Lectin-mediated, nonopsonic phagocytosis of type 1 Escherichia coli by human peritoneal macrophages of uremic patients treated by peritoneal dialysis

Human peritoneal macrophages isolated from uremic patients undergoing peritoneal dialysis bind type 1 fimbriated Escherichia coli in the absence of opsonins. The number of bacteria bound per macrophage was 6.9, as determined by microscopic examination. Methyl alpha-mannoside (0.1 mM) and p-nitrophenyl alpha-mannoside (0.01 mM) inhibited this binding by about 66%. The ability of peritoneal macrophages to bind E. coli in a mannose-specific manner was confirmed in further experiments using an enzyme-linked immunosorbent assay (ELISA) with an antibacterial antibody, radiolabelled E. coli, and counts of colony-forming units (CFU). The number of bacteria bound per macrophage was 7 to 12 in the ELISA and 5.5-8.5 in the CFU assay. Methyl alpha-mannoside caused 70% inhibition of binding in the ELISA and 84% in the CFU assay, whereas p-nitrophenyl alpha-mannoside showed inhibition of 79% and 90%, respectively. Most bound bacteria (76-80%) were subsequently killed. Nonfimbriated E. coli 827 bound poorly to the macrophages (approximately 22%) as compared to that of the fimbriated bacteria. Although this binding was not inhibited by methyl alpha-D-mannoside or p-nitrophenyl alpha-mannoside, the percentage of bacteria killed was similar to that of the fimbriated phenotype. The peritoneal macrophage is thus able to phagocytose E. coli in the absence of opsonins. This may explain the relative rarity of E. coli as an etiologic agent of peritoneal infections in the dialysed patient.