Natural IgM and innate immune collectin SP-D bind to late apoptotic cells and enhance their clearance by alveolar macrophages in vivo.
Innate immune collectin surfactant protein D (SP-D) and natural immunoglobulin M (IgM) are two soluble proteins. These opsonic proteins are good candidates for enhancing late apoptotic cell clearance. However, effects of these proteins on late apoptotic cell clearance in the lungs are not clearly established. We have recently shown that SP-D can bind several immunoglobulin isotypes, including IgM. Here we hypothesized that IgM and SP-D bind to late apoptotic cells and enhance their clearance from the lungs. We show that IgM and SP-D bind to late apoptotic secondary necrotic cells, and that IgM and SP-D either co-localize to the same regions or to different regions of late apoptotic Jurkat T cells. Mouse alveolar macrophages internalized late apoptotic cells, in vivo. We induced lung inflammation in mice using LPS and show that airway IgM and SP-D levels and the clearance of late apoptotic cells by alveolar macrophages increases under these conditions. We then coated late apoptotic cells with IgM, SP-D, or both and instilled them into the mouse airways. We found that alveolar macrophages internalize IgM- and SP-D-coated late apoptotic cells more effectively than uncoated late apoptotic cells, in vivo. None of these conditions cause inflammation in the naïve lungs. Therefore, these data suggest that both IgM and SP-D effectively opsonize late apoptotic cells and directly enhance their clearance by alveolar macrophages in the lungs.