CXCL9 and viral infectious disease: While the expression of CXCL9 and CXCL10 has been used as a surrogate for IFN-I expression, these ligands can be induced during acute and chronic viral infection or recall responses by both type I and II IFNs (Borriello et al., 2022; De Giovanni et al., 2020; Sung et al., 2012).