Abstract: Follistatin-like protein (FSTL-1) is a secreted glycoprotein of unknown function, first isolated from mouse osteoblastic cells as a transforming growth factor-?1-inducible gene. The inventors have discovered that FSTL-1 is a proinflammatory mediator. As such, the invention provides for composition and methods of using agents that bind to FSTL-1 to modulate various types of inflammation (e.g., autoimmune diseases). Inhibitors and antagonists of FSTL-1, particularly antibodies or antibody fragments, may be used to treat conditions related to inflammation, such as arthritis. In addition, the inventors have discovered that FSTL-1 has a role in the Th17 pathway. Accordingly, the invention provides for compositions and methods of using agents which bind to FSTL-1 to modulate the generation of Th17 cells. Such agents are useful for delaying development of and treating diseases associated with undesired production of Th17 cells, such as autoimmune diseases.