Antigen processing and presentation by EBV‐carrying cell lines: Cell‐phenotype dependence and influence of the EBV‐encoded LMP1

Burkitt's-lymphoma (BL) lines which have maintained in vitro the tumor-cell phenotype (group-1 BLs) are poor antigen- presenting cells (APC), in spite of a relatively high surface expression of MHC class II. In order to investigate the mechanism of this deficiency, we have compared group-1 BL lines, their sub-lines which have progressed in vitro towards an LCL-like phenotype (group-III BL), and EBV-transformed lymphoblastoid cell lines (LCLs), for their ability to bind and process tetanus toxoid (TT). The uptake and internalization of 125I-labelled TT was equivalent in the 3 cell types. Only LCLs and group-III BL lines were able to process the IT,as shown by the identification of discrete proteolytic products after separation of whole-cell extracts in tricine-SDS-polyacrylamide gels, and by the recovery of TCA-soluble radioactivity in the culture supernatant. Processing of TT was induced by expression of the EBV- encoded membrane protein LMPI in transfected group-I BLs. The present findings suggest that the inability of group-I BLs to act as APC is due to their failure to process exogenous antigens. This function appears to be related to phenotypic properties that can be modulated by the expression of LMPI.