CD4 and hemorrhagic fever with renal syndrome: Firstly, ex vivo granzyme B enzyme-linked immunospot (ELISPOT) assay showed that the HTNV-Gn/Gc epitope peptide pools could stimulate CD4+T cells of HFRS patients to produce granzyme B with the average magnitude of 450 SFC/106 cells (range, 21–1,226) (S6 Fig, n = 11 for mild/moderate and severe/critical patients, respectively).