Background

Hyperacute xenograft rejection is affected by activation of the complement cascade. Split products of early complement components influence the localization, activation, and effector function of platelets, granulocytes, and lymphocytes, whereas the formation of the membrane attack complex (C5b-9) leads to direct cellular injury. In a unique strain of PVG rats deficient in the C6 component of complement, the terminal membrane attack complex is not formed. However, production of the chemotactic and vasoactive components C3a and C5a proceeds normally. Guinea pig cardiac xenografts in these C6-deficient rats have prolonged survival, and at the time of rejection the inflammatory infiltrate is composed primarily of neutrophils. NPC 15669, a member of a class of antiinflammatory agents called leumedins, is known to inhibit neutrophil adhesion. The purpose of this study was to determine whether inhibition of neutrophil recruitment in animals incapable of membrane attack complex formation would prolong cardiac xenograft survival.

Methods

Cardiac xenografts from male Hartley guinea pigs were heterotopically grafted into PVG (C-) and PVG (C+) male rats. Experimental animals received 20 mg/kg of NPC 15669 iv before cross-clamp release and 10 mg/kg of NPC 15669 intravenously on postoperative day 1. Control animals received intravenous saline solution only.

Results

Complement sufficient PVG (C+) rats rejected cardiac xenografts hyperacutely despite mode of treatment: PVG (C+) rats which received saline solution (n= 5) rejected their xenografts at 10.8+/-2.6 minutes, and those receiving NPC 15669 (n= 5) rejected at 13.9+/-5.3 minutes. Histologic examination showed edema, platelet aggregation, and hemorrhage but no cellular inflammatory infiltrate. As expected, complement-deficient PVG (C-) rats had markedly longer xenograft survival in the saline solution-treated group (n= 5) with graft function being sustained 14.7+/-6.1 hours. NPC 15669 treatment (n= 4) further prolonged graft function to 61.0+/-4.7 hours. In addition to edema, platelet aggregation, and hemorrhage, histologic analysis of these grafts at the time of rejection was characterized by an infiltration of neutrophils.

Conclusions

We conclude that neutrophils play a critical role in cardiac xenograft rejection when complement activation is restricted. Combined inhibition of complement and neutrophil adhesion prolongs xenograft survival longer than inhibition of either component alone.