PCR analyses demonstrated that the increased expressions of IL-21, IL-6, and Bcl-6 mRNA in children with acute HSP were possible explanations for the increased number of circulating CXCR5+CD4+TFH cells and ICOS+CXCR5+CD4+TFH cells in these acute HSP children. Here, ICOS is linked to hereditary spastic paraplegia.