MX2 and viral infectious disease: In spite of the up-regulation of antiviral ISGs such as IFITM1, IFITM3 and MX2 by IFN-α treatment, why was the IFN-α-mediated antiviral effect canceled in the following two experiments: (1) multi-round virus infection in the shaking culture that only allows cell-free infection; and (2) single-round virus infection at relatively higher densities of cells and viruses?