Thromboembolic complications after treatment with monoclonal antibody against CD40 ligand

To the editor

Exciting results reporting that blockade of the CD40/CD40 ligand pathway by monoclonal antibody against CD40 ligand led to long-term acceptance of renal allografts in monkeys were recently reported in Nature Medicine¹ and elsewhere². Our own group has previously reported that mixed chimerism and renal allograft tolerance are achieved in monkeys conditioned with a multi-modality peritransplant regimen and without long-term maintenance immunosuppression^3,4. In an effort to simplify this therapeutic regimen, we tested a monoclonal antibody against CD40 ligand (American Type Culture Collection catalog number 5C8.33) in this protocol. In the first animals treated, we encountered an unusually high incidence of thromboembolic complications, which we thought worthy of reporting.

Using our standard regimen (non-myeloablative total body irradiation, thymic irradiation, anti-thymocyte globulin, donor bone marrow infusion and a 1-month course of cyclosporine), we found no thromboembolic complications in more than 50 animals. In contrast, after adding 20 mg/kg of monoclonal antibody against CD40 ligand to the regimen on days 0 and 2, we observed four thromboembolic complications in nine recipients. These included two renal artery thromboses, one renal vein thrombosis and one superior mesenteric artery thrombosis. In subsequent animals, the addition of 100 units/kg heparin immediately before the antibody (days 0 and 2) reduced the incidence of thrombotic complications to just two (one renal artery, one renal vein) in ten recipients. We then tried additional heparin treatments (100 units/kg, days 0,1,2,3) preceded by vigorous postoperative hydration. None of the five animals tested after this change have developed thrombotic complications. These observations may be relevant to ongoing clinical trials of monoclonal antibody against CD40 ligand (humanized version of 5C8) in which some thromboembolic complications have been reported (Vincent, J. Biogen News http://www.prnewswire.com, 11/2/99 ).

CD40 ligand was originally identified on activated CD4⁺ T cells⁵, later on stimulated mast cells and basophils⁶ and most recently on activated platelets in vitro and in vivo on platelets in the process of thrombus formation⁷. CD40 is constitutively expressed on the vascular endothelium of various organs, and its ligation can upregulate adhesion molecules such as E-selectin, VCAM-1 and ICAM-1. Ligation through CD40 has also been reported to upregulate tissue factor expression on endothelial cells⁸. CD40 ligand expression has also been seen in vivo on activated platelets by examining fresh thrombi formed during vessel injury. In all fresh thrombi analyzed, CD40 ligand was expressed on a large proportion of platelets in areas in which densely packed platelets had not yet formed an amorphous mass and on platelets directly adhering to the vessel endothelium. These results may be relevant to the mechanism of a possible increased risk factor for thromboembolism induced by monoclonal antibody against CD40 ligand. Our results indicate that the administration of heparin in conjunction with monoclonal antibody against CD40 ligand can reduce the incidence of thromboembolic complications.