GCOE global education seminar"The importance of IL-17 in the regulation of neutrophil homeostasis"

Blood neutrophil numbers are, in part, regulated by T lymphocytes. Normal neutrophil trafficking results in recruitment to tissues, apoptosis and uptake of apoptotic neutrophils by these phagocytes, which downregulates their IL-23 secretion. In normal mice, most IL-17A is produced by gd T cells. Granulocyte colony-stimulating factor (G-CSF), its receptor and interleukin-17 receptor A (IL-17RA) are all required to maintain baseline neutrophil counts in mice. Recombinase activating gene (Rag)1-/- mice cannot produce mature T cells, but maintain normal neutrophil counts, whereas nude mice lacking thymic epithelium are neutropenic. Supernatants from thymic explants of Rag1-/- mice contain as much IL-17A as those from wild-type (WT) mice. The IL-17A-producing cells are found in the double negative (DN)1 compartment of WT and Rag1-/- thymi. DN1 Rag1-/- IL-17A-producers express intracellular CD3, but not Gr-1 or NK1.1, suggesting that they are of T cell lineage. These cells colonize the spleen and MLN and secrete IL-17A in vitro following stimulation with IL-23, at a level similar to WT splenocytes. Adoptively transferred Rag1-/- thymocytes are equipotent to WT thymocytes in correcting neutropenia in nude mice, suggesting that the development of IL-17A-producing T-lineage cells requires an intact thymic epithelium, but not V(D)J recombination. We also tested whether IL-17F could compensate for IL-17A and maintain baseline neutrophil counts in the absence of IL-17A. Unlike the reduced neutrophil counts found in IL-17RA deficient mice, neutrophil counts were mildly increased in IL-17A deficient (Il17a-/-) animals. There was no evidence for infection or altered neutrophil activation. Plasma G-CSF and IL-17F levels were elevated in Il17a-/- compared to wild-type mice. IL-17F was mainly produced in the spleen and mesenteric lymph nodes. IL-23, which strongly promotes IL-17 secretion, was unaltered in Il17a-/- mice. However, Il17a-/- splenocytes differentiated with IL-6, TGF-b and IL-23 ex-vivo produced significantly more IL-17F in response to IL-23 than wild-type cells. Adding recombinant IL-17A to Il17a-/-splenocyte cultures reduced IL-17F mRNA and protein. These effects were also observed in wild-type but not IL-17RA deficient cells. We conclude that the IL-23 - IL-17A/F - IL-17RA axis is a major determinant of the normal set-point of blood neutrophil counts in vivo.