Characterization of integrins in cellular immunity of the oriental armyworm, Mythimna separata

Insect hemocytes eliminate foreign substances from the hemocoel through various immune reactions. Integrins, receptor proteins present on the cell membrane, are formed as a heterodimer from alpha and beta subunits and are known to be involved in various immune reactions. To elucidate the role of integrins in the immunity of the lepidoptera Mythimna separata, genes encoding integrins were screened from the genome, resulting in the identification of eight alpha and four beta integrin genes. The expression levels of the integrin genes did not change in response to the injection of small abiotic beads undergoing phagocytosis in M. separata larvae. However, significant inductions of some integrin gene expressions were observed in hemocytes that formed capsules around large abiotic beads during encapsulation, especially in MysInt alpha 2. Under biotic stimulation, induction of the MysInt alpha 2 was evident after exposures to Gram-negative bacteria (Escherichia coli) and entomopathogenic nematodes (Steinernema carpocapsae), but not to Gram-positive bacteria (Micrococcus luteus). Immunostaining analysis revealed that MysInt alpha 2 was specifically localized to hemocytes surrounding the beads during the encapsulation reaction. Furthermore, the spreading and encapsulation abilities of hemocytes were significantly inhibited by incubation with MysInt alpha 2 antibodies. Suppression of MysInt alpha 2 expression in M. separata larvae by injecting double-stranded RNA also resulted in a decrease in encapsulation activity. Collectively, these results indicate that MysInt alpha 2 plays pivotal roles in the cellular immune response of M. separata, particularly during encapsulation. This likely occurs through the regulation of hemocyte spreading activity, thereby facilitating the formation of multilayered capsules around large invaders.