Antibodies against functional leukocyte surface molecules in polyclonal antilymphocyte and antithymocyte globulins

Antilymphocyte or antithymocyte globulins were shown to be immunosuppressive when administered to recipients of organ transplants as prophylactic or rescue treatment of acute rejection or in patients with acute graft versus host reactions following bone marrow transplantation. Several monoclonal antibodies specific for activation or adhesion molecules of the T lymphocyte surface can also inhibit experimental or clinical allograft reactions. We have investigated the presence of some antibodies, of defined specificity and documented biological activity, in polyclonal antilymphocyte and antithymocyte globulins in order to get further insight into the mechanism of action of these polyclonal antibodies. Using a quantitative immunofluorescence assay by flow cytometry we could estimate the minimal amounts of antibodies to LFA-1 (CD11a and CD18), CD45, CD3, and CD5. Antibodies to HLA-DR, CD2, CD4, CD8, and CD25 were also demonstrated but could not be quantified. Antibodies to beta 2-microglobulin were determined by ELISA. These data suggest that interference with functional lymphocyte surface molecules may account at least in part for the immunosuppressive activity of antilymphocyte and antithymocyte globulins.