The high frequency of PD-L1 expression in feline fibrosarcoma suggests that, in the unique etiopathogenesis of feline FISS (where chronic inflammation is thought to trigger tumorigenesis [42]), PD-L1 induction by inflammatory signaling (IFNs, IL-6, TNF-α, etc.)[37] may play a pivotal role in the escape from adaptive immune responses. Here, TNF is linked to fibrosarcoma.