The organization and composition of the immune system change over time and vary depending on the tissue being studied. These changes can result from aging and hormonal influences or from adaptations to the cellular environment and antigenic challenges. Therefore, in this axe, our goal is to dissect the ancestor, location, and phenotype of the Tfr compartment across different age and human tissues (blood, tonsils, lymph nodes, GALT, BALT): our previous study indicates that the Tfr compartment is composed of two cell types with two reactivities, identifiable by clonal affiliation, phenotype, and location within the follicle. Experiments determine if circulating and lymph node-resident nTfr/iTfr cells are, as in tonsils, clonally, phenotypically, and locally distinct.

The biology of Tfr compartments across various tissues and ages are elucidated using cutting-edge technologies, such as single-cell omics, multiplex microscopy, and multicolor flow cytometry-areas in which our team has developed significant expertise.

In this axe, we aim to elucidate the biogenesis and biological role of the human Tfr compartment.

• Human Tfr cell function

For many years, Tfr cell role on humoral response has been limited to the control of GC reaction magnitude and the emergence of autoreactive B cell. Recently, a study has extended the role of IL-10⁺ Tfr cells to a positive regulatory function by promoting antigen-specific B cell response. Our data showed a suppressive function of tonsillar nTfr and iTfr cells on T-cell proliferation and a beneficial effect of iTfr cell on follicular B-cell, further supporting the observation from these recent studies. Therefore, Tfr cell diverse functional properties could be related to their polyclonal affiliation. Using in vitro co-culture approach, we characterize and compare nTfr and iTfr cell role(s) during the three main GC reaction steps: initiation, maintenance, and regression.

• Human Tfr cell development

During our previous study, we found in the Tfr pool, clones shared with Treg and Tfh population. Using in-silico transcriptomic trajectory projections, pseudotime projection and RNA velocity, we showed human Tfh cells are not terminally differentiated but some of them are predicted to feed the Tfr pool through a CD25^hi Tfh cell intermediary stage. To explore Tfr cell developmental trajectories predicted by bioinformatics, we are modellingthe formation of the Tfr compartment using tonsillar organoids, a structure mimicking a secondary lymphoid organ created from tonsillar cells. Alternatively, to explore a possible transcriptional signature of cells known to contribute to the Tfr pool, we are taking advantage of the single cell TCR repertoire and transcriptome libraries previously generated to track down Treg and Tfh cells sharing clones with Tfr cells.

Although the GC reaction is closely monitored, some exterior parameters, such as age, genetics, and/or the presence of pathogens, can alter its output and integrity. Indeed, systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA), multifactorial autoimmune diseases, are prototypical example of a GC autoantibody-driven autoimmune disease. On the other hand, age-declined immune system, also described as immuno-senescence, is associated with an increased vulnerability to infectious diseases and lower benefits of vaccination. Although ageing and SLE immune systems refer to two opposite types of defective immune responses, both immunopathologies are linked to changes happening within GCs impacting its efficacy and integrity. The forces affecting the efficiency, and the regulation of aged and SLE GCs are still under investigation.

In this axe, we aim to characterize Tfr compartment impact on GC integrity and output, in immunopathologic contexts.dysregulated GCs is the common trait of many immunopathologies. Follicular cell assessment from patients affected by two different types of defective immune responses, such as autoantibody production (SLE, RA) and poor vaccine response (ageing), will be used to determine the cellular and molecular environment necessary to mimic and correct the impairment using in vitro models.