19Arthritogenic alphaviruses such as Chikungunya virus and O'nyong nyong virus cause acute 20 and chronic crippling arthralgia associated with inflammatory immune responses. However, the 21 physiological functions of individual immune signaling pathways in the pathogenesis of 22 alphaviral arthritis remain poorly understood. Here we report that a deficiency in the stimulator-23 of-interferon-genes (STING) led to enhanced viral loads, exacerbated inflammation and 24 selectively elevated expression of CXCL10, a chemoattractant for monocytes/macrophages/T 25 cells, in mouse feet. Cxcl10 -/mice had the same viremia as wild-type animals, but fewer 26 immune infiltrates and lower viral loads in footpads at the peak of arthritic disease (6-8 days 27 post infection). Macrophages constituted the largest immune cell population in footpads 28 following infection, which were significantly reduced in Cxcl10 -/mice. The viral RNA loads in 29 neutrophils and macrophages were reduced in Cxcl10 -/compared to wild-type mice. In 30 summary, our results demonstrate that STING signaling represses, while CXCL10 signaling 31 promotes, pathogenesis of alphaviral disease. 32 33 34 35 36 37 38 42 worldwide, particularly in tropical/subtropical regions. Many alphaviruses are arthritogenic, 43 including Chikungunya (CHIKV), O'nyong-nyong (ONNV) and Ross River viruses (RRV) etc.
44CHIKV is the causative agent of acute and chronic crippling arthralgia that was initially identified 45 in Tanzania in 1952 1 . Since then, several major epidemics have been recorded on the Indian
46Ocean islands, India, Southeast Asia, which resulted in over 6 million cases 2 . In late 2013,
47CHIKV emerged on the Caribbean islands, and has now spread to more than 50 countries 48 across the Central and South America, including autochthonous infections in the United States 49 and caused over 2.5 million infection cases (Sources: Pan America Health Organization).
50Approximately 50% of CHIKV-infected patients suffer from rheumatic manifestations that last 6 51 months to years, with ~5% of the victims having rheumatoid arthritis-like illnesses 3,4 .
52During the acute phase of infection in humans (~ two weeks), CHIKV infects many 53 organs and cell types 2 , induces apoptosis and direct tissue damage 5-7 . The acute phase is also 54 characteristic of robust innate immune responses, including high levels of type I IFNs, 55 proinflammatory cytokines/chemokines and growth factors 5,8-12 . Immune cell infiltration is a 56 hallmark of acute CHIKV infection, including primarily macrophages, monocytes, but also 57 neutrophils, dendritic cells, NK cells and lymphocytes 2 . In the chronic phase, CHIKV arthritis 58 may progress without active viral replication, typified by elevated expression of cytokines and 59 immune cell infiltration 2,13 . In particular, human arthritic disease severity is associated with a 60 high level of serum chemoattractants for monocytes/macrophages/T cells, CXCL10 and CXCL9 61 14 . In mice, CHIKV infection leads to a low viremia lasting usually 5-7 ...