Provided herein are compositions which exhibit novel diagnostic capabilities and allow to rapidly add functionality to adoptive immunotherapy. The compositions include isolated nucleic acids encoding proteins including antibody regions capable of binding compounds including a peptidyl moiety (e.g., a meditope). The recombinant proteins provided herein are useful, inter alia, for a broad variety of therapeutic and diagnostic purposes. For example, the recombinant proteins provided herein including embodiments thereof may be used as non-invasive means to characterize chimeric antigen receptor (CAR) T cells before and/or during treatment of diseases (e.g., cancer).