The present invention relates to molecules that stabilize and/or enhance CD154 activity, compositions comprising these molecules, methods for using these molecules and compositions (e.g., pharmaceutical compositions) comprising them. Specifically, the molecules can be used for enhancing immune function (e.g., enhancing Th1 cytokine expression), stimulating immune responses, and/or treating certain disorders (e.g., cancer, infectious disease, and immune compromised states). The invention also relates to kits and compositions comprising the molecules.