Immunofluorescent titration of thymocyte reactive membrane antibodies as a suggested in vitro correlate of the immunosuppressive potency of anti-human thymocyte globulin.

Continued utilization of antilymphocyte and antithymocyte sera for clinical use as an immunosuppressant emphasizes the need for a reproducible in vitro assay that will correlate closely with graft protection in vivo. In this regard the in vitro serologic reactivity of antibodies in three preparations of horse anti-human thymocyte globulin (HAHTG) to cell surface antigens of human thymocytes (thymic lymphocytes) as determined by agglutination, cytotoxicity and in particular for the present study by immunofluorescence (IF) have been compared with their in vivo immunosuppressive potency as measured by prolongation of graft survival. Titration by IF of antibodies in HAHTG reactive with surface antigens of cells of the thymolymphatic system appeared to provide a reasonably simple, reliable and complement-independent in vitro correlate of immunosuppressive potency.