The MHC-encoded class I molecule, H-2Kk, 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 22degreesC. Immunoprecipitation experiments revealed that H-2K(k), H-2D(d) and beta(2)-microglobulin (beta(2)m) 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 beta(2)m 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 beta(2)m-inediated H-2D(d) levels. Secondly, Tapasin overexpression increases Tap2 transcripts and cooperates with TAP1 or human beta(2)m 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. (C) 2004 Elsevier Ltd. All rights reserved.