Thread: What could explain different effects of protein manipulation in two human cell lines?

If I knockdown a protein in two different human cells (both have the same relative levels of the protein), what could explain different effects seen after subsequently infecting them with a virus? For instance, if I knockdown the protein in both human cell types then try to infect them with a virus and in one cell I see antiviral effects coming from the knockdown but no effect in the other.

different lines have different genetics.
There could be a compensatory mechanism that is mutated/absent in the one that showed no antiviral effects.

The one that still showed antiviral effects could have a compensatory mechanism that meant that even when one protein was knocked out, the cell could still switch on other antiviral genes but if this was absent In the other line, knocking out the protein could mean that it loses it's only antiviral protein.