Streptavidin(SA)-4-1BBL and TLR agonists such as monophosphoryl lipid A (MPL) exhibit surprising synergy as adjuvants, inducing immune responses against weak antigens. Accordingly, there are provided adjuvant compositions comprising 4-1BBL and a toll-like receptor (TLR) agonist, such as MPL, methods of inducing an immune response against an antigen in a subject, comprising administering to the subject (a) the antigen, (b) a TLR agonist and, (c) 4-1BBL, and methods of treating a tumor or a cancer in a subject, comprising administering to the subject (a) an antigen associated with the tumor or cancer, (b) a TLR agonist, and (c) 4-1BBL.
