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. Early graft‐infiltrating lymphocytes played crucial roles for determining skin allograft fate depending on days post‐transplantation. Experimental models using immunodeficient mice revealed the in vivo immunological role of graft‐infiltrating cells alone.