The present study reports the molecular cloning of NKp44, a NK-specific triggering receptor involved in non-MHC–restricted natural cytotoxicity (3
). Different from NKp46 (2
), which is expressed by all resting and activated NK cells, NKp44 is selectively expressed by activated human NK cells. In this context, it is well known that culture in IL-2 greatly enhances the NK-mediated anti-tumor cytotoxicity both in vitro (4
) and in vivo (5
). It is conceivable that the increased cytolytic activity mediated by activated NK cells may be consequent, at least in part, to the de novo expression of triggering receptors. NKp44 may well represent one of these receptors since it is involved in triggering of activated NK cells in the process of tumor cell lysis (3
Molecular cloning revealed that NKp44 is a type I glycoprotein belonging to the Ig-SF, which does not display any major amino acid sequence homology with known proteins. NKp44 is characterized by an extracellular region containing an Ig-like domain of the V type. The transmembrane portion contains the charged amino acid lysine, possibly involved in the association with KARAP/DAP12 molecules. Indeed, biochemical data confirmed that NKp44 associates with KARAP/DAP12 molecules (3
). Moreover, molecular cloning of NKp44 demonstrated that the association with KARAP/DAP12 molecules is required for the surface expression of NKp44.
Interestingly, the cytoplasmic tail of NKp44 contains a classical ITIM (21
). In this context, it is of note that, under the experimental conditions resulting in a strong tyrosine phosphorylation of the KARAP/DAP12 molecules associated to the NKp44 receptor (3
), no tyrosine phosphorylation of the NKp44 molecule itself could be detected. Moreover, under the same experimental conditions, no evidence has been obtained so far that NKp44 may associate with phosphatases such as SHP-1, SHP-2, or SHIP, which have been reported to associate with ITIM-bearing receptors (21
). The functional role of this ITIM is presently under investigation.
Remarkably, unlike most of the genes encoding Ig-SF receptors involved in the regulation of NK cell–mediated cytolytic activity (including KIR , KAR , NKp46 , and ILT/LIR [26, 27]), which are on human chromosome 19, the NKp44 gene is on chromosome 6. Southern blot analysis of human genomic DNA revealed a restriction enzyme digestion pattern that is compatible with the existence of a single NKp44 gene. Moreover, the NKp44 gene appears to be conserved across species since the human NKp44 ORF cDNA probe cross-hybridized with genomic DNA from monkey and mouse. It will be of interest to analyze whether the murine counterpart of NKp44 exists and whether its expression is restricted to NK cells and to a subset of TCR-γ/δ+ T lymphocytes as it occurs in humans.