Involvement of a pattern recognition receptor C-type lectin 7 in enhancing cellular encapsulation and melanization due to its carboxyl-terminal CRD domain in the cotton bollworm, Helicoverpa armigera

• Both HaCTL7 and CRD2 own the agglutinate ability against various bacteria.
• Both HaCTL7 and CRD2 enhance encapsulation and melanization processes.
• Endogenous HaCTL7 binds to granular cells, plasmatocytes and oenocytoids.
• Recombinant HaCTL7 and rCRD2 could bind to both granular cells and oenocytoids.
• HaCTL7 enhances encapsulation and melanization likely through its C-terminal CRD2.

C-type lectins play important roles in innate immunity as pattern recognition receptors (PRRs). We have previously reported a novel C-type lectin HaCTL7 from the cotton bollworm (Helicoverpa armigera) which contains two carbohydrate-recognition domains (CRDs), namely N-terminal CRD1 and C-terminal CRD2. Interestingly, there are four but not six of conserved cysteine residues in CRD2 of HaCTL7, which is different from that of other dual CRD C-type lectins. In the current study, we expressed and purified recombinant HaCTL7 (rHaCTL7) as well as rCRD1 and rCRD2, and demonstrated that both rHaCTL7 and rCRD2, but not rCRD1, owned the agglutinate ability against both Gram-negative and Gram-positive bacteria in a calcium dependent manner. In addition, both rHaCTL7 and rCRD2, but not rCRD1, could bind to various bacteria, and enhanced haemocytes mediated encapsulation and melanization processes. HaCTL7 secreted from fat bodies is able to bind to granulocytes, plasmatocytes and oenocytoids, but not to spherulocytes. Recombinant HaCTL7 and rCRD2 are capable of binding to both granulocytes and oenocytoids, while rCRD1 can only bind to granulocytes. Our data suggest that as a PRR HaCTL7 enhances encapsulation and melanization likely through its C-terminal CRD2, but not N-terminal CRD1, which imply that the characteristic four cysteine structure of CRD2 plays key roles in innate immunity.