IDO2 and viral infectious disease: MSCs can produce a direct anti-viral effect by secreting antibacterial peptides and proteins (AMPs), indoleamine 2,3-dioxygenase (IDO), IL-17 and other molecules, and unlike somatic cells that produce interferon during virus invasion and then activate hundreds of genes that resist virus infection, MSCs can continuously activate a large number of anti-virus genes independent of interferon, such as IFITM gene, which can encode protein structures that prevent viruses from invading cells [19] (Table 1).