Antibodies to the common leukocyte antigen (T200) inhibit an early phase in the activation of resting human B cells.

T191, a monoclonal antibody reactive with the T200 common leukocyte antigen, profoundly inhibits an early event(s) associated with alpha-immunoglobulin M (alpha IgM)/T cell replacing factor (TRF) or alpha IgM/recombinant interleukin 1 and 2 (rIL 1 and rIL 2)-induced tonsillar B cell proliferation. Kinetic analysis of T191-mediated inhibition indicated that the antibody exerts its effect within 12 to 24 hr of the initiation of cultures and rapidly loses its activity thereafter. Small resting B cells are most sensitive to T191 inhibition, whereas B cells with increasing buoyant density (presumably reflecting stages of increased activation) become progressively T191 insensitive. Analysis of RNA synthesis subsequent to alpha IgM crosslinking of surface immunoglobulin demonstrated that T191 reduced [3H]uridine incorporation by up to 38% during the first 20 hr of culture. In contrast to the effects seen with alpha IgM stimulated B cells, T191 had no inhibitory effect upon phorbol myristate acetate-induced B cell proliferation. The inhibitory effect upon B cell proliferation observed with T191 is not unique among other alpha-T200 antibodies. Four of five previously described alpha-T200 monoclonal antibodies had similar inhibitory effects (82 to 57% maximum inhibition of [3H]thymidine incorporation). However, 13.3, an alpha-T200 monoclonal antibody previously shown to block natural killer (NK) cell-mediated killing was without effect. Likewise, those antibodies capable of inhibiting B cell proliferation failed to block NK-mediated cytolysis. Antibody binding experiments together with proliferation inhibition studies suggest that all of the monoclonal antibodies tested recognized distinct epitopes on the T200 antigen. Both observations are of significance because they demonstrate that the effects seen with anti-T200 antibodies represent an interference with highly specific functional regions on the T200 molecules.