Described herein are methods and compositions for treating autoimmunity and inflammatory conditions without non-specific suppression of the host immune system. In particular, the anti-OX40L antibodies described herein are unique in that they not only inhibit the differentiation of inflammatory T cells but also promote the generation and function of regulatory T cells by inducing IL-10 and inhibiting TNF-α and by reducing aberrant Th2 cell responses. Furthermore, the methods and compositions described herein eliminate or reduce aberrant T follicular helper cell—(Tfh) responses that may contribute to the pathogenicity of autoimmune disease.