Laboratory control in predicting clinical efficacy of T cell-depletion procedures used for prevention of graft-versus-host disease: importance of limiting dilution analysis.

In an attempt to assess the usefulness of partial elimination of T lymphocytes from histocompatible donor bone marrow as a sole method of graft-versus-host disease prophylaxis in patients undergoing bone marrow transplantation, we compared in vitro and clinically the efficacy of two depletion protocols, each resulting in a different degree of T cell reduction. The protocols were based on complement-dependent T cell lysis induced either by one (T10B9) or two (T10B9 and T12A10) monoclonal antibodies, contributing respectively to 95% and 99% depletion of T cells defined functionally by limiting dilution analysis. Phenotypic analysis was unsuitable for detecting differences in the efficiency of the two depletion modalities due to very low numbers of residual OKT3-positive cells generally present. Of the 34 patients with advanced leukemias transplanted with marrow from HLA-matched sibling donors, 26 (ages 15-52) received two-antibody depleted grafts (group I) with subsequent incidence of severe acute graft-versus-host disease of 8% (in two of 24 engrafted). The T cell dose for two patients of this group whose grafts were directly evaluated by limiting dilution analysis was less than 2 x 10(5)/kg and presumably did not exceed 3.2 x 10(5)/kg for others, based on the depletion efficiency of the protocol estimated in additional small marrow samples. The remaining eight patients (ages 8-55) received one-antibody depleted grafts (group II) with 5-18 x 10(5) T cells/kg defined functionally in five grafts. Severe acute graft-versus-host disease (grade 3-4) developed in all seven engrafted subjects. Although with phenotypic analysis several grafts in this group revealed no residual T lymphocytes, they apparently carried doses of donor T cells not tolerable in a HLA-identical host unprotected by additional immunosuppression. Careful evaluation by functional T cell analysis should be considered in choosing a depletion protocol as a method for reducing the risk of graft-versus-host disease in allogeneic bone marrow transplantation.