IRF-8 regulates expansion of myeloid-derived suppressor cells and Foxp3⁺regulatory T cells and modulates Th2 immune responses to gastrointestinal nematode infection

Interferon regulatory factor-8 (IRF-8) is critical for Th1 cell differentiation and negatively regulates myeloid cell development including myeloid-derived suppressor cells (MDSC). MDSC expand during infection with various pathogens including the gastrointestinal (GI) nematode Heligmosomoides polygyrus bakeri (Hpb). We investigated if IRF-8 contributes to Th2 immunity to Hpb infection. Irf8 expression was down-regulated in MDSC from Hpb-infected C57BL/6 (B6) mice. IRF-8 deficient Irf8^-/-and BXH-2 mice had significantly higher adult worm burdens than B6 mice after primary or challenge Hpb infection. During primary infection, MDSC expanded to a significantly greater extent in mesenteric lymph nodes (MLN) and spleens of Irf8^-/-and BXH-2 than B6 mice. CD4⁺GATA3⁺T cells numbers were comparable in MLN of infected B6 and IRF-8 deficient mice, but MLN cells from infected IRF-8 deficient mice secreted significantly less parasite-specific IL-4 ex vivo. The numbers of alternatively activated macrophages in MLN and serum levels of Hpb-specific IgG1 and IgE were also significantly less in infected Irf8^-/-than B6 mice. The frequencies of antigen-experienced CD4⁺CD11a^hiCD49d^hicells that were CD44^hiCD62L^-were similar in MLN of infected Irf8^-/-and B6 mice, but the proportions of CD4⁺GATA3⁺and CD4⁺IL-4⁺T cells were lower in infected Irf8^-/-mice. CD11b⁺Gr1⁺cells from naïve or infected Irf8^-/-mice suppressed CD4⁺T cell proliferation and parasite-specific IL-4 secretion in vitro albeit less efficiently than B6 mice. Surprisingly, there were significantly more CD4⁺T cells in infected Irf8^-/-mice, with a higher frequency of CD4⁺CD25⁺Foxp3⁺T (Tregs) cells and significantly higher numbers of Tregs than B6 mice. In vivo depletion of MDSC and/or Tregs in Irf8^-/-mice did not affect adult worm burdens, but Treg depletion resulted in higher egg production and enhanced parasite-specific IL-5, IL-13, and IL-6 secretion ex vivo. Our data thus provide a previously unrecognized role for IRF-8 in Th2 immunity to a GI nematode.