Interleukin-2 is a major growth factor for activated T lymphocytes, and antibodies reacting with the Tac-chain component of the interleukin-2 receptor can prevent allograft rejection in animals. Because Tac chains are expressed only on a small fraction of activated lymphocytes, monoclonal antibodies against the interleukin-2 receptor may offer a more specific means of immunosuppression than polyclonal antilymphocyte globulin in prophylaxis against graft rejection. Therefore, we compared the immunosuppressive effect of 33B3.1, a rat monoclonal antibody against the human Tac chain, with the effect of a rabbit polyclonal antithymocyte globulin in a randomized study of 100 recipients of first renal transplants.

Injections of 33B3.1 (10 mg per day) were tolerated well, whereas major side effects in 15 of 47 patients (32 percent) receiving antithymocyte globulin required discontinuation of treatment before day 14. The incidence of rejection episodes was not statistically different in the two groups at days 14, 30, 60, and 90 after transplantation. Patient and graft survival was also equal in the two groups at one year (96 and 85 percent, respectively, in both groups), and graft function was similar. The total number of infectious episodes within the first three months was lower in the 33B3.1 group than in the antithymocyte group (47 vs. 72). The drop in peripheral-blood lymphocyte concentrations was significantly larger in the patients treated with antithymocyte globulin. The level of circulating Tac-chain—bearing lymphocytes remained below 1 percent during 33B3.1 treatment, as compared with 4 to 5 percent during antithymocyte-globulin treatment (P not significant).

We conclude that 33B3.1 is as effective as antithymocyte globulin in the prevention of renal-transplant rejection, and its use results in fewer infections and side effects. (N Engl J Med 1990; 322:1175–82.)