ICOS and influenza: cTfh cells share phenotypic and functional properties to GC Tfh cells, and it has been shown that inactivated influenza vaccination (IIV) induces expansion and PD-1/ICOS-activation of CD4+CXCR5+CXCR3+ cTfh type-1 (cTfh1) cells, which correlated with antibody and CD19+CD27++CD38++ ASC responses4,32.