Characterization of integrins in cellular immunity of the oriental armyworm, <i>Mythimna separata</i>

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 α and β 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 α and four β 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 MysIntα2. Under biotic stimulation, induction of the MysIntα2 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 MysIntα2 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 MysIntα2 antibodies. Suppression of MysIntα2 expression in M. separata larvae by injecting double-stranded RNA also resulted in a decrease in encapsulation activity. Collectively, these results indicate that MysIntα2 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.