Many well-known immune-related C-type lectin-like receptors (CTLRs) such as NKG2D, CD69, and the Ly49 receptors are encoded in the natural killer gene complex (NKC). Recently, we characterized the orphan NKC gene CLEC2A encoding for KACL, a further member of the human CLEC2 family of CTLRs. In contrast to the other CLEC2 family members AICL, CD69, and LLT1, KACL expression is mostly restricted to skin. Here we show that KACL is a nondisulfide-linked homodimeric surface receptor and stimulates cytotoxicity by human NK92MI cells. We identified the corresponding activating receptor on NK92MI cells that is encoded adjacently to the CLEC2A locus and binds KACL with high affinity. This CTLR, termed NKp65, stimulates NK cytotoxicity and release of proinflammatory cytokines upon engagement of cell-bound KACL. NKp65, a distant relative of the human activating NK receptor NKp80, possesses an amino-terminal hemITAM that is required for NKp65-mediated cytotoxicity. Finally, we show that KACL expression is mainly restricted to keratinocytes. Freshly isolated keratinocytes express KACL and are capable of stimulating NKp65-expressing cells in a KACL-dependent manner. Thus, we report a unique NKC-encoded receptor-ligand system that may fulfill a dedicated function in the immunobiology of human skin.CLEC2 family | natural killer cells | NKRP1 receptors | skin

The Natural Killer Gene Complex (NKC) encodes for numerous C-type lectin-like receptors (CTLR) variably expressed on lymphocytes and myeloid cells. Well-known representatives of NKC-encoded CTLR are NKG2D, CD69, and Ly49 molecules. However, the NKC also contains a considerable number of genes which are barely characterized. Recently, we described the CLEC2A gene encoding for a new member of the human CLEC2 family of NKC-encoded CTLR. CLEC2A, in contrast to other human CLEC2 family members such as CD69/CLEC2C and AICL/CLEC2B, is virtually not expressed by peripheral leukocytes, but its expression appears restricted to skin. We now report that CLEC2A encodes for a non-disulfide-linked, homodimeric surface CTLR which is expressed on freshly isolated skin cells. Addressing a potential immune-associated function, we find that CLEC2A stimulates cytotoxicity and cytokine release of human NK cells. However, we failed to ascertain CLEC2A engagement by known activating NK receptors. Instead, we identified a hitherto undescribed NKC-encoded CTLR (NKp65) that specifically interacts with CLEC2A and activates NK cytotoxicity. In summary, we define CLEC2A as a skin-specific and NK cell-stimulating CTLR engaging NKp65, a novel activating CTLR which may allow for dedicated immunosurveillance of human skin. This work was supported by funds (SFB 685/A1) of the German Research Foundation (DFG).