Mechanism by which Bombyx mori hemocytes recognize microorganisms: direct and indirect recognition systems for PAMPs

Hemocytes play an important role in cellular reactions in the immune system. Although the recognition of pathogens is thought to involve pattern-recognition proteins (PRPs) in insects, the exact mechanisms by which insect hemocytes recognize pathogens are not clear. This study examined the mechanism by which Bombyx mori hemocytes recognize microorganisms and pathogen-associated molecular patterns (PAMPs) using flow cytometry and fluorescence microscopy. Fluorescence-labeled bacterial or fungal cells were observed to bind to hemocytes and this binding was inhibited by adding lipoteichoic acid (LTA) or beta-1,3-glucan. Lipopolysaccharide (LPS) bound to hemocytes directly. These results suggest that hemocytes have a mechanism that recognizes LPS, LTA, and beta-1,3-glucan directly. Previously, we identified two types of C-type lectin (BmLBP and BmMBP) and showed that they recognize a variety of PAMPs leading to the induction of nodule formation. These lectins enhanced hemocyte binding to microorganisms and their direct binding to hemocytes suggests that hemocytes have a mechanism for recognizing microorganisms using lectin receptors.