Cutting edge: Rapamycin augments pathogen-specific but not graft-reactive CD8+ T cell responses

Recent evidence demonstrating that exposure to rapamycin during viral infection increased the quantity and quality of Ag-specific T cells poses an intriguing paradox, because rapamycin is used in transplantation to dampen, rather than enhance, donor-reactive T cell responses. In this report, we compared the effects of rapamycin on the Ag-specific T cell response to a bacterial infection versus a transplant. Using a transgenic system in which the Ag and the responding T cell population were identical in both cases, we observed that treatment with rapamycin augmented the Ag-specific T cell response to a pathogen, whereas it failed to do so when the Ag was presented in the context of a transplant. These results suggest that the environment in which an Ag is presented alters the influence of rapamycin on Ag-specific T cell expansion and highlights a fundamental difference between Ag presented by an infectious agent as compared with an allograft.

Surgical correction of gastroesophageal reflux in lung transplant patients is associated with decreased effector CD8 cells in lung lavages: a case series

BACKGROUND

Lung transplantation is associated with a high incidence of gastroesophageal reflux disease (GERD). The presence of GERD is considered a risk factor for the subsequent development of obliterative bronchiolitis (OB), and surgical correction of GERD by gastric fundoplication (GF) may be associated with increased freedom from OB. The mechanisms underlying a protective effect from OB remain elusive. The objective of this study was to analyze the flow cytometric properties of BAL cells in patients who have undergone GF early after transplant.

METHODS

In a single-center lung transplant center, eight patients with GERD who were in the first transplant year underwent GF. Prior to and immediately following GF, BAL cells were analyzed by polychromatic flow cytometry. Spirometry was performed before and after GF.

RESULTS

GF was associated with a significant reduction in the frequency of BAL CD8 lymphocytes expressing the intracellular effector marker granzyme B, compared with the pre-GF levels. Twenty-six percent of CD8 cells were granzyme Bhi pre-GF compared with 12% of CD8 cells post-GF (range 8%-50% pre-GF, 2%-24% post-GF, P = .01). In contrast, GF was associated with a significant interval increase in the frequency of CD8 cells with an exhausted phenotype (granzyme Blo, CD127lo, PD1hi) from 12% of CD8 cells pre-GF to 24% post-GF (range 1.7%-24% pre-GF and 11%-47% post-GF, P = .05). No significant changes in spirometry were observed during the study interval.

CONCLUSIONS

Surgical correction of GF is associated with a decreased frequency of potentially injurious effector CD8 cells in the BAL of lung transplant recipients.

Decreased incidence of NSF in patients on dialysis after changing gadolinium contrast-enhanced MRI protocols

PURPOSE

To retrospectively determine the incidence of nephrogenic systemic fibrosis (NSF) in patients on dialysis administered either a lower dose high-relaxivity linear gadolinium-chelate, gadobenate dimeglumine (MultiHance, MH), compared to a standard dose linear gadolinium chelate, gadodiamide (Omniscan, OM).

MATERIALS AND METHODS

This study was Health Insurance Portability and Accountability Act (HIPAA)-compliant and Institutional Review Board (IRB)-approved. As per institution standardized contrast-enhanced magnetic resonance imaging (MRI) protocols, patients on dialysis were imaged using either MH, between 2/2007 to 9/2008, or OM between 10/2003 and 1/2007. Rates of NSF were compared using 95% score-based confidence intervals (CI). The Wilcoxon rank sum test was used to test similarity/difference between contrast doses given to each patient group.

RESULTS

Overall, 312 patients on dialysis received OM and eight (2.6%) developed NSF (95% CI: 1.30%-4.98%). In all, 784 patients on dialysis received MH at a mean cumulative dose of 0.11 mmol/kg (0.05-0.75 mmol/kg) and no cases of NSF were identified (upper 95% confidence bound of 0.45%). The mean cumulative dose of OM was 0.16 mmol/kg (0.1-0.9 mmol/kg) for all patients and 0.28 mmol/kg (0.1-0.8 mmol/kg) for the patients with NSF. The median OM dose was greater in patients who developed NSF (P = 0.03), and was greater than the median MH dose (P < 0.005).

CONCLUSION

NSF incidence in at-risk patients receiving contrast-enhanced MRI can be reduced after changing contrast administration protocols that includes changing the type and dose of contrast agent.

DHA promotes the neuronal differentiation of rat neural stem cells transfected with GPR40 gene

G-protein coupled receptor 40 (GPR40), which is expressed ubiquitously in the human brain and pancreas, is a member of the large family of seven-transmembrane receptors and can be activated by polyunsaturated fatty acid (PUFA), in particular docosahexaenoic acid (DHA). Recent studies have shown that the DHA/GPR40 signaling pathway may be closely related with adult neurogenesis in the hippocampus. Here, reconstructing pEGFP-N1 vector-expressing GPR40 gene in cultured rat neural stem cells, we demonstrated that DHA-induced neuronal differentiation, neurite growth and branching of adult rat stem cells is mediated at least in part through GPR40 and it remains effective even at low concentrations of DHA. Furthermore, we also revealed that DHA/GPR40 induced the PLC/IP3 signaling pathway and therefore modulated intracellular Ca(2+) mobilization independent of extracellular Ca(2+) concentration. This may be involved in neuronal differentiation and neurite growth in rat neural stem cells transfected with GPR40 gene. These data provide a new sight in the future utilization of neural stem cells transplantation.

A phase III study of belatacept-based immunosuppression regimens versus cyclosporine in renal transplant recipients (BENEFIT study)

Belatacept, a costimulation blocker, may preserve renal function and improve long-term outcomes versus calcineurin inhibitors in kidney transplantation. This Phase III study (Belatacept Evaluation of Nephroprotection and Efficacy as First-line Immunosuppression Trial) assessed a more intensive (MI) or less intensive (LI) regimen of belatacept versus cyclosporine in adults receiving a kidney transplant from living or standard criteria deceased donors. The co-primary endpoints at 12 months were patient/graft survival, a composite renal impairment endpoint (percent with a measured glomerular filtration rate (mGFR) <60 mL/min/1.73 m(2) at Month 12 or a decrease in mGFR > or =10 mL/min/1.73 m(2) Month 3-Month 12) and the incidence of acute rejection. At Month 12, both belatacept regimens had similar patient/graft survival versus cyclosporine (MI: 95%, LI: 97% and cyclosporine: 93%), and were associated with superior renal function as measured by the composite renal impairment endpoint (MI: 55%; LI: 54% and cyclosporine: 78%; p < or = 0.001 MI or LI versus cyclosporine) and by the mGFR (65, 63 and 50 mL/min for MI, LI and cyclosporine; p < or = 0.001 MI or LI versus cyclosporine). Belatacept patients experienced a higher incidence (MI: 22%, LI: 17% and cyclosporine: 7%) and grade of acute rejection episodes. Safety was generally similar between groups, but posttransplant lymphoproliferative disorder was more common in the belatacept groups. Belatacept was associated with superior renal function and similar patient/graft survival versus cyclosporine at 1 year posttransplant, despite a higher rate of early acute rejection.

Transient CD86 expression on hepatitis C virus-specific CD8+ T cells in acute infection is linked to sufficient IL-2 signaling

Costimulatory signals via B7/CD28 family molecules (signal 2) are critical for effective adaptive CD8(+) T cell immune responses. In addition to costimulatory signals, B7/CD28 family coinhibitory receptor/ligands that modulate immune responses have been identified. In acute hepatitis C virus (HCV) infection, programmed death receptor 1, an inhibitory receptor in the CD28 family, is highly expressed on virus-specific CD8(+) T cells, yet vigorous immune responses often develop. We hypothesized that other costimulatory signals present during the acute phase of HCV infection would be important to counter this negative signaling. In this study, we found that CD86 was highly expressed on HCV-specific CD8(+) T cells early in acute HCV infection and was lost on transition to chronic HCV infection; the expression of CD86 was different from other activation markers, because expression was delayed after in vitro TCR stimulation and required sufficient IL-2 signaling; and HCV-specific CD8(+) T cells in the liver of patients with chronic HCV infection were highly activated (CD69, CD38, and HLA-DR expression), but only a minority expressed CD86 or showed evidence of recent IL-2 signaling (low basal phosphorylated STAT5), despite persistent viremia. Our study identified B7 ligand expression on HCV-specific CD8(+) T cells as a distinct marker of effective T cell stimulation with IL-2 signaling in acute HCV infection. Expression of costimulatory molecules, such as CD86, early in HCV infection may be essential in overcoming inhibitory signals from the high level of programmed death receptor 1 expression also seen at this phase of infection.

Anti-CD40 monoclonal antibody synergizes with CTLA4-Ig in promoting long-term graft survival in murine models of transplantation

Blockade of the CD40/CD154 signaling pathway using anti-CD154 Abs has shown promise in attenuating the alloimmune response and promoting long-term graft survival in murine model systems, although side effects observed in humans have hampered its progression through clinical trials. Appropriately designed anti-CD40 Abs may provide a suitable alternative. We investigated two isoforms of a novel monoclonal rat anti-mouse CD40 Ab (7E1) for characteristics and effects mirroring those of anti-CD154: 7E1-G1 (an IgG1 isotype); and 7E1-G2b (an IgG2b isotype). In vitro proliferation assays to measure the agonist properties of the two anti-CD40 Abs revealed similar responses when plate bound. However, when present as a soluble stimulus, 7E1-G1 but not 7E1-G2b led to proliferation. 7E1-G2b was as effective as anti-CD154 when administered in vivo in concert with CTLA4-Ig in promoting both allogeneic bone marrow chimerism and skin graft survival, whereas 7E1-G1 was not. The protection observed with 7E1-G2b was not due to depletion of CD40-bearing APCs. These data suggest that an appropriately designed anti-CD40 Ab can promote graft survival as well as anti-CD154, making 7E1-G2b an attractive substitute in mouse models of costimulation blockade-based tolerance regimens.

Alefacept promotes co-stimulation blockade based allograft survival in nonhuman primates

Memory T-cells promote allograft rejection particularly in costimulation blockade (CoB)-based immunosuppressive regimens. Here we show that the CD2-specific fusion protein alefacept (LFA3-Ig) selectively eliminates memory T-cells and when combined with a CoB-based regimen utilizing CTLA4-Ig, prevents renal allograft rejection and alloantibody formation in primates. These results support the development of an immediately translatable regimen for the prevention of allograft rejection without the use of calcineurin inhibitors, steroids, or pan-T-cell depletion.

mTOR regulates memory CD8 T-cell differentiation

Memory CD8 T cells are a critical component of protective immunity and inducing effective memory T cell responses is a major goal of vaccines against chronic infections and tumors 1-3. Considerable effort has gone into designing vaccine regimens that will increase the magnitude of the memory response but there has been minimal emphasis on developing strategies to improve the functional qualities of memory T cells 4. In this study we show that mTOR, the mammalian target of rapamycin 5, is a major regulator of memory CD8 T cell differentiation and in contrast to what we expected the mTOR specific inhibitor rapamycin, an immunosuppressive drug, had surprising immunostimulatory effects on the generation of memory CD8 T cells. Treatment of mice with rapamycin following acute lymphocytic choriomeningitis virus (LCMV) infection enhanced not only the quantity but also the quality of virus specific CD8 T cells. Similar effects were seen after immunization of mice with a non-replicating VLP based vaccine. In addition, rapamycin treatment also enhanced memory T cell responses in non-human primates following vaccination with MVA (modified vaccinia virus - Ankara). Rapamycin was effective during both the expansion and contraction phases of the T cell response; during the expansion phase it increased the number of memory precursors and during the contraction phase (effector to memory transition) it accelerated the memory T cell differentiation program. Experiments using RNAi to inhibit mTOR, raptor or FKBP12 expression in antigen specific CD8 T cells showed that mTOR acts intrinsically through the mTORC1 pathway to regulate memory T cell differentiation. Thus, these studies identify a molecular pathway regulating memory formation and provide an effective strategy for improving the functional qualities of vaccine or infection induced memory T cells.

Translating costimulation blockade to the clinic: lessons learned from three pathways

SUMMARY

As the recognition that costimulatory signals are critical for optimal T-cell activation, proliferation, and differentiation, there has been an explosion in the study of costimulatory molecules and their roles in enhancing anti-donor T-cell responses following transplantation. Here, we focus on the bench-to-beside translation of blocking agents designed to target three critical costimulatory pathways: the CD28/CD80/CD86 pathway, the CD154/CD40 pathway, and the lymphocyte function associated antigen-1/intercellular adhesion molecule pathway. While blockade of each of these pathways proved promising in inhibiting donor-reactive T-cell responses and promoting long-term graft survival in murine models of transplantation, the progression of development of therapeutic agents to block these pathways has each taken a slightly different course. Both logistical and biological pitfalls have accompanied the translation of blockers of all three pathways into clinically applicable therapies, and the development of costimulatory blockade as a substitute for current standard-of-care calcineurin inhibitors has by no means reached completion. Collaboration between both the basic and clinical arenas will further propel the development of costimulation blockers currently in the pipeline, as well as of novel methods to target these critical pathways during transplantation.

IFN-gamma dictates allograft fate via opposing effects on the graft and on recipient CD8 T cell responses

CD8 T cells are necessary for costimulation blockade-resistant rejection. However, the mechanism by which CD8 T cells mediate rejection in the absence of major costimulatory signals is poorly understood. IFN-gamma promotes CD8 T cell-mediated immune responses, but IFN-gamma-deficient mice show early graft loss despite costimulation blockade. In contrast, we found that IFN-gamma receptor knockout mice show dramatically prolonged graft survival under costimulation blockade. To investigate this paradox, we addressed the effects of IFN-gamma on T cell alloresponses in vivo independent of the effects of IFN-gamma on graft survival. We identified a donor-specific CD8 T cell breakthrough response temporally correlated with costimulation blockade-resistant rejection. Neither IFN-gamma receptor knockout recipients nor IFN-gamma-deficient recipients showed a CD8 breakthrough response. Graft death on IFN-gamma-deficient recipients despite costimulation blockade could be explained by the lack of IFN-gamma available to act on the graft. Indeed, the presence of IFN-gamma was necessary for graft survival on IFN-gamma receptor knockout recipients, as either IFN-gamma neutralization or the lack of the IFN-gamma receptor on the graft precipitated early graft loss. Thus, IFN-gamma is required both for the recipient to mount a donor-specific CD8 T cell response under costimulation blockade as well as for the graft to survive after allotransplantation.

Expanded nonhuman primate tregs exhibit a unique gene expression signature and potently downregulate alloimmune responses

We have established two complementary strategies for purifying naturally occurring regulatory T cells (Tregs) from rhesus macaques in quantities that would be sufficient for use as an in vivo cellular therapeutic. The first strategy identified Tregs based on their being CD4+/CD25(bright). The second incorporated CD127, and purified Tregs based on their expression of CD4 and CD25 and their low expression of CD127. Using these purification strategies, we were able to purify as many as 1x10(6) Tregs from 120 cc of peripheral blood. Cultures of these cells with anti-CD3, anti-CD28 and IL-2 over 21 days yielded as much as a 450-fold expansion, ultimately producing as many as 4.7x10(8) Tregs. Expanded Treg cultures potently inhibited alloimmune proliferation as measured by a carboxyfluorescein succinimidyl ester- mixed lymphocyte reaction (CFSE-MLR) assay even at a 1:100 ratio with responder T cells. Furthermore, both responder-specific and third-party Tregs downregulated alloproliferation similarly. Both freshly isolated and cultured Tregs had gene expression signatures distinguishable from concurrently isolated bulk CD4+ T-cell populations, as measured by singleplex reverse transcriptase-polymerase chain reaction (RT-PCR) and gene array. Moreover, an overlapping yet distinct gene expression signature seen in freshly isolated compared to expanded Tregs identifies a subset of Treg genes likely to be functionally significant.

PD-1-dependent mechanisms maintain peripheral tolerance of donor-reactive CD8+ T cells to transplanted tissue

Peripheral mechanisms of self-tolerance often depend on the quiescent state of the immune system. To what degree such mechanisms can be engaged in the enhancement of allograft survival is unclear. To examine the role of the PD-1 pathway in the maintenance of graft survival following blockade of costimulatory pathways, we used a single-Ag mismatch model of graft rejection where we could track the donor-specific cells as they developed endogenously and emerged from the thymus. We found that graft-specific T cells arising under physiologic developmental conditions at low frequency were actively deleted at the time of transplantation under combined CD28/CD40L blockade. However, this deletion was incomplete, and donor-specific cells that failed to undergo deletion up-regulated expression of PD-1. Furthermore, blockade of PD-1 signaling on these cells via in vivo treatment with anti-PD-1 mAb resulted in rapid expansion of donor-specific T cells and graft loss. These results suggest that the PD-1 pathway was engaged in the continued regulation of the low-frequency graft-specific immune response and thus in maintenance of graft survival.

CD28/CD154 blockade prevents autoimmune diabetes by inducing nondeletional tolerance after effector t-cell inhibition and regulatory T-cell expansion

OBJECTIVE

Blocking T-cell signaling is an effective means to prevent autoimmunity and allograft rejection in many animal models, yet the clinical translation of many of these approaches has not resulted in the success witnessed in experimental systems. Improved understanding of these approaches may assist in developing safe and effective means to treat disorders such as autoimmune diabetes.

RESEARCH DESIGN AND METHODS

We studied the effect of anti-CD154 and CTLA4-Ig on diabetes development, and the requirements to induce tolerance in nod.scid mice after transfer of transgenic beta-cell reactive BDC2.5.NOD T-cells.

RESULTS

Nod.scid recipients of diabetogenic BDC2.5.NOD cells were protected indefinitely from diabetes by a short course of combined costimulation blockade, despite the continued diabetogenic potential of their T-cells. The presence of pathogenic T-cells in the absence of disease indicates peripheral immune tolerance. T-cell maturation occurred in protected recipients, yet costimulation blockade temporarily blunted early T-cell proliferation in draining pancreatic nodes. Tolerance required preexisting regulatory T-cells (Tregs), and protected recipients had greater numbers of Tregs than diabetic recipients. Diabetes protection was successful in the presence of homeostatic expansion and high T-cell precursor frequency, both obstacles to tolerance induction in other models of antigen-specific immunity.

CONCLUSIONS

Immunotherapies that selectively suppress effector T-cells while permitting the development of natural regulatory mechanisms may have a unique role in establishing targeted long-standing immune protection and peripheral tolerance. Understanding the mechanism of these approaches may assist in the design and use of therapies for human conditions, such as type 1 diabetes.

Expansion of effector memory TCR Vbeta4+ CD8+ T cells is associated with latent infection-mediated resistance to transplantation tolerance

Therapies that control largely T cell-dependent allograft rejection in humans also possess the undesirable effect of impairing T cell function, leaving transplant recipients susceptible to opportunistic viruses. Prime among these opportunists are the ubiquitous herpesviruses. To date, studies are lacking that address the effect of viruses that establish a true latent state on allograft tolerance or the effect of tolerance protocols on the immune control of latent viruses. By using a mixed chimerism-based tolerance-induction protocol, we found that mice undergoing latent infection with gammaHV68, a murine gamma-herpesvirus closely related to human gamma-herpesviruses such as EBV and Kaposi's sarcoma-associated herpesvirus, significantly resist tolerance to allografts. Limiting the degree of virus reactivation or innate immune response did not reconstitute chimerism in latently infected mice. However, gammaHV68-infected mice showed increased frequency of CD8+ T cell alloreactivity and, interestingly, expansion of virus-induced, alloreactive, "effector/effector memory" TCR Vbeta4+CD8+ T cells driven by the gammaHV68-M1 gene was associated with resistance to tolerance induction in studies using gammaHV68-M1 mutant virus. These results define the viral gene and immune cell types involved in latent infection-mediated resistance to allograft tolerance and underscore the influence of latent herpesviruses on allograft survival.

A critical precursor frequency of donor-reactive CD4+ T cell help is required for CD8+ T cell-mediated CD28/CD154-independent rejection

Ag-specific precursor frequency is increasingly being appreciated as an important factor in determining the kinetics, magnitude, and degree of differentiation of T cell responses, and recently was found to play a critical role in determining the relative requirement of CD8(+) T cells for CD28- and CD154-mediated costimulatory signals during transplantation. We addressed the possibility that variations in CD4(+) T cell precursor frequency following transplantation might affect CD4(+) T cell proliferation, effector function, and provision of help for donor-reactive B cell and CD8(+) T cell responses. Using a transgenic model system wherein increasing frequencies of donor-reactive CD4(+) T cells were transferred into skin graft recipients, we observed that a critical CD4(+) T cell threshold precursor frequency was necessary to provide help following blockade of the CD28 and CD154 costimulatory pathways, as measured by increased B cell and CD8(+) T cell responses and precipitation of graft rejection. In contrast to high-frequency CD8(+) T cell responses, this effect was observed even though the proliferative and cytokine responses of Ag-specific CD4(+) T cells were inhibited. Thus, we conclude that an initial high frequency of donor-reactive CD4(+) T cells uncouples T cell proliferative and effector cytokine production from the provision of T cell help.

Measuring symptom experience of side-effects of immunosuppressive drugs: the Modified Transplant Symptom Occurrence and Distress Scale

Measurement of the patients' subjective experience of side-effects of immunosuppressants is a critical post-transplant outcome. This study aimed to update and validate the 45-item Modified Transplant Symptom Occurrence and Symptom Distress Scale (MTSOSD) for novel immunosuppressive regimens. We used four steps: (1) literature review to identify immunosuppressant-related symptoms; (2) screening of adverse event forms; (3) international experts' evaluation of the appropriateness of each symptom; and (4) a pilot study in 24 renal transplant patients to test the clarity of instructions and items, and a pilot study in 84 lung transplant patients, to determine content and discriminant validity. Steps 1 and 2 produced a list of 76 symptoms. Clinical experts deemed 59 symptoms as being relevant for assessing symptom experience (step 3). Based on the first pilot testing, items and instructions were adapted to improve clarity. The second pilot testing showed that the updated MTSOSD-59R was easy to complete, that items and instructions were understandable, and that symptom profiles differed between males and females, and between depressed and nondepressed patients (step 4). The MTSOSD-59R is an instrument with established content and discriminant validity for assessing transplant patients' symptom experience of side-effects stemming from currently available immunosuppressive regimens.

Glial cell line-derived neurotrophic factor increases beta-cell mass and improves glucose tolerance

BACKGROUND & AIMS

Pancreatic beta-cell mass increases in response to increased demand for insulin, but the factors involved are largely unknown. Glial cell line-derived neurotrophic factor (GDNF) is a growth factor that plays a role in the development and survival of the enteric nervous system. We investigated the role of GDNF in regulating beta-cell survival.

METHODS

Studies were performed using the beta-TC-6 pancreatic beta-cell line, isolated mouse pancreatic beta cells, and in vivo in transgenic mice that overexpress GDNF in pancreatic glia. GDNF receptor family alpha1 and c-Ret receptor expression were assessed by reverse-transcription polymerase chain reaction and immunofluorescence microscopy. Apoptosis was evaluated by assessing caspase-3 cleavage. Phosphoinositol-3-kinase signaling pathway was analyzed by Akt phosphorylation. Glucose homeostasis was assessed by performing intraperitoneal glucose tolerance tests. Insulin sensitivity was assessed using intraperitoneal injection of insulin.

RESULTS

We demonstrate the presence of receptors for GDNF, GFRalpha1, and c-Ret on beta cells. GDNF promoted beta-cell survival and proliferation and protected them from thapsigargin-induced apoptosis (P<.0001) in vitro. Exposure of beta-cells to GDNF also resulted in phosphorylation of Akt and GSK3beta. Transgenic mice that overexpress GDNF in glia exhibit increased beta-cell mass, proliferation, and insulin content. No differences in insulin sensitivity and c-peptide levels were noted. Compared with wild-type mice, GDNF-transgenic mice have significantly lower blood glucose levels and improved glucose tolerance (P<.01). GDNF-transgenic mice are resistant to streptozotocin-induced beta-cell loss (P<.001) and subsequent hyperglycemia.

CONCLUSIONS

We demonstrate that over expression of GDNF in pancreatic glia improves glucose tolerance and that GDNF may be a therapeutic target for improving beta-cell mass.

Immune responsiveness and protective immunity after transplantation

The growing success of solid organ transplantation poses unique challenges for the implementation of effective immunization strategies. Although live attenuated vaccines have proven benefits for the general population, immunosuppressed patients are at risk for unique complications such as infection from the vaccine because of lack of both clearance and containment of a live attenuated virus. Moreover, while vaccination strategies using killed organisms or purified peptides are believed to be safe for immunosuppressed patients, they may have reduced efficacy in this population. The current lack of knowledge of the basic safety and efficacy of vaccination strategies in the immunosuppressed has limited the development of guidelines regarding vaccination in this population. Recent fears of influenza pandemics and potential attacks by weaponized pathogens such as smallpox heighten the need for increased knowledge. Herein, we review the current understanding of the effects of immunosuppressants on the immune system and the ability of the suppressed immune system to respond to vaccination. This review highlights the need for systematic, longitudinal studies in both humans and nonhuman primates to understand better the defects in innate and adaptive immunity in transplant recipients, thereby aiding the development of strategies to vaccinate these individuals.

Engraftment of adult porcine islet xenografts in diabetic nonhuman primates through targeting of costimulation pathways

Recent advances in human allogeneic islet transplantation have established beta-cell replacement therapy as a potentially viable treatment option for individuals afflicted with Type 1 diabetes. Two recent successes, one involving neonatal porcine islet xenografts transplanted into diabetic rhesus macaques treated with a costimulation blockade-based regimen and the other involving diabetic cynomolgus monkeys transplanted with adult porcine islet xenografts treated with an alternative multidrug immunosuppressive regimen have demonstrated the feasibility of porcine islet xenotransplantation in nonhuman primate models. In the current study, we assessed whether transplantation of adult porcine islet xenografts into pancreatectomized macaques, under the cover of a costimulation blockade-based immunosuppressive regimen (CD28 and CD154 blockade), could correct hyperglycemia. Our findings suggest that the adult porcine islets transplanted into rhesus macaques receiving a costimulation blockade-based regimen are not uniformly subject to hyperacute rejection, can engraft (2/5 recipients), and have the potential to provide sustained normoglycemia. These results provide further evidence to suggest that porcine islet xenotransplantation may be an attainable strategy to alleviate the islet supply crisis that is one of the principal obstacles to large-scale application of islet replacement therapy in the treatment of Type 1 diabetes.

Viral targeting of fibroblastic reticular cells contributes to immunosuppression and persistence during chronic infection

Many chronic viral infections are marked by pathogen persistence and a generalized immunosuppression. The exact mechanisms by which this occurs are still unknown. Using a mouse model of persistent lymphocytic choriomeningitis virus (LCMV) infection, we demonstrate viral targeting of fibroblastic reticular cells (FRC) in the lymphoid organs. The FRC stromal networks are critical for proper lymphoid architecture and function. High numbers of FRC were infected by LCMV clone 13, which causes a chronic infection, whereas few were infected by the acute strain, LCMV Armstrong. The function of the FRC conduit network was altered after clone 13 infection by the action of CD8(+) T cells. Importantly, expression of the inhibitory programmed death ligand 1, which was up-regulated on FRC after infection, reduced early CD8(+) T cell-mediated immunopathology and prevented destruction of the FRC architecture in the spleen. Together, this reveals an important tropism during a persistent viral infection. These data also suggest that the inhibitory PD-1 pathway, which likely evolved to prevent excessive immunopathology, may contribute to viral persistence in FRC during chronic infection.

Fully MHC-Disparate Mixed Hemopoietic Chimeras Show Specific Defects in the Control of Chronic Viral Infections

The establishment of mixed allogeneic chimerism can induce donor-specific transplantation tolerance across full MHC barriers. However, a theoretical disadvantage of this approach is the possibility that the state of mixed chimerism might negatively affect the recipient's immune competence to control pathogens. Previous studies using murine models have not supported this hypothesis, because they indicate that acute viral infections are cleared by chimeric animals with similar kinetics to that of unmanipulated controls. However, chronic or persistent viral infections often require a more complex and sustained response with cooperation between CD4 Th cells, CTL, and B cells for effective control. The current study indicates that profound defects become manifest in the control of chronic pathogenic infections in MHC-disparate mixed allogeneic chimeric mice. Furthermore, we show that ineffective priming of the donor-restricted CTL response leads to virus persistence, as well as severe T cell exhaustion. Our results further suggest that either T cell adoptive immunotherapy or selected MHC haplotype matching partially restore immune competence. These approaches may facilitate the translation of mixed chimerism therapeutic regimens.

NK cells rapidly reject allogeneic bone marrow in the spleen through a perforin- and Ly49D-dependent, but NKG2D-independent mechanism

We have used a sensitive and specific in vivo killing assay to monitor the kinetics, anatomic location and mechanisms controlling NK-mediated rejection of Balb/c bone marrow by C57BL/6 natural killer (NK) cells. We find that NK killing of fully allogeneic bone marrow is a rapid, highly efficient process, leading to substantial rejection of transplanted marrow within 6 h of transplant and elimination of 85% of the transplanted cells within 2 days. NK-mediated rejection occurred predominantly in the spleen, with sparing of rejection in the bone marrow and lymph nodes. Rejection was dependent on Perforin gene function, but was independent of interferon-gamma. Finally, rejection of Balb/c bone marrow by B6 NK cells required signaling through the Ly49D receptor, but occurred despite blockade of NKG2D, which distinguishes these results from previous studies using semiallogeneic transplant pairs. These results identify NK cells as highly active mediators of bone marrow rejection, and suggest that inhibiting NK function early during transplantation may increase the efficiency of engraftment and allow successful engraftment of limiting doses of donor bone marrow.

Infusion of Stably Immature Monocyte-Derived Dendritic Cells Plus CTLA4Ig Modulates Alloimmune Reactivity in Rhesus Macaques

BACKGROUND

Immature dendritic cells (DC) can promote long-term transplant survival in rodents. We assessed the impact of stably immature, donor-derived DC on alloimmune reactivity in rhesus macaques.

METHODS

CD14 monocytes isolated from leukapheresis products of Macacca mulatta were cultured in granulocyte-macrophage colony stimulating factor plus interleukin (IL)-4+/-vitamin (vit) D3, and IL-10. Major histocompatibility complex class II and cosignaling molecule expression was determined on CD11c cells by flow cytometry. T-cell allostimulatory capacity of the DC, including DC exposed to proinflammatory cytokines, was determined in mixed leukocyte reaction. To test their influence in vivo, purified DC were infused intravenously into allogeneic recipients, either alone or followed by CTLA4Ig, 24 hr later. Proliferative responses of recipient CFSE-labeled T cells to donor or third party DC, cytokine production by stimulated T cells, and circulating alloantibody levels were determined by flow cytometry, up to 100 days postinfusion.

RESULTS

VitD3/IL-10-conditioned, monocyte-derived DC were resistant to maturation and failed to induce allogeneic T cell proliferation in vitro. After their infusion, an increase in anti-donor and anti-third party T-cell reactivity was observed, that subsequently subsided to fall significantly below pretreatment levels (by day 56) only in animals also given CTLA4Ig. No increase in circulating immunoglobulin (Ig) M or IgG anti-donor alloantibody titers compared with pretreatment values was detected. With DC+CTLA4Ig infusion, alloreactive IL-10-producing T cells were prevalent in the circulation after day 28.

CONCLUSIONS

Maturation-resistant rhesus DC infusion is well-tolerated. DC+CTLA4Ig infusion modulates allogeneic T-cell responses and results in hyporesponsiveness to donor and third party alloantigens.

Transplantation Tolerance

Tolerance following organ transplantation was first described in experimental models over 50 years ago. Reports of tolerance in clinical transplantation have appeared in the literature sporadically for decades. Despite this long-standing fascination with transplantation tolerance, the ability to reproducibly induce tolerance in humans undergoing organ transplantation has remained elusive. Recent advances in our knowledge of the mechanisms that contribute to the induction and maintenance of tolerance as well as those factors that oppose tolerance may allow the design of clinical trials aimed at introducing tolerance-inducing strategies into clinical transplantation.

Antigenic disparity impacts outcome of agonism but not blockade of costimulatory pathways in experimental transplant models

Treatment regimens consisting of CTLA-4 Ig/anti-CD154 or agonistic anti-CD28/rapamycin have both been shown to prevent GVHD in fully allogeneic murine model systems. Using a transgenic approach to track the fate of host-reactive T cells in a minor antigen disparity model of GVHD, we found that while treatment with CTLA-4 Ig/anti-CD154 retained efficacy, the costimulation agonist anti-CD28 combined with rapamycin failed to prevent GVHD. Analysis of the host-reactive CD4(+) and CD8(+) T-cell responses revealed that in contrast to CTLA-4Ig/anti-CD154-treated recipients, host-reactive T cells in recipients treated with agonistic anti-CD28/rapamycin displayed enhanced and accelerated T-cell proliferation and failed to undergo activation-induced cell death. An increase in systemic levels of inflammatory cytokines was observed in the anti-CD28/rapamycin-treated recipients of the minor but not major antigen disparity bone marrow transplants. Our results demonstrate the intricacies of costimulatory receptor signaling pathways, and reveal how agonism of the CD28 pathway can have opposing outcomes depending on the degree of antigenic disparity.

Role of CD28 in fatal autoimmune disorder in scurfy mice

Scurfy mice develop CD4 T-cell-mediated lymphoproliferative disease leading to death within 4 weeks of age. The scurfy mutation causes loss of function of the foxp3 gene (foxp3(sf)), which is essential for development and maintenance of naturally occurring regulatory CD4 T cells (nTregs). In humans, mutations of the foxp3 gene cause immune dysregulation, polyendocrinopathy, enteropathy, and X-linked syndrome (IPEX). In most patients with IPEX and also in scurfy mice, T cells show hyperreactivity and levels of Th1- and Th2-associated cytokines are substantially elevated. We report that removal of CD28 expression rescued scurfy mice from early death. Longer-term surviving CD28-deficient scurfy mice still had lymphoproliferative disorder, but their CD4 T cells showed decreased interferon-gamma and no sign of interleukin-4 or interleukin-10 hyperproduction. Furthermore, injection of CTLA4-Ig to block CD28-B7 interactions substantially improved the survival of scurfy mice by blocking effector T-cell differentiation. These data support the hypothesis that CD28-B7 interactions play a critical role in the etiology of lethal autoimmune disease in scurfy mice by stimulating the differentiation of antigen-activated naive T cells into effector T cells.

Functionally impaired T cell-dependent immunopathology in FK506-treated mice with LCMV infection (46.10)

Lymphocytic choriomeningitis virus (LCMV) was recently shown to contaminate transplanted organs with lethal consequences in transplant recipients. The diagnosis of LCMV as a cause of death under circumstances of immunosuppression is somewhat paradoxical since LCMV is a non-cytolytic virus and is the classic example of immune-mediated disease. To understand how treatment with immunosuppressive drugs given to transplant recipients results in LCMV disease, we have initiated studies using the calcineurin inhibitor FK506. Surprisingly, FK506-treated LCMV-infected mice showed high lethality. The drug treatment resulted in a failure of infection control and interestingly induced functionally impaired virus-specific T cells. Lesions and death appeared to be the consequence of overproduction of cytokines, such as TNF-alpha. This response was dependent on the presence of T cells but these cells themselves appeared not to be the source of the disease-producing cytokines. Our results suggest that functionally impaired T cells in drug treated animals signal cells in LCMV infected tissues to overproduce the cytokines responsible for lesions.

Apoptotic donor leukocytes limit mixed-chimerism induced by CD40-CD154 blockade in allogeneic bone marrow transplantation

"Mini" allogeneic bone marrow transplants using non-myeloablative conditioning have reduced early treatment-related mortalities, but graft-versus-host disease (GVHD) and graft rejection remain clinical problems. Our preliminary studies indicated that low-dose busulfan conditioning and costimulatory blockade using anti-CD154 monoclonal antibody (mAb) in combination with a pretransplantation "tolerating" dose of bone marrow (BM) cells were sufficient to establish stable mixed-chimerism without GVHD when transplanting moderate doses of T cell depleted (TCD)-BM from major histocompatibility complex (MHC) fully-mismatched donors (Adams AB, Durham MM, Kean L, et al. J Immunol. 2001;167:1103-1111). In this study, donor splenocytes were administered before transplantation as a tolerating cell infusion with a conditioning regimen consisting of low-dose busulfan and anti-CD154 mAb. We compared the ability of viable and apoptotic donor cells of different ex vivo treatments and purified different donor cell populations (CD3(+), CD3(-), CD11b(+), and CD11b(-) splenocytes) to induce tolerance and enhance donor chimerism in a MHC mismatched model of murine bone marrow transplantation. We found that mixed chimerism without GVHD was enhanced by pretransplantation administration of viable allogeneic splenocytes and diminished in mice with prior exposure to apoptotic/necrotic donor splenocytes. CD11b(+)-enriched splenocytes more potently enhanced donor chimerism compared to unfractionated splenocytes or other splenocyte subsets. Mixed lymphocyte cultures demonstrated that apoptotic stimulators overcame the immune-tolerating activity of anti-CD154 mAb and led to increased interferon gamma and tumor necrosis factor alpha synthesis, increased proliferation of responder T cells, and decreased production of interleukin-10. In conclusion, viable donor splenocytes administered before transplantation in combination with costimulatory blockade induced tolerance and enhanced donor chimerism, whereas pretransplantation administration of apoptotic/necrotic donor cells led to host T cell activation and decreased overall donor engraftment.

Induction of chimerism in rhesus macaques through stem cell transplant and costimulation blockade-based immunosuppression

A strategy for producing high-level hematopoietic chimerism after non-myeloablative conditioning has been established in the rhesus macaque. This strategy relies on hematopoietic stem cell transplantation after induction with a non-myeloablative dose of busulfan and blockade of the IL2-receptor in the setting of mTOR inhibition with sirolimus and combined CD28/CD154 costimulation blockade. Hematopoietic stem cells derived from bone marrow and leukopheresis products both were found to be successful in inducing high-level chimerism. Mean peripheral blood peak donor chimerism was 81% with a median chimerism duration of 145 days. Additional immune modulation strategies, such as pre-transplant CD8 depletion, donor-specific transfusion, recipient thymectomy or peritransplant deoxyspergualin treatment did not improve the level or durability of chimerism. Recipient immunologic assessment suggested that chimerism occurred amidst donor-specific down-regulation of alloreactive T cells, and the reappearance of vigorous T-mediated alloreactivity accompanied rejection of the transplants. Furthermore, viral reactivation constituted a significant transplant-related toxicity and may have negatively impacted the ability to achieve indefinite survival of transplanted stem cells. Nevertheless, this chimerism-induction regimen induced amongst the longest-lived stem cell chimerism reported to date for non-human primates and thus represents a platform upon which to evaluate emerging tolerance-induction strategies.

Antigen-specific precursor frequency impacts T cell proliferation, differentiation, and requirement for costimulation

After a brief period of antigenic stimulation, T cells become committed to a program of autonomous expansion and differentiation. We investigated the role of antigen-specific T cell precursor frequency as a possible cell-extrinsic factor impacting T cell programming in a model of allogeneic tissue transplantation. Using an adoptive transfer system to incrementally raise the precursor frequency of antigen-specific CD8⁺ T cells, we found that donor-reactive T cells primed at low frequency exhibited increased cellular division, decreased development of multifunctional effector activity, and an increased requirement for CD28- and CD154-mediated costimulation relative to those primed at high frequency. The results demonstrated that recipients with low CD4⁺ and CD8⁺ donor-reactive T cell frequencies exhibited long-term skin graft survival upon CD28/CD154 blockade, whereas simultaneously raising the frequency of CD4⁺ T cells to ∼0.5% and CD8⁺ T cells to ∼5% precipitated graft rejection despite CD28/CD154 blockade. Antigenic rechallenge of equal numbers of cells stimulated at high or low frequency revealed that cells retained an imprint of the frequency at which they were primed. These results demonstrate a critical role for initial precursor frequency in determining the CD8⁺ T cell requirement for CD28- and CD154-mediated costimulatory signals during graft rejection.

Continuous recruitment of naive T cells contributes to heterogeneity of antiviral CD8 T cells during persistent infection

Numerous microbes establish persistent infections, accompanied by antigen-specific CD8 T cell activation. Pathogen-specific T cells in chronically infected hosts are often phenotypically and functionally variable, as well as distinct from T cells responding to nonpersistent infections; this phenotypic heterogeneity has been attributed to an ongoing reencounter with antigen. Paradoxically, maintenance of memory CD8 T cells to acutely resolved infections is antigen independent, whereas there is a dependence on antigen for T cell survival in chronically infected hosts. Using two chronic viral infections, we demonstrate that new naive antigen-specific CD8 T cells are primed after the acute phase of infection. These newly recruited T cells are phenotypically distinct from those primed earlier. Long-lived antiviral CD8 T cells are defective in self-renewal, and lack of thymic output results in the decline of virus-specific CD8 T cells, indicating that newly generated T cells preserve antiviral CD8 T cell populations during chronic infection. These findings reveal a novel role for antigen in maintaining virus-specific CD8 T cells during persistent infection and provide insight toward understanding T cell differentiation in chronic infection.

Tolerance Assays: Measuring the Unknown

Distinguishing transplant recipients who will benefit from a reduction in, or even the withdrawal of, immunosuppression from those who require intensive, lifelong immunosuppression will be dependent on developing strategies for immune monitoring. Currently, no assays have been shown to accurately predict the development or presence of donor-specific tolerance in humans after transplantation. In this overview we describe and discuss those assays that we believe may be useful for identifying tolerant transplant recipients. Validation of "tolerance" assays will be critical for the safe development of tolerance regimens in humans.

Transplant tolerance in non-human primates: progress, current challenges and unmet needs

Given the significant morbidity associated with current post-transplant immunosuppressive regimens, induction of immune tolerance continues to be an important goal of clinical organ transplantation. While many strategies for inducing tolerance have been successfully applied in murine models, significant barriers are faced when translating these approaches to the clinic. This has necessitated pre-clinical studies in the more closely related model system, the non-human primates (NHP). In this review, we will discuss the four most prominent strategies for inducing transplantation tolerance and highlight their relative success and shortcomings in NHP. These strategies are: (1) T-cell costimulation blockade (2) mixed chimerism induction (3) T-cell depletion and (4) tolerance induction through regulatory T-cells. After discussing the progress that has been made with each of these strategies, we will identify this field's most pressing unmet needs and discuss how we may best overcome the resulting barriers to tolerance induction.

A mouse model for polyomavirus-associated nephropathy of kidney transplants

Polyomavirus-associated nephropathy is an important cause of dysfunction and failure of renal transplants. BK virus is an ubiquitous human polyoma virus that persistently infects the kidney. This otherwise silent infection can reactivate in immunosuppressed individuals, resulting in renal complications. Because polyoma viruses are highly species-specific, we developed a mouse polyoma virus-renal transplant model in order to investigate the pathogenesis of polyomavirus-associated nephropathy. Using this model, we found that polyoma virus preferentially replicates in the allogeneic kidney grafts, accelerating graft failure; thus, this animal model is able to mimic the polyomavirus-associated nephropathy seen in human renal transplant patients. Acute polyoma virus infection of mouse allograft recipients augmented the alloreactive CD8+ T-cell response, while maintaining the anti-viral CD8+ T-cell response. In addition to the known virus-induced cytopathology, these findings demonstrate a potential role for an enhanced anti-donor T-cell response in the pathogenesis of polyomavirus-associated nephropathy.

A new look at blockade of T-cell costimulation: a therapeutic strategy for long-term maintenance immunosuppression

Activated T cells orchestrate the immune response that results in graft rejection; therefore, a common goal among current immunosuppressive therapies is to block T-cell activation, proliferation and function. Current immunosuppressive regimens that inhibit T cells and immune cells have greatly reduced the incidence of acute rejection following solid-organ transplant. However, the expected improvements in long-term outcomes have not been realized. This may be related to the non-immune side effects of current maintenance immunosuppressants, which target ubiquitously expressed molecules. The focus in transplantation research is shifting in search of maintenance immunosuppressive regimens that might offer improved long-term outcomes by providing efficacy in prevention of acute rejection combined with reduced toxicities. An emerging therapeutic strategy involves an immunoselective maintenance immunosuppressant that inhibits full T-cell activation by blocking the interaction between costimulatory receptor-ligand pairs. This review describes costimulatory pathways and the development of molecules, which inhibit them in the context of transplantation research. Recent clinical data using the selective costimulation blocker, belatacept (LEA29Y), as a part of a CNI-free maintenance immunosuppressive regimen in renal transplantation is highlighted.

Costimulation requirements for antiviral CD8+ T cells differ for acute and persistent phases of polyoma virus infection

The requirement for costimulation in antiviral CD8+ T cell responses has been actively investigated for acutely resolved viral infections, but it is less defined for CD8+ T cell responses to persistent virus infection. Using mouse polyoma virus (PyV) as a model of low-level persistent virus infection, we asked whether blockade of the CD40 ligand (CD40L) and CD28 costimulatory pathways impacts the magnitude and function of the PyV-specific CD8+ T response, as well as the humoral response and viral control during acute and persistent phases of infection. Costimulation blockade or gene knockout of either CD28 or CD40L substantially dampened the magnitude of the acute CD8+ T cell response; simultaneous CD28 and CD40L blockade severely depressed the acute T cell response, altered the cell surface phenotype of PyV-specific CD8+ T cells, decreased PyV VP1-specific serum IgG titers, and resulted in an increase in viral DNA levels in multiple organs. CD28 and CD40L costimulation blockade during acute infection also diminished the memory PyV-specific CD8+ T cell response and serum IgG titer, but control of viral persistence varied between mouse strains and among organs. Interestingly, we found that CD28 and CD40L costimulation is dispensable for generating and/or maintaining PyV-specific CD8+ T cells during persistent infection; however, blockade of CD27 and CD28 costimulation in persistently infected mice caused a reduction in PyV-specific CD8+ T cells. Taken together, these data indicate that CD8+ T cells primed within the distinct microenvironments of acute vs persistent virus infection differ in their costimulation requirements.

Long-term survival of neonatal porcine islets in nonhuman primates by targeting costimulation pathways

We evaluated the ability of neonatal porcine islets to engraft and restore glucose control in pancreatectomized rhesus macaques. Although porcine islets transplanted into nonimmunosuppressed macaques were rapidly rejected by a process consistent with cellular rejection, recipients treated with a CD28-CD154 costimulation blockade regimen achieved sustained insulin independence (median survival, >140 days) without evidence of porcine endogenous retrovirus dissemination. Thus, neonatal porcine islets represent a promising solution to the crucial supply problem in clinical islet transplantation.

Patients, Pathogens, and Protective Immunity: The Relevance of Virus-Induced Alloreactivity in Transplantation

Successful transplantation requires the establishment of an ongoing state in which there is simultaneous inhibition of the undesired T cell-dependent rejection response and yet retention of the ability to develop effective cell-mediated primary and memory responses to pathogens. The complexity of attaining such a precarious state is underscored by the growing body of evidence that alloreactivity can be profoundly influenced by infections that occur before, concurrent with, or subsequent to an organ transplant. In this review, we explore the growing list of mechanisms that have been identified by which pathogen-host interactions might influence rejection, including the degeneracy of TCR recognition leading to cross-reactive immune responses, the effects of pathogens on innate immune mechanisms, and the potential impact of virally induced lymphopenia.

Immunosuppressive and Trafficking Properties of Donor Splenic and Bone Marrow Dendritic Cells

BACKGROUND

Infusion of donor dendritic cells (DC) has been shown to prolong allograft survival in a number of models. However, many regimens that utilize donor DC do not consistently produced tolerance or long-term allograft survival. We hypothesized that one factor limiting the therapeutic effect of donor DC is their relative inability to traffic to recipient peripheral lymph nodes and inhibit the function of resident alloreactive T cells.

METHODS

Donor strain DC isolated from the spleens or bone marrow of Flt3L-treated mice were transferred intravenously into recipients at the time of skin grafting. Where indicated, recipients were treated with an anti-CD40L antibody and CTLA4-Ig.

RESULTS

Infusion of donor DC together with costimulatory blockade promoted donor-specific prolongation of skin allograft survival in mice. Perhaps due to their more immature phenotype, bone marrow DC trafficked more effectively to the spleen, bone marrow, and thymus and were associated with significantly longer allograft survival than were splenic DC. Neither population of DC trafficked well to peripheral lymph nodes. Consistent with our hypothesis, splenic but not lymph node T cells from DC-treated recipients displayed donor-specific hyporesponsiveness in vitro.

CONCLUSION

These data suggest that one factor contributing to rejection following treatment with donor DC plus costimulation blockade is the persistence of donor-reactive T cells within the recipient's secondary lymphoid structures. Strategies to improve DC trafficking to these structures may enhance their therapeutic effect.

NK cells mediate costimulation blockade-resistant rejection of allogeneic stem cells during nonmyeloablative transplantation

Although T-cell CD28/CD40 costimulation blockade represents a powerful mechanism to promote immune tolerance during murine allotransplantation, it has not yet been successfully translated to clinical transplantation. We determined the impact of natural killer (NK) cells on costimulation blockade-resistant rejection of donor bone marrow. We found that NK cells represent a potent barrier to engraftment: host NK depletion led to increased donor stem cell survival, increased mixed hematopoietic chimerism and to engraftment of low doses of donor marrow (1 x 10(8)/kg) that were otherwise rejected. To understand the mechanisms of NK alloreactivity, we employed an in vivo NK-specific cytotoxicity assay. We found that an increased proportion of target cells were killed between days 2 and 8 after cell transfer, and that NK killing of parental targets was inducible: NK cells preprimed with allotargets were more efficient at their elimination upon reexposure. Finally, both transplant and in vivo NK-killing models were used to determine the contribution of LFA-1 to NK alloreactivity. Blockade of LFA-1 led to decreased NK-mediated killing, and increased alloengraftment. These results identify NK alloreactivity as an integral component to costimulation blockade-resistant rejection, and suggest that its inhibition may represent an important target in the clinical translation of tolerance-induction transplantation.

Transplant Tolerance: Converging on a Moving Target

Enthusiasm for tolerance induction has been tempered by the realization that it is more difficult to achieve clinically than was predicted by experimental models. Unlike the view that the immune response to an allograft is ordered and thus predictable, we view alloimmunity as highly plastic and molded by previous and ongoing experiences with allogeneic and environmental antigens. This implies that an individual's response to an allograft changes over time and that responses of seemingly similar individuals may vary greatly. This variability highlights the need to develop assays for monitoring the recipient immune response as well as individualized methods for therapeutic immune modulation.

Late Priming and Variability of Epitope-Specific CD8+T Cell Responses during a Persistent Virus Infection

Control of persistently infecting viruses requires that antiviral CD8(+) T cells sustain their numbers and effector function. In this study, we monitored epitope-specific CD8(+) T cells during acute and persistent phases of infection by polyoma virus, a mouse pathogen that is capable of potent oncogenicity. We identified several novel polyoma-specific CD8(+) T cell epitopes in C57BL/6 mice, a mouse strain highly resistant to polyoma virus-induced tumors. Each of these epitopes is derived from the viral T proteins, nonstructural proteins produced by both productively and nonproductively (and potentially transformed) infected cells. In contrast to CD8(+) T cell responses described in other microbial infection mouse models, we found substantial variability between epitope-specific CD8(+) T cell responses in their kinetics of expansion and contraction during acute infection, maintenance during persistent infection, as well as their expression of cytokine receptors and cytokine profiles. This epitope-dependent variability also extended to differences in maturation of functional avidity from acute to persistent infection, despite a narrowing in TCR repertoire across all three specificities. Using a novel minimal myeloablation-bone marrow chimera approach, we visualized priming of epitope-specific CD8(+) T cells during persistent virus infection. Interestingly, epitope-specific CD8(+) T cells differed in CD62L-selectin expression profiles when primed in acute or persistent phases of infection, indicating that the context of priming affects CD8(+) T cell heterogeneity. In summary, persistent polyoma virus infection both quantitatively and qualitatively shapes the antiviral CD8(+) T cell response.