Low CD7 expression in benign and malignant cutaneous lymphocytic infiltrates: experience with an antibody reactive with paraffin-embedded tissue.

Loss of CD7 expression by neoplastic lymphocytes is considered a distinguishing characteristic of mycosis fungoides (MF) and cutaneous T-cell lymphoma. Reports to date examining for the CD7 immunophenotype in MF have been performed on fresh-frozen tissue. In this study, we used a paraffin-reactive antibody directed against CD7 to determine its range of expression in MF and to compare these results with those in controls. Examining 22 cases of MF and 61 controls, we found minimal CD7 expression by lymphocytes in MF and in a few cases of benign inflammatory dermatosis (BID). The lowest mean CD7 counts (as a percentage of total lymphocytes) were found in MF (patch stage: 5% +/- 5%, range: 0-10; plaque and tumor stages: 15% +/- 5%, range: 5-25), and these counts were significantly lower than those for BID (35% +/- 20%, range: 5-80; p = 0.001). By logistic regression analysis, low CD7 expression (<10% lymphocytes labeling) had sensitivity and positive predictive values of 80% and 72%, respectively, and specificity and negative predictive values of 93% and 96%, respectively, for the diagnosis of patch stage MF. False-positive results were found for spongiotic dermatitis. Moreover, spongiotic dermatitides exhibited a progressive decrease in mean CD7 counts from acute to subacute to chronic stages (50% versus 35% versus 30%, respectively). In conclusion, minimal CD7 expression is a specific finding for MF. Benign inflammatory infiltrates can also show low CD7 expression, however, which rarely matches that of patch stage MF. Progressive loss of CD7 expression in BID is the likely consequence of expansion of antigen-selected CD3+CD4+CD7- T cells. These inflammatory CD4+CD7- T cells may represent the physiologic counterpart to the neoplastic lymphocyte of MF.