Regulation of the Functions of Natural Cytotoxicity Receptors by Interactions with Diverse Ligands and Alterations in Splice Variant Expression

Abstract

The natural cytotoxicity receptor (NCR) family is constituted by NKp46, NKp44, and NKp30 in humans, which are expressed mainly on natural killer (NK) cells and are encoded by the ncr1, ncr2, and ncr3 genes, respectively. NCRs have classically been defined as activating receptors that trigger cytotoxicity and cytokine responses by NK cells upon engaging with ligands on tumor cells. Several new findings, however, have challenged this model and identified alternative mechanisms regulating the function of NCRs. Recent reports indicate that ligand matters, since the interaction of NKp44 with distinct ligands on target cells can either activate or inhibit NK cells. Also, the NCRs have been found to interact with distinct specificities to various heparan sulfate glycosaminoglycans, which are complex polysaccharides found in extracellular matrix or on cell surface heparan sulfate proteoglycans (HSPGs). The NCRs can engage with HSPGs in trans as a co-ligand on the target cells or in cis on the NK cell surface to regulate receptor-ligand interactions and NK cell activation. A number of splice variants of ncr2 and ncr3 have also been identified, and a predominant expression of certain variants results in inhibitory signaling through NKp44 and NKp30. Several recent studies have found that the selective expression of some of these inhibitory splice variants can significantly influence outcome in the contexts of cancer, infection, and pregnancy. These findings establish that NCR functions are more diverse than originally thought, and better understanding of their splice variant expression profiles and ligand interactions are needed to establish their functional regulation in the context of human health.

Keywords: RNA splice variants; cancer immunology; cytotoxicity; human immunology; natural cytotoxicity receptors; natural killer cells; pregnancy; virus immunity.

Figure 1

Ligands for natural cytotoxicity receptors (NCRs). Schematic representation of NCR ligands on tumor cell and their interaction with NKp30, NKp44, and NKp46 on natural killer (NK) cells. B7-H6 is an activating ligand for NKp30 upregulated on tumor cells and absent on normal cells. HLA-B-associated transcript 3 (BAT3)/Bcl2-associated anthogene 6 (BAG6) expressed in the nucleus moves to the plasma cell membrane or is released in exosomes. NKp44L is a splice variant isoform of the nuclear protein Mixed-lineage leukemia-5 protein that localizes to the tumor cell plasma membrane to serve as an activating ligand for NKp44. Proliferating cell nuclear antigen (PCNA) is a nuclear protein involved in DNA replication and repair mechanisms that relocalizes to the plasma membrane to serve as an NKp44 inhibitory ligand. Cytoskeleton type III filamentous vimentin is an intracellular protein but can be upregulated on the cell surface of infected cells, where it serves as a ligand for NKp46. Heparan sulfate proteoglycans (HSPGs) can interact with all NCRs. Heparan sulfate (HS) expressed on NK cell surface (cis interaction) can mask interactions with HSPG or other ligands on target cells (trans interactions).

Figure 2

Differential functions of different isoforms of NKp30 and NKp44. Immunostimulatory NKp30a and NKp30b isoforms interaction with ligands on tumor target cells or immature DCs (iDCs) to stimulate degranulation responses and interferon (IFN)-γ production. Inhibitory NKp30c isoform instead produces inhibitory cytokine IL-10 upon engagement. NKp30c is also less tightly associated with TCR-ζ and triggers stronger activation of p38 MAP kinase. The NKp44-1 isoform contains a cytoplasmic ITIM that can transduce inhibitory signals when engaged with proliferating cell nuclear antigen (PCNA) ligand on target cell surface (pathway “A”), leading to decreased cytotoxicity and IFN-γ production. In contrast, all three NKp44 isoforms can engage with NKp44L to transduce activation signaling through DAP12 (pathway “B” for NKp44-1) that triggers cytotoxicity and IFN-γ production.