Cellular mechanisms of adjuvant action of bacterial lipopolysaccharide in anti-sheep red blood cell antibody response.

Abstract

Adjuvant action of lipopolysaccharide (LPS) extracted from Salmonella typhimurium LT2 on anti-sheep red blood cell (sRBC) antibody response in mouse spleen was studied at the cellular level by the technique of localized hemolysis in agar. Injection of LPS caused significant increase in direct 19S and indirect 7S plaque-forming cells (PFC) in mouse spleen after the primary antigenic stimulation, while the adjuvant effect of LPS could hardly be assessed in the secondary PFC response. The adjuvant effect on the primary PFC response was most conspicuous when LPS was injected at the same time as the sRBC, and no effect could be observed when LPS was injected 1 or 5 hr before PFC determination. The mice who had received LPS with the primary sRBC stimulation showed much higher PFC level after the secondary sRBC stimulation, indicating the increase in the memory cells. Cell population consisting of lymphoid cells and macrophages was separated into ‘adherent’ (macrophage-rich) and ‘non-adherent’ (lymphoid cell-rich) fractions by incubating in petri dishes, and effect of LPS on each fraction was examined. Transfer of non-adherent lymphoid cells treated with LPS in vitro and simultaneous sRBC injection resulted in a significant increase of PFC in the spleen of the syngeneic recipient mice, but transfer of LPS-treated macrophages did not show any effect, suggesting that LPS acts on lymphocytes but not on macrophages. This idea appeared also to be supported by the results that LPS exerted antagonistic effect on the immune suppression caused by heterologous anti-lymphocyte serum, anti-sRBC serum and cortisone which are currently supposed to suppress the antigen reactive cells.