C-type lectins (CTLs) are widely distributed in mammals, insects, and plants, which act as pattern recognition receptors (PRRs) to recognize pathogens and initiate immune responses. In this study, we identified a C-type lectin gene called BmIML-2 from the silkworm Bombyx mori. Its open reading frame (ORF) encodes 314 amino acids, which contain dual tandem C-type lectin-like domain (CTLD). BmIML-2 is highly expressed in the fat body and is significantly induced at 24 h after BmNPV infection. Moreover, overexpression of BmIML-2 dramatically inhibited the proliferation of BmNPV, and knockdown assay via siRNA further validated the inhibition of BmIML-2 on viral proliferation. In addition, transcript level detection of apoptosis-related genes and observation of apoptosis bodies implied that overexpression of BmIML-2 promoted BmNPV-induced apoptosis. Immunofluorescence analysis indicated that BmIML-2 distributed throughout the cytoplasm and was slightly concentrated in the cell membrane. Taken together, our results suggest that BmIML-2 could inhibit in the proliferation of BmNPV by facilitating cell apoptosis.

Insect innate immunity is initiated by the special recognition and binding of the foreign pathogens, which is accomplished by the pattern recognition receptors (PRRs). As an important type of PRRs, C-type lectins (CTLs) play various roles in insect innate immunity, including pathogen recognition, stimulation of prophenoloxidase, regulation of cellular immunity and so on. In this study, we have cloned the full-length cDNA of a CTL gene named CTL-S6 from the silkworm, Bombyx mori. The open reading frame (ORF) of B. mori CTL-S6 encodes 378 amino acids, which contain a secretion signal peptide. The mRNA of CTL-S6 exhibited the highest transcriptional level in the midgut. Its transcriptional level increased dramatically in fat body and hemocytes upon Escherichia coli or Micrococcus luteus challenge. Purified recombinant CTL-S6 could bind to bacterial cell wall components, including peptidoglycan (PGN, from Bacillus subtilis) and lipopolysaccharide (LPS, from E. coli 0111:B4), and recombinant CTL-S6 was involved in the encapsulation and melanization of hemocytes. Furthermore, the addition of recombinant CTL-S6 to the hemolymph of silkworm resulted in a significant increase in phenoloxidase activity. Overall, our results indicated that B. mori CTL-S6 may serve as a PRR for the recognition of foreign pathogens, prophenoloxidase pathway stimulation and involvement in the innate immunity.

The term cellular immune response refers to haemocyte-mediated responses, including phagocytosis, nodulation, and encapsulation. In the present study, we identified five types of circulating haemocytes in larvae of the haemolymph of the Asian corn borer, Ostrinia furnacalis (Guen?e), including granulocytes, oenocytoids, plasmatocytes, prohaemocytes, and spherulocytes. The relative number of total free haemocytes per larva decreased significantly 0.5, 24, and 36 h after the injection of Beauveria bassiana conidia. Upon conidia challenge, both phagocytosis and nodulation were observed in the collected haemolymph from O. furnacalis larvae. In addition, plasma was found to be necessary for both phagocytosis and nodulation. Therefore, we here confirm that phagocytosis and nodulation are involved in O. funacalis larvae during their fight against infection by B. bassiana, and further, that the cellular immune response of O. furnacalis helps eliminate the invading organisms despite the fact that not all the fungal conidia are killed.

The insect immune response is initiated by the recognition of invading microorganisms. Peptidoglycan recognition proteins (PGRPs) function primarily as pattern recognition receptors by specifically binding to peptidoglycans expressed on microbial surfaces. We cloned a full-length cDNA for a PGRP from the Asian corn borer Ostrinia furnacalis (Guenée) and designated it as PGRP1. PGRP1 mRNA was mainly detected in the fat bodies and hemocytes. Its transcript levels increased significantly upon bacterial and fungal challenges. Purified recombinant PGRP1 exhibited binding activity to the gram-positive Micrococcus luteus, gram-negative Escherichia coli, entomopathogenic fungi Beauveria bassiana, and yeast Pichia pastoris. The binding further induced their agglutination. Additionally, PGRP1 preferred to bind to Lys-type peptidoglycans rather than DAP-type peptidoglycans. The addition of recombinant PGRP1 to O. furnacalis plasma resulted in a significant increase in phenoloxidase activity. The injection of recombinant PGRP1 into larvae led to a significantly increased expression of several antimicrobial peptide genes. Taken together, our results suggest that O. furnacalis PGRP1 potentially recognizes the invading microbes and is involved in the immune response in O. furnacalis.