AN IL-5/EOSINOPHIL DEPENDENT PATHWAY PLAYS A CRUCIAL ROLE IN THE ACUTE REJECTION OF MHC CLASS-II INCOMPATIBLE SKIN GRAFTS.

Abstract

54 Skin grafts from MHC class II antigen disparate bm12 mice are acutely rejected by C57BL/6 mice. Cytotoxic CD4+ cells are known to be responsible for acute rejection in this setting through Fas/Fas-ligand (Fas-L) interactions. We performed skin grafts between bm12 mice and C57BL/6 Fas-L deficient (gld) mice in order to investigate the pattern of rejection in mice lacking CD4 cytotoxicity. The majority of gld mice acutely rejected bm12 grafts with a kinetic similar to that observed in wild-type mice (median survival time, Fas-L deficient vs wild-type mice: 15.6 vs 16.5 days, P=NS). In vitro CTL assays confirmed that CD4+ cells from gld animals, in contrast to wild type mice, could not mount a cytotoxic response against bm12 targets. On histology, bm12 grafts rejected by gld mice displayed a massive infiltrate by eosinophils. This led us to search for Interleukin-5, the major eosinophil growth and differentiation factor, in Fas-L deficient mice with acute rejection. T cells from bm12 sensitized gld mice secreted much higher levels of IL-5 than cells from naive animals in mixed lymphocyte reaction with donor alloantigens (median: 5063 pg/ml vs 572 pg/ml in naive animals, P<0.001). In addition, rejected grafts expressed IL-5 mRNA by RT-PCR, while no signal was present in syngeneic grafts. The effector role of IL-5 and eosinophils in the acute rejection process was demonstrated by a significant increase of skin graft survival in Fas-L deficient mice injected with a neutralizing anti-IL-5 mAb (% graft survival at day 35: 83% vs 27% in mice injected with a control mAb; P=0.027). The IL-5/eosinophil effector pathway also plays a role in the rejection of MHC class II incompatible skin grafts by recipients able to generate a normal CD4 cytotoxic activity. Indeed, the rejection of bm12 skin grafts was significantly delayed by IL-5 KO recipients as compared to wild-type animals (22.1 days vs 12.5 in wild-type recipients, p<0.001). In conclusion: 1) mice lacking alloreactive cytotoxic CD4+ cells reject MHC class II disparate allografts through an IL-5 and eosinophil dependent mechanism; 2) this process also contributes to acute rejection in normal mice.