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Two patients with histologically proven mycosis fungoides, a malignancy of phenotypically mature T cells, received a topical challenge with mechlorethamine to areas of clinically uninvolved skin to exclude possible hypersensitivity reactions to this chemotherapeutic agent. In both patients, allergic contact dermatitis (ACD) developed at the sites of the application and resolved completely after withdrawal of mechlorethamine. The lesions were biopsied and analyzed for the presence of clonal T-cell receptor (TCR)-γ gene rearrangements using two polymerase chain reaction (PCR)-based assays involving denaturing gradient gel electrophoresis (PCR/DGGE) and ribonuclease protection analysis (PCR/RPA). The former method has a clonal detection threshold of 10-3-10-2, while the latter has a sensitivity of 10-5. In both cases, the ACD lesions were polyclonal by PCR/ DGGE. In contrast, PCR/RPA detected tumor-specific TCR-γ gene rearrangements in these same lesions. This indicates that the ACD lesions contained tumor cells at a density within the 10-5-102 range. Analysis of peripheral blood mononuclear cells from both patients failed to detect the malignant done and showed the same result as blood from four normal individuals. The normal skin from one patient also lacked detectable TCR-γ gene rearrangements. These results Indicate that mycosis fungoides tumor cells are present within ACD lesions Induced In mycosis fungoides patients and that this phenomenon does not appear to be due to the ubiquitous presence of detectable levels of these tumor cells in the blood or skin. These findings might be explained by nonspecific recruitment of malignant T cells to sites of local inflammation mediated by non-neoplastic antigen-specific T cells. Alternatively, they might be due to the local proliferation of very rare tumor cells in apparently normal skin in response to cytokines generated during the ACD reaction. In either case, the present study offers evidence that the malignant cells in mycosis fungoides retain at least some capability of responding in vivo to physiologic stimuli. [ABSTRACT FROM AUTHOR]