Quantitative flow cytometric identification of aberrant T cell clusters in erythrodermic cutaneous T cell lymphoma. Implications for staging and prognosis

Aims: Assessment of peripheral blood tumour burden for staging of cutaneous T cells lymphoma is most often accomplished by flow cytometry (FC) using various nonstandarised strategies. We report the results of calculating absolute Sezary cell counts (SCCs) by FC, based on the identification of aberrant T cell clusters on a virtual 6-dimensional space and independently of the expected immunophenotype (6D-FC SCC). Methods: 6D-FC SCCs were calculated on 65 peripheral blood specimens from 28 patients with erythrodermic cutaneous T cells lymphoma (stage III or IV). Comparisons were made with recommended FC strategies and correlations with overall mortality were studied. Results: At first visit, 17 of 28 patients (61%) had 6D-FC SCCs meeting current criteria for Stage IV disease (=1000 SC/μL); while only 2 patients (7%) met Stage IV criteria on other tissues. As defined by comprehensive staging using clinicomorphological criteria and 6D-FC SCCs, Stage IV disease identified a subgroup of patients with worse overall survival (p=0.0227). Residual nonaberrant CD4 T cells were markedly decreased in Stage IV disease ( p=0.018). Among 65 specimens, discrepancies were observed between 6D-FC SCCs and usual FC thresholds for Stage IV disease, namely a CD4:CD8 ratio =10:1 (9 discrepancies, 14%), and =40% aberrant CD4 T cells (4 discrepancies, 6%). Surprisingly, 8 cases (12%) from 6 patients exhibited two distinctively separate clusters of aberrant CD4 T cells with different CD7 and/or CD26 expression. Conclusions: Visual 6-dimensional identification of aberrant T cell clusters by FC allows for the calculation of clinically significant SCCs. Simplified gating strategies and relative quantitative values might underestimate the immunophenotypical complexity of Sezary cells.