How is evasion of immune surviellence in cancer?

There are tumor antigens (may be mutated or increased normal proteins like tyrosine, or antigens expressed in inappropriate place like cancer-testis Ag, or viral proteins like EBV/HPV) Normally cells should get destroyed by CD8 molecules attacking MHC I, which is evaded by PDL1 acting on PD1 of CTL leading to inhibition of CTL/ or CTLA4 also inhibits CTL interacting w/B7 Ag, so we are using Ab against those Ag in therapy