Disclosed herein are chimeric antigen receptor (CAR) polypeptides, which can be used with adoptive cell transfer to target and kill cancers, that comprise a costimulatory signaling region having a mutated form of a cytoplasmic domain of CD28 with altered phosphorylation at Y206 and/or Y218. Also disclosed are immune effector cells, such as T cells or Natural Killer (NK) cells, that are engineered to express these CARs. Therefore, also disclosed are methods of providing an antitumor immunity in a subject with a tumor associated antigen-expressing cancer that involves adoptive transfer of the disclosed immune effector cells engineered to express the disclosed CARs.