Tumor evasion mechanisms in PTLD. (A) Donor EBV CTLs are generated by stimulation with donor EBV-LCLs transformed with the B95 laboratory strain of EBV. TCRs in clones in the donor CTLs will recognize EBV peptides derived from antigens in B95 in the context of different donor HLA molecules. (B) Recipient tumors may fail to be susceptible to donor EBV CTLs if (1) the tumor cells are transformed with an EBV strain variant that differs from B95 so that the activity of the line is directed at epitopes found in B95 but not found in the tumor variant, (2) the activity in the line is restricted by an HLA type not found in the recipient that may occur if the recipient and the donor are mismatched and the PTLD is of recipient origin or if the donor is third-party and EBV activity is mediated through nonshared antigens. Professional illustration by Debra T. Dartez.