move into the clinic, there is recognition that little is known as to their safety and efficacy when confronted with environmental perturbants such as virus infection. In animal models, it has been reported that virus infection can prevent the induction of tolerance by costimulation blockade and, in at least one experimental protocol, can lead to significant morbidity and mortality. In this review, we discuss how viruses modulate the induction and maintenance of transplantation tolerance.

Differential effects of donor-specific alloantibody

Alloantigen exposure typically provokes an adaptive immune response that can foster rejection of transplanted organs, and these responses present the most formidable biological barrier to kidney transplantation. Although most cellular alloimmune responses can be therapeutically controlled with T-cell-specific immunosuppressants, humoral alloimmune responses remain relatively untamed. Importantly, humoral immunity, typically manifesting as allospecific antibody production, is increasingly recognized for its variable appearance after kidney transplantation. Indeed, the appearance of alloantibody can herald the onset of rapid and destructive antibody-mediated rejection or have no demonstrable acute effects. The factors determining the end result of alloantibody formation remain poorly understood. This review will discuss the breadth of alloantibody responses seen in clinical kidney transplantation and provide an overview of potential factors explaining the phenotypic variability associated with humoral alloimmunity. We propose several avenues ripe for future investigation including the influence of innate immune components and the potential influence of heterologous immune responses in determining the ultimate clinical import of an alloantibody response.

Alloreactive (CD4-Independent) CD8+ T cells jeopardize long-term survival of intrahepatic islet allografts

Despite success of early islet allograft engraftment and survival in humans, late islet allograft loss has emerged as an important clinical problem. CD8+ T cells that are independent of CD4+ T cell help can damage allograft tissues and are resistant to conventional immunosuppressive therapies. Previous work demonstrates that islet allografts do not primarily initiate rejection by the (CD4-independent) CD8-dependent pathway. This study was performed to determine if activation of alloreactive CD4-independent, CD8+ T cells, by exogenous stimuli, can precipitate late loss of islet allografts. Recipients were induced to accept intrahepatic islet allografts (islet 'acceptors') by short-term immunotherapy with donor-specific transfusion (DST) and anti-CD154 mAb. Following the establishment of stable long-term islet allograft function for 60-90 days, recipients were challenged with donor-matched hepatocellular allografts, which are known to activate (CD4-independent) CD8+ T cells. Allogeneic islets engrafted long-term were vulnerable to damage when challenged locally with donor-matched hepatocytes. Islet allograft loss was due to allospecific immune damage, which was CD8- but not CD4-dependent. Selection of specific immunotherapy to suppress both CD4- and CD8-dependent immune pathways at the time of transplant protects islet allografts from both early and late immune damage.

Clinical transplantation tolerance: many rivers to cross

Modern immunosuppressive regimens for organ transplantation have resulted in excellent short-term results but less dramatic improvements in long-term outcomes. Moreover, they are associated with significant deleterious effects. One solution that should avoid the adverse drug effects and result in improved graft and patient longevity as well as positively impacting on the organ shortage is the establishment of transplantation tolerance. Ever since the original description of transplantation tolerance in rodent allografts, there have been significant efforts made to translate tolerance-promoting strategies to the clinical arena. However, >50 years later, we are still faced with significant barriers that are preventing such a goal from being widely attained. Nonetheless, pilot clinical tolerance protocols are underway in selected transplant recipients. In this review, we discuss the scientific and nonscientific issues that must be overcome for successful transplantation tolerance to become a clinical reality.

Activation and maturation of alloreactive CD4-independent, CD8 cytolytic T cells

The goal of this study was to determine the in vivo conditions that promote activation of the (CD4-independent) CD8+ T cell-mediated rejection pathway. We have previously noted that hepatocellular but not islet allografts readily activate this rejection pathway. In the current study, we utilized these two cell transplant models to investigate whether differences in host cell recruitment to the graft site, expression of T-cell activation markers by CD8+ graft infiltrating cells (GICs), and/or development of delayed-type hypersensitivity (DTH) and cytotoxic T lymphocyte cell-mediated effector functions could account for the differential transplant outcomes. The collective results demonstrate that recruitment of CD8+ T cells to the site of transplant, CD103 or CD69 expression on CD8+ GICs, and activation of alloreactive DTH responses are insufficient to initiate CD4-independent, CD8-dependent transplant rejection. Instead, rejection by alloreactive (CD4-independent) CD8+ T cells correlated with expression of CD25, CD154 and CD43 by CD8+ GICs, in vitro alloproliferation by recipient CD8+ T cells, and the development of in vivo allospecific cytolytic effector function. These results suggest that tissue-derived factors influence the activation and maturation of (CD4-independent) CD8+ T cells into cytolytic effectors, which correlates with transplant rejection.

Rat cytomegalovirus infection interferes with anti-CD4 mAb-(RIB 5/2) mediated tolerance and induces chronic allograft damage

In order to assess the role of heterologous immunity on tolerance induction (TI) by signal 1 modification, the influence of rat cytomegalovirus infection (RCMVI) on TI by a non-depleting monoclonal anti-CD4 mAb (monoclonal antibody) (RIB 5/2) in a rat kidney transplant (KTx) model was investigated. Orthotopic rat KTx (Dark Agouty (DA)-->Lewis (LEW)) was performed after TI with RIB 5/2 [10 mg/kg body weight (BW); day -1, 0, 1, 2, 3; i.p. (intraperitoneal route)]. RCMVI (5x10E5 Plaque forming units [PFU] i.p.) was simultaneously conducted to KTx, 50 days after KTx, and 14 days before and after KTx. RIB 5/2 induced robust allograft tolerance even across the high-responder strain barrier. RCMVI broke RIB 5/2-induced tolerance regardless of the time of RCMVI but did not induce acute graft failure during the 120 days follow-up. RCMVI induced a significant chronic deterioration of allograft function (p<0.01) and enhanced morphological signs of chronic allograft damage (p<0.05). Cellular infiltrates and major histo-compatibility complex (MHC)-expression were more pronounced (p<0.05) in the infected groups. RCMVI induced not only RCMV-specific T-cell response but also enhanced the frequency of alloreactive T cells. RCMV interferes with anti-CD4 mAb-induced tolerance and leads to chronic allograft damage. The data we presented suggest a potentially important role of viral infections and their prophylaxis in clinical TI protocols.

Determination of the functional status of alloreactive T cells by interferon-gamma kinetics

BACKGROUND

A fundamental limitation of in vitro immunologic tests in the field of transplantation is that existing functional tests poorly correlate with in vivo immune responses such as rejection, tolerance, or absence of rejection due to immunosuppression. It would be helpful to have a measure of T lymphocyte responsiveness that reliably reflects these conditions.

METHODS

C57BL/6J mice received skin transplants from BALB/c donors with: a) no treatment, b) treatment with CsA, or c) treatment with CTLA-4Ig, alpha-CD40L mAb, and alpha-CD25 mAb. Syngeneic skin transplants served as controls. Recipient splenocytes were co-cultured with irradiated donor splenocytes and culture supernatant was harvested once a day for 5 consecutive days. IFN-gamma levels were measured by ELISA.

RESULTS

Splenocytes obtained from non-transplanted mice responded to specific alloantigen stimulation (primary response) at least 2 days later than the splenocytes from mice which had rejected skin grafts (effector/memory response). Splenocytes from mice treated with CsA after skin transplants had no response to third-party alloantigen, but showed an effector/memory pattern of IFN-gamma elaboration with donor cell stimulation (immunosuppression), although the IFN-gamma levels were not as high as those mice with unmodified graft rejection. Mice treated with combined CTLA4Ig, alpha-CD40L and alpha-CD25 accepted skin grafts without further immunosuppression. Splenocytes from these tolerant mice showed a primary response to the third-party and failed to secrete detectable IFN-gamma in the presence of donor cells (tolerance).

CONCLUSION

This assay clearly differentiated the functional status of the alloreactive T cells, including primary alloimmune response, effector/memory response, immunosuppressed T cell response, and donor specific tolerance.

Rapid conversion of effector mechanisms from NK to T cells during virus-induced lysis of allogeneic implants in vivo

Viral infections can strongly stimulate both NK cell and allospecific CD8 T cell responses, and these same effector cells can lyse allogeneic cell lines in vitro. However, the impact of viral infections on the effector systems mediating rejection of allogeneic tissues in vivo has not been fully explored. Using in vivo cytotoxicity assays, we evaluated the effector systems mediating the rejection of CFSE-labeled allogeneic splenocytes after an infection of C57BL/6 (B6) mice with lymphocytic choriomeningitis virus. Naive B6 mice predominantly used a NK cell-effector mechanism to reject allogeneic splenocytes because they rejected BALB/C (H2(d)) splenocytes but not CBA (H2(k)) splenocytes, and the rejection was prevented by immunodepletion of NK1.1(+) or Ly49D(+) NK cells. This rapid and efficient in vivo cytotoxicity assay recapitulated the specificity of NK cell-mediated rejection seen in longer duration in vivo assays. However, as early as 1 day after infection with lymphocytic choriomeningitis virus, a CD8 T cell-dependent mechanism participated in the rejection process and a broader range of tissue haplotypes (e.g., H2(k)) was susceptible. The CD8 T cell-mediated in vivo rejection process was vigorous at a time postinfection (day 3) when NK cell effector functions are peaking, indicating that the effector systems used in vivo differed from those observed with in vitro assays measuring the killing of allogeneic cells. This rapid generation of allospecific CTL activity during a viral infection preceded the peak of viral epitope-specific T cell responses, as detected by in vivo or in vitro cytotoxicity assays.

CD8 memory T cells: cross-reactivity and heterologous immunity

Virus-specific memory T cell populations demonstrate plasticity in antigen recognition and in their ability to accommodate new memory T cell populations. The degeneracy of T cell antigen recognition and the flexibility of diverse antigen-specific repertoires allow the host to respond to a multitude of pathogens while accommodating these numerous large memory pools in a finite immune system. These cross-reactive memory T cells can be employed in immune responses and mediate protective immunity, but they can also induce life-threatening immunopathology or impede transplantation tolerance and graft survival. Here we discuss examples of altered viral pathogenesis occurring as a consequence of heterologous T cell immunity and propose models for the maintenance of a dynamic pool of memory cells.

Transplantation tolerance in pediatric recipients: lessons and challenges

Clinical transplantation tolerance has remained an elusive goal in the 50 yr since it was first described in experimental animals. Greater understanding of the molecular mechanisms responsible for allorecognition have allowed for the development of promising immunosuppressive strategies that may bring us closer to reproducible induction of tolerance; consideration of past successes and failures from both clinical and basic science is required to define future challenges facing this field. This article reviews mechanisms of self and transplantation tolerance, translation of basic science research to clinical protocols in animals and human beings, the changing role of immunosuppression, complications following tolerance induction and controversies surrounding the choice of patients for tolerance trials with a focus on issues relevant to pediatric patients. The role of the Immune Tolerance Network is discussed along with realistic goals for tolerance induction in human beings over the next decade.

In vivo helper functions of alloreactive memory CD4+ T cells remain intact despite donor-specific transfusion and anti-CD40 ligand therapy

Memory T cells have specific properties that are beneficial for rapid and efficient protection from pathogens previously encountered by a host. These same features of memory T cells may be deleterious in the context of a transplanted organ. Consistent with this contention is the accumulating evidence in experimental transplantation that previously sensitized animals are resistant to the effects of costimulatory blockade. Using a model of murine cardiac transplantation, we now demonstrate that alloreactive memory CD4(+) T cells prevent long-term allograft survival induced through donor-specific cell transfusion in combination with anti-CD40 ligand Ab (DST/anti-CD40L). We show that memory donor-reactive CD4(+) T cells responding through the direct or indirect pathways of allorecognition provide help for the induction of antidonor CD8(+) T effector cells and for Ab isotype switching, despite DST/anti-CD40L. The induced pathogenic antidonor immunity functions in multiple ways to subsequently mediate graft destruction. Our findings show that the varied functions of alloreactive memory CD4(+) T cells remain intact despite DST/anti-CD40L-based costimulatory blockade, a finding that will likely have important implications for designing approaches to induce tolerance in human transplant recipients.

In remembrance of things past: memory T cells and transplant rejection

A cardinal feature of the adaptive immune response is its ability to generate long-lived populations of memory T lymphocytes. Memory T cells are specific to the antigen encountered during the primary immune response and react rapidly and vigorously upon re-encounter with the same antigen. Memory T cells that recognize microbial antigens provide the organism with long-lasting protection against potentially fatal infections. On the other hand, memory T cells that recognize donor alloantigens can jeopardize the survival of life-saving organ transplants. We review here the immunobiology of memory T cells and describe their role in the rejection of solid organ allografts.

Clinical transplantation tolerance: The promise and challenges

Organ transplantation is now well established as a preferred option for the treatment of end-stage organ failure. However, there is a severe shortage of donor organs and continued loss of a significant number of organ grafts due to chronic allograft dysfunction. Induction of tolerance of a transplant recipient toward their foreign organ graft, therefore, remains the "Holy Grail" of transplantation immunobiologists. Recently, clinical trials to explore pilot tolerance protocols in humans have been initiated. Defining the ideal strategy(ies) and the role of immunosuppressive drugs, developing tolerance assay(s), and enhancing cooperation between transplant professionals, industry, and the government are some of the challenges to achieving clinical transplantation tolerance. This article reviews the promise and the challenges of achieving clinical transplantation tolerance in human organ transplant recipients.

Induction of tolerance for islet transplantation for type 1 diabetes

Type 1 diabetes is an autoimmune disorder characterized by selective destruction of pancreatic b cells and absolute insulin deficiency. Even when treated well, control is imperfect and complications inevitable. Advances in immunosuppressive drugs and preparation of donor islets have recently made curative islet transplantation a reality for type 1 diabetes. Unfortunately, short-term side effects and long-term health risks of lifelong systemic immunosuppression compromise the otherwise extraordinary benefits that accrue from a successful graft. Our current goal is to obviate the need for immunosuppression and achieve islet graft tolerance. New protocols based on costimulation blockade have brought us close to that goal, inducing states of both peripheral and central transplantation tolerance. These have overcome both allograft rejection and recurrent autoimmunity, but potentially detrimental effects of environmental agents on tolerance are not yet fully understood. Studies of the underlying mechanisms have provided new insights into the nature of both tolerance and autoimmunity.

T cells primed by Leishmania major infection cross-react with alloantigens and alter the course of allograft rejection

Alloreactive T lymphocytes can be primed through direct presentation of donor MHC:peptide complexes on graft cells and through indirect presentation of donor-derived determinants expressed by recipient APCs. The large numbers of determinants on an allograft and the high frequency of the alloreactive repertoire has further led to speculation that exposure to environmental Ags may prime T cells that cross-react with alloantigens. We sought to develop a model in which to test this hypothesis. We found that CD4(+) T cells obtained from C57BL/6 (B6) mice that clinically resolved Leishmania major infection exhibited statistically significant cross-reactivity toward P/J (H-2(p)) Ags compared with the response to other haplotypes. B6 animals that were previously infected with L. major specifically rejected P/J skin grafts with second set kinetics compared with naive animals. Although donor-specific transfusion combined with costimulatory blockade (anti-CD40 ligand Ab) induced prolonged graft survival in naive animals, the same treatment was ineffective in mice previously infected with L. major. The studies demonstrate that cross-reactive priming of alloreactive T cells can occur and provide direct evidence that such T cells can have a significant impact on the outcome of an allograft. The results have important implications for human transplant recipients whose immune repertoires may contain cross-reactively primed allospecific T cells.

Mechanistic perspectives on sulfonamide-induced cutaneous drug reactions

PURPOSE OF REVIEW

Idiosyncratic drug reactions continue to limit the therapeutic utility of sulfonamide drugs because of their associated morbidity and mortality. Cutaneous reactions are the predominant reasons for withdrawal of such drugs from use in patients. As a consequence of the recognized metabolic and immunologic capability of the skin, an understanding of the pathogenic role of this tissue in the development of sulfonamide-induced cutaneous drug reactions may provide insight into the mechanisms and risk factors for these and other adverse drug events.

RECENT FINDINGS

In the present review we discuss currently available mechanistic information, including issues related to drug bioactivation and adduct formation, immunoresponsiveness, and immune dysregulation, for the development of sulfonamide-induced (delayed-type) cutaneous drug reactions. The potential application of findings from several related areas of research are also discussed within the context of the pathogenesis of these cutaneous reactions.

SUMMARY

Despite progress, numerous unresolved issues support the testing of novel hypotheses, the search for additional risk factors, and the need for a global approach, including links between laboratory and clinical paradigms. These issues must be addressed if we are to gain an understanding of the mechanistic bases for these cutaneous drug reactions.

Viral abrogation of stem cell transplantation tolerance causes graft rejection and host death by different mechanisms

Tolerance-based stem cell transplantation using sublethal conditioning is being considered for the treatment of human disease, but safety and efficacy remain to be established. We have shown that mouse bone marrow recipients treated with sublethal irradiation plus transient blockade of the CD40-CD154 costimulatory pathway develop permanent hematopoietic chimerism across allogeneic barriers. We now report that infection with lymphocytic choriomeningitis virus at the time of transplantation prevented engraftment of allogeneic, but not syngeneic, bone marrow in similarly treated mice. Infected allograft recipients also failed to clear the virus and died. Postmortem study revealed hypoplastic bone marrow and spleens. The cause of death was virus-induced IFN-alphabeta. The rejection of allogeneic bone marrow was mediated by a radioresistant CD8(+)TCR-alphabeta(+)NK1.1(-) T cell population. We conclude that a noncytopathic viral infection at the time of transplantation can prevent engraftment of allogeneic bone marrow and result in the death of sublethally irradiated mice treated with costimulation blockade. Clinical application of stem cell transplantation protocols based on costimulation blockade and tolerance induction may require patient isolation to facilitate the procedure and to protect recipients.

Characterization of virus-mediated inhibition of mixed chimerism and allospecific tolerance

Simultaneous blockade of the CD28 and CD40 T cell costimulatory pathways has been shown to effectively promote skin allograft survival in mice. Furthermore, blockade of one or both of these pathways has played a central role in the development of strategies to induce mixed hematopoietic chimerism and allospecific tolerance. It has recently been observed that the beneficial effects of CD40 blockade and donor splenocytes in prolonging skin graft survival can be abrogated by some viral infections, including lymphocytic choriomeningitis virus (LCMV). In this study, we show that LCMV infection prevents prolonged allograft survival following CD28/CD40 combined blockade. We further show that LCMV prevents the induction of allospecific tolerance and mixed hematopoietic chimerism, while delay of infection for 3-4 wk posttransplant has no effect on tolerance induction. Because of reports of anti-H-2(d) activity following LCMV infection, we assayed the ability of LCMV-specific T cells to respond to alloantigen at a single cell level. Although we confirm that LCMV infection induces the generation of alloreactive cells, we also demonstrate that LCMV-specific T cells do not divide in response to alloantigen. The alloresponse suppressed by costimulation blockade is restored by LCMV infection and correlates with increased dendritic cell maturation. We hypothesize that the costimulation blockade-resistant rejection mediated by LCMV could be partly attributable to the up-regulation of alternative costimulatory pathways subsequent to LCMV-induced dendritic cell maturation.