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

Diverse Routes of Allograft Tolerance Disruption by Memory T Cells

Memory T lymphocytes constitute a significant problem in tissue and organ transplantation due their contribution to early rejection and their relative resistance to tolerance-promoting therapies. Memory cells generated by environmental antigen exposure, as with T cells in general, harbor a high frequency of T cell receptors (TCR) spontaneously cross-reacting with allogeneic major histocompatibility complex (MHC) molecules. This phenomenon, known as ‘heterologous’ immunity, is thought to be a key barrier to transplant tolerance induction since such memory cells can potentially react directly with essentially any prospective allograft. In this review, we describe two additional concepts that expand this commonly held view of how memory cells contribute to transplant immunity and tolerance disruption. Firstly, autoimmunity is an additional response that can comprise an endogenously generated form of heterologous alloimmunity. However, unlike heterologous immunity generated as a byproduct of indiscriminate antigen sensitization, autoimmunity can generate T cells that have the unusual potential to interact with the graft either through the recognition of graft-bearing autoantigens or by their cross-reactive (heterologous) alloimmune specificity to MHC molecules. Moreover, we describe an additional pathway, independent of significant heterologous immunity, whereby immune memory to vaccine- or pathogen-induced antigens also may impair tolerance induction. This latter form of immune recognition indirectly disrupts tolerance by the licensing of naïve alloreactive T cells by vaccine/pathogen directed memory cells recognizing the same antigen-presenting cell in vivo. Thus, there appear to be recognition pathways beyond typical heterologous immunity through which memory T cells can directly or indirectly impact allograft immunity and tolerance.

Dynamics of Sendai Virus Spread, Clearance, and Immunotherapeutic Efficacy after Hematopoietic Cell Transplant Imaged Noninvasively in Mice

There are no approved vaccines or virus-specific treatments for human parainfluenza viruses (HPIVs), which have recently been reclassified into the species Human respirovirus 1, Human respirovirus 3, Human rubulavirus 2, and Human rubulavirus 4 These viruses cause morbidity and mortality in immunocompromised patients, including those undergoing hematopoietic cell transplant (HCT). No small-animal models for noninvasive imaging of respiratory virus infection in the HCT host exist, despite the utility that such a system would offer to monitor prolonged infection, its clearance, and treatment options. We used a luciferase-expressing reporter virus to noninvasively image in mice the infection of murine respirovirus (strain Sendai virus [SeV]), the murine counterpart of HPIV1. Independent of disease severity, the clearance of infection began approximately 21 days after HCT, largely due to the recovery of CD8+ T cells. Immunotherapy with granulocyte colony-stimulating factor (G-CSF) and adoptive transfer of natural killer (NK) cells provided a limited therapeutic benefit. Treatment with a fusion (F) protein-specific monoclonal antibody arrested the spread of lung infection and reduced the disease severity even when treatment was delayed to up to 10 days postinfection but had little observable effect on upper respiratory tract infection. Adoptive transfer of virus-specific T cells at 10 days postinfection accelerated the clearance by 5 days, reduced the extent of infection throughout the respiratory tract, and reduced the disease severity. Overall, the results support investigation of the clinical treatment of respiratory virus infection in the HCT host with monoclonal antibodies and adoptive T-cell transfer; the imaging system should be extendable to other respiratory viruses, such as respiratory syncytial virus and influenza virus.IMPORTANCE Parainfluenza viruses are a major cause of disease and death due to respiratory virus infection in the immunocompromised host, including those undergoing bone marrow transplantation. There are currently no effective treatment measures. We noninvasively imaged mice that were undergoing a bone marrow transplant and infected with Sendai virus, a murine parainfluenza virus (respirovirus). For the first time, we show the therapeutic windows of adoptive T-cell therapy and treatment with a monoclonal antibody to the fusion (F) protein in clearing Sendai virus from the respiratory tract and reducing disease severity. Mice tolerated these treatments without any detectable toxicity. These findings pave the way for studies assessing the safety of T-cell therapy against parainfluenza virus in humans. Adoptive T-cell therapy against other blood-borne viruses in humans has been shown to be safe and effective. Our model of noninvasive imaging in mice that had undergone a bone marrow transplant may be well suited to track other respiratory virus infections and develop novel preventive and therapeutic strategies.

Disruption of Transplant Tolerance by an "Incognito" Form of CD8 T Cell-Dependent Memory

Several approaches successfully achieve allograft tolerance in preclinical models but are challenging to translate into clinical practice. Many clinically relevant factors can attenuate allograft tolerance induction, including intrinsic genetic resistance, peritransplant infection, inflammation, and preexisting antidonor immunity. The prevailing view for immune memory as a tolerance barrier is that the host harbors memory cells that spontaneously cross-react to donor MHC antigens. Such preexisting "heterologous" memory cells have direct reactivity to donor cells and resist most tolerance regimens. In this study, we developed a model system to determine if an alternative form of immune memory could also block tolerance. We posited that host memory T cells could potentially respond to donor-derived non-MHC antigens, such as latent viral antigens or autoantigens, to which the host is immune. Results show that immunity to a model nonself antigen, ovalbumin (OVA), can dramatically disrupt tolerance despite undetectable initial reactivity to donor MHC antigens. Importantly, this blockade of tolerance was CD8⁺ T cell-dependent and required linked antigen presentation of alloantigens with the test OVA antigen. As such, this pathway represents an unapparent, or "incognito," form of immunity that is sufficient to prevent tolerance and that can be an unforeseen additional immune barrier to clinical transplant tolerance.

Alloimmune T cells in transplantation

Alloimmune T cells are central mediators of rejection and graft-versus-host disease in both solid organ and hematopoietic stem cell transplantation. Unique among immune responses in terms of its strength and diversity, the T cell alloresponse reflects extensive genetic polymorphisms between allogeneic donors and recipients, most prominently within the major histocompatibility complex (MHC), which encodes human leukocyte antigens (HLAs) in humans. The repertoire of alloreactive T cell clones is distinct for every donor-recipient pair and includes potentially thousands of unique HLA/peptide specificities. The extraordinary magnitude of the primary alloresponse and diversity of the T cell population mediating it have presented technical challenges to its study in humans. High-throughput T cell receptor sequencing approaches have opened up new possibilities for tackling many fundamental questions about this important immunologic phenomenon.

T-cell activation and transplantation tolerance

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

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

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.

Effects of lipopolysaccharide on the induction of mixed chimerism in cyclophosphamide-induced tolerance

Cyclophosphamide (CP)-induced tolerance is a mixed chimerism-based tolerance and is one of the strategies used to induce transplant tolerance. Toll-like receptor (TLR) agonists are reportedly able to abrogate the induction of tolerance by activating alloreactive T cells, or by inhibiting Treg cells. However, little is known about the effect of the immune response mediated by TLR on mixed chimerism-based tolerance protocols. In this study, we evaluated the influence of lipopolysaccharide (LPS), which is best known as an TLR4 agonist, on CP-induced tolerance. BALB/c (H-2(d)) mice received a conditioning regimen consisting of 10(8) donor DBA/2 (H-2(d)) spleen cells (SC) on day 0 and 200 mg/kg CP on day 2. A single dose of 20 microg LPS was injected on day -2, 0, 7, or 35. Our results showed that LPS infusion at any time point resulted in chronic rejection of donor skin grafts and the abrogation of mixed chimerism in 33-60% of recipients. We found a correlation between skin graft acceptance and higher levels of mixed chimerism. Flow cytometric analysis revealed that donor-reactive T cells were permanently eliminated, regardless of LPS infusion. In conclusion, LPS-infusion had little influence on the immune response of donor-reactive T cells, but had a significant effect on the induction and maintenance of mixed chimerism in CP-induced tolerance.

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.

Clinical experience with mixed chimerism to induce transplantation tolerance

Lymphohematopoietic chimerism was first shown to be associated with donor-specific allograft tolerance more than 60 years ago. However, early clinical experience with bone marrow transplantation soon revealed that conventional, myeloablative approaches were far too toxic and the risk of graft-versus-host disease too great to justify using this technology for the purpose of organ allograft tolerance induction in the absence of malignant disease. In this review, we discuss a step-wise approach that has been applied by several centers to establish less toxic approaches to using hematopoietic cell transplantation (HCT) for tolerance induction. These steps include (i) feasibility and efficacy data for tolerance induction in large animal models; (ii) safety data in clinical trials for patients with hematologic malignancies; and (iii) pilot trials of combined HCT and kidney transplantation for tolerance induction. Thus far, only one published trial conducted at the Massachusetts General Hospital in Boston has achieved long-term acceptance of human leukocyte antigen-mismatched kidney allografts without chronic immunosuppressive therapy. Alternative protocols have been successful in large animals, but long-term organ allograft tolerance has not been reported in patients. Thus, proof-of-principle that nonmyeloablative induction of mixed chimerism can be used intentionally to induce organ allograft tolerance has now been achieved. Directions for further research to make this approach applicable for a broader patient population are discussed.

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.

Manipulating the immune system for anti-tumor responses and transplant tolerance via mixed hematopoietic chimerism

SUMMARY

Stem cells (SCs) with varying potentiality have the capacity to repair injured tissues. While promising animal data have been obtained, allogeneic SCs and their progeny are subject to immune-mediated rejection. Here, we review the potential of hematopoietic stem cells (HSCs) to promote immune tolerance to allogeneic and xenogeneic organs and tissues, to reverse autoimmunity, and to be used optimally to cure hematologic malignancies. We also review the mechanisms by which hematopoietic cell transplantation (HCT) can promote anti-tumor responses and establish donor-specific transplantation tolerance. We discuss the barriers to clinical translation of animal studies and describe some recent studies indicating how they can be overcome. The recent achievements of durable mixed chimerism across human leukocyte antigen barriers without graft-versus-host disease and of organ allograft tolerance through combined kidney and bone marrow transplantation suggest that the potential of this approach for use in the treatment of many human diseases may ultimately be realized.

Type 1 IFN mediates cross-talk between innate and adaptive immunity that abrogates transplantation tolerance

TLR activation of innate immunity prevents the induction of transplantation tolerance and shortens skin allograft survival in mice treated with costimulation blockade. The mechanism by which TLR signaling mediates this effect has not been clear. We now report that administration of the TLR agonists LPS (TLR4) or polyinosinic:polycytidylic acid (TLR3) to mice treated with costimulation blockade prevents alloreactive CD8(+) T cell deletion, primes alloreactive CTLs, and shortens allograft survival. The TLR4- and MyD88-dependent pathways are required for LPS to shorten allograft survival, whereas polyinosinic:polycytidylic acid mediates its effects through a TLR3-independent pathway. These effects are all mediated by signaling through the type 1 IFN (IFN-alphabeta) receptor. Administration of IFN-beta recapitulates the detrimental effects of TLR agonists on transplantation tolerance. We conclude that the type 1 IFN generated as part of an innate immune response to TLR activation can in turn activate adaptive immune responses that abrogate transplantation tolerance. Blocking of type 1 IFN-dependent pathways in patients may improve allograft survival in the presence of exogenous TLR ligands.

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

Established memory T cell responses to a previously encountered pathogen can have a major impact on the course and outcome of a subsequent infection with an unrelated pathogen. This phenomenon, known as heterologous immunity, is dependent on the sequence of infections and can be either beneficial or detrimental to the host. Examples of heterologous immunity between unrelated viruses and alloantigens are mounting, and the role of cross-reactive T cells both in the pathogenesis of infections and in transplant rejection is now being explored. Memory T cells seem to be part of a continually evolving interactive network in which with each new infection, an alteration in the frequencies, distributions, and activities of memory cells is generated in response to previous infections and alloantigens.

Different mechanisms control peripheral and central tolerance in hematopoietic chimeric mice

Regulatory T cells (Treg) are important in peripheral tolerance, but their role in establishing and maintaining hematopoietic mixed chimerism and generating central tolerance is unclear. We now show that costimulation blockade using a donor-specific transfusion and anti-CD154 antibody applied to mice given bone marrow and simultaneously transplanted with skin allografts leads to hematopoietic chimerism and permanent skin allograft survival. Chimeric mice bearing intact skin allografts fail to generate effector/memory T cells against allogeneic targets as shown by the absence of IFNgamma-producing CD44(high)CD8+ T cells and in vivo cytotoxicity. Depletion of Tregs by injection of anti-CD4 or anti-CD25 antibody prior to costimulation blockade prevents chimerism, shortens skin allograft survival and leads to generation of effector/memory cytotoxic T cells. Depletion of Tregs by injection of anti-CD4 or anti-CD25 antibody two months after transplantation leads to loss of skin allografts even though mice remain chimeric and exhibit little in vivo cytotoxicity. In contrast, chimerism is lost, but skin allografts survive following naïve T-cell injection. We conclude that hematopoietic chimerism and peripheral tolerance may be maintained by different mechanisms in mixed hematopoietic chimeras.

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.

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.

How can we measure immunologic tolerance in humans?

Prevention and treatment of allograft rejection in organ transplant recipients relies primarily on non-antigen-specific immunosuppression, with all its associated potential hazards and costs. Currently, the status of the recipient immune response is measured by monitoring pharmacologic drug levels and clinical/pathologic evaluation of graft function. Development of reliable assays that can measure accurately the status of the immune response not only would help clinicians customize the prescription of immunosuppressive drugs in individual patients but also may allow their complete withdrawal in some patients with immunologic tolerance. Furthermore, these assays would facilitate the safe evaluation of novel tolerogenic regimens. Achieving this goal has proved to be very difficult because it requires both a more in-depth understanding of complex mechanisms of tolerance and also identification of transplant patients with acquired tolerance to an allograft that can be studied. This review discusses the current understanding of tolerance mechanisms and outlines the unique and specific challenges in development of tolerance/monitoring assays in the field of transplantation. In addition, several of the most promising candidate assays are discussed in detail.

Anti-mouse CD154 antibody treatment facilitates generation of mixed xenogeneic rat hematopoietic chimerism, prevents wasting disease and prolongs xenograft survival in mice

BACKGROUND

The induction of xenogeneic hematopoietic chimerism is an attractive approach for overcoming the host response to xenografts, but establishing xenogeneic chimerism requires severe myeloablative conditioning of the recipient. The goal of this study was to determine if co-stimulation blockade would facilitate chimerism and xenograft tolerance in irradiation-conditioned concordant recipients.

METHODS

Wistar Furth rat bone marrow (BM) cells were injected into irradiation-conditioned C57BL/6 mice with or without co-administration of anti-mouse CD154 monoclonal antibody (mAb). Chimerism was quantified by flow cytometry, and mice were transplanted with WF rat skin and islet xenografts.

RESULTS

Blockade of CD40-CD154 interaction facilitated establishment of xenogeneic chimerism in mice conditioned with 600 cGy irradiation. Anti-CD154 mAb was not required for establishment of chimerism in mice treated with 700 cGy. However, mice irradiated with 700 cGy but not treated with anti-CD154 mAb developed a "graft-versus-host disease (GVHD)-like" wasting syndrome and died, irrespective of their development of chimerism. Xenogeneic chimeras established with irradiation and anti-CD154 mAb treatment exhibited prolonged skin and, in many cases, permanent islet xenograft survival. Chimerism was unstable and eventually lost in most recipients. Skin xenografts were rejected even in mice that remained chimeric, whereas most islet xenografts survived to the end of the observation period.

CONCLUSIONS

Blockade of host CD40-CD154 interaction facilitates the establishment of xenogeneic chimerism and prevents wasting disease and death. Chimerism permits prolonged xenograft survival, but chimerism generated in this way is unstable over time. Skin xenografts are eventually rejected, whereas most islet xenografts survive long term and perhaps permanently.

TLR agonists abrogate costimulation blockade-induced prolongation of skin allografts

Costimulation blockade protocols are effective in prolonging allograft survival in animal models and are entering clinical trials, but how environmental perturbants affect graft survival remains largely unstudied. We used a costimulation blockade protocol consisting of a donor-specific transfusion and anti-CD154 mAb to address this question. We observed that lymphocytic choriomeningitis virus infection at the time of donor-specific transfusion and anti-CD154 mAb shortens allograft survival. Lymphocytic choriomeningitis virus 1) activates innate immunity, 2) induces allo-cross-reactive T cells, and 3) generates virus-specific responses, all of which may adversely affect allograft survival. To investigate the role of innate immunity, mice given costimulation blockade and skin allografts were coinjected with TLR2 (Pam3Cys), TLR3 (polyinosinic:polycytidylic acid), TLR4 (LPS), or TLR9 (CpG) agonists. Costimulation blockade prolonged skin allograft survival that was shortened after coinjection by TLR agonists. To investigate underlying mechanisms, we used "synchimeric" mice which circulate trace populations of anti-H2b transgenic alloreactive CD8+ T cells. In synchimeric mice treated with costimulation blockade, coadministration of all four TLR agonists prevented deletion of alloreactive CD8+ T cells and shortened skin allograft survival. These alloreactive CD8+ T cells 1) expressed the proliferation marker Ki-67, 2) up-regulated CD44, and 3) failed to undergo apoptosis. B6.TNFR2-/- and B6.IL-12R-/- mice treated with costimulation blockade plus LPS also exhibited short skin allograft survival whereas similarly treated B6.CD8alpha-/- and TLR4-/- mice exhibited prolonged allograft survival. We conclude that TLR signaling abrogates the effects of costimulation blockade by preventing alloreactive CD8+ T cell apoptosis through a mechanism not dependent on TNFR2 or IL-12R signaling.

Homeostatic T cell proliferation as a barrier to T cell tolerance

The maintenance of T cell numbers in the periphery is mediated by distinct homeostatic mechanisms that ensure the proper representation of naïve and memory T cells. Homeostatic proliferation refers to the process by which T cells in lymphopenic hosts divide in the absence of cognate antigen to reconstitute the peripheral lymphoid compartment. During this process T cells acquire effector-memory like properties, including the ability to respond to low doses of antigen in the absence of CD28 costimulation. Furthermore, this capacity is retained long after proliferation has ceased. Accumulating data implicates homeostatic proliferation in autoimmune diseases and transplant rejection, and suggests that it may represent a barrier to tolerance in protocols that use T cell depletion. Implementing combination therapies that aim to promote the development and expansion of regulatory T cell populations while specifically targeting alloresponsive T cells may be the soundest approach to attaining allograft tolerance in the aftermath of T cell depletion and homeostatic proliferation.

Vaccine-enhanced donor lymphocyte infusion (veDLI)

Hematopoietic stem cell or bone marrow transplantation (HSCT/BMT) is curative in many cases of hemato-logical malignancy, but the post-transplant course is often complicated by delayed immune reconstitution that predisposes to opportunistic infections and disease recurrence. Furthermore, since HLA-matched donors cannot be found for almost half of all patients that would benefit from HSCT, donors mismatched at 2-3 HLA loci are increasingly being used, which is associated with elevated rates of opportunistic infections. Donor lymphocyte infusion (DLI) is a powerful and direct approach to improve post-transplant immune function. For example, DLI using enriched antiviral cytolytic effectors (CTLs) has been shown to reconstitute cellular immunity to cytomega-lovirus (CMV) and Epstein-Barr virus (EBV) and prevent viral disease following HSCT. However, because in vitro expansion and purification of CTLs is lengthy, labor-intensive, and costly, it is rarely used clinically to prevent and treat viral infections following HSCT. Active vaccination after allogeneic transplantation to stimulate in vivo expansion of donor and/or recipient CTLs has been proposed as an alternative method to rapidly reconstitute antiviral immunity, prevent viral disease, and reduce adverse sequelae of antiviral drugs. Fortunately, recent progress has been made in developing vaccines and methodologies that are both safe and effective when administered to immunocompromised HSCT recipients.

Induction of donor-specific tolerance in sublethally irradiated recipients by gene therapy

Donor-specific transplantation tolerance can be established through the induction of molecular chimerism following reconstitution of lethally irradiated mice with autologous bone marrow expressing retrovirally transduced allogeneic MHC antigens. Here, we set out to define nonmyeloablative host conditioning regimens that would allow for establishment of molecular chimerism and the induction of donor-specific tolerance. Recipient mice received various doses of whole-body irradiation, together with costimulatory blockade using anti-CD154 monoclonal antibody prior to reconstitution with syngeneic bone marrow cells transduced with retroviruses carrying the gene encoding H-2K(b). Conditioning consisting of 3 Gy whole-body irradiation and treatment with anti-CD154 was sufficient to induce molecular chimerism resulting in stable multilineage expression of K(b) on hematopoietic cells. T cells from molecular chimeras were unable to lyse allogeneic targets expressing K(b) and contained substantially fewer K(b)-reactive IL-2- and IFN-gamma-producing CD4 T cells than controls receiving mock-transduced bone marrow. Induction of molecular chimerism using nonmyeloablative host conditioning allowed for permanent survival of K(b)-disparate allogeneic skin grafts. These data suggest that nonmyeloablative host conditioning can be used effectively to induce molecular chimerism resulting in transplantation 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.

The Generation of CD25+CD4+Regulatory T Cells That Prevent Allograft Rejection Does Not Compromise Immunity to a Viral Pathogen

In all but a small minority of cases, continued survival of solid organ grafts after transplantation depends on lifelong, nonselective immunosuppression that, although effective, results in increased rates of infection, cancer, and vascular disease. Therapeutic strategies that engage or mimic self-tolerance may allow prolonged allograft survival without the disadvantages of nonspecific immunotherapy. Pretreatment of recipient mice with donor alloantigen combined with transient modulation of the peripheral T cell pool with anti-CD4 Ab leads to the indefinite survival of MHC-incompatible cardiac allografts without further therapy. Tolerance is dependent on CD25+ CD4+ regulatory T cells that arise from naive CD25- precursors and regulate rejection via both IL-10 and CTLA-4. Although these cells are clearly effective at controlling rejection, the proven ability of recently activated CD25+ cells to mediate bystander regulation raises the possibility that tolerized individuals might also have a reduced capacity to respond to environmental pathogens. We have examined anti-influenza responses in tolerized primary heart recipients, secondary recipients following adoptive transfer of regulatory populations, and tolerized mice in which bystander regulation has been deliberately induced. Neither virus-specific CTL activity in vitro nor the clearance of virus in vivo was significantly diminished in any of these treatment groups compared with infected unmanipulated controls. The data suggest that the induction of dominant allograft tolerance dependent on regulatory T cells does not necessarily result in attenuated responses to pathogens providing further support for the development of tolerance induction protocols in clinical transplantation.

Immunocompetent T-cells with a memory-like phenotype are the dominant cell type following antibody-mediated T-cell depletion

T-cell depletion facilitates reduced immunosuppression following organ transplantation and has been suggested to be pro-tolerant. However, the characteristics of post-depletional T cells have not been evaluated as they relate to tolerance induction. We therefore studied patients undergoing profound T-cell depletion with alemtuzumab or rabbit anti-thymocyte globulin following renal transplantation, evaluating the phenotype and functional characteristics of their residual cells. Naive T cells and T cells with potential regulatory function (CD4+CD25+) were not prevalent following aggressive depletion. Rather, post-depletion T cells were of a single phenotype (CD3+CD4+CD45RA-CD62L-CCR7-) consistent with depletion-resistant effector memory T cells that expanded in the first month and were uniquely prevalent at the time of rejection. These cells were resistant to steroids, deoxyspergualin or sirolimus in vitro, but were calcineurin-inhibitor sensitive. These data demonstrate that therapeutic depletion begets a limited population of functional memory-like T cells that are easily suppressed with certain immunosuppressants, but cannot be considered uniquely pro-tolerant.

Hematopoietic chimerism and central tolerance created by peripheral-tolerance induction without myeloablative conditioning

Allogeneic hematopoietic chimerism leading to central tolerance has significant therapeutic potential. Realization of that potential has been impeded by the need for myeloablative conditioning of the host and development of graft-versus-host disease (GVHD). To surmount these impediments, we have adapted a costimulation blockade-based protocol developed for solid organ transplantation for use in stem cell transplantation. The protocol combines donor-specific transfusion (DST) with anti-CD154 mAb. When applied to stem cell transplantation, administration of DST, anti-CD154 mAb, and allogeneic bone marrow leads to hematopoietic chimerism and central tolerance with no myeloablation and no GVHD. Tolerance in this system results from deletion of both peripheral host alloreactive CD8+ T cells and nascent intrathymic alloreactive CD8+ T cells. In the absence of large numbers of host alloreactive CD8+ T cells, the transfusion that precedes transplantation need not be of donor origin, suggesting that both allospecific and non-allospecific mechanisms regulate engraftment. Agents that interfere with peripheral transplantation tolerance impair establishment of chimerism. We conclude that robust allogeneic hematopoietic chimerism and central tolerance can be established in the absence of host myeloablative conditioning using a peripheral transplantation tolerance protocol.

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.

Mechanisms of tolerance induction through the transplantation of donor hematopoietic stem cells: central versus peripheral tolerance

The transplantation of donor hematopoietic stem cells has been used successfully in numerous experimental settings to induce donor-specific tolerance. After appropriate host conditioning, hematopoietic stem-cell transplantation leads to a lasting state of donor macrochimerism that is associated with a robust form of tolerance. One of the key factors in the success of this approach is its reliance on intrathymic clonal deletion to ensure lifelong tolerization of newly developing T cells. Evidence for ongoing central deletion comes from studies following superantigen-reactive T cells and from experiments using mice transgenic for an alloreactive T-cell receptor. In protocols inducing tolerance through macrochimerism, the preexisting mature T-cell repertoire is controlled by either globally destroying all T cells before the hematopoietic cell transplantation or, in more recent models, by tolerizing it through co-stimulation blockade. The peripheral mechanisms induced by hematopoietic stem-cell transplantation and co-stimulation blockade include both extrathymic clonal deletion and the nondeletional mechanisms anergy, suppression, or both. In addition to these immunologic hurdles, a physiologic engraftment barrier has to be surmounted for the successful induction of mixed chimerism. This can be achieved by cytoreductive host treatment or by the infusion of high numbers of donor hematopoietic cells. A detailed delineation of the mechanisms responsible for tolerance induction after hematopoietic stem-cell transplantation is expected to help in the translation of these experimental protocols to clinical organ transplantation.

Blockade of CD40-mediated signaling is sufficient for inducing islet but not skin transplantation tolerance

Treatment of mice with a single donor-specific transfusion (DST) plus a brief course of anti-CD154 mAb to block CD40-mediated signaling uniformly induces donor-specific transplantation tolerance. Survival of islet allografts in treated mice is permanent, but skin grafts eventually fail unless recipients are thymectomized. The nature of the cellular mechanisms involved and the basis for the difference in survival of islet vs skin allografts are not known. In this study, we used CD40 knockout mice to investigate the role of CD40-mediated signaling in each component of the tolerance induction protocol: the DST, the graft, and the host. When CD40-mediated signaling was eliminated in only the DST or the graft, islet allografts were rapidly rejected. However, when CD40 signaling was eliminated in the host, approximately 40% of the islet allografts survived. When CD40 signaling was eliminated in the DST, the graft, and the host, islet grafts survived long term (>84 days), whereas skin allografts were rapidly rejected ( approximately 13 days). We conclude that transplantation tolerance induction in mice treated with DST and anti-CD154 mAb requires blockade of CD40-mediated signaling in the DST, the graft, and the host. Blockade of CD40-mediated signaling is necessary and sufficient for inducing islet allograft tolerance and is necessary but not sufficient for long-term skin allograft survival. We speculate that a requirement for regulatory CD4(+) T cells in skin allograft recipients could account for this differential response to tolerance induction.

Cutting edge: persistent viral infection prevents tolerance induction and escapes immune control following CD28/CD40 blockade-based regimen

A continuing concern with CD28 and/or CD40 blockade-based strategies to induce tolerance and mixed chimerism is their potential to disrupt protective immunity to preexisting infections. In this report, we find that preexisting persistent infection with lymphocytic choriomeningitis virus (LCMV) clone 13 prevents the induction of tolerance, mixed chimerism, and donor-reactive T cell deletion. Mice continue to be refractory to tolerance induction even after viremia has been resolved and virus is present only at very low levels in peripheral tissues. Conversely, we find that the full tolerance regimen, or costimulation blockade alone, specifically inhibits already ongoing antiviral immune responses, leading to an inability to control viremia. These findings suggest that ongoing T cell responses continue to depend on costimulatory interactions in the setting of a chronic infection and provide insight into potential risks following costimulation blockade posed by chronic or latent viral infections such as hepatitis C, EBV, and CMV.