S100A4 and viral infectious disease: Mechanically, it was demonstrated that interferon-γ secreted by S100A4 + macrophages induced the tight junction protein Claudin-1 to translocate from the plasma membrane into the nuclei, thus increasing the permeability of the blood–testis barrier (an indispensable structure surrounding the seminiferous tubules and protecting the spermatogenic cells inside from viral infection and immune attack).