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955 MONOCLONAL ANTI-ACTIN ANTIBODIES BIND TO DIFFERENTIATION ANTIGEN ON NORMAL HUMAN LYMPHOCYTES AND PREVENT REACTIVITY TO MITOGENS

There have been conflicting claims to the presence of the cytoskeletal protein actin on the surface membrane of the normal human lymphocyte. We now report our present investigations using monoclonal murine anti-actin antibodies suggesting the presence of non-muscle cell actin on mitogen transformed human lymphocytes. This monoclonal antibody repeatedly inhibited the stimulation of normal lymphocytes by mitogens (PHA, ConA, PWM) by >90% (as measured by the incorporation of 3HdThd) when added to the culture system in a final dilution of 1:12 to 1:12,000 (p<0.0001), however a control myeloma cell line supernatant did not. The addition of purified rabbit thymus actin (RTA) to the culture system reversed this inhibition of lymphocyte proliferation induced by the anti-actin antibody. However, it was not reversed by the addition of purified chicken muscle actin (CMA), nor bovine serum albumin (BSA).These studies establish the presence of surface actin on mitogen activated lymphocytes and suggest that the surface actin molecule plays a key role in the activation of normal lymphocytes.