Differential requirement for tapasin in the presentation of leader- and insulin-derived peptide antigens to Qa-1^b-restricted CTLs

The loading of MHC class I molecules with peptides involves a variety of accessory proteins, including TAP-associated glycoprotein (tapasin), which tethers empty MHC class I molecules to the TAP peptide transporter. We have evaluated the role of tapasin for the assembly of peptides with the class Ib molecule Qa-1^b. In normal cells, Qa-1^b is predominantly bound by a peptide, the Qa-1 determinant modifier (Qdm), derived from the signal sequence of class Ia molecules. Our results show that tapasin links Qa-1 ^b to the TAP peptide transporter, and that tapasin facilitates the delivery of Qa-1^b molecules to the cell surface. Tapasin was also required for the presentation of endogenous Qdm peptides to Qdm-specific, Qa-1^b-restricted CTLs. In sharp contrast, tapasin expression was dispensable for the presentation of an insulin peptide to insulin-specific, Qa-1^b-restricted CTL isolated from TCR transgenic mice. However, tapasin deficiency significantly impaired the positive selection of these insulin-specific, Qa-1^b-restricted transgenic CD8⁺ T cells. These findings reveal that tapasin plays a dilferential role in the loading of Qdm and insulin peptides onto Qa-1^b molecules, and that tapasin is dispensable for retention of empty Qa-1^b molecules in the endoplasmic reticulum, and are consistent with the proposed peptide-editing function of tapasin.