AJP - Lung Cellular and Molecular Physiology, Vol 267, Issue 5 578-L584, Copyright © 1994 by American Physiological Society
Binding of surfactant protein A to C1q receptors mediates phagocytosis of Staphylococcus aureus by monocytes
M. F. Geertsma, P. H. Nibbering, H. P. Haagsman, M. R. Daha and R. van Furth
Department of Infectious Diseases, University Hospital Leiden, The Netherlands.
During both steady-state conditions and inflammatory reactions in the lower
airways, monocytes migrate to the alveoli where they come into contact with
surfactant. Surfactant is composed of phospholipids, neutral lipids, and
specific proteins, and its main function is to reduce surface tension in
the alveoli. The most abundant glycoprotein surfactant protein A (SP-A)
affects the structure, function, and metabolism of pulmonary surfactant.
The aim of the present study was to determine whether SP-A plays a role in
the antibacterial activities of human monocytes and whether this is
mediated by a receptor for SP-A on these cells. The results showed that
SP-A binds to both Staphylococcus aureus and monocytes and mediates the
phagocytosis of the bacteria by these cells. SP-A does not stimulate the
intracellular killing of bacteria by monocytes, and SP-A-opsonized S.
aureus do not induce the production of reactive oxygen intermediates. SP-A
binds to the C1q receptor (C1qR) on monocytes, since its binding was
inhibited by C1q and the SP-A-enhanced association of S. aureus with these
cells was completely abolished when monocytes were adherent to surfaces
coated with C1q or anti-C1qR monoclonal antibody. Furthermore, the binding
of SP-A to monocytes results in an increased intracellular concentration of
adenosine 3',5'-cyclic monophosphate. Together, these results demonstrate
that C1qR mediates the phagocytosis of SP-A-opsonized S. aureus by
monocytes.
