IL-17 production elicited by allo-major histocompatibility complex class II recognition depends on CD25posCD4pos T cells.

BACKGROUND: Interleukin (IL)-17 is involved in autoimmune inflammatory disorders and naturally occurring CD25pos regulatory T cells were shown to promote IL-17 synthesis. Because IL-17 is overproduced in certain types of allograft rejection, it is important to characterize the cells responsible for IL-17 synthesis and to define how IL-17 is regulated during alloimmune responses.

METHODS: Splenic CD4pos T cells were isolated from C57BL/6 mice and fractionated according to CD25 expression. T cells were stimulated by major histocompatibility complex class II-mismatched bone marrow-derived dendritic cells from bm12 mice, either immature or made mature by exposure to lipopolysaccharide. To track T cell populations, CD25negCD4pos and CD25posCD4pos were isolated from Thy1.1 and congenic Thy1.2 mice, respectively. Cell proliferation was quantified by CFSE dilution. IL-17-producing cells and FOXP3pos cells were enumerated by intracytoplasmic staining and cytokine levels in culture supernatants were measured by ELISA.

RESULTS: Addition of CD25posCD4pos T cells to CD25negCD4pos T cells inhibited IL-2, interferon-[gamma], and IL-13 production but promoted IL-17 synthesis on stimulation by allogenic immature DC. In this setting, IL-17 originated from CD25intCD4posFOXP3neg memory T cells, which depend on IL-2 to produce IL-17. Alloreactive CD25negCD4pos T cells were also induced to produce IL-17 when stimulated by mature DC in the presence of CD25highCD4posFOXP3pos T cells.

CONCLUSIONS: We conclude that (1) the cellular source of IL-17 during an antiallo major histocompatibility complex class II response depends on the maturation status of allogenic DC, (2) whereas suppressing Th1 and Th2 cytokine synthesis, naturally occurring regulatory T cells, allow IL-17 production by alloreactive CD4pos T cells.