Properties of a new complement-dependent bactericidal factor specific for Ra chemotype salmonella in sera of conventional and germ-free mice.

A complement-dependent bactericidal factor, Ra-reactive factor, that acts specifically on Ra chemotype strains of Salmonella was found in sera of eighteen strains of nonimmune mice, including a thymusless strain. The specificity of binding of this factor to the cells of Ra chemotype bacteria was evidenced by absorption studies. The factor was inactivated by treatment with 0.02 M 2-mercaptoethanol or heating for 30 min at 55 degrees C. This factor was present in mice reared by feeding with a chemically defined, antigen-free diet under germ-free conditions. Fetal and newborn mice from a conventional or germ-free mother had the factor, although the titer was lower than the adults'. The titer in the newborn mice reached adult level 1 wk after birth. The titer in the adult mice did not increase markedly after immunization with heat-killed bacteria or administration of Freund's complete adjuvant and turpentine. These results indicate that this factor is actively produced by mice that have never been stimulated with exogenous Ra antigen.