In many experimental models, heart, pancreas and kidney allografts are accepted long-term following costimulation-targeting therapies, whereas skin, lung and intestine resist the induction of tolerance under the same regimens. We noted that a common feature of the resistant organs is their constant exposure to commensal microbes and hypothesized that these microorganisms may stimulate Toll-like receptors (TLRs), promote alloresponses and prevent tolerance induction. This hypothesis prompts the predictions that TLR engagement at the time of transplantation should avert tolerance to heart allografts in animals treated with costimulation-targeting therapies, whereas inhibition of TLR signaling should promote tolerance to skin allografts under the same conditions. Indeed, engagement of a single TLR was sufficient to prevent anti-CD154-mediated long-term cardiac allograft acceptance and correlated with abolished intragraft recruitment of CD4 + /FoxP3 + regulatory T cells and the development of linked-suppression. Conversely, a lack of donor and recipient MyD88-dependent signaling led to successful skin allograft acceptance in anti-CD154-treated animals. Thus, the status of TLR signaling contributes to the resistance versus susceptibility of organs to transplantation tolerance.

Normal immune responses stimulated by pathogenic and environmental antigens generate memory T cells that react with donor antigens and no currently used immunosuppressive drug completely inhibits memory T-cell function. While donor-reactive memory T cells clearly compromise graft outcomes, mechanisms utilized by memory T cells to promote rejection are largely unknown. In this study, we investigated how early endogenous memory cells infiltrate and express effector function in cardiac allografts. Endogenous CD8 memory T cells in nonsensitized recipients distinguish syngeneic versus allogeneic cardiac allografts within 24 h of reperfusion. CD8-dependent production of IFNc and CXCL9/Mig was observed 24 to 72 h posttransplant in allografts but not isografts. CXCL9 was produced by donor cells in response to IFN-c made by recipient CD8 T cells reactive to donor class I major histocompatibility complex (MHC) molecules. Activated CD8 T cells were detected in allografts at least 3 days before donor-specific effector T cells producing IFN-c were detected in the recipient spleen. Early inflammation mediated by donor-reactive CD8 memory T cells greatly enhanced primed effector T-cell infiltration into allografts. These results suggest that strategies for optimal inhibition of alloimmunity should include neutralization of infiltrating CD8 memory T cells within a very narrow window after transplantation.

Increasing detection of acute humoral rejection (AHR)of renal allografts has generated the need for appropriate animal models to investigate underlying mechanisms. Murine recipients lacking the chemokine receptor CCR5 reject cardiac allografts with marked C3d deposition in the parenchymal capillaries and high serum donor-reactive antibody titers, features consistent with AHR. The rejection of MHC-mismatched renal allografts from A/J (H-2 a ) donors by B6.CCR5 -/-(H-2 b ) recipients was investigated. A/J renal allografts survived longer than 100 days in wild-type C57BL/6 recipients with normal blood creatinine levels (28 ± 7 lmol/L). All CCR5 -/-recipients rejected renal allografts within 21 days posttransplant (mean 13.3 ± 4 days) with elevated creatinine (90 ± 31 lmol/L). The rejected allografts had neutrophil and macrophage margination and diffuse C3d deposition in peritubular capillaries, interstitial hemorrhage and edema, and glomerular fibrin deposition. Circulating donor-reactive antibody titers were 40-fold higher in B6.CCR5 -/-versus wild-type recipients. Depletion of recipient CD8 T cells did not circumvent rejection of the renal allografts by CCR5-deficient recipients. In contrast, lMT -/-/CCR5 -/-recipients, incapable of producing antibody, did not reject most renal allografts. Collectively, these results indicate the rapid rejection of renal allografts in CCR5 -/-recipients with many histopathologic features observed during AHR of human renal allografts.

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