In vivo, SGN-BB228 improved the quality and magnitude of the cytotoxic T cell response within CD228-expressing tumors, augmenting anti-tumor immunity. In vitro, 4-1BB costimulation provided by SGN-BB228 elicited proliferation, and cytokine production from functionally exhausted human T cells when in culture with CD228-expressing tumor cell lines engineered to engage the T cell receptor....Together these data highlight SGN-BB228, a first-in-class, investigational CD228 x 4-1BB costimulatory Antibody Anticalin® bispecific with potent and CD228-conditional 4-1BB costimulatory activity with therapeutic potential in multiple solid tumor types.