Arthritogenic alphaviruses such as Ross River virus (RRV) and Chikungunya virus (CHIKV) are mosquito-borne viruses that have been responsible for outbreaks of arthritic disease in Australia and worldwide, respectively. Characteristic disease of arthritogenic alphaviruses include the development of joint inflammation and arthritic disease which can last months to years after infections. Arthritic disease caused by RRV and CHIKV shares similar characteristics with rheumatoid arthritis (RA), such as the development of incapacitating, joint inflammation, as well as the upregulation of proinflammatory mediators. While most studies, using experimental animal models of viral infection, have shown a role played by innate immune cells in alphaviral inflammation, the involvement of adaptive immune cells remains poorly understood. Studies investigating RA mouse models and RA patients have identified distinct subsets of T lymphocytes as important contributors in joint inflammation and disease. In particular, CD4⁺ T cells that produce IL-17 act by upregulating proinflammatory cytokines promoting inflammatory cell recruitment to the joints. The role of IL-17-producing cells in alphavirus-induced arthritis has not been investigated, we therefore aimed to investigate the role of IL-17⁺ cell subsets in a mouse model of RRV-induced joint inflammation. Mice were infected with RRV and ankle joints were harvested at the peak of inflammation (10 days postinfection). Flow cytometry analysis revealed the presence of IL-17A⁺ and IL-17F⁺ cell subsets produced by CD4⁺ and CD8⁺ T cells in the joints of infected mice. Further, gene expression analysis and protein analysis of joint tissue homogenates revealed elevated mRNA expression and protein levels of IL-17A at 10 days post-infection (dpi) in RRV-infected mice. In addition, RRV-infected joints showed a significant increase in pro-inflammatory cytokines such as IFNγ, TNF and IL-6. We next sought to determine whether in vivo neutralisation of IL-17 could reduce joint inflammation in RRV-infected mice. Using a monoclonal antibody targeting IL17A, we found that antibody-treated mice showed a reduction in disease severity from 8-10 dpi, compared to untreated RRV-infected mice. Blocking IL-17A had no significant effect on cell subsets in joint infiltrates, and did not reduce RRV viral load in joint tissue. However, protein analysis showed a significant reduction in IFN-γ in antibody-treated mice compared to untreated mice. This indicates that although blockade of IL-17A did not have an effect on infiltrating cells or viral load, disease severity was substantially reduced. Finally, we also showed that targeting the IL-17A/F heterodimer using a specific anti-IL-17A/F antibody reduced disease severity, indicating that both isoforms of IL-17 may play a role in RRVD pathogenesis.

In this study, we characterise for the first time two distinct subsets of IL-17-producing cells in an animal model of alphavirus-induced joint inflammation. In addition, we show that inhibiting IL-17A, as well as IL-17A/F heterodimers in vivo prior to the onset of RRV disease can help alleviate joint inflammation in RRV-infected mice. Thus, we provide evidence that IL-17⁺ T cell subsets can contribute to joint inflammation in RRV infection, and that targeting IL-17 is likely to be a promising approach in the development of future therapies to treat alphaviral infections.