Lipoprotein(a), an Opsonin, Enhances the Phagocytosis of Nontypeable Haemophilus influenzae by Macrophages

We recently showed that both nontypeable Haemophilus influenzae (NTHi) and its surface plasminogen- (Plg-) binding proteins interact with lipoprotein(a) (Lp(a)) in a lysine-dependent manner. Because Lp(a) can be taken up by macrophages, we postulated that it serves as an opsonin to enhance phagocytosis of NTHi by macrophages. Based on colony-forming unit (CFU) counts, Lp(a) was found to increase U937 macrophage-mediated phagocytosis of NTHi49247 and NTHi49766 by 34% and 43%, respectively, after 120 min. In contrast, Lp(a) did not enhance phagocytosis of Escherichia coli BL21 or E. coli JM109, which were unable to bind to Lp(a). As with U937 macrophages, Lp(a) was capable of increasing phagocytosis of NTHi49247 by peripheral blood mononuclear cell-derived macrophages. Opsonic phagocytosis by Lp(a) was inhibited by the addition of recombinant kringle IV type 10 (rKIV(10)), a lysine-binding competitor; moreover, Lp(a) did not increase phagocytosis of NTHi by U937 macrophages that were pretreated with a monoclonal antibody against the scavenger receptor CD36. Taken together, our observation suggests that Lp(a) might serve as a lysine-binding opsonin to assist macrophages in rapid recognition and phagocytosis of NTHi.