Functional roles of graft-infiltrating lymphocytes during early-phase post-transplantation in mouse cardiac transplantation models

Early graft-infiltrating lymphocytes are not associated with graft rejection in a mouse model of skin transplantation

Graft-infiltrating lymphocytes (GILs) play an important role in promoting rejection after organ transplantation. We recently reported that GILs that accumulated up to 3 days post-transplantation did not promote rejection, whereas GILs present 3-5 days post-transplantation promoted rejection in a mouse heart transplantation model. However, the immunological behaviour of GILs in murine skin transplantation remains unclear. GILs were isolated on days 3, 5 or 7 post-transplantation from C57BL/6 (B6) allogeneic skin grafts transplanted onto BALB/c mice. BALB/c Rag2-/- γc-/- mice (BRGs) underwent B6 skin graft transplantation 10 weeks after adoptive transfer of day 3, 5, or 7 GILs. BRGs reconstituted with day 5 or 7 GILs completely rejected B6 grafts. However, when B6 grafts harvested from recipient BALB/c mice on day 5 or 7 were re-transplanted into BRGs, half of the re-transplanted day 5 grafts established long-term survival, although all re-transplanted day 7 grafts were rejected. BRGs reconstituted with day 3 GILs did not reject B6 grafts. Consistently, re-transplantation using day 3 skin grafts resulted in no rejection. Administration of anti-CD25 antibodies did not prevent the phenomenon observed for the day 3 skin grafts. Furthermore, BRGs reconstituted with splenocytes from naïve BALB/c mice immediately rejected the naïve B6 skin grafts and the re-transplanted day 3 B6 grafts, suggesting that day 3 GILs were unable to induce allograft rejection during the rejection process. In conclusion, the immunological role of GILs depends on the time since transplantation. Day 3 GILs had neither protective nor alloreactive effects in the skin transplant model.

TCF1highPD-1+Ly108+CD8+ T Cells Are Associated with Graft Preservation in Sensitized Mice Treated with Non-Fc Receptor-Binding CD3 Antibodies

The non-Fc-binding anti-CD3 Ab [anti-CD3F(ab')2] can induce graft acceptance depending on the therapeutic window in a rodent heart transplant model. The delayed protocol allows for early graft infiltration of lymphocytes, which may behave in an inhibitory manner. We investigated the most effective protocol for anti-CD3F(ab')2 in sensitized conditions to confirm the evidence for clinical application. C57BL/6 mice were sensitized with BALB/c tail skin grafts and transplanted with BALB/c heart grafts at 8-12 wk after sensitization. Fifty micrograms of anti-CD3F(ab')2 was administered daily for 5 consecutive days on days 1-5 (day 1 protocol) or days 3-7 (delayed protocol). In nonsensitized mice, the delayed protocol significantly prolonged graft survival after transplantation from BALB/c to naive B6 (median survival time [MST], >100 d). In contrast, the delayed protocol was unable to prevent graft rejection in sensitized mice (MST, 5 d). A significantly increased percentage of granzyme B+ CD8+ T cells was observed in the graft on day 3 posttransplantation in sensitized conditions. Further, the day 1 protocol significantly prolonged graft survival (MST, 18 d), even in sensitized conditions. Day 1 treatment significantly increased the percentage of Foxp3+CD25+CD4+ T cells and phenotypically changed CD8+ T cells in the graft (i.e., caused a significant increase in the proportion of Ly108+TCF1highPD-1+CD8+ T cells). In conclusion, different timings of delayed anti-CD3F(ab')2 treatment promoted allograft preservation in association with phenotypic changes in CD4+ and CD8+ T cells in the graft under sensitized conditions.

Pathways of Antigen Recognition by T Cells in Allograft Rejection

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.