Mitogenic properties of the thymus‐independent antigen pneumococcal polysaccharide siii

Abstract

Abstract Pneumococcal polysaccharide type SIII was found to stimulate DNA synthesis in nonsensitized spleen cells cultivated in vitro , peak activation occurring 2 days after the addition of 150 μg/ml of SIII. Cortisone‐resistant thymocytes did not respond, whereas spleen cells from congenitally athymic (nude) mice reacted as normal spleen cells. SIII induced polyclonal antibody synthesis in normal spleen cells, which was detected against a variety of red cell antigens and haptens. It was able to substitute for T cells, as demonstrated by the ability of sheep red cells to induce a primary immune response in cultures of nude spleen cells in the presence, but not in the absence, of SIII. It is concluded that SIII is a B cell mitogen capable of directly activating DNA and antibody synthesis in these cells. This property is presumably responsible for the thymus‐independent nature of the immune response to SIII and lipopolysac‐charide. It is suggested that other thymus‐independent antigens also are B cell mitogens, although the degree of mitogenicity may vary.