Isolation and characterization of an outer membrane protein of Salmonella paratyphi B: a mitogen and polyclonal activator of human B lymphocytes

Salmonella paratyphi B (S. paratyphi B) has been previously characterized as a human T-independent polyclonal B cell activator. To define further the nature of the bacterial structure responsible for these properties, we studied the effects of autoclaving and enzyme treatment of S. paratyphi B on its stimulatory capacity. We found that both autoclaving and papain treatment decreased the ability of S. paratyphi B to induce B cell activation, while trypsin treatment did not affect this capacity. Neither type of treatment affected the binding of S. paratyphi B to lymphocytes, suggesting that binding and B cell stimulation are mediated by different structures. The observation that B cell stimulation was significantly reduced by papain treatment led us to attempt to purify membrane proteins so that we could investigate whether they shared the stimulating capacity of S. paratyphi B. A water-insoluble, 43-45,000 mol. wt. protein, rich in aspartic acid, glutamine, glycine, alanine and leucine, similar in mol. wt. and physicochemical chemical properties to the porins of other gram negative bacteria, was isolated and designated as outer membrane protein (OMP). This protein was equally efficient to S. paratyphi B in inducing T-independent B cell activation. By performing time-course studies of [3H]-thymidine incorporation we observed a burst of mitogenic activity after stimulation of PBL or purified B cells with both S. paratyphi B and OMP peaking at 48-96 hr of culture (compared to 96-120 hr for the PWM proliferation peak), and with a magnitude of roughly 10% of that observed after PWM stimulation. Given the fact that the proportion of B lymphocytes in PBL is 4-12%, it appears likely that the proliferation burst seen with S. paratyphi B and OMP corresponds to a mitogenic effect mainly restricted to the B cell population.