Macrophages present pinocytosed exogenous antigen via MHC class I whereas antigen ingested by receptor-mediated endocytosis is presented via MHC class II

Macrophages present exogenous Ag either via MHC class I or MHC class II molecules. We investigated whether the mode of hemagglutinin (HA) uptake influences the class of MHC molecule by which this Ag is presented. Normally, HA is ingested by receptor-mediated endocytosis, but this may be switched to macropinocytosis and pinocytosis by adding phorbol esters to the cells. This switch resulted in altered intracellular routing of ingested Ag and a transition from Ag presentation via MHC class II molecules to presentation via MHC class I molecules. Similarly, inhibition of receptor-mediated HA endocytosis, by treating the cells with the HA receptor destroying enzyme neuraminidase, abrogated Ag presentation via MHC class II molecules and induced presentation via MHC class I molecules. If, however, under these conditions, receptor-mediated uptake of HA was restored, by virtue of HA/anti-HA Ab interaction and subsequent uptake of HA via the Fc receptor, presentation via MHC class II was restored as well, whereas presentation of HA via MHC class I molecules was no longer detectable. We conclude that in macrophages the mode of Ag uptake is decisive in determining via which class of MHC molecules Ag is presented: pinocytosis and macropinocytosis produce exclusive presentation of exogenous Ag via MHC class I molecules whereas receptor-mediated endocytosis leads exclusively to presentation via class II molecules.

Lipopolysaccharide regulates macrophage fluid phase pinocytosis via CD14-dependent and CD14-independent pathways

Lipopolysaccharide (LPS) is a mediator of inflammation and septic shock during bacterial infection. Although monocytes and macrophages are highly responsive to LPS, the biological effects of LPS in these cell types are only partially understood. We decided, therefore, to investigate the influence of LPS on macrophage pinocytosis and Fc receptor-mediated endocytosis, two prominent and related macrophage effector functions. We observed that LPS did not greatly influence endocytosis in either macrophages or monocytes, but did exert a dual action on pinocytosis: at lower concentrations (0.1 to 100 ng/mL), LPS caused a decrease in pinocytosis in both macrophages and monocytes, whereas at higher LPS concentrations, enhanced pinocytosis in macrophages was observed. Detoxified LPS was two orders of magnitude less potent in producing these effects. After inhibition of the LPS receptor CD14, the LPS-induced decrease in pinocytosis was absent, and stimulation of pinocytosis at lower LPS concentrations was unmasked. We conclude that LPS can influence pinocytosis via CD14-dependent and CD14-independent signaling pathways. Furthermore, as addition of LPS to macrophages effected pinocytosis but not Fc receptor-mediated endocytosis, these two processes are independently regulated in macrophages.