IL-23 is required for neutrophil homeostasis in normal and neutrophilic mice

IL-23 is secreted by macrophages and dendritic cells in response to microbial products and inflammatory cytokines. IL-23 is a heterodimer composed of the unique IL-23p19 subunit linked to the common p40 subunit that it shares with IL-12. IL-23 is implicated in autoimmune diseases, where it supports the expansion of IL-17A-producing CD4⁺ Th17 cells. IL-23 also regulates granulopoiesis in a neutrostat regulatory feedback loop through IL-17A-producing neutrophil regulatory (Tn) cells, most of which express γδ TCR. This homeostatic system is disrupted in mice lacking adhesion molecules like β₂-integrins (Itgb2^-/-) which have defective neutrophil trafficking and neutrophilia. To test the role of IL-23 in the homeostatic regulation of circulating neutrophil numbers, we measured blood neutrophil numbers in p40-deficient (IL12b^-/-) mice and found them reduced compared with wild-type mice. IL12b^-/-Itgb2^-/- mice, lacking β₂-integrins, IL-12, and IL-23 showed significantly blunted neutrophilia compared with Itgb2^-/- mice. Treatment of both IL12b^-/- and IL12b^-/-Itgb2^-/- mice with IL-23, but not IL-12, restored circulating neutrophil counts. Serum levels of IL-17A were readily detectable in Itgb2^-/- mice, but not in IL12b^-/-Itgb2^-/- mice, suggesting that IL-17A production is reduced when IL-23 is absent. Similarly, tissue mRNA expression of IL-17A was reduced in IL12b^-/-Itgb2^-/- mice compared with Itgb2 ^-/- controls. The total number of CD3⁺ IL-17A-producing Tn cells were significantly reduced in the spleen and lamina propria of IL12b ^-/-Itgb2^-/- mice, with the largest reduction found in γδ⁺ T cells. Our results suggest a prominent role of IL-23 in the regulation of granulopoiesis and the prevalence of IL-17A-producing Tn cells.