Described are antagonistic TNFR2 polypeptides, such as antibodies and antigen-binding fragments thereof, and the use of these polypeptides to inhibit the proliferation of regulatory T cells (T-regs) and/or myeloid-derived suppressor cells (MDSCs), to expand T effector cell populations or function, and to reduce the proliferation of, or directly kill, tumor cells, such as tumor cells that express TNFR2 antigen. The polypeptides, such as antibodies and antigen-binding fragments thereof, are TNFR2 antagonists, such as dominant TNFR2 antagonists. The polypeptides can be used to suppress the T-reg- or MDSC-mediated deactivation of tumor reactive T lymphocytes, expand populations of tumor-reactive cytotoxic T cells, and/or to directly kill TNFR2+ tumor cells. The antagonistic TNFR2 polypeptides described herein can be used to treat a wide variety of cancers and infectious diseases.
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