Rabbit anti-thymocyte globulin (ATG-Fresenius) is a polyclonal anti-serum raised against the lymphoblastic T cell line Jurkat. It is used for in vivo depletion of host and donor T cells for allogeneic stem cell transplantation. After administration of 90 mg/kg prior to transplant, rabbit immunoglobulin G (IgG) remains present for 4-5 weeks, but it is unknown how long T cell-reactive antibodies persist. Therefore, we measured anti-Jurkat antibodies by flow cytometry. The detection limit for Jurkat-reactive antibodies was 0.1 microg/ml rabbit IgG; half-maximal labeling of Jurkat cells required 183 microg/ml rabbit ATG. The mean half-life of Jurkat-reactive antibodies in 7 patients was 4 days. Detectable levels persisted up to 3 weeks with antibody levels equivalent to 0.2-4.1 microg/ml rabbit ATG. Jurkat-reactive antibodies were eliminated two-fold faster than rabbit IgG, as assessed by enzyme-linked immunosorbent assay (ELISA). The results suggest that in patients pretreated with ATG before transplantation, residual anti T-cell antibodies may effectively modulate recovery of T cells generated after transplantation, thereby lowering the incidence of severe GVHD.
