Methods and compositions for treating uveitis are provided. The methods include administering to a subject a single-domain antibody (sdAb) comprising the amino acid sequence set forth in SEQ ID NO:1. In certain embodiments, the sdAb is administered systemically at a therapeutically effective dose and inhibits STAT3 activation in CD4+ T-cells, thereby reducing Th17-mediated inflammation. The methods may be used to treat autoimmune uveitis and may improve retinal function as measured by electroretinogram (ERG) amplitude. Also provided are pharmaceutical compositions comprising the sdAb and a pharmaceutically acceptable carrier. Further methods of modulating immune responses associated with uveitis are disclosed, including reducing cytokine production and T-cell-mediated inflammation.