Cellular and molecular requirements for rejection of B16 melanoma in the setting of regulatory T cell depletion and homeostatic proliferation

We have recently demonstrated that adoptive transfer of regulatory T cell-depleted polyclonal T cells into lymphopenic mice leads to rejection of B16 melanoma, which generated an opportunity to study host requirements for tumor rejection when it effectively occurred. CD8(+) T cell priming and tumor rejection required tumor Ag cross-presentation, as evidenced by tumor outgrowth in Kb(-/-) bone marrow chimeric or B71/2(-/-) mice. CD4(+) T cells were additionally required for optimal tumor control, although not through classical CD4 "help," as the frequency of primed CD8(+) T cells was similar in the absence of CD4(+) T cells, and tumor rejection did not depend upon CD40-CD40L interactions or on IL-2 production by CD4(+) T cells. Rather, CD4(+) T cells appeared to act at the effector phase of tumor rejection and responded to B16-derived Ags in vitro. At the effector phase, IFN-γ production by transferred T cells, but not host cells, was necessary. IFN-γ acted either on host or tumor cells and was associated with reduced tumor vascularity. Finally, tumor rejection occurred after transfer of TNF-α, perforin, or FasL-deficient T cells. However, perforin/FasL double-knockout T cells failed to reject, arguing that the killing of B16 melanoma cells could occur either via the cytotoxic granule or Fas pathways. Collectively, these results support a model in which host tumor Ag cross-presentation primes adoptively transferred T cells, which remain functional in the setting of homeostatic proliferation and regulatory T cell depletion, and which promote tumor rejection via IFN-γ and lysis via cytotoxic granules and/or FasL.     

Fas ligand is required for resolution of granulomatous experimental autoimmune thyroiditis

We previously suggested that CD8(+) T cells promoted resolution of granulomatous experimental autoimmune thyroiditis (G-EAT) at least in part through regulation of Fas ligand (FasL) expression on thyroid epithelial cells. To directly evaluate the role of the Fas pathway in G-EAT resolution, Fas- and FasL-deficient mice on the NOD.H-2h4 background were used as recipients of activated G-EAT effector cells. When MTg-primed wild-type (WT) donor splenocytes were activated and transferred to WT recipients, thyroid lesions reached maximal severity on day 20 and resolved on day 50. Fas, FasL, and FLIP were up-regulated, and many apoptotic inflammatory cells were detected in recipient thyroids on day 20. Fas was predominantly expressed by inflammatory cells, and FasL and FLIP were mainly expressed by thyroid epithelial cells. After depletion of CD8(+) T cells, G-EAT resolution was delayed, FLIP and FasL were predominantly expressed by inflammatory cells, and few inflammatory cells were apoptotic. When WT donor splenocytes were transferred to gld recipients, disease severity on day 20 was similar to that in WT recipients, but resolution was delayed. As in CD8-depleted WT recipients, there were few apoptotic inflammatory cells, and FLIP and FasL were expressed primarily by inflammatory cells. These results indicated that the expression of functional FasL in recipient mice was critical for G-EAT resolution. WT cells induced minimal disease in lpr recipients. This was presumably because donor cells were eliminated by the increased FasL on lpr recipient cells, because donor cells were not eliminated, and the mice developed G-EAT if lpr recipients were given anti-FasL mAb.     

CD4+ T lymphocytes are not necessary for the acute rejection of vascularized mouse lung transplants

Acute rejection continues to present a major obstacle to successful lung transplantation. Although CD4(+) T lymphocytes are critical for the rejection of some solid organ grafts, the role of CD4(+) T cells in the rejection of lung allografts is largely unknown. In this study, we demonstrate in a novel model of orthotopic vascularized mouse lung transplantation that acute rejection of lung allografts is independent of CD4(+) T cell-mediated allorecognition pathways. CD4(+) T cell-independent rejection occurs in the absence of donor-derived graft-resident hematopoietic APCs. Furthermore, blockade of the CD28/B7 costimulatory pathways attenuates acute lung allograft rejection in the absence of CD4(+) T cells, but does not delay acute rejection when CD4(+) T cells are present. Our results provide new mechanistic insight into the acute rejection of lung allografts and highlight the importance of identifying differences in pathways that regulate the rejection of various organs.     

Expression and regulation of Fas and Fas ligand on thyrocytes and infiltrating cells during induction and resolution of granulomatous experimental autoimmune thyroiditis.

Granulomatous experimental autoimmune thyroiditis (G-EAT) is induced by mouse thyroglobulin-sensitized spleen cells activated in vitro with mouse thyroglobulin, anti-IL-2R, and IL-12. G-EAT lesions reach maximal severity 19-21 days after cell transfer, and lesions almost completely resolve by day 35. Depletion of CD8+ cells delays resolution and reduces Fas ligand (FasL) mRNA expression in thyroids. This study was undertaken to analyze Fas and FasL protein expression in the thyroid during induction and resolution of G-EAT and to determine whether CD8+ cells might regulate Fas or FasL expression in the thyroid. Fas and FasL expression was analyzed by immunohistochemical staining or in situ hybridization in thyroids of mice with or without depletion of CD8+ cells. Fas and FasL proteins were not detectable in normal thyroids, but expression of both proteins increased during development of G-EAT. Fas was expressed primarily by inflammatory cells; some enlarged thyrocytes were also Fas+. Thyrocytes had intense FasL immunoreactvity, and many CD8+ cells were also FasL positive. Depletion of CD8+ cells resulted in decreased FasL expression by thyrocytes and inflammatory cells, but had no effect on Fas expression. TUNEL assay detected many apoptotic inflammatory cells in proximity to thyrocytes. CD8-depleted thyroids had ongoing inflammation with fewer apoptotic infiltrating cells at day 35. Administration of a neutralizing anti-FasL mAb had no apparent effects on development of G-EAT, but anti-FasL was as effective as anti-CD8 in preventing G-EAT resolution. These results suggested that CD8+ T cells and thyrocytes may kill inflammatory cells through the Fas pathway, contributing to G-EAT resolution.     

T cell-activated macrophages are capable of both recognition and rejection of pancreatic islet xenografts

Macrophages have been proposed as the major effector cell in T cell-mediated xenograft rejection. To determine their role in this response, NOD-SCID mice were transplanted with fetal pig pancreas (FPP) before reconstitution with CD4(+) T cells from BALB/c mice. Twelve days after CD4(+) T cell reconstitution, purified macrophages (depleted of T cells) were isolated from CD4(+) T cell-reconstituted FPP recipient mice and adoptively transferred to their nonreconstituted counterparts. After adoptive macrophage transfer, FPP recipient mice transferred with macrophages from CD4(+) T cell-reconstituted mice demonstrated xenograft destruction along with massive macrophage infiltration at day 4 and complete graft destruction at day 8 postmacrophage transfer. By contrast, FPP recipients that received macrophages from nonreconstituted mice showed intact FPP xenografts with few infiltrating macrophages at both days 4 and 8 after macrophage transfer. The graft-infiltrating macrophages showed increased expression of their activation markers. Depletion of endogenous macrophages or any remaining CD4(+) T cells did not delay graft rejection in the macrophage-transferred FPP recipients, whereas depletion of transferred macrophages with clodronate liposomes prevented graft rejection. Our results show that macrophages primed by FPP and activated by CD4(+) T cells were attracted from the peripheral circulation and were capable of specific targeting and destruction of FPP xenografts. This suggests that in xenograft rejection, there are macrophage-specific recognition and targeting signals that are independent of those received by T cells.     

Effector and memory CD8+ T cells can be generated in response to alloantigen independently of CD4+ T cell help

There is now considerable evidence suggesting that CD8(+) T cells are able to generate effector but not functional memory T cells following pathogenic infections in the absence of CD4(+) T cells. We show that following transplantation of allogeneic skin, in the absence of CD4(+) T cells, CD8(+) T cells become activated, proliferate, and expand exclusively in the draining lymph nodes and are able to infiltrate and reject skin allografts. CD44(+)CD8(+) T cells isolated 100 days after transplantation rapidly produce IFN-gamma following restimulation with alloantigen in vitro. In vivo CD44(+)CD8(+) T cells rejected donor-type skin allografts more rapidly than naive CD8(+) T cells demonstrating the ability of these putative memory T cells to mount an effective recall response in vivo. These data form the first direct demonstration that CD8(+) T cells are able to generate memory as well as effector cells in response to alloantigen during rejection in the complete absence of CD4(+) T cells. These data have important implications for the design of therapies to combat rejection and serve to reinforce the view that CD8(+) T cell responses to allografts require manipulation in addition to CD4(+) T cell responses to completely prevent the rejection of foreign organ transplants.     

"Pruning" of alloreactive CD4+ T cells using 5- (and 6-)carboxyfluorescein diacetate succinimidyl ester prolongs skin allograft survival

Removal of alloreactive cells by either thymic deletion or deletion/anergy in the periphery is regarded as crucial to the development of tolerance. Dyes, such as CFSE, that allow monitoring of cell division suggest that in vitro proliferation could be a used as a way of "pruning" alloreactive cells while retaining a normal immune repertoire with retention of memory to previously encountered pathogens. This would overcome the problems occurring as a result of therapies that use massive depletion of T cells to allow acceptance of organ transplants or bone marrow grafts. We therefore used a skin graft model of CD4-mediated T cell rejection across a major H-2 mismatch (C57BL/6 (H-2(b)) to BALB/c (H-2(d)) mice) to evaluate whether nondividing CD4(+) T cells derived from a mixed lymphocyte culture would exhibit tolerance to a skin graft from the initial stimulator strain. We demonstrate that selective removal of dividing alloreactive CD4(+) T cells resulted in marked specific prolongation of allogeneic skin graft survival, and that the nondividing CD4(+) T cells retained a broad TCR repertoire and the ability to maintain memory. This novel way of depleting alloreactive T cells may serve as a useful strategy in combination with other mechanisms to achieve transplant tolerance.     

Mature but not immature Fas ligand (CD95L)-transduced human monocyte-derived dendritic cells are protected from Fas-mediated apoptosis and can be used as killer APC

Several in vitro and animal studies have been performed to modulate the interaction of APCs and T cells by Fas (CD95/Apo-1) signaling to delete activated T cells in an Ag-specific manner. However, due to the difficulties in vector generation and low transduction frequencies, similar studies with primary human APC are still lacking. To evaluate whether Fas ligand (FasL/CD95L) expressing killer APC could be generated from primary human APC, monocyte-derived dendritic cells (DC) were transduced using the inducible Cre/Loxp adenovirus vector system. Combined transduction of DC by AdLoxpFasL and AxCANCre, but not single transduction with these vectors, resulted in dose- and time-dependent expression of FasL in >70% of mature DC (mDC), whereas <20% of immature DC (iDC) expressed FasL. In addition, transduction by AdLoxpFasL and AxCANCre induced apoptosis in >80% of iDC, whereas FasL-expressing mDC were protected from FasL/Fas (CD95/Apo-1)-mediated apoptosis despite coexpression of Fas. FasL-expressing mDC eliminated Fas(+) Jurkat T cells as well as activated primary T cells by apoptosis, whereas nonactivated primary T cells were not deleted. Induction of apoptosis in Fas(+) target cells required expression of FasL in DC and cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. Induction of apoptosis in Fas(+) target cells required expression of FasL in DC, cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. The present results demonstrate that FasL-expressing killer APC can be generated from human monocyte-derived mDC using adenoviral gene transfer. Our results support the strategy to use killer APCs as immunomodulatory cells for the treatment of autoimmune disease and allograft rejection.     

NKT cells inhibit antigen-specific effector CD8 T cell induction to skin viral proteins

We recently demonstrated that CD1d-restricted NKT cells resident in skin can inhibit CD8 T cell-mediated graft rejection of human papillomavirus E7-expressing skin through an IFN-γ-dependent mechanism. In this study, we examined the role of systemically derived NKT cells in regulating the rejection of skin grafts expressing viral proteins. In lymph nodes draining transplanted skin, Ag-specific CD8 T cell proliferation, cytokine production, and cytotoxic activity were impaired by NKT cells. NKT cell suppression was mediated via CD11c(+) dendritic cells. Inhibition of CD8 T cell function did not require Foxp3(+) regulatory T cells or NKT cell-secreted IFN-γ, IL-10, or IL-17. Thus, following skin grafting or immunization with human papillomavirus-E7 oncoprotein, NKT cells reduce the capacity of draining lymph node-resident APCs to cross-present Ag to CD8 T cell precursors, as evidenced by impaired expansion and differentiation to Ag-specific CD8 T effector cells. Therefore, in the context of viral Ag challenge in the skin, systemic NKT cells limit the capacity for effective priming of adaptive immunity.     

The Nef-mediated AIDS-like disease of CD4C/human immunodeficiency virus transgenic mice is associated with increased Fas/FasL expression on T cells and T-cell death but is not prevented in Fas-, FasL-, tumor necrosis factor receptor 1-, or interleukin-1beta-converting enzyme-deficient or Bcl2-expressing transgenic mice

CD4(+)- and CD8(+)-T-cell death is a frequent immunological dysfunction associated with the development of human AIDS. We studied a murine model of AIDS, the CD4C/HIV transgenic (Tg) mouse model, to assess the importance of the apoptotic pathway in human immunodeficiency virus type 1 (HIV-1) pathogenesis. In these Tg mice, Nef is the major determinant of the disease and is expressed in immature and mature CD4(+) T cells and in cells of the macrophage/myeloid lineage. We report here a novel AIDS-like phenotype: enhanced death, most likely by apoptosis (as assessed by 7-aminoactinomycin D and annexin V/propidium iodide staining), of Tg thymic and peripheral CD4(+) and CD8(+) T cells. The Tg CD4(+) and CD8(+) T cells were also more susceptible to cell death after activation in vitro in mixed lymph node (LN) cultures. However, activation-induced cell death was not higher in Tg than in non-Tg-purified CD4(+) T cells. In addition, expression of Fas and FasL, assessed by flow cytometry, was increased in CD4(+) and CD8(+) T cells from Tg mice compared to that of non-Tg littermates. Despite the enhanced expression of Fas and FasL on Tg CD4(+) and CD8(+) T cells, Fas (lpr/lpr) and FasL (gld/gld) mutant CD4C/HIV Tg mice developed an AIDS-like disease indistinguishable from lpr/+ and gld/+ CD4C/HIV Tg mice, including loss of CD4(+) T cells. Similarly, CD4C/HIV Tg mice homozygous for mutations of two other genes implicated in cell death (interleukin-1beta-converting enzyme [ICE], tumor necrosis factor receptor 1 [TNFR-1]) developed similar AIDS-like disease as their respective heterozygous controls. Moreover, the double-Tg mice from a cross between the Bcl2/Wehi25 and CD4C/HIV Tg mice showed no major protection against disease. These results represent genetic evidence for the dispensable role of Fas, FasL, ICE, and TNFR-1 on the development of both T-cell loss and organ disease of these Tg mice. They also provide compelling evidence on the lack of protection by Bcl2 against Tg CD4(+)-T-cell death. In view of the high resemblance between numerous phenotypes observed in the CD4C/HIV Tg mice and in human AIDS, our findings are likely to be relevant for the human disease.     

Molecular mechanisms of induction of antigen-specific allograft tolerance by intranasal peptide administration

We have previously shown that intranasal (i.n.) administration of a single MHC class II-restricted HY peptide to female mice induces tolerance to up to five additional epitopes expressed on test male grafts, a phenomenon known as linked suppression. In this study, we investigated the molecular mechanisms involved both in the induction phase following peptide administration and during linked suppression after grafting. We report that following initial i.n. administration, peptide is widely disseminated and is presented by functionally immature dendritic cells. These fail to cause optimal stimulation of the responding HY-specific CD4(+) T cells that express genes characteristic of regulatory T cells. Following i.n. peptide plus LPS administration, causing immunization, HY-specific CD4(+) T cells express genes characteristic of activated T cells. We further find that following male skin grafting, HY-specific CD8(+) T cells from peptide-treated tolerant mice display both quantitative and qualitative differences compared with similar cells from untreated mice that reject their grafts. In tolerant mice there are fewer HY-specific CD8(+) cells and they express several genes characteristic of exhausted T cells. Furthermore, associated with specific chemokine receptor and integrin expression, HY-specific CD8(+) T cells show more limited migration from the graft draining lymph node into other tissues.     

CD8+ T cells induce thyroid epithelial cell hyperplasia and fibrosis

CD8(+) T cells can be important effector cells in autoimmune inflammation, generally because they can damage target cells by cytotoxicity. This study shows that activated CD8(+) T cells induce thyroid epithelial cell hyperplasia and proliferation and fibrosis in IFN-γ(-/-) NOD.H-2h4 SCID mice in the absence of CD4(+) T cells. Because CD8(+) T cells induce proliferation rather than cytotoxicity of target cells, these results describe a novel function for CD8(+) T cells in autoimmune disease. In contrast to the ability of purified CD8(+) T cells to induce thyrocyte proliferation, CD4(+) T cells or CD8 T cell-depleted splenocytes induced only mild thyroid lesions in SCID recipients. T cells in both spleens and thyroids highly produce TNF-α. TNF-α promotes proliferation of thyrocytes in vitro, and anti-TNF-α inhibits development of thyroid epithelial cell hyperplasia and proliferation in SCID recipients of IFN-γ(-/-) splenocytes. This suggests that targeting CD8(+) T cells and/or TNF-α may be effective for treating epithelial cell hyperplasia and fibrosis.     

CD4+/CD25+ regulatory cells inhibit activation of tumor-primed CD4+ T cells with IFN-gamma-dependent antiangiogenic activity, as well as long-lasting tumor immunity elicited by peptide vaccination

CD25(+) regulatory T (T reg) cells suppress the activation/proliferation of other CD4(+) or CD8(+) T cells in vitro. Also, down-regulation of CD25(+) T reg cells enhance antitumor immune responses. In this study, we show that depletion of CD25(+) T reg cells allows the host to induce both CD4(+) and CD8(+) antitumoral responses following tumor challenge. Simultaneous depletion of CD25(+) and CD8(+) cells, as well as adoptive transfer experiments, revealed that tumor-specific CD4(+) T cells, which emerged in the absence of CD25(+) T reg cells, were able to reject CT26 colon cancer cells, a MHC class II-negative tumor. The antitumoral effect mediated by CD4(+) T cells was dependent on IFN-gamma production, which exerted a potent antiangiogenic activity. The capacity of the host to mount this antitumor response is lost once the number of CD25(+) T reg cells is restored over time. However, CD25(+) T reg cell depletion before immunization with AH1 (a cytotoxic T cell determinant from CT26 tumor cells) permits the induction of a long-lasting antitumoral immune response, not observed if immunization is conducted in the presence of regulatory cells. A study of the effect of different levels of depletion of CD25(+) T reg cells before immunization with the peptide AH1 alone, or in combination with a Th determinant, unraveled that Th cells play an important role in overcoming the suppressive effect of CD25(+) T reg on the induction of long-lasting cellular immune responses.     

Gammadelta T cells promote a Th1 response during coxsackievirus B3 infection in vivo: role of Fas and Fas ligand

Fas/Fas ligand (FasL) interactions regulate disease outcome in coxsackievirus B3 (CVB3)-induced myocarditis. MRL(+/+) mice infected with CVB3 develop severe myocarditis, a dominant CD4(+) Th1 (gamma interferon [IFN-gamma(+)]) response to the virus, and a predominance of gammadelta T cells in the myocardial infiltrates. MRL lpr/lpr and MRL gld/gld mice, which lack normal expression of Fas and express a mutated FasL, respectively, have minimal myocarditis and show a dominant CD4(+) Th2 (interleukin-4 [IL-4(+)]) phenotype to CVB3. Spleen cells from virus-infected wild-type, lpr, and gld animals proliferate equally to virus in vitro. Adoptive transfer of gammadelta T cells from hearts of CVB3-infected MRL(+/+) mice (FasL(+)) into infected MRL gld/gld recipients (FasL(-)/Fas(+)) restores both disease susceptibility and Th1 cell phenotype. However, transfer of these cells into MRL lpr/lpr recipients (FasL(+)/Fas(-)) did not promote myocarditis and the viral response remained Th2 biased. This paralleled the expression of very high surface levels of FasL by myocardial gammadelta T cells, as well as their propensity to selectively lyse Th2 virus-specific CD4(+) T cells. These results demonstrate that Fas/FasL interactions conferred by gammadelta T cells on lymphocyte subpopulations may regulate the cytokine response to CVB3 infection and pathogenicity.     

Rejection of mouse cardiac allografts by costimulation in trans

The activation of T cells by B7 costimulation in trans has been demonstrated in vitro, but the in vivo relevance is unknown. To study costimulation in trans of CD4(+) T cells in vivo, we performed cardiac transplants from B7-1/B7-2-deficient mice to recipients that do not express MHC class II molecules on peripheral APCs, but do have functional CD4(+) T cells (II(-)/4(+) mice). This model restricts the B7-dependent activation of CD4(+) T cells to costimulation in trans and excludes any contribution from indirect Ag presentation. We find that II(-)/4(+) recipients reject B7-deficient grafts as rapidly as wild-type grafts, suggesting that costimulation in trans can mediate rejection as potently as costimulation in cis. Treatment of II(-)/4(+) recipients of B7-deficient grafts with depleting Abs to CD4 or CD8 demonstrates that indirect Ag presentation to CD8(+) cells does not significantly contribute to rejection. This is the first demonstration that costimulation in trans can mediate an immune response in vivo and has important therapeutic implications.     

Characterization of primary T cell subsets mediating rejection of pancreatic islet grafts

The cellular mechanisms by which pancreatic islet grafts are rejected have not been clearly defined. In order to address the roles of CD4+ and CD8+ T cells in pancreatic islet rejection, we used an adoptive transfer model in which H-2b nude mice were reconstituted with negatively selected H-2b CD4+ or CD8+ T cell subpopulations and engrafted with fully allogeneic pancreatic islet grafts. We found that primary (unprimed) CD4+ T cells mediated the rejection of pancreatic islet grafts, whereas, primary CD8+ T cells failed to do so, even though both T cell subpopulations were competent to reject skin allografts. These data indicate that primary CD4+ T cells are necessary for rejection of allogeneic pancreatic islet grafts, whereas primary CD8+ T lymphocytes are not. Implications concerning the nature of the APC involved in the initiation of the rejection response to islet allografts and the expression of MHC Ag by pancreatic islet cells are discussed.     

B lymphocytes differentially influence acute and chronic allograft rejection in mice

The relative contributions of B lymphocytes and plasma cells during allograft rejection remain unclear. Therefore, the effects of B cell depletion on acute cardiac rejection, chronic renal rejection, and skin graft rejection were compared using CD20 or CD19 mAbs. Both CD20 and CD19 mAbs effectively depleted mature B cells, and CD19 mAb treatment depleted plasmablasts and some plasma cells. B cell depletion did not affect acute cardiac allograft rejection, although CD19 mAb treatment prevented allograft-specific IgG production. Strikingly, CD19 mAb treatment significantly reduced renal allograft rejection and abrogated allograft-specific IgG development, whereas CD20 mAb treatment did not. By contrast, B cell depletion exacerbated skin allograft rejection and augmented the proliferation of adoptively transferred alloantigen-specific CD4(+) T cells, demonstrating that B cells can also negatively regulate allograft rejection. Thereby, B cells can either positively or negatively regulate allograft rejection depending on the nature of the allograft and the intensity of the rejection response. Moreover, CD19 mAb may represent a new approach for depleting both B cells and plasma cells to concomitantly impair T cell activation, inhibit the generation of new allograft-specific Abs, or reduce preexisting allograft-specific Ab levels in transplant patients.     

Modulation of tissue-specific immune response to cardiac myosin can prolong survival of allogeneic heart transplants

The role of immune response to tissue-specific Ags in transplant rejection is poorly defined. We have previously reported that transplantation of cardiac allografts triggers a CD4(+) Th1 cell response to cardiac myosin (CM), a major contractile protein of the heart, and that pretransplant activation of proinflammatory CM-specific T cells accelerates rejection. In this study, we show that administration of CM together with IFA (CM/IFA) can prevent acute rejection of an allogeneic heart transplant. Prolongation of cardiac graft survival is associated with activation of CM- and allo-specific T cells secreting type 2 cytokines (IL-4, IL-5) and reduction of the frequency of proinflammatory IFN-gamma-secreting (type 1) alloreactive T cells. Blocking of IL-4 cytokine with Abs abrogates the prolongation. CM/IFA treatment prevents acute rejection of MHC class I-mismatched, but not fully mismatched grafts. However, if donor heart is devoid of MHC class II expression, CM-IFA administration delays rejection of fully allogeneic cardiac transplants. This finding suggests that the effect of CM modulation depends on the type (direct vs indirect) and strength of recipient's CD4(+) T cell alloresponse. Our results underscore the important role of host immunity to tissue-specific Ags in the rejection of an allograft. This study demonstrates that modulation of the immune response to a tissue-specific Ag can significantly prolong cardiac allograft survival, an observation that may have important implications for the development of novel selective immune therapies in transplantation.     

Lymphopenia-induced homeostatic proliferation of CD8+ T cells is a mechanism for effective allogeneic skin graft rejection following burn injury

Burn patients are immunocompromised yet paradoxically are able to effectively reject allogeneic skin grafts. Failure to close a massive burn wound leads to sepsis and multiple system organ failure. Immune suppression early (3 days) after burn injury is associated with glucocorticoid-mediated T cell apoptosis and anti-inflammatory cytokine responses. Using a mouse model of burn injury, we show CD8+ T cell hyperresponsiveness late (14 days) after burn injury. This is associated with a CD8+ T cell pro- and anti-inflammatory cytokine secretion profile, peripheral lymphopenia, and accumulation of a rapidly cycling, hyperresponsive memory-like CD8+CD44+ IL-7R- T cells which do not require costimulation for effective Ag response. Adoptive transfer of allospecific CD8+ T cells purified 14 days postburn results in enhanced allogeneic skin graft rejection in unburned recipient mice. Chemical blockade of glucocorticoid-induced lymphocyte apoptosis early after burn injury abolishes both the late homeostatic accumulation of CD8+ memory-like T cells and the associated enhanced proinflammatory CD8+ T cell response, but not the late enhanced CD8+ anti-inflammatory response. These data suggest a mechanism for the dynamic CD8+ T cell response following injury involving an interaction between activation, apoptosis, and cellular regeneration with broad clinical implications for allogeneic skin grafting and sepsis.