Surfactant Protein A Heightens Macrophage Mannose Receptor Activity

U.S. researchers have shed new light on the early immune response to M. tuberculosis and other bacterial pathogens.

"Inhaled particulates and microbes are continually cleared by a complex array of lung innate immune determinants, including alveolar macrophages (AMs)," explained Alison A. Beharka and colleagues at the University of Iowa and the Veterans Affairs Medical Center in Iowa City, the University of Colorado Health Sciences Center in Denver, and the University of Cincinnati. "AMs are unique cells with an enhanced capacity for phagocytosis that is due, in part, to increased activity of the macrophage mannose receptor (MR)." Surfactant protein A increases macrophage MR activity, which can enhance or, in some cases, degrade immune responses to infection, Beharka and coauthors found.

SP-A, a key component of lung surfactant, is known to augment macrophages' ability to kill infected cells through phagocytosis, the investigators said. Their study shows that SP-A selectively upregulates MR on human monocyte-derived macrophages. Study data showed that SP-A upregulation occurred quickly, even in the absence of new protein synthesis, suggesting the extra MR was drawn from intracellular sources.

However, the siphoning of intracellular MR triggered by SP-A could impair immune defenses against intracellular microbes, the authors warned. Their full report, "Pulmonary Surfactant Protein A Up-Regulates Activity of the Mannose Receptor, a Pattern Recognition Receptor Expressed on Human Macrophages" was published in the Journal of Immunology (2002;169(7):3565-3573).

"SP-A upregulation of MR activity provides a mechanism for enhanced phagocytosis of microbes by AMs, thereby enhancing lung and host defense against extracellular pathogens or, paradoxically, enhancing the potential for intracellular pathogens to enter their intracellular niches," Beharka and colleagues concluded.

Back to other CDC news for December 20, 2002

Previous Updates