For the association between Tfh cell proportion and anti-PGA antibody, PGA-IgA was correlated with the CD4+ CXCR5+ (r = 0.380 and P = 0.042) and CD4+ CXCR5+ ICOS+ Tfh in the HSP group (r = 0.906 and P < 0.01, Figure 2a and b). This evidence concerns the gene ICOS and hereditary spastic paraplegia.