CD4+ T cells responsive through the indirect pathway can mediate skin graft rejection in the absence of interferon-gamma

BACKGROUND: T cells responding through the indirect pathway can induce allograft rejection, but mechanisms of rejection are not known. Interferon-y (IFN-gamma) may be an important mediator of rejection under these circumstances. METHODS: We transferred CD4+ T cells from IFN-gamma-deficient (IFN-gamma-/-) mice into SCID recipients of MHC II-deficient (MHC II-/-) skin grafts. Under these conditions, rejection can only occur via the indirect pathway and cannot be mediated by T-cell production of IFN-gamma. RESULTS: IFN-gamma-/- CD4+ T cells rejected MHC II -/- skin grafts. Flow cytometry revealed only CD4+ T cells in the recipients. Cytokine enzyme-linked immunosorbent spot assays confirmed only indirect recognition with an associated expansion of an alloreactive population of IL-2-, IL-4-, and IL-5-secreting T cells. CONCLUSION: CD4+ T cells recognizing alloantigens via the indirect pathway can mediate skin graft rejection in the absence of IFN-gamma.     

Further Analysis of the T-Cell Subsets and Pathways of Murine Cardiac Allograft Rejection

The present study examined the role of CD4+ and CD8+ T cells in cardiac allograft rejection when either the direct or indirect pathway was eliminated for the CD4+ portion of the response. To study the pathways in vivo, we used genetically altered mouse strains that lack class II antigens as either the donors or recipients for cardiac transplantation. In contrast to earlier published studies, which used different strain combinations, we found that either CD4- or CD8-depletion prolonged cardiac allograft survival moderately, but not indefinitely, in an MHC-mismatched, minor-matched combination. When the CD4+ indirect pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present and when either CD4+ or CD8+ T cells were depleted. When the CD4+ direct pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present, there was slow rejection when CD4+ T cells were eliminated, and no rejection was seen for more than 100 days when CD8+ T cells were eliminated. However, the long-surviving allografts on the recipients with only CD4+ cells and an indirect pathway did show evidence of chronic vasculopathy. Thus, either CD4+ or CD8+ T cells can mediate acute cardiac allograft rejection in these experiments when both pathways are available. In addition, CD4+ T cells can provide help for acute rejection through either the direct or indirect pathway. Finally, recipients who have only CD4+ cells and an indirect pathway do not demonstrate acute rejection, but do show evidence of chronic rejection.     

The replacement of graft endothelium by recipient-type cells conditions allograft rejection mediated by indirect pathway CD4+ T cells

BACKGROUND: Whereas the participation of alloreactive T cells sensitized by indirect allorecognition in graft rejection is well documented, the nature of recipient antigen presenting cells recognized by indirect pathway CD4+ T cells within the graft has yet to be identified. The purpose of this study was to determine the role played by graft endothelium replacement in the immune recognition of cardiac allografts rejected by indirect pathway CD4+ T cells. METHODS: Transgenic RAG2-/- mice expressing I-Ab-restricted male antigen H-Y-specific TcR were studied for their capacity to reject H-2k male cardiac allografts. Chronic vascular rejection in this model was due to the indirect recognition of H-Y antigen shed from H-2k male allograft and presented by the recipient's own I-Ab APC to transgenic T cells. RESULTS: Immunohistochemical analysis of rejected grafts revealed the presence of numerous microvascular endothelial cells (EC) that expressed recipient's I-Ab MHC class II molecules. This observation suggested that graft endothelium replacement by I-Ab-positive cells of recipient origin could stimulate the rejection of male H-2k graft by I-Ab-restricted H-Y-specific T cells. To investigate further this possibility, hearts from H-2b-into-H-2k irradiation bone marrow (BM) chimera were transplanted in transgenic recipients. A direct correlation was observed between the presence of I-Ab-positive EC within myocardial microvessels and the induction of acute rejection of chimeric H-2k male cardiac allografts transplanted in transgenic recipients. CONCLUSIONS: We conclude that graft endothelium replacement by recipient-type cells is required for the rejection of cardiac allograft mediated by indirect pathway alloreactive CD4+ T cells.     

In vivo blockade of macrophage migration inhibitory factor prevents skin graft destruction after indirect allorecognition.

BACKGROUND: The effector mechanisms that ultimately destroy transplanted tissues are poorly understood. In particular, it is not clear how CD4+ T cells primed to donor-derived determinants expressed on recipient MHC molecules (the indirect pathway) can mediate graft destruction in the absence of cognate recognition of peptide: MHC on the graft cells themselves. Macrophage migration inhibitory factor (MIF) inhibits macrophage movement and is a proinflammatory and regulatory cytokine known to be essential for development of delayed-type hypersensitivity reactions. METHODS: To test whether MIF participates in graft destruction following indirect recognition, we studied rejection of MHC-II-deficient skin grafts placed on allogeneic SCID recipients adoptively transferred with na?ve CD4+ T cells, and the recipients were treated with neutralizing anti-MIF monoclonal antibody or isotype control IgG. In this model graft rejection can only occur indirectly as the graft cells lack MHC II for recognition by the recipient CD4+ T cells. RESULTS: We found that in vivo blockade of MIF inhibited indirect CD4+ cell-mediated skin graft destruction, and markedly reduced detectable macrophages within the grafts. The neutralizing anti-MIF antibody significantly inhibited alloreactive DTH but did not prevent T cell priming or interferon-gamma release by primed T cells. CONCLUSIONS: The results strongly implicate MIF as an active participant in skin graft destruction after indirect recognition and suggest that this effect is mediated through an inhibition of macrophage migration and/or function.     

Allele-specific and peptide-dependent recognition of swine leukocyte antigen class I by human cytotoxic T-cell clones.

BACKGROUND: The T-cell mediated immune responses play a major role in xenograft rejection. However, the mechanisms behind human T-cell recognition of porcine xenoantigens remain to be elucidated. METHODS: Human CD8+ T-cell lines were generated against porcine aortic endothelial cells (PAECs) from y/y and z/z haplotypes of Yucatan inbred swine. T-cell clones were obtained by limiting dilution. The human T-cell receptor (TCR)-swine leukocyte antigen (SLA) class I interaction was characterized. RESULTS: The human CD8+ T-cell mediated direct recognition of PAECs was SLA haplotype-specific. The haplotype specificity was restricted by the SLA class I allelic polymorphism. To characterize the role of SLA-bound peptides in the human TCR-SLA class I interaction, we stripped peptides from SLA molecules by a brief acid treatment. Using z/z-specific CD8+ T cells as effectors, we demonstrated that the acid-treatment, which stripped SLA molecules of bound peptides, decreased the lysis of PAECs by 72%. Addition of peptides eluted from affinity purified z/z SLA class I molecules, but not from the irrelevant y/y SLA class I, restored the lysis of acid-treated z/z PAECs. In addition, the lysis of a human HLA class I negative cell line, 721.221, transfected with a relevant SLA class I allele derived from the z/z haplotype, was significantly increased with the addition of relevant z/z peptides. These experiments indicated that both SLA class I and bound peptides were required for recognition by human CD8+ T cells. Cloning studies identified two groups of xenoreactive T-cell clones. Group I clones recognized distinct porcine peptides in the context of SLA class I molecules, whereas group II clones recognized human endogenous cross-reactive peptides presented by SLA class I. CONCLUSIONS: Our results demonstrated that, despite the differences in MHC molecules between species, human T-cell recognition of porcine MHC is similar to direct allo-recognition, that is, human TCR recognizes xenogeneic SLA-peptide complexes.     

Stimulation of CD4+ T lymphocytes by allogeneic MHC peptides presented on autologous antigen-presenting cells. Evidence of the indirect pathway of allorecognition in some strain combinations.

A preliminary analysis of the alloantibody response to free, unconjugated class I and class II MHC peptides in several rat and mouse strains was performed, to screen for an effective interaction between the allogeneic MHC peptides and recipient MHC molecules. The PVG rat strain was noted to produce very strong, MHC-restricted, primary and secondary responses to a synthetic peptide derived from the alpha helical region of the alpha 2 domain of an RT1.C/E class I MHC molecule of the DA strain. In vitro proliferation studies demonstrated that CD4+ but not CD8+ T cells of the PVG strain responded in a recipient APC-dependent manner to the peptide, whereas the BN strain (which showed no antibody response to this peptide) gave no T cell proliferation. Immunization of PVG rats with the peptide did not influence the rejection of DA skin allografts. The relevance of these studies to the possible mechanisms of allograft rejection by an indirect pathway are discussed.     

Development of autoimmunity after skin graft rejection via an indirect alloresponse

BACKGROUND: T cell allorecognition occurs through direct contact with donor peptide: MHC complexes on graft cells and through indirect recognition of donor-derived determinants expressed by recipient MHC molecules. As both indirect allorecognition and autoantigen recognition are self-restricted, we hypothesized that chronic activation of indirectly primed T cells might result in determinant spreading to involve autoantigens, analogous to that which occurs during chronic autoimmune diseases. METHODS: We placed C57BL/6 MHC II knockout (B6 II-/-) skin grafts onto BALB/c SCID mice reconstituted with wild-type (WT) CD4+ T cells. Under these conditions the CD4+ cells could not recognize any antigen on the graft, but could respond through the indirect pathway. CD4+ cell-mediated rejection of WT B6 skin was studied to determine if autoreactivity was induced after direct allorecognition. Recall immune responses against donor- and self-stimulator cells were determined by ELISPOT and animals were tested for their ability to reject second isografts. RESULTS: WT allografts were rejected by day 14 although B6 II-/- grafts underwent delayed rejection over 4-5 weeks. CD4+ cells reisolated from the recipients of the MHC II-/- grafts, but not from the recipients of WT grafts, vigorously produced interferon-gamma and interleukin-2 in response to self, BALB/c stimulators. These autoreactive CD4+ T cells mediated rejection of a second isogenic BALB/c skin graft, demonstrating that the autoimmune response was pathogenic. CONCLUSION: Autoreactivity can develop after transplant rejection via the indirect pathway. Although the direct alloresponse is likely to be the driving force in acute graft rejection, posttransplantation induced autoimmune responses may be important elements of delayed or chronic rejection.     

The replacement of graft endothelium by recipient-type cells conditions allograft rejection mediated by indirect pathway CD4(+) T cells

BACKGROUND: Whereas the participation of alloreactive T cells sensitized by indirect allorecognition in graft rejection is well documented, the nature of recipient antigen presenting cells recognized by indirect pathway CD4(+) T cells within the graft has yet to be identified. The purpose of this study was to determine the role played by graft endothelium replacement in the immune recognition of cardiac allografts rejected by indirect pathway CD4(+) T cells. METHODS: Transgenic RAG2(-/-) mice expressing I-A(b)-restricted male antigen H-Y-specific TcR were studied for their capacity to reject H-2(k) male cardiac allografts. Chronic vascular rejection in this model was due to the indirect recognition of H-Y antigen shed from H-2(k) male allograft and presented by the recipient's own I-A(b) APC to transgenic T cells. RESULTS: Immunohistochemical analysis of rejected grafts revealed the presence of numerous microvascular endothelial cells (EC) that expressed recipient's I-A MHC class II molecules. This observation suggested that graft endothelium replacement by I-A(b)-positive cells of recipient origin could stimulate the rejection of male H-2(k) graft by I-A(b)--restricted H-Y--specific T cells. To investigate further this possibility, hearts from H-2(b)--into--H-2(k) irradiation bone marrow (BM) chimera were transplanted in transgenic recipients. A direct correlation was observed between the presence of I-A(b)-positive EC within myocardial microvessels and the induction of acute rejection of chimeric H-2(k) male cardiac allografts transplanted in transgenic recipients. CONCLUSIONS: We conclude that graft endothelium replacement by recipient-type cells is required for the rejection of cardiac allograft mediated by indirect pathway alloreactive CD4(+) T cells.     

Modulation of the major histocompatibility complex antigen and the immunogenicity of islet allografts

Modulation of major histocompatibility complex (MHC) antigen by parenchymal cells and "passenger leukocytes" is a common feature of allograft rejection. To assess its significance we have examined the fate of antigen-presenting cell (APC)-depleted pancreatic islet allografts subsequent to increasing their expression of MHC antigens by in vitro exposure to the lymphokine interferon-gamma (gIFN). While most untreated grafts survived indefinitely, gIFN-exposed grafts were acutely rejected. Using in vitro islet cell-lymphocyte coculture assays, we attempted to dissect the underlying mechanism of enhanced islet cell immunogenicity resulting from gIFN treatment. We determined that gIFN exposure did not affect the capacity of islet cells to serve as APC for T lymphocytes, however islet cell exposure to gIFN was associated with enhanced vulnerability to allogeneic cytotoxic T lymphocyte (CTL) lysis in vitro by an CD5+ (OX-19+), CD8+ (OX-8+), CD4- (W3/25-), class I-restricted CTL. On the basis of these findings, we conclude that antigenic modulation can be a decisive factor in the survival of engrafted tissues by augmenting the interaction of the graft antigens with cytolytic effector T lymphocytes.     

Allorecognition and effector pathways of islet allograft rejection in normal versus nonobese diabetic mice

Islet transplantation is becoming an accepted therapy to cure type I diabetes mellitus. The exact mechanisms of islet allograft rejection remain unclear, however. In vivo CD4(+) and CD8(+) T cell-depleting strategies and genetically altered mice that did not express MHC class I or class II antigens were used to study the allorecognition and effector pathways of islet allograft rejection in different strains of mice, including autoimmunity-prone nonobese diabetic (NOD) mice. In BALB/c mice, islet rejection depended on both CD4(+) and CD8(+) T cells. In C57BL/6 mice, CD8(+) T cells could eventually mediate islet rejection by themselves, but they produced rejection more efficiently with help from CD4(+) T cells stimulated through either the direct or indirect pathway. In C57BL/6 mice, CD4(+) T cells alone caused islet rejection when only the direct pathway was available but not when only the indirect pathway was available. In contrast, in NOD mice, CD4(+) T cells alone, with only the indirect pathway, could mediate islet and cardiac allograft rejection. These findings indicate that different mouse strains can make use of different pathways for T cell-mediated rejection of islet allografts. In addition, they demonstrate that NOD mice, which develop autoimmunity and are known to be resistant to tolerance induction, have an unusually powerful CD4(+) cell indirect mechanism that can cause rejection of both islet and cardiac allografts. These data shed light on the mechanisms of islet allograft rejection in different responder strains, including those with autoimmunity.     

Coexpression of Donor Peptide/Recipient MHC Complex and Intact Donor MHC: Evidence for a Link between the Direct and Indirect Pathways

T lymphocytes recognize foreign antigens presented by donor or recipient cells through the direct and indirect pathways respectively. This raises the question of how directly and indirectly activated T cells interact. A 4‐cell model involving the interaction of CD4⁺ and CD8⁺ T cells recognizing major histocompatibility complex (MHC) class II on recipient antigen presenting cell (APC), and MHC class I on donor APC, has been proposed. However, this would require complex co‐ordination between all the participating cell types. The semidirect pathway of alloantigen presentation suggests a simpler mechanism. Although exchange of MHC class II molecules between donor and recipient cells has been described, coexpression of recipient MHC molecules presenting donor derived allopeptides (indirect presentation) and donor MHC (direct presentation) on the same cell, a key requirement for the semidirect alloantigen presentation pathway, has not been demonstrated. We have used a mouse transplantation model to demonstrate the presence of cells expressing both donor MHC class I/II molecules, and a donor MHC class II peptide in the context of a recipient MHC class II molecule. This would allow indirectly activated CD4⁺ T cells to regulate directly activated CD4⁺ T cells, or to help directly activated CD8⁺ T cells, thus providing physical evidence for the semidirect pathway.     

Indirect Allorecognition of Mismatched Donor HLA Class II Peptides in Lung Transplant Recipients with Bronchiolitis Obliterans Syndrome

A correlation between indirect allorecognition of mismatched donor HLA class I peptides and development of bronchiolitis obliterans syndrome (BOS) after lung transplantation has been previously observed. The aim of this study was to determine whether there was a correlation between indirect allorecognition of mismatched donor HLA class II peptides and development of BOS after lung transplantation. Peripheral blood mononuclear cells from nine BOS+ and nine BOS-lung transplant recipients were cultured with synthetic peptides corresponding to the beta-chain hypervariable region of a mismatched donor HLA-DR molecule. Then, proliferative alloreactivity as well as frequency of alloreactive T cells were determined. In addition, the immunodominant epitopes from the donor HLA-DR molecules were identified in selected patients. T cells from BOS+ patients showed a dose-dependent proliferative alloreactivity against donor HLA-DR peptides that was significantly higher than that observed in BOS- patients (p=0.001). Similarly, the frequency of HLA-DR alloreactive T cells was significantly higher in BOS+ patients than in BOS- patients (p=0.001). This T-cell alloreactivity was directed against a single immunodominant HLA-DR peptide. These results suggest that indirect alloreactivity to donor HLA class II molecules may play a role in the pathogenesis of BOS after lung transplantation.     

CD8 T Cells Can Reject Major Histocompatibility Complex Class I-Deficient Skin Allografts

Following transplantation, recipient T cells can recognize and respond to donor antigens expressed directly on donor cells, and can respond to donor-derived peptides that have been processed and presented in the context of recipient MHC through the indirect pathway. Indirectly primed CD4⁺ T cells have been well studied in transplantation, but little information is available regarding whether indirectly primed CD8⁺ T cells participate in rejection. To address this, we placed MHC class I-deficient D^bK^b knockout skin grafts onto allogeneic H-2 ^k SCID recipients followed by adoptive transfer of purified H-2 ^k CD8⁺ T cells. The MHC class I-deficient grafts were rejected and only CD8⁺ T cells were detectable in the recipient lymphoid organs and in the skin grafts. Immunohistochemical analysis showed that CD8⁺ T cells were found in close proximity to vascular endothelial cells and to recipient infiltrating macrophages, suggesting specific interactions. The data demonstrate that cross-primed polyclonal CD8⁺ T cells can function as active participants in the effector phase of rejection. The findings confirm and extend previous studies using a monoclonal TCR transgenic T cell and shed light on mechanisms of acute and chronic graft injury that are potentially relevant to human transplant recipients.     

HLA class II antigen presentation by prostate cancer cells

Prostate cancer is the second most commonly diagnosed cancer in men. Recent evidence suggests that reduced expression of target protein antigens and human leukocyte antigen (HLA) molecules is the predominant immune escape mechanism of malignant prostate tumor cells. The purpose of this study was to investigate the prospect of antigen specific immunotherapy against prostate cancer via the HLA class II pathway of immune recognition. Here, we show for the first time that prostate cancer cells express HLA class II proteins that are recognized by CD4+ T cells. Prostate tumor cells transduced with class II molecules efficiently presented tumor-associated antigens/peptides to CD4+ T cells. This data suggests that malignant prostate tumors can be targeted via the HLA class II pathway, and that class II-positive tumors could be employed for direct antigen presentation, and CD4+ T-cell mediated tumor immunotherapy.Prostate Cancer and Prostatic Diseases (2008) 11, 334-341; doi:10.1038/sj.pcan.4501021; published online 16 October 2007.     

Mechanism of action of donor-specific transfusion in inducing tolerance: role of donor MHC molecules, donor co-stimulatory molecules, and indirect antigen presentation

Donor-specific transfusion (DST) can synergize with T cell co-stimulatory blockade in inducing tolerance in several transplant models, but the mechanism of action of DST is poorly characterized. This study used genetically altered mice in an established model of cardiac transplantation to study the role of MHC and co-stimulatory molecule expression on DST cells in mediating the immunomodulatory effects of DST. In addition, to examine the role of indirect antigen presentation in the effect of DST, experiments used recipient mice that do not express MHC class II molecules on peripheral antigen-presenting cells, but do have functional CD4(+) T cells (II(-)4(+)). As previously reported, treatment with DST from wild-type donors in combination with CD154 blockade induced tolerance in wild-type recipients of cardiac allografts. Tolerance in this model is also induced despite the absence of MHC class I and II, CD40, or B7 molecules on transfused cells. In contrast, eliminating the indirect pathway using II(-)4(+) recipients blocked the induction of long-term cardiac allograft survival by DST. These results indicate that the indirect antigen recognition pathway mediates the immunomodulatory effect of DST in inducing transplantation tolerance in vivo.     

Role of three quantitatively dominant endogenous peptides from HLA-DRB10401 molecules in class II specific alloreactivity

Alloreactivity remains an important barrier to organ transplantation and is caused by T cell recognition of foreign histocompatibility antigens (HAg) in two ways: (1) indirect recognition, in which processed HAg peptides are presented by self MHC like any other foreign antigen, and (2) direct recognition, where the foreign MHC itself is recognized in contravention of the T cell recognition rule of self restriction. Whereas the role of endogenous peptides in direct MHC class I specific recognition is now established, their role in class II specific direct alloreactivity remains controversial, since no defined endogenous peptide has been shown to be required for alloreactivity. That mutations resulting in defective antigen processing impair class II specific allostimulation, however, suggests that the endogenous pathway is important for class II as well as class I alloreactivity. We attempted to establish the importance of endogenous peptides for alloreactivity by identifying common sequences of peptides bound by DR molecules of an HLA-DRB10401 homozygous B cell line. Peptides corresponding to three of these (calreticulin, HLA class I and an unidentified molecule) were used to restimulate established allospecific HLA-Dw4 reactive T cell clones, as well as to sensitize allogeneic T cells de novo in vitro. Xenogeneic chinese hamster ovary (CHO) cells coexpressing the relevant DR allele together with CD80 were used as antigen presenting cells. The role of CD80 could be determined on these cells because (1) they are xenogeneic and (2) they do not express B7 family members bound by CTLA-4Ig. However, we were unable to obtain evidence for allorecognition of any of these three peptides, although alloreactive T cell clones could be obtained only following stimulation of naive T cells by CHO cells in the presence of synthetic peptide, and not after stimulation by DR/CD80-bearing transfectants alone. Therefore, it seems unlikely that the most common endogenous peptides bound to DR molecules of B cell lines are directly relevant to allorecognition and, consequently, that attempts to influence alloreactivity by peptide blockade are unlikely to succeed.     

Mechanism of allorecognition and skin graft rejection in CD28 and CD40 ligand double-deficient mice

BACKGROUND: It has been shown that simultaneous blockade of CD28- and CD40-mediated costimulatory signals significantly prolongs allograft survival. Although these results led to an expectation of the establishment of specific immunotolerant therapy for organ transplantation, it became evident that these treatments rarely resulted in indefinite allograft survival. To uncover the mechanisms underlying these costimulation blockade-resistant allograft rejections, we studied the process of allogenic skin graft rejection in CD28 and CD40 ligand (L) double-deficient (double-knockout [dKO]) mice. METHODS: Skin grafts from BALB/c or BALB.B mice were transplanted to C57BL/6 background dKO mice. The frequency of CD4+ and CD8+ T cells responding to alloantigens presented by direct or indirect pathways were defined by the use of a cytostaining assay. RESULTS: BALB/c skin grafts were rapidly rejected by dKO mice. This CD28 and CD40L independent allograft rejection was inhibited by the depletion of CD8+ T cells. In vitro studies indicated that CD8+ T cells from BALB/c skin-grafted dKO mice responded to donor antigen presented only by the direct pathway. Unlike major histocompatibility complex (MHC)-mismatched donors, allogenic skin grafts from MHC-matched donors were accepted by dKO mice. CONCLUSION: In the absence of CD28 and CD40 costimulatory signals, CD8+ T cells recognize MHC antigens by the direct pathway, resulting in the rejection of skin grafts from MHC-mismatched donors. In contrast, MHC-matched and non-MHC-mismatched donor skin grafts indefinitely survive in dKO mice. These results indicated that donor-host MHC matching may still be critical to costimulation blockade therapy for organ transplantation.     

Altered intragraft immune responses and improved renal function in MHC class II-deficient mouse kidney allografts

BACKGROUND: During renal allograft rejection, expression of MHC class II antigens is up-regulated on the parenchymal cells of the kidney. This up-regulation of MHC class II proteins may stimulate the intragraft alloimmune response by promoting their recognition by recipient CD4+ T cells. In previous studies, absence of donor MHC class II antigens did not affect skin graft survival, but resulted in prolonged survival of cardiac allografts. METHODS: To further explore the role of MHC class II antigens in kidney graft rejection, we performed vascularized kidney transplants using donor kidneys from A(beta)b-deficient mice that lack MHC class II expression. RESULTS: At 4 weeks after transplant, GFR was substantially depressed in control allografts (2.18+/-0.46 ml/min/kg) compared to nonrejecting isografts (7.98+/-1.62 ml/min/kg; P<0.01), but significantly higher in class II- allografts (4.38+/-0.60 ml/min/kg; P<0.05). Despite the improvement in renal function, class II- allograft demonstrated histologic features of acute rejection, not unlike control allografts. However, morphometric analysis at 1 week after transplantation demonstrated significantly fewer CD4+ T cells infiltrating class II- allografts (12.8+/-1.2 cells/mm2) compared to controls (25.5+/-2.6 cells/mm2; P=0.0007). Finally, the intragraft profile of cytokines was altered in class II- allografts, with significantly reduced expression of Th2 cytokine mRNA compared to controls. CONCLUSIONS: These results support a role of MHC class II antigens in the kidney regulating immune cells within the graft. Further, effector pathways triggered by class II antigens promote renal injury during rejection.