Cloning And Characterization Of KLRL1, A Novel C-type Lectin-like Inhibitory Receptor Of Natural Killer Cell

NK cells are crucial for innate host defense against certain tumor cells and pathogens, and in particular, against viral infections. The susceptibility of tumor targets to natural killing is inversely related to target-cell expression of MHC class I molecules, which formed the basis for the "missing-self hypothesis. "Missing-self is now explained by the expression of NK cell inhibitory receptors specific for MHC class I molecules. In humans, there are three distinct families of inhibitory receptors for HLA class I molecules: (1) killer cell Ig-like receptors (KIR), which are type I transmembrane molecules belonging to the immunoglobulin (Ig) superfamily; (2) immunoglobulin like transcripts (ILT), which are expressed mainly on B, T and myeloid cells, although some members are also expressed on NK cells; (3) killer cell lectin-like receptors (KLR), which are encoded by the NK gene complex (NKC), which are type II transmembrane glycoproteins. Inhibitory receptors mediate their effects through the immunoreceptor tyrosine-based inhibitory motifs (ITIM) present in their cytoplasmic domain, which becomes tyrosine phosphorylated by a src-family tyrosine kinase on ligand binding. The src-family tyrosine kinases include SH2-domain-containing protein tyrosine phosphatase 1 (SHP-1), SHP-2 and SH2-domain-containing inositol polyphosphate 5' phosphatase (SHIP1). SHP-1, in particular, has been demonstrated to associate with phosphorylated ITIMs, and to mediate inhibition of NK cell cytotoxicity.Several different NKC-encoded KLR families have thus far been identified; members are generally activating, inhibitory or co-stimulatory receptors. With the exception of NKG2 (natural killer group 2), most KLRs are orphan receptors, whose physiological ligands and/or functions remain undefined or have not been directly determined. The human NKG2A-CD94 and NKG2C-CD94 heterodimers recognizethe non-classical MHC class I molecule, HLA-E (Qa-1 in mice), which primarily displays peptides derived from the signal peptides of classical MHC class I molecules. The interactions of NKG2-CD94 heterodimers with HLA-E or Qa-1 molecules allow NK cells to monitor indirectly the expression of classical MHC class I molecules. Human and mouse KLRs orthologues have a broad expression pattern, which includes both NK and T cell subsets. Following polyoma-virus infection antigen-specific T cells involve in the response expressing of NKG2A-CD94 heterodimers, and gain the ability to bind Qa-1 tetramers. T cell antigen-specific function is inhibited by NKG2-CD94 during virus clearance and oncogenesis. Although this functional inhibition might not be evident in responses to every virus, marked T-cell expression of NKG2-CD94 can be found in a variety of viral infections. Emerging data also indicate that primed T cells might express other NKC molecules, including the Ly49 family and KLRG1; the role of these molecules modulating the function of antigen-specific T cells requires further evaluation. Similarly, NKG2D molecules are also expressed by T cells, where they associate with DAP 10. NKG2D can provide co-stimulatory function under conditions when peptide-MHC class I stimulation might be down-regulated.These studies showed important functions of KLRs in immune system. But our knowledge about these receptors has remained quite limited until recently. In order to further study the functions of this kind of receptors, we obtain a full-length cDNA with complete open-reading frame (ORF) potentially encoding a 265 aa peptide by PCR from human dendritic cells (DC) after large-scale random sequencing. The 1566 bp cDNA contained a single open reading frame (ORF) of 798 bp with 3 in-frame stop codons upstream of initial codon and a putative polyadenylation signal located 15 bp upstream of the poly (A) stretch. The 3'untranslated region also contained a number of potential rapid degradation signals including two repeats of the consensus sequence ATTTA. The ORF encoded a 265-amino-acid protein with a theoretical molecular mass of 30.8 kDa and an isoelectric point of 8.77. No signal...