T-CELL INDEPENDENCE OF MACROPHAGE AND NATURAL-KILLER-CELL INFILTRATION, CYTOKINE PRODUCTION, AND ENDOTHELIAL ACTIVATION DURING DELAYED XENOGRAFT REJECTION

Rejection of guinea pig cardiac grafts in rats depleted of complement takes place in 3-4 days and involves progressive mononuclear cell infi ltration and cytokine expression, fibrin and antibody deposition, and endothelial cell up-regulation of adhesion and procoagulant molecules, a process termed delayed xenograft rejection (DXR). The relative cont ribution of each effector mechanism and the role of T cells in this co mplex process are unknown, although small numbers of interleukin (IL) 2 receptor-positive T cells are present at the time of rejection, We i nvestigated the importance of T cells in DXR by comparing discordant x enograft responses of nude rats, which lack T cell receptor (TCR)-alph a/beta(+) cells, with those of normal Lewis rats, Nude or Lewis rats r eceiving guinea pig cardiac grafts were assigned to one of three group s: no therapy, daily administration of cobra venom factor (CVF), or sp lenectomy plus daily CVF. All untreated rats rejected their xenografts within 10-15 min, whereas grafts in complement-depicted recipients su rvived a further 3-4 days; splenectomy had no significant additional e ffect upon graft survival. Immunohistologic analysis in CVF-treated nu de recipients with or without splenectomy showed: ii) considerable leu kocyte infiltration of xenografts (mean +/- SD, 76+/-14 and 71+/-16 le ukocytes/field, respectively, at 72 hr, compared with 68+/-17 in Lewis rats), consisting largely of macrophages (>75% of total leukocytes) p lus small numbers of natural killer cells (10-20%) with no detectable B or T cells (TCR-alpha/beta or TCR-gamma/delta); (2) at least 10-fold lower levels of intragraft IgM or IgG deposition than in correspondin g Lewis recipients; and (3) considerable cytokine expression by intrag raft macrophages (IL-12, tumor necrosis factor-alpha, monocyte chemoat tractant protein-1, IL-1 beta, IL-6, IL-7, IL-12) and natural killer c ells (interferon-gamma), as well as up-regulation of tissue factor exp ression and dense fibrin deposition. Analysis of recipient sera of bot h control and nude rats by ELISA, for the binding of IgG or IgM to gui nea pig platelets, showed a rapid rise after transplantation in the ti ters of IgM and IgG antibodies, which was abrogated by prior splenecto my; i.e., data from splenectomized xenograft recipients reflect the pr esence of only basal levels of IgM and IgG. Thus, our data in nude rat s show rejection times and intragraft features of DXR comparable to th ose in immunocompetent Lewis recipients, despite a lack of detectable host T cells, and, in the case of splenectomized rats, only about one tenth of normal xenoreactive antibody levels. Our data document a new model in which to analyze the immunopathogenesis of DXR.