Uniquely Conformed Peptide-Containing β2-Microglobulin- Free Heavy Chains of HLA-B2705 on the Cell Surface1

The human class I MHC molecules are known to generally exist on the cell surface either as peptide-containing complexes of H chain (α-chain) and β2-microglobulin (β2m) or as β2m-free H chains incapable of binding peptides. In this study, a uniquely conformed peptide-containing β2m-free HLA-B2705 H chain has been isolated using the recently described highly efficient perfusion-affinity chromatography system for purification of class I MHC protein molecules. This form recognized by the mAb MARB4 is very closely associated with the remainder of the peptide containing HLA-B2705/β2m complex reactive with mAb ME1 and is present to ∼1–10% of mAb ME1 reactive forms on the cell surface. Also, HLA-B2705 purified using the mAb ME1 affinity column includes this unique mAb MARB4-reactive, unusually stable peptide-containing β2m-free form. A peptide nonamer GRWRGWYTY was isolated and identified from this β2m-free HLA-B2705 H chain and was used to assemble the mAb MARB4 reactive form efficiently on the surface of cells expressing HLA-B2705. The discovery of this form opens new avenues for further investigation of the role of HLA-B27 in spondyloarthropathies.