GM-CSF-activated human dendritic cells promote type1 T follicular helper cells (Tfh1) polarization in a CD40-dependent manner

T follicular helper (Tfh) cells are specialized CD4+ T cells that regulate humoral immunity by providing B cell help. Tfh1 sub-population was recently identified and associated with severity in infection and autoimmune diseases. The cellular and molecular requirements to induce human Tfh1 differentiation are unknown. Our work investigated the role of human dendritic cells (DC) in promoting Tfh1 differentiation and their physiopathological implication in mycobacterium tuberculosis and mild COVID-19 infection. Activated human blood CD1c+ DC were cocultured with allogeneic naive CD4+ T cells. Single-cell RNA sequencing was then used alongside protein validation to define the induced Tfh lineage. DC signature and correlation with Tfh1 cells in infected patients was established through bioinformatic analysis. Our results show that GM-CSF-activated DC drove the differentiation of Tfh1 cells, displaying typical Tfh molecular features, including 1) high levels of PD-1, CXCR5, and ICOS expression; 2) BCL6 and TBET co-expression; 3) IL-21 and IFN-γ secretion. Mechanistically, GM-CSF triggered the emergence of two distinct DC sub-populations defined by their differential expression of CD40 and ICOS-ligand (ICOS-L), and distinct phenotype, morphology, transcriptomic signature, and function. We showed that Tfh1 differentiation was efficiently and specifically induced by CD40highICOS-Llow DC in a CD40-dependent manner. Tfh1 cells were positively associated with a CD40highICOS-LLow DC signature in patients with latent mycobacterium tuberculosis and mild COVID-19 infection. Our study uncovers a novel CD40-dependent human Tfh1 axis. Immunotherapy modulation of Tfh1 activity might contribute to control diseases where Tfh1 are known to play a key role, such as infections. Significance Statement Dendritic cells (DC) play a central role in triggering the adaptive immune response due to their T cell priming functions. Among different T cell subsets, it is still not clear how human type1 T follicular helper cells (Tfh1) differentiate. Tfh1 cells are implicated in several physiopathological conditions, including infections. Here we show that GM-CSF induces diversification of human DC. Only CD40highICOS-LLow DC were able to drive Tfh1 cell differentiation. We found that CD40highICOS-LLow DC signature was associated to Tfh1 cells in mycobacterium tuberculosis and COVID-19 patients. Our data reveal a previously undescribed pathway leading to human Tfh1 cell differentiation and highlight the importance of GM-CSF and CD40 as potential targets for the design of anti-infective therapies. ### Competing Interest Statement The authors have declared no competing interest.