Abstract
Increasing evidence now identifies surfactant protein D (SP-D) as an important element of the innate immune system of the lung. In this study, we examined the interactions of rat and human SP-D with the human pathogen, Mycoplasma pneumoniae. Rat and human SP-D bound the organism with high affinity in a reaction that required Ca(2+) and was inhibited by EGTA. Membranes derived from the organism bound the proteins in a similar manner, except the rat SP-D also exhibited a significant level of Ca(2+)-independent binding. Pretreatment of membranes with proteases did not alter the Ca(2+)-dependent SP-D binding of membranes by either protein. Mannose, glucose, maltose, and inositol, at millimolar concentrations, competed for human SP-D binding to the bacterial membrane. Lipids extracted from membranes and separated by two-dimensional thin layer chromatography bound human SP-D with high affinity in a Ca(2+)-dependent reaction. A tandem mutant of SP-D with E321Q and N323D substitutions, failed to bind M. pneumoniae lipids, directly implicating the carbohydrate recognition domain in the interaction. The interaction of rat and human SP-D with M. pneumoniae was unaffected by the presence of surfactant lipids and the hydrophobic surfactant proteins. These findings demonstrate that M. pneumoniae is likely to be recognized by SP-D in the alveolar environment and that primary determinants recognized on the organism are lipid components of the cell membrane.
Highlights
Mycoplasmas, the smallest self-replicating microorganisms, are pathogens capable of causing a wide variety of diseases
These findings demonstrate that M. pneumoniae is likely to be recognized by surfactant protein D (SP-D) in the alveolar environment and that primary determinants recognized on the organism are lipid components of the cell membrane
SP-D Binds M. pneumoniae with High Affinity—M. pneumoniae is a human pathogen that resides in the same extracellular alveolar and bronchiolar compartment as SP-D
Summary
Mycoplasmas, the smallest self-replicating microorganisms, are pathogens capable of causing a wide variety of diseases. Microbial targets for SP-D include both Gram-positive and Gram-negative respiratory pathogens, influenza, and respiratory syncytial viruses, Cryptococcus neoformans, Pneumocystis carinii, and Aspergillus fumigatus [9]. Both monocytes/macrophages and neutrophils express surface receptors that can interact with SP-D [13,14,15,16,17,18,19,20]. Our findings indicate that SP-D binds M. pneumoniae with high affinity and the principal ligands are lipids Both the specificity and avidity of the SP-D binding indicate that microbial recognition by the protein is likely to occur in vivo
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