Natural killer cell- and macrophage-mediated rejection of concordant xenografts in the absence of T and B cell responses.

Hyperacute, complement-mediated xenograft (Xg) rejection is a first major hurdle for xenotransplantation. Thereafter, cellular immunity, including T and B lymphocytes, NK cells, and macrophages may become involved as well. In the present study, hamster heart Xgs were performed in Leflunomide (LF)-treated nude rats. These animals, which are genetically T cell deficient, are known to have a strong NK activity. Moreover, xenoantibody (XAb) formation by B lymphocytes was previously shown to be blocked in nude rats by LF. Hence, this model was well suited to study the role of NK cells and macrophages in Xg rejection. Despite a total suppression of XAb formation, LF-treated nude rats rejected hamster heart Xgs at a same speed (3 days) as untreated nude rats. NK cells played a major role in this rejection process. Indeed, an NK cell-dominated cellular infiltration was noticed in the rejected Xgs and the addition of an anti-NK cell serum, anti-Asialo GM-1 (ASGM-1), to the LF treatment resulted in a significant delay of Xg rejection (from 3 days to 6 days; p < 0.0001) that seemed to be mediated by activated macrophages because 1) spleens from rejecting animals showed an increased percentage of macrophages (ED1+ or ED2+) and 2) rejected Xgs were infiltrated predominantly by macrophages. Moreover, splenocytes taken from rats that rejected Xgs despite a treatment with LF + ASGM-1 provoked an acceleration of Xg rejection when transferred to newly transplanted LF + ASGM-1-treated nude rats, and finally, the latter accelerating effect disappeared when macrophages were first depleted from the adoptively transferred splenocytes. This study thus shows that NK cells and macrophages can be activated in the absence of T cells or XAbs to directly reject Xgs.