CD3γ/δ+ T cells and MCSFR+ macrophages are activated to produce IL-26 after bacterial infection in grass carp.

Interleukin-26 (IL-26) belongs to the IL-10 cytokine family and exerts diverse biological functions in regulating immune responses in vertebrates. Although IL-26 has been extensively studied in mammals, the functions of IL-26 remain largely unexplored in lower vertebrates. In this study, we determined the tissue and cell sources of IL-26 using a monoclonal antibody (mAb) generated against grass carp (Ctenopharyngodon idella, Ci) IL-26, and investigated the responses of IL-26 producing cells to bacterial infection. We showed that the CiIL-26 mAb specifically recognized the recombinant CiIL-26 proteins expressed in the Escherichia coli and HEK293 cells. Flow cytometry analysis revealed that the CiIL-26 mAb could detect the intracellular CiIL-26 expressed in the HEK293 cells and CIK cells stimulated with inactivated Aeromonas hydrophila (A. hydrophila). Using confocal microscopy, we analyzed IL-26+ cells in various tissues of grass carp following infection with A. hydrophila. It was shown that the IL-26+ cells were significantly increased in the gills, head kidney, posterior intestine and spleen. Remarkably, for the first time, we observed that most IL-26+ cells were CD3γ/δ+ T cells and MCSFR+ monocytes/macrophages, which could be induced by A. hydrophila. Our findings highlight the essential roles of CD3γ/δ+/IL-26+ T cells and MCSFR+/IL-26+ macrophages in the immune defense against bacterial infections in fish.