A 24-year-old woman with leukemia receives an allogeneic bone marrow transplant. Three weeks later, she develops a skin rash and diarrhea. Liver function tests show elevated serum levels of AST and ALT. A skin biopsy discloses a sparse lymphocytic infiltrate in the dermis and epidermis, as well as apoptotic cells in the epidermal basal cell layer. Skin rash and diarrhea in this patient are caused primarily by which of the following cells?