An extra molecule in addition to human tapasin is required for surface expression of beta2m linked HLA-B4402 on murine cell.

Murine MHC class I can be readily expressed on the surface of human cell lines, but human class I molecules are expressed on mouse cells at a reduced level. Both human beta-2-microglobulin (beta(2)m) and tapasin (Tpn) have been demonstrated to be required for proper human MHC class I surface expression. Here we report that besides beta(2)m and tapasin, an extra unidentified component is also critical for the expression of certain human class I alleles. By covalently linking HLA-B4402 heavy chain to beta(2)m (beta(2)m-B44) a pre-assembled class I molecule has been created, which can be efficiently expressed and travel to the surface in human cells. In spite of being able to express inside cells, the linked beta(2)m-B44 molecule does not express on the surface of a murine fibroblast. Further investigation shows that lack of appearance on the surface is not due to quick degradation of unloaded class I, since provision of HLA-B4402 binding peptide could not rescue impaired surface expression. Co-expression with human tapasin does not rescue the defect excluding tapasin as the critical component for expression and indicating that a novel component of human origin is required for efficient surface expression of beta(2)m-B44 in murine cells. Surprisingly, not only did the beta(2)m-B44 construct fail to express on murine cells but also the surface expression of native murine MHC class I Kb was greatly reduced in transfected cells. It is likely that the expressed linked chain competitively associates with a component of class I processing in murine cells, reducing the exit rate of assembled mouse class I molecules. The results together suggest an unknown mechanism, which leads to the trapping of class I molecules in the ER.