Deep immunophenotypic dissection and clinical impact of T cells in the follicular lymphoma microenvironment

Follicular lymphoma (FL) is an indolent B-cell lymphoma with a heterogenous disease course, and patients may not require immediate treatment upon diagnosis. Scrutiny of its microenvironment may provide key insights into lymphomagenesis and enhancement of therapeutic options. We analyzed the T-cell composition of a large, well-annotated follicular hyperplasia (FH; N=43) cohort utilizing standardized high dimensionality flow cytometry (>150,000 cells analyzed/sample) and a novel reproducible analytical pipeline leading to identification of even minor T-cell subsets. This baseline reference set was compared to prospectively collected FL samples (N=91) from untreated patients (FL-UT) and patients with relapsed/refractory disease (FL-RR). Compared to FH, both FL-UT and FL-RR specimens exhibited depletion of CD4+ and CD8+ naïve subsets and were characterized by an immune suppressive microenvironment enriched in specific inhibitory T cells, along with exhausted memory T cells overexpressing varying combinations of immune checkpoint receptors. FL specimens showed enrichment of T-follicular regulatory cells (TFR) and two highly suppressive regulatory T-cell (T-reg) populations expressing TIGIT and CTLA4 (TC) and PD1, TIGIT, CTLA4, and TIM3 (PTCTi). FL-UT cases with either increased T-reg TC or increased T-follicular helper cells (TFH) showed reduced time to first treatment (</= 3 months). Our study suggests that changes in the balance between TFR, T-reg and TFH may lead to greater tumor growth and identifies factors that are associated with earlier time to treatment in FL-UT. We also identified specific combinations of immune checkpoint receptors that may be used to target specific inhibitory T-cell subsets and regulatory cells in FL to increase anti-tumor immune response.