Surfactant protein D inhibition of human macrophage uptake of Mycobacterium tuberculosis is independent of bacterial agglutination

The innate immune system in the lung is essential for controlling infections due to inhaled pathogens. Mycobacterium tuberculosis (M.tb) encounters components of the innate immune system when inhaled into the lung, but the consequences of these interactions are poorly understood. Surfactant protein D (SP-D) binds to and agglutinates M.tb bacilli, and reduces the uptake of the bacteria by human macrophages. In the current studies, we utilized a recombinant SP-D variant (CDM) that lacks the collagen domain to further characterize the interaction of SP-D with M.tb, and determine the effects of agglutination on bacterial uptake by human monocyte-derived macrophages. These studies demonstrate that the binding of SP-D and CDM to M.tb is saturable and inhibited by carbohydrate competition and Ca2+ chelation, implicating the carbohydrate recognition domain in the interaction. Fluorescence microscopy reveals that dodecameric SP-D leads to agglutination of the bacilli, whereas the trimeric CDM does not, demonstrating that the multivalent nature of SP-D is essential for agglutination of M.tb. However, preincubation of MA with increasing concentrations of SP-D or CDM leads to a concentration-dependent reduction in the uptake of the bacteria by macrophages, indicating that agglutination does not play a direct role in this observation. Finally, the reduced uptake of M.tb by SP-D is associated with reduced growth of M.tb in monocyte-derived macrophages. These studies provide direct evidence that the inhibition of phagocytosis of M.tb effected by SP-D occurs independently of the aggregation process.