Cutting Edge: Murine UL16-Binding Protein-Like Transcript 1: A Newly Described Transcript Encoding a High-Affinity Ligand for Murine NKG2D

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Abstract
Murine NKG2D is known to recognize H60 and five RAE1 variants. The human homologue recognizes both inducible MHC class I chain-related gene and constitutive (UL16-binding protein (ULBP)) ligands. Widely expressed, the latter are thought to mark transformed or infected cells for destruction by NK cells in the context of down-regulated cell surface class I (i.e., the “missing self”-response). Unlike MIC and ULBP however, mRNA for the murine ligands appears only in very limited contexts in the mature animal. In this study, we describe a NKG2D ligand termed “murine ULBP-like transcript 1 (MULT1) whose mRNA appears to be widely expressed in adult parenchyma. This molecule possesses MHC class I-like α1 and α2 domains as well as a large cytoplasmic domain. Recombinant MULT1 binds NKG2D with relatively high affinity (KD ≈ 6 nM) and low koff (∼0.006s−1). Expression of MULT1 by normally resistant RMA cells results in their susceptibility to lysis by C57BL/6 splenocytes.