Molecular Circuits Driving Human iTfr Cell Development

Recent work has identified and reported a distinct Tfr developmental pathway in which germinal-center resident Tfr cells are derived from Tfh cells [1,2]. These Tfh-induced regulatory cells are referred to as induced Tfr (iTfr) cells [2]. While the existence of a Tfh to iTfr developmental arc has been established in mice and humans, the cell-extrinsic stimuli and cell-intrinsic molecular machinery required for this transition are unclear. Our preliminary data suggest that when cultured alone, human tonsillar Tfh appear terminally differentiated, but when incorporated in a autologous tonsil organoid (TO), around half of these cells spontaneously divide and differentiate into iTfr cells [2]. TOs are an all-human model system containing a range of cell types and stimulatory molecules that recapitulates key features of human follicular biology [2,3]. By intravitally labeling Tfh cells incorporated into TO systems, we have mapped the transcriptome of differentiating cells on a single cell, per-division bases. We have found that expression levels of key Tfh and Tfr transcription factors, including BCL6, BLIMP1, and FOXP3 change on a per-division basis as Tfh differentiate into iTfr cells. These data indicate that a key molecular event in the Tfh-to-iTfr transition is the downregulation of the transcription factor BCL6 and reciprocal upregulation of first BLIMP1 and then FOXP3. On this basis, we hypothesize that cellular and molecular components of TOs induce Tfh cells to adopt a BLIMP1-associated transcriptional program in Tfh cells, which licenses their transition into iTfr cells. Obstruction in this differentiation pathway contributes to autoantibody-production in CVID and potentially other immune conditions [4]. Thus, identifying the specific molecular levers controlling Tfh vs. iTfr cell fate will create translational opportunities to modulate humoral immunity.