Thermolabile H-2K^b molecules expressed by transporter associated with antigen processing-deficient RMA-S cells are occupied by low-affinity peptides

RMA-S cells do not express functional TAP, yet they express MHC class I molecules at the cell surface, especially at reduced temperatures (26°C). It is generally assumed that such class I molecules are 'empty,' devoid of any associated peptide. A radiochemical approach was used to label class I- associated peptides and to determine the extent to which K^b molecules in RMA-S cells are associated with peptides. These studies revealed that at 26°C K^b molecules in RMA-S cells are occupied with self-peptides. Such peptides stably associate with K^b at 26°C but easily dissociate from them at 37°C, suggesting low-affinity interactions between K^b and the associated peptides. At 26°C, at least some of these K^b molecules are stably expressed in a peptide-receptive state on the cell surface, whereas at 37°C they are short lived and are only transiently capable of binding and presenting exogenously supplied OVA 257-264 peptide for presentation to CD8⁺ K^b- restricted T lymphocytes. Thus contrary to current models of class I assembly in TAP-deficient RMA-S cells, the presumably 'empty' molecules are in fact associated with peptides at 26°C. Together, our data support the existence of an alternative mechanism of peptide binding and display by MHC class I molecules in TAP-deficient cells that could explain their ability to present Ag.