Cell-surface binding by natural antibody (NAb) places it well for controlling cell function directly through signalling. Flow cytometry revealed an instability of syngeneic NAb binding to C3H 10T½ fibroblast variants at 37°, which could be partially reduced by H7, an inhibitor of the pivotal signalling serine/threonine kinase, protein kinase C (PKC). Cells coated with purified NAb at 4° followed by a rise in temperature to 37° showed an increase in membrane expression of introduced rat PKC-β1 and endogenous PKC-α, in the PKC-β1-overexpressing PKC-4 and v-H-ras-producing I3T2.1, respectively. Tyrosine phosphorylation of membrane-associated 60 000 MW protein including the tyrosine kinase src was markedly reduced. In addition, both the precoated NAb and numerous membrane molecules ranging from 20 000 to 220 000 MW were released into the supernatant, including the receptor-like protein tyrosine phosphatase α (RPTP-α). Furthermore, purified NAb reduced the growth of I3T2.1 cells in culture assessed as a decrease in total cell numbers and an increase in the proportion of cells in the G0/G1 phase of the cell cycle. Together, these data argue that the interaction of NAb with cell surface structures initiated a series of intracellular signalling events leading to the release of membrane molecules and over time the suppression of cell proliferation. This process could provide a biological mechanism for direct NAb control of activated cells in both physiological and pathological conditions
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