To determine the biological and clinical relevance of programmed death 1 (PD-1) in follicular lymphoma (FL), we characterized PD-1+ T-cell subsets and assessed their biological function as well as potential clinical impact. We found that PD-1 is expressed on intratumoral CD4+ T cells with both bright and dim intensity, representing two different sub-populations of cells. By immunohistochemistry, we found that CD4+PD-1high T cells predominantly reside in the lymph node follicles, while PD-1low T cells are mainly located in an interfollicular pattern. Intratumoral CD4+PD-1high T cells have a TFH cell phenotype, express CXCR5, secrete IL-21 and are BCL-6 positive with no TIM-3 expression. In contrast, CD4+PD-1low T cells have an exhausted phenotype, express TIM-3 and do not express BCL-6 and CXCR5. Functionally, CD4+PD-1high T cells actively supported B-cell growth, while CD4+PD-1low T cells displayed a reduced cytokine production and cell-signal transduction. Clinically, we observed that the numbers of CD4+ or CD8+ PD-1low T cells significantly correlate with a reduced overall survival in FL patients (P = 0.007 and 0.04 respectively; n = 32). In contrast, the number of CD4+PD-1high T cells was not associated with patient outcome. Taken together, these results indicated that PD-1 expression defines two sub-populations with distinct functions that differentially impact patient outcome in FL.
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