P1215: CHARACTERIZATION AND CLINICAL SIGNIFICANCE OF T FOLLICULAR HELPER SUBSETS IN FOLLICULAR LYMPHOMA

Topic: 20. Lymphoma Biology & Translational Research Background: T follicular helper (TFH) cells are a subset of CD4+ T cells that reside within the follicles of secondary lymphoid organs and essential in the germinal center (GC) reaction. Follicular lymphoma (FL) is a GC-derived malignancy and malignant B cells play a crucial role in the development of the tumor microenvironment. Aims: Given TFH cells are a major subpopulation in the TME and phenotypically heterogenous, in this study, we wanted to characterize intratumoral TFH cells and to examine its clinical significance in FL. Methods: Single cell analysis of mass cytometry (CyTOF) and the Indexing of Transcriptomes and Epitopes by Sequencing (CITE-seq) were performed. Antibody staining list includes 37 surface markers for both CyTOF and CITE-seq. Results: Transcriptomic and proteomic profiling analysis with CITE-seq (n=8) and CyTOF (n=82) revealed that TFH cells are phenotypically more heterogenous in FL than benign tonsil tissues (Fig.1A). Clustering analysis showed that TFH cells form subsets with unique phenotypes. The expression of many common markers including CXCR5, ICOS, IL-21, TOX2, CD57, GZMK, XCL1, XCL2, CXCL13 and CD40L varies on TFH subsets (Fig. 1B). Each FL patient showed different dominant TFH subsets, in contrast to tonsil donors which were highly homogenous. Some TFH subsets were only present in FL and absent in tonsil tissues, suggesting the existence of FL-specific TFH cells. Cytokines (IL-2, IL-12, IL-17 and IL-21) and activated B cells upregulated TFH markers including PD-1, CXCR5 and IL-21 to induce the development of TFH subsets such as IL-21+ or IL-21+IFN-γ+ TFH cells, which was inhibited by rapamycin, an immunosuppressor (Fig. 1C). We observed that the TFH cell frequency significantly correlated with the number of lymphoma B cells. Futhermore, while TFH cells as a whole showed no correlation with prognosis, TFH subsets such as CD57+ TFH cells were significantly associated with inferior patient outcomes. Using CITRUS analysis, we found that the abundance of CD57+ TFH clusters was higher in patients failed event-free survival at 24 months (EFS24) than those achieved EFS24 as well as patients with progression (Prog) than those with complete remission (CR) (Fig. D-E). Summary/Conclusion: Taken together, our results indicate that TFH cells are highly heterogenous in FL with some TFH subsets being tumor specific. We observed that CD57+ TFH subset is associated with inferior patient outcomes, suggesting a detrimental role of this subset in FL.Keywords: T cell, Lymphoma, Tumor immunology, Follicular lymphoma