Recognition of secretory IgA by DC-SIGN: implications for immune surveillance in the intestine.

Secretory IgA (SIgA), the predominant class of antibody in intestinal secretions, serves as the first line of defense against enteric infections. SIgA has also been proposed to function in immune surveillance, given that both SIgA and SIgA-antigen complexes are actively transported by Peyer's patch M cells from the intestinal lumen to sub-epithelial dendritic cells (DCs). The goal of the present study was to identify the receptor(s) potentially utilized by mucosal DCs to recognize and internalize SIgA. We demonstrate that human colostral SIgA is recognized by purified recombinant human DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN) in a solid phase binding assay, as well as by DC-SIGN ectopically expressed on the surface of Chinese hamster ovary (CHO-S) cells. The interaction between SIgA and DC-SIGN was specific, given that it was Ca(2+)-dependent and inhibited by mannan. Moreover, SIgA bound to, and was internalized by, endogenous DC-SIGN expressed on THP-1 cells following monocyte to macrophage-like cell differentiation by stimulation with phorbol ester and interleukin-4. These data identify DC-SIGN as a putative receptor for SIgA, and reveal a mechanism by which DCs could collaborate with M cells in immune surveillance at mucosal surfaces.