Endoplasmic reticulum signal sequence facilitated transport of peptide epitopes restores immunogenicity of an antigen processing defective tumour cell line.

The identification of MHC-restricted and tumour-specific cytotoxic T lymphocytes (CTLs) provides strong evidence in support of T cell-mediated immune surveillance against human tumour cells. These CTLs recognize short peptides derived from tumour-associated antigens in conjunction with class I molecules expressed on tumour cells. In contrast to these observations there are now numerous examples to suggest that a number of tumours escape this CTL-mediated control either by down-regulating accessory molecules or by blocking the intracellular processing of tumour-specific antigens. Recently a number of tumour cell lines have been identified which display a transcriptional deficiency of transporters associated with antigen processing (also referred to as TAP). The Epstein-Barr virus (EBV)-associated tumour, Burkitt's lymphoma (BL), is a classic example in this category. In the present study we have restored class I-restricted antigen processing in a BL cell line by transfecting a minigene expression vector encoding a CTL epitope derived from EBV linked to an endoplasmic reticulum translocation signal sequence. These minigene transfected BL cells were not only susceptible to lysis by virus-specific CTL but were also capable of efficiently activating an antigen-specific CTL response. Interestingly, the immunogenicity of these BL cells was not affected by the significantly down-regulated expression of adhesion molecules such as LFA1 alpha, LFA1 beta and LFA3. These findings suggest that resistance of tumour cells to CTL-mediated immune control can be reversed if the relevant peptide epitopes are appropriately presented on the cell surface.