Brucella abortus lipopolysaccharide in murine peritoneal macrophages acts as a down-regulator of T cell activation

Macrophages play a central role in host immune responses against pathogens by acting as both professional phagocytic cells and as fully competent APCs . We report here that the LPS from the facultative intracellular Gram-negat ive bacteria Brucella abortus interferes with the MHC class II Ag presentat ion pathway. LPS inhibits the capacity of macrophages to present hen egg ly sozyme (HEL) antigenic peptides to specific CD4(+) T cells but not those of OVA to specific CD8(+) T cells, This defect was neither related to a decre ase of MHC class II surface expression nor to a deficient uptake or process ing of HEL, In addition, B. abortus EPS did not prevent the formation of SD S-resistant MHC class II complexes induced by HEL peptides, At the cell sur face of macrophages, we observed the presence of LPS macrodomains highly en riched in MHC class II molecules, which may be responsible for the signific ant down-regulation of CD4(+) T cell activation, This phenomenon may accoun t for the avoidance of the immune system by certain bacterial pathogens and may explain the immunosuppression observed in individuals with chronic bru cellosis.