Summary of Methods & Results
In the current paper, Wedebye Schmidt et al. examine the Th17 cytokines IL-17A and IL-17F and their effects on the induction and maintenance of experimental colitis. The authors transferred CD4CD25 T cells to severe combined immune deficiency mice to induce colitis and used flow cytometry to examine Th-cell populations from lamina propria (LP), spleen and mesenteric lymph nodes (MLNs) in mice with varying degrees of colitis. They measured IFN-γ (Th1) and IL-17A (Th17) single-positive (SP) cells, as well as IFN-γIL-17A (Th1/17) double-positive (DP) cells as a proportion of total CD4 cells. Mice with more severe colitis scores had nearly twice as many IL-17A SP cells in MLNs compared with mice with no evidence of disease. The proportion of IL-17A SP cells present in the spleen also increased with worsening colitis, but less dramatically. IFN-γ SP cells present in MLNs and the spleen remained relatively unchanged in mice with and without colitis; however, IFN-γIL-17A DP cells present in MLNs and the spleen decreased with worsening colitis.
The proportion of IL-17A SP cells present in the LP, spleen or MLNs increased at day 40 when compared with day 10, although these cells accounted for only approximately 10% of the CD4 cells present. During this period, the proportion of IFN-γ SP cells, which accounted for between 20 and 40% of the CD4 cells present, did not differ significantly in the LP, spleen or MLNs. The proportion of IFN-γIL-17A DP cells present in the spleen and MLNs decreased significantly, dropping from approximately 10 to less than 5%. There was no significant change in the proportion of IFN-γIL-17A DP cells present in cells obtained from the LP. As expected, when compared with wild-type controls, there was an increased proportion of IL-17A and IFN-γ SP CD4 cells in the LP from colitic mice.
IL-17F was expressed by both IL-17A SP cells and IL-17AIFN-γ DP cells, but not by IFN-γ SP cells. Sustained expression of IL-17F in IL-17A cells was noted in MLNs from mice with colitis, suggesting that IL-17A cells and IL-17F contribute to the development of experimental colitis.
Immunohistochemistry was performed in order to confirm IL-17A and IL-17F expression in colonic tissue of mice with colitis. Few IL-17A SP cells were present in mice without colitis. IL-17A staining was increased in mice with colitis and was concentrated in the LP, as well as areas with inflammatory infiltrates. IL-17F was distributed in a similar pattern to IL-17A.
In order to determine the role of IL-17A and IL-17F in colitis development, T-cell transplanted severe combined immune deficiency mice were treated with antibodies directed against IL-17A, IL-17F or both IL-17A and IL-17F. Treatment with an IL-17A antibody alone or antibodies against both IL-17A and IL-17F at the time of T-cell transfer was associated with a significant decrease in rectum weight when compared with mice treated with a control antibody. Treatment with an antibody directed against either IL-17A or IL-17F alone did not improve colitis scores when compared with mice treated with a control antibody. Treatment with antibodies against both IL-17A and IL-17F significantly improved colitis scores when compared with mice treated with a control antibody.
In order to more closely mimic patients with pre-existing IBD, antibodies were administered 18 days after colitis induction. Simultaneous administration of antibodies against IL-17A and IL-17F was associated with a significant decrease in rectum weight when compared with treatment with a control antibody. A decrease in the colitis score in mice treated simultaneously with antibodies against IL-17A and IL-17F was also reported when compared with mice treated with a control antibody; however, this difference was not statistically significant (p = 0.0979). A significant decrease in colitis score was observed in mice treated with antibodies against both IL-17A and IL-17F when compared with colitic mice that did not receive a control antibody. Treatment with antibodies against either IL-17A or IL-17F alone was not associated with a significant decrease in colitis score when compared with mice treated with a control antibody.