Antigen presentation in Syrian hamster cells: Substrate selectivity of TAP controlled by polymorphic residues in TAP1 and differential requirements for loading of H2 class I molecules

Expression of mouse major histocompatibility complex (MHC) class I molecules in different cell lines derived from Syrian hamsters has revealed antigen presentation deficiencies of some H2 allelic products in two cell lines (BHK and NIL-2) which were overcome by transient expression of the rat transporter associated with antigen processing. Here we show that in both cell lines the endogenous MHC class I cell surface expression was completely down-regulated. Lymphokine treatment induced endogenous and recombinant mouse MHC class I cell surface expression to levels similar to that in other Syrian hamster cell lines competent for antigen presentation through transduced H2 molecules. Accordingly, constitutive downregulation of expression of accessory molecules of the MHC class I pathway can reveal differences between H2 class I alleles in antigen presentation not encountered when the expression levels are augmented. In addition to the differential expression of MHC class I pathtway genes, two cell lines representing competent (FF) and defective (BHK) antigen presentation phenotypes for mouse class I MHC restriction elements demonstrated substantial sequence polymorphism in Tap1 but not Tap2. Cytokine-treated FF or BHK cells and human TAP-deficient T2 cells transfected with FF or BHK TAP1 in combination with FF TAP2 differed in their preference for C-terminal peptide residues, as shown by an in vitro peptide transport assay. Thus, polymorphic residues in TAP1 can influence the substrate selectivity of the Syrian hamster peptide transporter.