Natural killer (NK) cells can restrict tumour growth by secreting proinflammatory cytokines and direct lysis of malignant cells. The NKp44 receptor is upregulated in IL-2 or IL-15 activated NK cells and recognises the secreted mitogen Platelet Derived Growth Factor-D (PDGF-D). Normally tightly regulated, PDGF-D can be over-produced by aggressive carcinomas. Contrary to evidence of secreted ligands inhibiting cellular activation, the binding of PDGF-D to NKp44 triggers NK cell secretion of TNF and IFNγ. Furthermore, we report that recombinant PDGF-D also enhances NK cell lysis of tumour cell lines in vitro. These observations indicate our understanding of how NKp44 regulates NK cell anti-tumour functions is incomplete.

The expression and signalling properties of NKp44-isoforms remain controversial. NKp44 isoform-1 (NKp44-1), but not NKp44-2 or NKp44-3, encodes a cytoplasmic tyrosine-based motif that conforms to a putative immunoreceptor tyrosine-based inhibition motif (ITIM) and YXXØ internalisation motif. Intriguingly, we observed that all NKp44-isoforms activate green fluorescent protein reporter cells upon binding to PDGF-D. However, lower surface expression of NKp44-1 suggests that this cryptic ITIM may mediate receptor endocytosis rather than inhibit cellular activation.

In the absence of antibodies that can discriminate between NKp44-isoforms, attempts to characterise the expression of these proteins in different NK cell subsets have been limited to transcriptional analyses. We have specifically raised novel monoclonal antibodies against NKp44-1 and NKp44-3 and provide the first evidence that they are expressed as proteins in IL-2 activated NK cells.

We will use these novel antibodies to investigate the signalling properties of NKp44-isoforms and their role in NK cell cytotoxicity upon binding to PDGF-D. Overall, this research aims to build our understanding of how NK cell surveillance of soluble factors contributes to anti-tumour immunity.