Germinal responses in murine Peyer’s Patches are regulated by a subpopulation of microbiota-induced IL-21high Tfh cells

IL-21 is the signature cytokine produced by T follicular helper (Tfh) cells and is vital in driving both the germinal centre (GC) reaction and formation of high affinity antibodies. However, the degree of Tfh cell heterogeneity and function is not fully understood. By utilising a novel IL-21eGFP reporter mouse carrying a diphtheria toxin receptor (DTR)-enhanced green fluorescent protein (eGFP) fusion gene, we identified a subpopulation of highly differentiated Tfh cells within Peyer’s Patches (PPs). This subpopulation demonstrated highest expression of the key Tfh molecules - Bcl6, ICOS and IL-21, and was found within germinal centres and surrounding B cell areas. TCRb repertoire analysis of GFP + Tfh cells revealed that these were polyclonal and closely related to GFP − Tfh cells, indicating selection by common antigens. In vitro s timulation of IL-21eGFP CD4 + cells in the presence of IL-6, TGFb and retinoic acid led to the induction of GFP + cells that did not co-express IL-17 or Foxp3. Treatment of IL-21eGFP mice with antibiotics led to an ablation of PP GFP + CD4 + cells meaning that the presence of these cells relies on an intact bacterial microbiota. Furthermore, both polyclonal CD4 + T cell and B cell activation is necessary to support the differentiation of GFP + Tfh cells. Finally, GFP + cell depletion resulted in a reduced frequency of PP GC B cells and IgG1 + cells and small but significant shifts in gut microbiome composition. Therefore, using a new IL-21-reporter mouse we have identified a subpopulation of Tfh cells induced by the gut microbiota and necessary for optimal PP GC and IgG1 responses.