Blocking of mononuclear cell accumulation, cytokine production, and endothelial activation within rat cardiac allografts by CD4 monoclonal antibody therapy.

CD4 monoclonal antibody therapy prolongs allograft survival in a variety of experimental models and is currently undergoing clinical trials, though surprisingly little is known about the effects of CD4 mAb therapy on intragraft effector mechanisms that mediate rejection. We previously reported the significantly improved survival of (LEWxBN)F1 cardiac allografts in LEW rats treated for 10 days with the new CD4 mAb, BWH-4, at a dose of 700 micrograms/day, i.v., starting at the time of engraftment. Thus, CD4-treated rats showed prolongation of allograft survival to a median of 37 days (range 22 to greater than 100 days) post-Tx, compared with rejection at 7 days in untreated controls. We now report the results of detailed immunohistologic studies of allografts collected from these rats. Comparison of acutely rejecting allografts in untreated rats with well-functioning allografts collected at day 7 post-Tx from CD4-treated rats showed that CD4 mAb: (1) significantly reduced mononuclear cell infiltration, interstitial edema, hemorrhage formation and vascular and extravascular thrombosis; (2) inhibited mononuclear cell induction of receptors for IL-2 and transferrin, and upregulation of class II antigens and ICAM-1 on leukocytes and endothelial cells; (3) suppressed intragraft mononuclear cell and/or endothelial production of the cytokines IL-1, IL-2, IL-6, IFN-gamma, and TNF; and (4) blocked upregulation of endothelial tissue factor and downregulation of thrombomodulin, and consequently inhibited fibrin deposition. Studies of allografts from CD4-treated rats collected at day 30 post-Tx, prior to clinical rejection, showed a resurgence of CD4+ cells within allografts and a dense cellular immune response. We conclude that short-term CD4 mAb therapy has potent and extensive inhibitory effects on cytokine-related mononuclear cell and endothelial activation in vivo, blocking multiple afferent and efferent steps of the alloresponse.