CXCR5 and COVID-19: Furthermore, low frequencies of CD45RA-PD-1+CXCR5+cTfh cells were also observed, but elevated frequencies of activated cTfh (CD38+ICOS+) cells were positively correlated with anti-SARS-CoV-2 IgM and IgG titers in hospitalized COVID-19 patients (154).