Effects of an acidic polysaccharide procuced by Serratia piscatorum on immune responses in mice. I. mitogenicity and stimulation of plaque-forming cells (PFC) in vitro.

An acidic polysaccharide of Serratia piscatorum, consisting of L-rhamnose, D-galactose and D-galacturonic acid, stimulated the primary IgM PFC response of spleen cell cultures against sheep erythrocytes. The polysaccharide (PS) caused dose-dependent stimulation of 3H-thymidine incorporation into DNA of spleen cell cultures and the maximum response was seen after about 48 hr. Nonadherent B (ATXBM) cells were strongly reactive to PS, whereas adherent spleen cells were less reactive. Higher doses of PS suppressed the peak PFC response, but enhanced the earlier PFC responses. Lower doses of PS stimulated the PFC responses significantly throughout the period of PFC production. The optimal dose (0.1 mug/ml) of PS increased the PFC responses in mixed cell cultures containing spleen cells and either B cells, T cells activated with a different antigen, or hydrocortisone-resistant thymus cells of syngeneic origin. However, this dose of PS interfered with the enhanced PFC response of spleen cells due to addition of antigen-primed T cells. Since these characteristic activities of PS were like those of lipopolysaccharides (LPS), this acidic polysaccharide may be useful for further elucidating the modes of LPS action.