Contribution of activated macrophages to the process of delayed xenograft rejection.
BACKGROUND
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Lin
Y
Y
Laboratory for Experimental Transplantation, University of Leuven, Belgium.
Vandeputte
M
M
Waer
M
M
D000818:Animals; D003183:Complement C6 / Q000378:metabolism; D006224:Cricetinae; D005677:G(M1) Ganglioside / Q000502:physiology; D006084:Graft Rejection / Q000276:immunology*; D007111:Immunity, Cellular; D007075:Immunoglobulin M / Q000276:immunology; D007555:Isoxazoles / Q000627:therapeutic use; D007694:Killer Cells, Natural / Q000276:immunology; D000077339:Leflunomide; D008262:Macrophage Activation; D008297:Male; D008647:Mesocricetus; D009569:Nitric Oxide / Q000502:physiology; D051381:Rats; D011923:Rats, Nude; D013601:T-Lymphocytes / Q000276:immunology; D013997:Time Factors; D014183:Transplantation, Heterologous
D003183:Complement C6; D007075:Immunoglobulin M; D007555:Isoxazoles; D009569:Nitric Oxide; D005677:G(M1) Ganglioside; C018835:asialo GM1 ganglioside; D000077339:Leflunomide