An endotoxin induced serum factor that causes enhancement of antibody response to heterologous erythrocytes.

The sera obtained from blood of the mice, which had been intravenously injected with LPS several hours in advance, contained some active substance capable of enhancing anti-sheep red blood cell (SRBC) antibody responses in mice. Activity of the sera was still retained after passage through a rabbit anti-LPS antibody-coated Sepharose 4B column, but greatly reduced by passage through a rabbit anti-mouse thymocyte antibody-coated Sepharose 4B column. The active substance in the sera was eluted through a Sephadex G-200 column at the same position as the serum albumin. The addition of this substance to B cell rich spleen cell cultures in vitro in the presence of SRBC generated tremendous numbers of antibody forming cells 4 days after the incubation, suggesting that this substance was able to take over the helper function of T cells in thymus dependent antibody responses. However, this substance was not capable of stimulating 3H-thymidine-uptake into cultured spleen cells. The possible role of this substance in the adjuvant effect of LPS is discussed.