Expression of the W6/32 HLA epitope by cells of rat, mouse, human and other species: critical dependence on the interaction of specific MHC heavy chains with human or bovine β₂-microglobulin

The HLA class I epitope W6/32 is conformationally dependent on both heavy chain and β₂-microglobulin (β₂M). Previously, the W6/32 epitope has been detected in humans and other primates as well as from bovine sources. Two controversial reports suggest the W6/32 epitope is constitutively expressed by either normal or transformed murine cells expressing the D^b allele. Here we show that the appearance of the W6/32 epitope in murine cells results from the association of either the D^b or K^d gene products with either bovine or human β₂M. We use congenic mouse strains and hybrid H-2 class I genes between D^b and K^b to map the W6/32 epitope to particular amino acid residues in the α₂ domain. Subsequently, we show that β₂M exchange is not confined to murine or human cells in vitro but can be detected after β₂M injection into a mouse. The data presented suggests that β₂M exchange takes place at the cell surface under physiological conditions and indicates that MHC class I heavy chains are in an equilibrium between the bound and unbound form of β₂M.