Cutting edge: Murine UL16-binding protein-like transcript 1: A newly described transcript encoding a high-affinity ligand for murine NKG2D

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 al and α2 domains as well as a large cytoplasmic domain. Recombinant MULT1 binds NKG2D with relatively high affinity (K_D ≈ 6 nM) and low k_off (∼0.006s^-1). Expression of MULT1 by normally resistant RMA cells results in their susceptibility to lysis by C57BL/6 splenocytes.