Follicular Lymphoma Regulatory T Cell Function and Origin

Follicular Lymphoma (FL) is the most frequent indolent lymphoma and results from the malignant transformation of germinal center (GC) B cells. Among FL recurrent genetic alterations, some are not only oncogenic per se but favor the interactions of FL B cells with their microenvironment including in particular surrounding follicular helper T cells (Tfh). During normal GC reaction Tfh support early GC B-cell selection and differentiation whereas follicular regulatory T cells (Tfr) expand later and inhibit both Tfh activity and B-cell antibody secretion. Whereas the role of Tfh in human diseases is an intense matter of interest, very few data are available concerning the origin and function of human Tfr. We previously demonstrated that Tfh and Tfr proportions are increased within FL lymph nodes and FL-Tfh contribute, through a specific polarization profile, to FL B cell survival and drug resistance. Interestingly FL-Tfr expansion is correlated to the one of FL-Tfh but not FL-Treg. The aim of this work was to characterize human Tfr in non-malignant (chronically inflamed tonsils) and malignant (FL) settings through transcriptomic, DNA methylation, TCR repertoire, and functional studies. We demonstrated for the first time that human Tfr purified from FL samples and tonsils directly inhibit autologous Tfh proliferation and trigger a downregulation of the co-stimulatory markers CD86 and HLA-DR on autologous normal and malignant GC B cells. In agreement FL-Tfh proliferated less than tonsil-Tfh in situ and after in vitro restimulation. Moreover, we found that Tfr clustered with Treg at both transcriptomic and methylome levels including for the high demethylation level of the FOXP3 TSDR, suggesting a stable regulatory phenotype. However, TCR sequencing demonstrated that tonsil-Tfr shared a common repertoire only with tonsil-Tfh unlike tonsil-Treg, arguing for their peripheral unlike thymic origin. Conversely in FL, even if the majority of common clones were found between Tfr and Tfh, a small proportion of TCR repertoire was shared between Tfr and Treg. Altogether, these results are the first detailed characterization of human Tfr and challenge our current understanding of Tfr commitment pathway. FL-Tfr retain their capacity to inhibit Tfh thus questioning their role in FL and their potential as therapeutic target in this fatal malignancy.