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In vivo characterisation of Follicular Helper T cells

Final Report Summary - TFHCELLS (In vivo characterisation of Follicular Helper T cells)

Project Objectives
After initial priming with an antigen (Ag)-experienced Dendritic cell (DC), Ag-specific T Helper (Th) cells are clonally selected and expand drastically. These events result in the development of effector Th cells with a variety of functions determined by the quality and context of Ag priming. Follicular helper T (Tfh) cells are considered as the critical regulators of the B cell response. Until recently, Tfh cells were considered fully differentiated cells prone to apoptosis once the germinal center (GC) reaction resolved. However, we obtained compelling evidence that effector TFH cells become memory Tfh cells that remain in draining lymphoid tissue. Recent evidence suggest that effector TFH cells constitute a distinct lineage of Th cells with unique developmental program mediated by IL-21and ICOS engagement and regulated by the transcription factor Bcl-6. Our first objective was to assess the genetic program imprinted in TFH cells by unique transcription factor(s), the role of cytokines or co-stimulatory signals and the impact of the inflammatory context in controlling TFH differentation in vivo. In our second objective, we addressed study the mechanisms that underpin memory TFHcell organization at the cellular and molecular level and more precisely, how pMHCII impact the basis of memory TFH cell reservoir, maintenance and organization and how one can modulate this pool of memory cells.

Work performed
All the studies concerning this proposal were performed using mouse models. In order to get insights into the difficult facets of T-dependent B cell responses after protein immunization, we developed Ag models that allow the tracking by Flow cytometry and microscopy of Ag-specific Th cells, Ag-specific B cells and Ag-presenting cells in wild-type non modified animals.

Main results
We found that adjuvantation with CpG, the Toll-like Receptor 9 agonist, increased the differentiation of Tfh cells without changing the overall magnitude of the T cell response. This phenomenon correlated with an enhancement of the GC reaction and memory B cells. We comprehensively demonstrated that, in addition to conventional DC that mediated the classical Tfh differentiation pathway, monocyte-derived DC (moDC) orchestrated this enhancing effect as an alternative pathway through Ag presentation and IL-6 secretion. Furthermore, we showed that BCR affinity regulated B cell fate through the regulation of the expression of co-stimulatory molecules important for the crosstalk between Ag-primed B cells and TFH cells. Finally, we assessed the diversity of memory TFH cell subsets as well as their cellular interactions with memory B cells and found that circulating memory TFH cells largely emerged from the pool of the local ones.

Expected Results
By getting fundamental aspects on how effector TFH cells and memory TFH cells develop and how their crosstalk with B cells impact memory B cell formation we have more informations into these complicated mechanisms that would permit to manipulate them either in physiological conditions (vaccine) or in pathological conditions such as certain lymphoma or antibody-mediated autoimmune diseases.