The present invention provides antibody compositions, including, e.g., antibodies, engineered antibodies and antibody fragments that bind to a tumor necrosis factor receptor superfamily member (i.e., 18). Provided compositions are useful in enhancing CD4+ and CD8+ T cell responses, and in the treatment, amelioration and prevention of diseases that can be counteracted with an augmented immune response, e.g., cancers. Also provided in the invention are polynucleotides and vectors that encode such molecules and host cells that harbor the polynucleotides or vectors; as well as pharmaceutical compositions that comprise such molecules and methods of use thereof.