Polypeptides and agents that bind a TNF receptor superfamily protein are disclosed, particularly agents that specifically bind GITR, OX40, or CD40. The polypeptides or agents may include fusion polypeptides, particularly polypeptides comprising GITRL, OX40L, or CD40L and/or bispecific agents. Also disclosed are methods of using the polypeptides or agents for inducing and/or enhancing the immune response, as well as methods for the treatment of diseases such as cancer.