Without CD4 help, CD8 rejection of pig xenografts requires CD28 costimulation but not perforin killing

HLA-G1+ Expression in GGTA1KO Pigs Suppresses Human and Monkey Anti-Pig T, B and NK Cell Responses

The human leukocyte antigen G1 (HLA-G1), a non-classical class I major histocompatibility complex (MHC-I) protein, is a potent immunomodulatory molecule at the maternal/fetal interface and other environments to regulate the cellular immune response. We created GGTA1-/HLAG1+ pigs to explore their use as organ and cell donors that may extend xenograft survival and function in both preclinical nonhuman primate (NHP) models and future clinical trials. In the present study, HLA-G1 was expressed from the porcine ROSA26 locus by homology directed repair (HDR) mediated knock-in (KI) with simultaneous deletion of α-1-3-galactotransferase gene (GGTA1; GTKO) using the clustered regularly interspersed palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) (CRISPR/Cas9) gene-editing system. GTKO/HLAG1+ pigs showing immune inhibitory functions were generated through somatic cell nuclear transfer (SCNT). The presence of HLA-G1 at the ROSA26 locus and the deletion of GGTA1 were confirmed by next generation sequencing (NGS) and Sanger's sequencing. Fibroblasts from piglets, biopsies from transplantable organs, and islets were positive for HLA-G1 expression by confocal microscopy, flow cytometry, or q-PCR. The expression of cell surface HLA-G1 molecule associated with endogenous β2-microglobulin (β2m) was confirmed by staining genetically engineered cells with fluorescently labeled recombinant ILT2 protein. Fibroblasts obtained from GTKO/HLAG1+ pigs were shown to modulate the immune response by lowering IFN-γ production by T cells and proliferation of CD4+ and CD8+ T cells, B cells and natural killer (NK) cells, as well as by augmenting phosphorylation of Src homology region 2 domain-containing phosphatase-2 (SHP-2), which plays a central role in immune suppression. Islets isolated from GTKO/HLA-G1+ genetically engineered pigs and transplanted into streptozotocin-diabetic nude mice restored normoglycemia, suggesting that the expression of HLA-G1 did not interfere with their ability to reverse diabetes. The findings presented here suggest that the HLA-G1+ transgene can be stably expressed from the ROSA26 locus of non-fetal maternal tissue at the cell surface. By providing an immunomodulatory signal, expression of HLA-G1+ may extend survival of porcine pancreatic islet and organ xenografts.

CD86 Blockade in Genetically Modified Porcine Cells Delays Xenograft Rejection by Inhibiting T-Cell and NK-Cell Activation

Porcine xenografts transplanted into primates are rejected in spite of immunosuppression. Identification of the triggering mechanisms and the strategies to overcome them is crucial to achieve long-term graft survival. We hypothesized that porcine CD86 (pCD86) contributes to xenograft rejection by direct activation of host T cells and NK cells. Formerly, we designed the human chimeric molecule hCD152-hCD59 to block pCD86 in cis. To test the efficacy in vivo, we have utilized a pig-to-mouse xenotransplant model. First, we showed that hCD152-hCD59 expression prevents the binding of murine CD28Ig to pCD86 on porcine aortic endothelial cells (PAEC) and dramatically reduces IL-2 secretion by Con A-stimulated mouse splenocytes in coculture. Moreover, IFN-γ secretion by IL-12-stimulated mouse NK cells was averted after coculture with hCD152-hCD59 PAEC. In vivo, control PAEC implanted under the kidney capsule were rapidly rejected (2–4 weeks) in BALB/c and BALB/c SCID mice. Rejection of hCD152-hCD59 PAEC was significantly delayed in both cases. Signs of immune modulation in the hCD152-hCD59-PAEC BALB/c recipients were identified such as early hyporesponsiveness and diminished antibody response. Thus, simply modifying the donor xenogeneic cell can diminish both T cell and NK cell immune responses. We specifically demonstrate that pCD86 contributes to rejection of porcine xenografts.

A theranostic small interfering RNA nanoprobe protects pancreatic islet grafts from adoptively transferred immune rejection

Islet transplantation has recently emerged as an acceptable clinical modality for restoring normoglycemia in patients with type 1 diabetes mellitus (T1DM). The long-term survival and function of islet grafts is compromised by immune rejection-related factors. Downregulation of factors that mediate immune rejection using RNA interference holds promise for improving islet graft resistance to damaging factors after transplantation. Here, we used a dual-purpose therapy/imaging small interfering (si)RNA magnetic nanoparticle (MN) probe that targets β(2) microglobulin (B2M), a key component of the major histocompatibility class I complex (MHC I). In addition to serving as a siRNA carrier, this MN-siB2M probe enables monitoring of graft persistence noninvasively using magnetic resonance imaging (MRI). Human islets labeled with these MNs before transplantation into B2M (null) NOD/scid mice showed significantly improved preservation of graft volume starting at 2 weeks, as determined by longitudinal MRI in an adoptive transfer model (P < 0.05). Furthermore, animals transplanted with MN-siB2M-labeled islets demonstrated a significant delay of up to 23.8 ± 4.8 days in diabetes onset after the adoptive transfer of T cells relative to 6.5 ± 4.5 days in controls. This study demonstrated that our approach could protect pancreatic islet grafts from immune rejection and could potentially be applied to allotransplantation and prevention of the autoimmune recurrence of T1DM in islet transplantation or endogenous islets.

TNF, Pig CD86, and VCAM-1 Identified as Potential Targets for Intervention in Xenotransplantation of Pig Chondrocytes

Xenotransplantation of genetically engineered porcine chondrocytes may benefit many patients who suffer cartilage defects. In this work, we sought to elucidate the molecular bases of the cellular response to xenogeneic cartilage. To this end, we isolated pig costal chondrocytes (PCC) and conducted a series of functional studies. First, we determined by flow cytometry the cell surface expression of multiple immunoregulatory proteins in resting conditions or after treatment with human TNF-α, IL-1α, or IL-1β, which did not induce apoptosis. TNF-α and to a lesser extent IL-1α led to a marked upregulation of SLA I, VCAM-1, and ICAM-1 on PCC. SLA II and E-selectin remained undetectable in all the conditions assayed. Notably, CD86 was constitutively expressed at moderate levels, whereas CD80 and CD40 were barely detected. To assess their function, we next studied the interaction of PCC with human monoblastic U937 and Jurkat T cells. U937 cells adhered to resting and in a greater proportion to cytokine-stimulated PCC. Consistent with its expression pattern, pig VCAM-1 was key, mediating the increased adhesion after cytokine stimulation. We also conducted coculture experiments with U937 and PCC and measured the release of pig and human cytokines. Stimulated PCC secreted IL-6 and IL-8, whereas U937 secreted IL-8 in response to PCC. Finally, coculture of PCC with Jurkat in the presence of PHA led to a marked Jurkat activation as determined by the increase in IL-2 secretion. This process was dramatically reduced by blocking pig CD86. In summary, CD86 and VCAM-1 on pig chondrocytes may be important triggers of the xenogeneic cellular immune response. These molecules together with TNF could be considered potential targets for intervention in order to develop xenogeneic therapies for cartilage repair.

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

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

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.

In vitro and in vivo prevention of human CD8+ CTL-mediated xenocytotoxicity by pig c-FLIP expression in porcine endothelial cells

Overcoming cell-mediated immunity, especially of human CD8(+) CTLs, is important for the success of xenotransplantation. Our group has previously reported that the cytotoxicity of human CD8(+) CTLs against pig endothelial cells (PEC) is highly detrimental and mediated in major part by the Fas/FasL apoptotic pathway. Cellular FLICE inhibitory protein (c-FLIP) was originally identified as an inhibitor of death-receptor signaling through binding competition with caspase-8 for recruitment to Fas-associated via death domain (FADD). Two major c-FLIP variants result from alternative mRNA splicing: a short, 26-KDa protein (c-FLIP(S)) and a long, 55-KDa form (c-FLIP(L)). The cytoprotective effects of c-FLIP(S/L) in xenograft cells remain controversial. This study demonstrates that the overexpression of c-FLIP(S/L) genes markedly suppress human CD8(+) CTL-mediated xenocytotoxicity and, in addition, the cytoprotective effects of c-FLIP(L) appear to be significantly stronger than those of c-FLIP(S). Furthermore, to prove the prolonged effects of xenograft survival, PEC transfectants with c-FLIP(S/L) genes were transplanted under rat kidney capsules. Prolonged survival was elicited from FLIP(S/L) transfectants, whereas parental PEC was completely rejected through day 5, posttransplant. Thus, intracellular remodeling with the overexpression of c-FLIP(S/L) in xenograft cells may avoid innate cellular attacks against xenografts and facilitate long-term xenograft survival.

CTLA-4.FasL inhibits allogeneic responses in vivo

CTLA-4.Fas ligand (CTLA-4.FasL), a paradigmatic 'trans signal converter protein (TSCP)', can attach to APC (via CTLA-4 binding to B7) and direct intercellular inhibitory signals to responding T cells (via FasL binding to Fas receptor), converting an activating APC-to-T cell signal into an inhibitory one. Our previous studies established that CTLA-4.FasL inhibits human primary mixed lymphocyte reactions (MLR) and induces alloantigen-specific hyporesponsiveness ex vivo. The present study extends this to an in vivo context. Using splenocytes from MHC-mismatched C57BL/6 and Balb/c mice, we demonstrated that his(6)CTLA-4.FasL, effectively inhibits murine MLR. Moving in vivo, we demonstrated that subcutaneously administered his(6)CTLA-4.FasL modulates the in vivo response of infused allogeneic splenocytes. his(6)CTLA-4.FasL reduces the number of cells in each cell division, and increases the percentage of dead cells in each division. These findings are consistent with an antigen-induced cell death of the alloreactive cells, and bolsters recombinant TCSP promise as a therapeutic for transplantation diseases.

CD4+ T-Cell–Independent Rejection of Corneal Allografts

BACKGROUND

Several studies suggest that a significant number of corneal allografts undergo rejection in the absence of CD4 T cells. This study examined the role of CD4 T cell-independent mechanisms of corneal allograft rejection.

METHODS

BALB/c corneal allografts were transplanted to C57BL/6 beige nude mice that received either CD8 or CD8 T cells from C57BL/6 CD4 knockout (KO) mice that had rejected BALB/c corneal allografts. Immune effector functions of CD8 or CD8 T cells from C57BL/6 CD4 KO mice were assessed using delayed-type hypersensitivity assays and Annexin V apoptosis assays respectively. RESULTS.: Both CD8 and CD8 T cells from CD4 KO corneal allograft rejector mice mediated corneal allograft rejection following adoptive transfer to nude mice. CD8 T cells, but not CD8 T cells, from CD4 KO mice adoptively transferred donor-specific DTH and induced apoptosis of BALB/c corneal endothelial cells in vitro. Apoptosis of BALB/c corneal endothelial cells was mediated by double negative (DN) T cells, as treatment of CD8 cells from CD4 KO mice with anti-Thy 1.2 plus complement abolished their effector function.

CONCLUSION

The results support the proposition that CD4 T cell-independent rejection of corneal allografts can be mediated by either CD8 or CD8 T cells. The CD8 T cells represent a unique DN T cell population that might mediate rejection by either direct cytolysis or by inducing apoptosis of the donor corneal endothelium.

Significant Inhibition of Human CD8+ Cytotoxic T Lymphocyte-Mediated Xenocytotoxicity by Overexpression of the Human Decoy Fas Antigen

BACKGROUND

Human CD8(+) CTL-mediated killing may be important for xenograft rejection. The purpose of this study was to explore the preventing methods for CTL-mediated xenocytotoxicity by overexpression of human decoy Fas, which lacks a death domain in its cytoplasmic region, by binding competition with endogenous pig Fas. Moreover, the cytoprotective effect of this CTL-killing of membrane-bound human FasL, which is resistant to metalloproteolytic cleavage, was also assessed.

METHODS

Human CTL were generated by the stimulation of human PBMC with swine endothelial cells (SEC) and human IL-2, subsequently a CD8(+) population were selected by magnetic beads and employed as the effector cells. Stable SEC transfectants expressing either decoy Fas or membrane-bound FasL were established. Double-transfectants were also created. The amelioration of cytotoxicity to these transfectants was examined with Cr release assay. RESULTS.: Human CD8(+) CTL were highly detrimental against parental SEC. This CTL-killing was strongly inhibited by anti-FasL mAb treatment, however partial suppression was observed by Concanamycin A treatment. The overexpression of either decoy Fas or membrane-bound FasL in SEC markedly inhibited CTL-xenocytotoxicity. The double expressions of these molecules also significantly reduced this xenocytotoxicity despite the low levels of expression of either decoy Fas or membrane-bound FasL.

CONCLUSION

These findings indicate that the strong xenocytotoxicity of human CD8(+) CTL is mediated mainly by the Fas/FasL pathway. The overexpression of either decoy Fas or membrane-bound FasL were quite effective in preventing CTL-killing. Furthermore, the combined expression of both molecules in pig cells may create a window of opportunity for prolonging xenograft survival.

Antigen delivery via two molecules on the CD8- dendritic cell subset induces humoral immunity in the absence of conventional “danger”

Targeting antigen to dendritic cells (DC) in vivo might be an effective method of modulating immune responses. Given the functional specializations among DC subsets, we investigated how targeting different receptors on different DC subsets may influence antibody (Ab) production. We show here that targeting FIRE (F4/80-like receptor) or CIRE (C-type lectin receptor), two molecules expressed on the surface of immature CD8- DC in the mouse, increases Ab production 100-1000-fold over a non-targeted control. This response was equivalent to that achieved with CpG adjuvant. In contrast, targeting CD205, which is primarily expressed on CD8+ DC, did not elicit an Ab response unless an adjuvant was added. Strong Ab responses in FcRgamma-/- mice, and with the use of F(ab')2 fragments, confirmed that FIRE and CIRE targeting was due to specific rather than FcR or complement binding. Our findings may reflect differences in the ability of CD8+ and CD8- DC subsets to stimulate immune responses in vivo. Although the consensus view is that Ag presentation on DC in their steady state leads to tolerance, the Ab enhancement from FIRE and CIRE targeting in the apparent absence of any "danger" or inflammatory signal would suggest that targeting certain DC molecules can supplant the need for external adjuvants for eliciting immune responses.

Donor Lymphocyte Infusion Induces Long-Term Donor-Specific Cardiac Xenograft Survival through Activation of Recipient Double-Negative Regulatory T Cells

Previous studies have shown that pretransplant donor lymphocyte infusion (DLI) can enhance xenograft survival. However, the mechanism by which DLI induces xenograft survival remains obscure. Using T cell subset-deficient mice as recipients we show that CD4+, but not CD8+, T cells are necessary to mediate the rejection of concordant cardiac xenografts. Adoptive transfer of naive CD4+ T cells induces rejection of accepted cardiac xenografts in CD4-/- mice. This rejection can be prevented by pretransplant DLI in the absence of any other treatment. Furthermore, we demonstrate that DLI activates alphabeta-TCR+CD3+CD4-CD8- double-negative (DN) regulatory T (Treg) cells in xenograft recipients, and that DLI-activated DN Treg cells can inhibit the proliferation of donor-specific xenoreactive CD4+ T cells in vitro. More importantly, adoptive transfer of DLI-activated DN Treg cells from xenograft recipients can suppress the proliferation of xenoreactive CD4+ T cells and their ability to produce IL-2 and IFN-gamma in vivo. Adoptive transfer of DLI-activated DN Treg cells also prevents CD4+ T cell-mediated cardiac xenograft rejection in an Ag-specific fashion. These data provide direct evidence that DLI can activate recipient DN Treg cells, which can induce donor-specific long-term cardiac xenograft survival by suppressing the proliferation and function of donor-specific CD4+ T cells in vivo.

The efficacy of CD40 ligand blockade in discordant pig-to-rat islet xenotransplantation is correlated with an immunosuppressive effect of immunoglobulin

BACKGROUND

The authors' aim was to evaluate the efficacy of immunosuppression with monoclonal anti-CD40 ligand antibodies (aCD40L) or nonspecific polyclonal intravenous immunoglobulin (IVIG) in the pig-to-rat islet xenotransplantation model.

METHODS

Fetal porcine islet-like cell clusters were transplanted under the kidney capsule of nondiabetic rats. All antibodies were administered alone or in combination with cyclosporine A (CsA). In addition, some animals were administered antibodies plus tacrolimus (TAC) or sirolimus (SIR). Twelve days after transplantation, islet xenograft survival and rejection were evaluated using immunohistochemistry.

RESULTS

aCD40L plus CsA had a pronounced inhibitory effect on islet xenograft rejection for up to 12 days after transplantation. Unexpectedly, treatment with a monoclonal control antibody (anti-keyhole limpet hemocyanin [aKLH]) plus CsA had a similar inhibitory effect. Furthermore, a similar inhibition of islet xenograft rejection was observed also in animals administered IVIG plus CsA. Monotherapy with aCD40L, aKLH, IVIG, or CsA had no effect on the rejection process. Also, when aCD40L or aKLH was administered together with TAC, islet xenograft rejection was inhibited. There was no marked difference compared with rats treated with aCD40L or aKLH and CsA. Immunosuppression with aCD40L or aKLH in combination with SIR also inhibited pig-to-rat islet xenograft rejection, but the protective effect was not as pronounced.

CONCLUSIONS

Immunosuppression with high doses of antibodies, monoclonal or polyclonal, in combination with CsA or TAC inhibits pig-to-rat islet xenograft rejection. No specific effect of co-stimulatory blockade with aCD40L could be observed. Instead, the results indicate a nonspecific immunosuppressive effect of high doses of antibodies in this model.

Responses against complex antigens in various models of CD4 T-cell deficiency: surprises from an anti-CD4 antibody transgenic mouse

The most common models of CD4 T-cell deficiency are mice exogenously injected with anti-CD4 antibody (Ab), CD4 knockout (CD4-/-) and major histocompatibility complex (MHC) class II knockout (class II-/-) mice. We recently described the anti-CD4 Ab transgenic mouse (GK) as an improved CD4 cell-deficient model. This review compares this new GK mouse model with the widely available class II-/- and CD4-/- mice, when exposed to complex antigens (foreign grafts and during bacterial or viral infection). We highlight here the cytometric and functional differences (including Ab isotype, viral or bacterial clearance, and graft survival) among these CD4 cell-deficient models. For example, whereas grafts are generally rejected in class II-/- and CD4-/- mice as quickly as in wild-type mice, they survive longer in GK mice. Also, CD4-/- mice produce IgG against both simple model and complex antigens, but class II-/- and GK mice produce small amounts of IgG2a against complex antigens but not simple model antigens. These differences harbinger the caveats in the use of these various mice.

Activated macrophages require T cells for xenograft rejection under the kidney capsule

Transplantation of tissues from other species has been advocated as a way to overcome the extreme shortage of human donors. Rejection, however, remains a major hurdle for clinical xenotransplantation. Although activation of macrophages by T cells is critical for the cellular rejection of xenografts, what other important interactions between these two types of cells remain less defined. When we activated macrophages of immuno-deficient mice (SCID or Rag-/-) using interferon-gamma and lipopolysacharide, xenogeneic cells were rejected by activated macrophages in the peritoneal cavity (which has an abundance of resident macrophages), but were not rejected under the kidney capsule (which requires the recruitment of effectors). This difference between the two sites implies that activated macrophages are inefficient for self-recruitment to peripheral graft sites and that T cells may still be required for the process. To test this hypothesis further, immunodeficient mice that had received xenogeneic cells were infused with peritoneal exudate cells (containing activated macrophages and activated T cells) from preimmunized immunocompetent mice. Xenogeneic cells at both the kidney capsule and peritoneal sites were rejected soon after cell transfer. However, when the exudate cells were transferred into SCID recipients that first had been injected with T cell depleting antibodies, xenograft rejection was only prominent at the peritoneal site but not kidney capsule site. These results argue that activated macrophages (as the result of T cell activation) still require T cells for xenograft rejection at peripheral sites.

Pancreatic islet xenotransplantation: barriers and prospects

Dramatic clinical advances indicate that pancreatic islet transplants can reliably restore euglycemia in insulin-dependent patients. However, clinical success actually highlights the pronounced deficiency of allogeneic pancreata available for islet isolation. This pressing issue has revitalized ongoing efforts to develop surrogate donor sources. Xenogeneic donors form a potential alternative tissue source because they can be generated in large numbers and are amenable to genetic engineering. However, there is less understanding of the innate and adaptive immune barriers to islet xenografts relative to those encountered by allografts. Presented evidence indicates that both innate and antigen-specific adaptive immune responses significantly contribute to islet xenograft rejection. Recent evidence suggests that the capacity to induce tolerance to islet xenografts may not differ markedly from strategies used to induce allograft tolerance.

Expression of a novel murine type I IFN in the pancreatic islets induces diabetes in mice

IFN-kappa belongs to a recently identified subclass of type I IFNs. In this study, we report the cloning and preliminary characterization of the murine homologue of IFN-kappa. The gene encodes a 200-aa protein which is 38.5% homologous to human IFN-kappa. Murine IFN-kappa contains four cysteines in analogous positions to those observed in the IFN-alpha and an additional fifth unique cysteine, C174. The murine gene is located on chromosome 4, where other type I murine IFN genes, IFN-alpha and IFN-beta, are clustered. This region is syntenic with human chromosome 9 where the gene encoding IFN-kappa and the type I IFN gene cluster are found. Mouse IFN-kappa is expressed at low levels in peritoneal macrophages and its expression is up-regulated by dsRNA and IFN-gamma. Similar to previously reported transgenic mice carrying type I and type II IFNs, transgenic mice overexpressing murine IFN-kappa in the beta cells of the pancreas develop overt diabetes with hyperglycemia. Histological characterization of pancreatic islets from these transgenic mice showed inflammatory infiltrates with corresponding destruction of beta cells.

Rat nigral xenografts survive in the brain of MHC class II-, but not class I-deficient mice

We have examined the role of the indirect pathway of antigen recognition and T cells in neural xenografts rejection by using major histocompatibility complex (MHC) class II-deficient mice as xenograft recipients. Dissociated embryonic ventral mesencephalic tissue from Sprague-Dawley rats was stereotaxically injected as a cell suspension into the striatum of MHC class II-deficient adult mice as well as MHC class I-deficient and wild-type mice as controls. All of the MHC class II-deficient mice had surviving grafts in the striatum 4 weeks post-grafting. In contrast, only a few of the MHC class I-deficient mice exhibited very small grafts and none of the wild-type mice had any surviving grafts. The mean number of surviving transplanted dopamine neurons in the MHC class II-deficient group was significantly larger than that observed in the other two groups. Moderate levels of MHC class I antigen expression were seen in the transplantation sites of some animals in the MHC class II-deficient group. No helper or cytotoxic T cells were observed infiltrating into the graft sites of this group. However, there were markedly increased levels of expression of MHC class I and class II antigens, and a number of T cells infiltrating in the graft sites in both the MHC class I-deficient and wild-type groups. These results show that rat embryonic nigral tissue can survive transplantation in the brain of the MHC class II-deficient mice for at least 4 weeks without any overt signs of rejection, suggesting that the indirect pathway of foreign antigen recognition mediated by host MHC class II molecules and helper T cells plays an important role in the rejection responses to intracerebral xenografts.

Islet xenograft rejection in absence of CD8+ T cells does not require either interferon-gamma or interleukin-5

We recently demonstrated that interleukin-5 and eosinophils mediate rejection of skin allografts when CD8+ T cell-dependent and Th1-type CD4+ T cell-dependent pathways are not functional. The purpose of this study was to determine whether a similar mechanism might be operative during rejection of rat islet xenografts in mice. First, we observed that eosinophils indeed infiltrate rejected islet grafts together with CD4+ and CD8+ T cells. CD8+ T cell depletion significantly enhanced graft survival and a further prolongation of islet function was obtained in combination with interferon-gamma neutralization. However, islet rejection characterized by prominent eosinophil and macrophage infiltration still occurred in this setting. Although eosinophil infiltrates were dramatically reduced in interleukin-5 deficient mice, the ability of these animals to reject islet xenografts under CD8+ T cell depletion and interferon-gamma neutralization was similar to that of wild-type mice. We conclude that in absence of CD8+ T cells and interferon-gamma, macrophages, but not eosinophils, mediate rejection of rat-to-mouse islet xenografts.