Prevention by a platelet-derived factor (platelet factor 4) of induction of low dose tolerance to pneumococcal polysaccharides.

It was previously shown that human or mouse serum, and platelet factor 4 (PF4) prepared from human platelet releasate, counteracts nonspecific immunosuppression induced in mice by injection of concanavalin A or syngeneic gamma-irradiated lymphoma cells. The present studies show that PF4 prepared from normal mouse or human serum by absorption to heparin-agarose and elution between 0.5 and 1.5 M NaCl is also active in this respect. The ability of PF4 to counteract antigen-specific suppression of the antibody response to pneumococcal polysaccharide (pps) was now studied. PF4 derived from human or mouse serum as well as recombinant PF4 interferes with induction of antigen-specific low dose tolerance when they are injected at the same time as a low dose (0.2 microgram) of type 14 pps 3 days before an optimal immunizing dose (25 micrograms). Furthermore, injection of platelet releasate at the time of an optimal primary immunizing dose of pps type 14 enhances the secondary response to killed bacteria injected 2 weeks later, but not the primary response itself. Both effects are interpreted as due to interference with antigen-specific suppressor cell induction during primary immunization. Injection of PF4 is much less effective in reversing low dose tolerance to an optimal immunizing dose (0.1 microgram) of type 3 pps induced by injection of 0.005 microgram of this antigen. Differences in the mechanism of tolerance induction for the two pps types that might be responsible for this are discussed.