Quantitation of intraepidermal T-cell subsets in formalin-fixed, paraffin-embedded tissue helps in the diagnosis of mycosis fungoides

Differentiation between mycosis fungoides (MF) and cutaneous inflammatory p rocesses can usually be made on clinical and histologic grounds. In difficu lt cases, immunohistochemical studies can be helpful since MF infiltrates u sually contain a predominance of CD4+ lymphocytes, while most inflammatory lesions usually have a mixture of CD4+ and CD8+ lymphocytes. However, this determination has traditionally required the use of frozen tissue, thus sev erely limiting its usefulness. Recently, antibodies that differentially lab el CD LS and CD8+ lymphocytes in formalin-fixed, paraffin-embedded tissue h ave become available (OPD4 and C8/144B respectively, DAKO (Carpinteria, CX, USA). This study tests the utility of these antibodies in the pathologic d iagnosis of MF and inflammatory lesions with significant exocytosis, In 9 c ases of MF for which both frozen and fixed tissues were available for compa rison, the OPD4+ cell count in fu;ed tissue was significantly lower than th e Leu-3a+ cell count in frozen tissue, Also, the C8/144B+ cell count in fix ed tissue was higher than the Leu-2a+ cell count in frozen tissue, although this difference was not significant statistically. In a larger series for which only fixed tissue was available, epidermal CD4:CD8 ratios were signif icantly greater in 23 MF cases (mean 4.0+/-4.76) than in 35 inflammatory ca ses (mean 0.6+/-0.42; p=0.001). Thus, although the studied antibodies appea r to detect different epitopes in frozen versus paraffin-embedded tissue, d emonstration of an elevated CD4:CD8 ratio in fixed tissue supports the diag nosis of MF, and is a helpful adjunct to routine histopathology.