PD-1 Expression Defines Two Distinct T-Cell Subpopulations That Differentially Impact Patient Outcomes In Follicular Lymphoma

The role of intratumoral PD-1⁺ T cells and their contribution to patient outcome in follicular lymphoma (FL) has been controversial. While some studies have found that increased numbers of PD-1⁺ T cells correlate with an inferior prognosis or have no impact on patient outcomes, other studies observed that the numbers of intratumoral PD-1⁺ T cells are an indicator of a favorable outcome in FL. In previous work, we observed that PD-1 can be expressed by different subsets of intratumoral T cells in FL including T follicular helper (T_FH) cells and exhausted T effector cells. However, the function, prevalence, distribution and clinical importance of the respective intratumoral PD-1⁺ T subsets in FL patients are largely unknown.

To characterize PD-1⁺ T cell subsets and to determine the biological and clinical relevance of these T cell subsets in FL.

In an analysis of biopsy specimens from 33 previously untreated FL patients, we found that PD-1 is abundantly expressed on intratumoral T cells. A median of 54.9% (range: 17.2-92.6, n=33) of CD4⁺ or 45.8% (range: 12.8-81.7, n=33) of CD8⁺ T cells express PD-1 on the cell surface. We found by flow cytometry that PD-1 is expressed on intratumoral CD4⁺ T cells with bright or dim intensity that represents two different subpopulations in FL. By immunohistochemistry, we identified that CD4⁺PD-1^bright T cells predominantly reside in the lymph node follicles while PD-1^dim T cells are mainly located in an inter-follicular pattern. Intratumoral CD4⁺PD-1^bright T cells have a T_FH cell phenotype and express CXCR5, secret IL-21 and are BCL-6 positive with no TIM-3 expression. In contrast, CD4⁺PD-1^dim T cells have an exhausted phenotype, express TIM-3 and do not express BCL-6 and CXCR5. Functionally, CD4⁺PD-1^dim T cells displayed reduced proliferation, cytokine production and cell signal transduction. When the CD8⁺ T cells were analyzed, we found that intratumoral CD8⁺ T cells, primarily reside in the inter-follicular regions, express PD-1 with low intensity and exhibit similar phenotypic and functional changes to inter-follicular CD4⁺PD-1^dim T cells. These cells express TIM-3 but lack CXCR5 and BCL-6. They also displayed reduced proliferation and cytokine production. Clinically, we observed that both the numbers of CD4⁺PD-1^dim and CD8⁺PD-1^dim exhausted T cells significantly correlate with a reduced progression-free survival in FL patients treated with single agent rituximab (p=0.007 and p=0.04 respectively; n=31). Although it did not reach statistical significance, CD4⁺PD-1^bright T_FH cells appeared to correlate with a favorable survival, suggesting that T_FH and exhausted T cells differentially impact patient outcome in FL.

Taken together, these results indicated that PD-1 expression defines two subpopulations with distinct functions that differentially impact patient outcome in FL. This finding provides not only an explanation for the previously discrepant clinical observations, but is also important in the design of future anti-PD-1 antibody-based therapy in FL.

No relevant conflicts of interest to declare.