The mechanisms controlling the recognition of tumor and virus infected cells by natural killer cells

TI Arnon^a, A Porgador PhD^b, O Mandelboim PhD^a

^aThe Hebrew University of Jerusalem, Jerusalem, Israel, ^bBen-Gurion University, Beer-Sheva, Israel

Natural killer (NK) cells are bone marrow derived lymphocytes that constitute a key frontline defense against a range of hazardous conditions, including viral infection and tumor transformation. Although NK cells can kill target cells spontaneously without prior stimulation, a delicate balance between inhibitory and activating signals tightly regulates their activation.A significant breakthrough in the understanding of NK cells specific activation was achieved following the recent identification of three novel NK specific triggering receptors collectively termed natural cytotoxic receptors (NCR), which include NKp46, NKp44 and NKp30. AIMS: The NCRs has been demonstrated as key players in NK cells activity against tumor and virally infected cells. However, little is known about the nature and distribution of the NCR ligands. The focus of our research is to identify and characterize viral and cellular ligands that are recognized by the NCRs. METHODS: We used fusion proteins that contain the extracellular portion of various NK killing receptors to isolate the viral ligand of NKp46 and NKp44. We next used truncated fusion proteins and point mutation analysis to identify the NKp46 binding domain and the precise amino acids that are involved in the recognition of various NK cells targets by NKp46. RESULTS: We identify the hemagglutinin (HA) of influenza as a novel functional ligand for NKp46 and NKp44, but not NKp30. We demonstrate that these interactions are functional and are largely mediated via sialylated residues of NKp46 and NKp44. We further establish that this recognition is direct and mainly mediated via 2,6-linked sialic acid carried by NKp46. In addition, we show that the ability to recognize virally infected and tumor cells is confined to the membrane proximal domain of the receptor and primarily depends on the highly conserved sugar-carrying residue, Thr 225. CONCLUSIONS: We suggest that NK cells have developed a general mechanism to recognize and kill virally infected cells based on the natural conserved tendency of HAs to bind sialic acids, manipulated to directly and specifically activate two sialylated lysis receptors NKp46 and NKp44. Furthermore, the fact that NKp46 binding to viral as well as tumor ligands is mediated via the same domain and involves the same amino acid (Thr225) demonstrates the ability of the innate immune system to react against such a broad spectrum of target cells using a relatively limited variety of receptors.

Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in oral session 1091 (Immunobiology).