The MHC-encoded class I molecule, H-2K^k, demonstrates distinct requirements of assembly factors for cell surface expression: Roles of TAP, Tapasin and β₂-microglobulin

Major histocompatibility complex encoded class I (MHC-I) molecules display peptides derived from endogenous proteins for perusal by CD8⁺ T lymphocytes. H6, a mouse hepatoma cell line, expresses low levels of surface H-2D^d but not H-2K^k. Surface H-2D^d molecules are unstable and their levels, but not H-2K^k, are induced at 22°C. Immunoprecipitation experiments revealed that H-2K^k, H-2D^d and β₂-microglobulin (β₂m) are expressed intracellularly; however no conformed MHC-I are present. Transcriptional profiling of factors required for MHC-I assembly demonstrated greatly reduced levels of the Transporter associated with antigen processing (Tap)2 subunit. The role of key assembly molecules in the MHC-I pathway was investigated by ectopic expression studies. Overexpression of β₂m enhanced surface H-2D^d, but not H-2K^k, levels whereas overexpression of TAP2 rescued surface H-2K^k, but not H-2D^d, levels. Interestingly, Tapasin plays a dual role: first, in quality control by reducing the induced surface expression of TAP2-mediated H-2K^k and β₂m-mediated H-2D^d levels. Secondly, Tapasin overexpression increases Tap2 transcripts and cooperates with TAPl or human β₂m to enhance surface H-2K^k expression; this synergy is TAP-dependent as demonstrated by infected cell protein 47 (ICP47) inhibition studies. Unlike the well studied H-2 MHC-I alleles, H-2K^b, H-2D^b, H-2K^d and H-2D ^d, a functional TAP is "essential" for H-2K^k cell surface expression.