Characterization and Localization of Lipopolysaccharides Following the Ingestion of E. Coli by Murine Macrophages in Vitro

Lipopolysaccharides (LPS) isolated from the outer membranes of gram negative bacteria have been demonstrated to modulate a spectrum of pathological and immunological events both in vitro and in vivo (14,15,23). Several lines of evidence have suggested that macrophages are not only stimulated by bacterial LPS (3,4,7,16) but may also be involved in the detoxification of this bacterial product (9,17,18). In recent studies designed to follow the fate of E. coli LPS after the uptake and catabolism of whole E. coli by macrophages, we demonstrated that LPS are exocytosed slowly from the macrophage. Furthermore, the LPS remaining within the macrophage at 72 hours, and the LPS released from the macrophage over that period, retain endotoxic activity (5,6). In this chapter, we provide evidence that the LPS remaining within the macrophage at 48–s72 hours is associated with macrophage phagocytic vacuoles, and that both the retained LPS and the exocytosed LPS have a significantly enhanced capacity to stimulate splenocytes. Parenthetically, our results also indicate that, as assessed by SDS-PAGE, macrophage processed LPS is enriched for higher molecular weight subunits relative to control LPS. Althrough the precise macrophages-induced biochemical altertions in LPS which contribute to enhances immunostimulatory activity remain to be elucidated, these data nevertheless provide evidence that macrophage processing of LPS may contribute to the local amplification of immunologicallly relevant events.