Contribution of activated macrophages to the process of delayed xenograft rejection

When hyperacute rejection, involving natural xenoreactive antibodies (XAb) and/or complement (C), can be prevented, xenografts (Xgs) undergo delayed xenograft rejection associated with a progressive mononuclear cell infiltration. We have previously shown that XAb formation can be totally suppressed in leflunomide (LF)-treated, T-deficient nude rats receiving hamster hearts. Hence, this model was well-suited to study a role played by other factors, e.g., natural killer (NK) cells and macrophages (Mphi). The relative contribution of Mphi to delayed xenograft rejection was investigated.

In addition to LF (20 mg/kg/24 hr p.o.), anti-asialoGM-1 serum (1 mg/48 hr i.v.) and N omega-nitro-L-arginine methyl ester (L-NAME; 100 mg/kg/24 hr i.v.) were given. Graft-infiltrating cells, deposition of cytokines (interferon-gamma [IFN-gamma] and tumor necrosis factor-alpha [TNF-alpha]), IgM and C, and expression of endothelial cell (EC) P- and E-selectins were investigated by immunohistochemistry. In some cases, rat rTNF-alpha or anti-TNF-alpha antibodies were injected intravenously.

Xgs rejected after 3 days by LF-treated rats showed an absence of IgM, C, and T cells, but the infiltration of NK cells and Mphi, together with the presence of IFN-gamma and TNF-alpha. Addition of NK cell depletion resulted in a significantly prolonged survival of Xgs (6 days; P<0.001) in which NK cells and IFN-gamma had disappeared, but Mphi were still prominent. Additional blockade of Mphi nitric oxide (NO) with L-NAME further prolonged Xg survival (11 days; P<0.001). In these rejected Xgs, Mphi, TNF-alpha, and EC expression of P- and E-selectins was still found, together with platelet thrombi, neutrophil-EC adhesion, and vessel intima lesions. The role of TNF-alpha in initiating this Xg rejection was further demonstrated by the acceleration of Xg rejection after injection of rTNF-alpha and by a synergism between L-NAME and anti-TNF-alpha antibodies in hampering the acceleration of Xg rejection seen after transfer of sensitized Mphi.

In the absence of XAb, T cells, and NK cells, Mphi can still reject Xgs. Both NO-dependent and NO-independent mechanisms are involved. In the latter case, Mphi-derived, TNF-alpha-associated EC activation may play a role.