The NKp44-1 Isoform Is an Activating Receptor for PDGF-DD Expressed on Natural Killer Cells

Natural killer (NK) cells are key innate lymphoid cells that restrict tumour progression by secreting proinflammatory cytokines and directly lysing malignant cells, with their activity tightly regulated by a balance of activating and inhibitory surface receptors. The natural cytotoxicity receptor NKp44 is induced on NK cells following stimulation with IL-2 or IL-15 and recognizes platelet-derived growth factor D (PDGF-DD) as a ligand. Mechanistic interpretation of NKp44 signalling upon PDGF-DD engagement is confounded by the existence of three distinct NKp44 isoforms (NKp44-1, -2, and -3), each capable of initiating divergent intracellular signalling cascades. Unlike NKp44-2 and -3, NKp44-1 encodes a cytoplasmic tyrosine residue (Y238) that conforms to a putative immunoreceptor tyrosine-based inhibition motif (ITIM) and has been reported to suppress NK cell effector functions in some contexts. However, it remains unclear whether the NKp44 isoforms are translated and expressed in NK cells, and formal evidence defining NKp44-1 signalling in response to engagement by PDGF-DD is lacking.

In this study, we used C-terminal targeting monoclonal antibodies (mAbs) and a NFAT-GFP reporter system to define the expression and signalling properties of NKp44 isoforms in response to PDGF-DD.

We demonstrate protein expression of NKp44-1 and NKp44-2-/3 receptors in IL-2 expanded NK cells. We further show that NKp44-1 transduces activating rather than inhibitory signals when engaged by PDGF-DD ligand, albeit weaker than NKp44-3. Intriguingly, we find that Y238 is dispensable for NKp44-1 activating signalling and instead functions as a YXXΦ internalisation motif.

Collectively, these findings provide the first evidence that the NKp44-1 and NKp44-2/3 isoforms are expressed in NK cells and establish that PDGF-DD activates signalling through NKp44-1 independently of Y238. This work lays the foundations for future studies investigating how PDGF-DD sensing by the different NKp44 isoforms shapes immune functions in different physiological and pathological contexts.