Interaction of C1q receptor with lung surfactant protein A

Earlier we reported the purification of C1q receptor (C1qR) from U937 cells and human tonsil lymphocytes (Malhotra, R. and Sim, R. B., Biochem. J. 1989. 218: 625) and showed that C1qR interacts with the ligands C1q, mannose-binding protein, conglutinin and lung surfactant protein A (SP-A) (Malhotra, R., Thiel, S., Reid, K. B. M. and Sim, R. B., J. Exp. Med. 1990. 172: 955). C1qR was characterized as an acidic glycoprotein, which, when solubilized, exists as a dimer of Mr 115,000 under non-denaturing conditions. In this article we provide evidence for binding of radioiodinated SP-A to U937 cells and show that binding of radioiodinated SP-A to U937 cells is specific, saturable, salt dependent and is inhibited by purified C1qR and by C1q. The interaction of SP-A with U937 cells was found to up-regulate the surface expression of C1qR. Incubation of SP-A with U937 cells at 37 degrees C for 80 min was found to increase the receptor number per cell. Increase in receptor number was inhibited in the presence of sodium azide and monensin. Incubation of cells with calcium ionophore A23187 induced increased surface expression in the absence of SP-A. The results indicate that interaction of SP-A with U937 cells triggers the expression of an intracellular pool of C1qR.