Calnexin expression does not enhance the generation of MHC class I-peptide complexes

We investigated the requirement for calnexin in the biogenesis of MHC class I molecules. Mutant human cells lacking calnexin were infected with recombinant vaccinia viruses encoding mouse MHC class I molecules, K ^d , K^b , K^k , D ^d , D^b , and L^d . Flow cytometry indicated that each of the six MHC class I allomorphs was transported to the cell surface at similar rates in calnexin-deficient cells and transfectants expressing calnexin. For K^b and K ^d , the calnexin-independent biogenesis occurred regardless of whether the MHC class I molecules contained human or mouse β2-microglobulin. Also addressed was the effect of calnexin on the surface expression of K^b molecules bearing the immunodominant peptide from ovalbumin (OVA_{257 – 264} ). This was detected with a recently described monoclonal antibody specific for the K^b/peptide complex. Calnexin expression had no significant effect on the formation of K^b /peptide complexes generated from full-length OVA, cytosolic OVA_{257 – 264} , or endoplasmic reticulum-targeted OVA_{257 – 264} , which was expressed in the presence of the herpes simplex virus ICP47 protein to ensure detection of TAP-independent peptide-MHC class I complexes. Complementary results were obtained with TAP-independent formation of K ^d /peptide complexes. These findings indicate that calnexin is not required for the efficient assembly of MHC class I molecules with TAP-dependent or independent peptides.