KIR2DS4

KIR2DS4 is an activating natural killer (NK) cell receptor that recognizes HLA class I ligands and triggers NK cell effector responses (PMID:19858347, PMID:31138701). Its full-length membrane-bound form binds subsets of C1+ and C2+ HLA-C alleles and HLA-A*11 (HLA-A*1102), with specificity dictated by a gene-conversion-derived proline-valine motif at positions 71-72; crystallographic and swap-mutagenesis analysis showed this motif, together with displacement of contact loop L2, distinguishes KIR2DS4 from inhibitory KIR2DL receptors (PMID:19858347). Ligand recognition is peptide-selective: KIR2DS4 strongly prefers HLA-C*05:01 presenting 9-mer peptides bearing tryptophan at position 8, and a conserved bacterial RecA epitope presented in this context is sufficient to activate primary KIR2DS4+ NK cells independently of licensing or other activating receptors (PMID:31138701). KIR2DS4 also engages a non-MHC class I ligand on melanoma cells through a binding interface distinct from its HLA-Cw4 contacts (PMID:15265913). Engagement drives functional outputs including degranulation and pro-inflammatory cytokine/chemokine secretion (IFN-γ, MIP-1β, GM-CSF, XCL1, CCL1) (PMID:27815424, PMID:24901871), trogocytic uptake of CCR7 from target cells (PMID:25961063), and, in uterine NK cells, secretion of factors that promote placental trophoblast invasion (PMID:27815424). A common 22-bp exon 5 deletion produces a frameshifted, truncated protein lacking transmembrane and cytoplasmic domains that is secreted rather than membrane-anchored (PMID:12445308, PMID:17321903). KIR2DS4 surface expression is controlled epigenetically through promoter CpG-island methylation, with hypomethylation correlating with increased expression following hematopoietic transplantation and after acute exercise (PMID:30508863, PMID:22939905).

1. 2002 Medium

   Establishing that a common KIR2DS4 allele cannot function as a membrane receptor reframed the gene as existing in functional and non-functional forms across populations.

   Evidence cDNA cloning and sequencing of the 22-bp exon 5 deletion variant with population screening

   PMID:12445308

   Open questions at the time

   • Protein-level secretion was predicted, not demonstrated, at this stage
   • Functional consequence of the secreted form for NK activity not addressed
2. 2004 Medium

   Demonstrating a non-MHC melanoma ligand engaged through residues distinct from the HLA-Cw4 contacts showed KIR2DS4 recognition is not restricted to classical MHC class I.

   Evidence Functional binding assays, site-directed mutagenesis, and cytotoxicity assays against melanoma lines

   PMID:15265913

   Open questions at the time

   • Identity of the non-class I ligand not defined
   • In vivo relevance of MHC-independent recognition unknown
3. 2007 Medium

   Direct protein-expression studies confirmed the deletion variant yields a soluble, non-membrane-anchored protein, validating the predicted secretion.

   Evidence In vitro cell line model and in vivo protein expression analysis of the deletion variant

   PMID:17321903

   Open questions at the time

   • Biological function of the secreted form not established
   • Whether secreted protein retains any ligand binding not tested
4. 2009 High

   Crystal structure and mutagenesis defined the molecular basis of KIR2DS4's HLA specificity, explaining how a gene-conversion-derived motif diverges it from inhibitory KIR2DL receptors.

   Evidence Site-directed swap mutagenesis, crystallography, and NK activation assays using A*1102 as ligand

   PMID:19858347

   Open questions at the time

   • Peptide selectivity of HLA recognition not yet resolved at this stage
   • Downstream signaling adaptor not characterized in the corpus
5. 2012 Medium

   Linking promoter CpG-island demethylation to KIR2DS4 mRNA levels established epigenetic control of receptor expression and connected it to NK clonal expansion after transplantation.

   Evidence Bisulfite sequencing and mRNA-cDNA quantitative PCR in transplant donor/recipient cells

   PMID:22939905

   Open questions at the time

   • Transcription factors acting at the demethylated promoter not identified
   • Causality between methylation change and expression not directly tested
6. 2014 Medium

   Associating full-length surface KIR2DS4 with polyfunctional NK responses in HIV infection indicated the receptor promotes a pro-inflammatory effector state in chronic disease.

   Evidence Degranulation and IFN-γ/MIP-1β secretion assays stratified by surface KIR2DS4 with longitudinal modeling

   PMID:24901871

   Open questions at the time

   • Correlative, not causal, link to polyfunctionality
   • Ligand driving activation in vivo not identified
7. 2015 Medium

   Showing KIR2DS4 can drive CCR7 trogocytosis upon HLA-Cw4/Cw6 recognition, yet not always override NKG2A inhibition, defined how its activating signal integrates with co-expressed inhibitory receptors.

   Evidence Trogocytosis and cytotoxicity assays with NK clones against HLA-C-transfected targets

   PMID:25961063

   Open questions at the time

   • Mechanism balancing activation against NKG2A inhibition not resolved
   • Functional consequence of acquired CCR7 for NK trafficking untested
8. 2016 High

   Identifying KIR2DS4 on uterine NK cells and its triggering of trophoblast-invasion-promoting chemokines extended its role from immune defense to reproductive tissue remodeling.

   Evidence Flow cytometry and functional stimulation of primary uNK cells with a 120-cytokine screen

   PMID:27815424

   Open questions at the time

   • In vivo contribution to placentation not demonstrated
   • Peptide/ligand presented by trophoblast HLA-C not defined
9. 2019 High

   Defining a tryptophan-at-p8 peptide preference on HLA-C*05:01 and a cross-reactive bacterial RecA epitope established KIR2DS4 as a peptide-specific activating receptor with potential pathogen-sensing function.

   Evidence Peptide library screening, HLA-C*05:01 presentation assays, and primary KIR2DS4+ NK degranulation assays with bacterial RecA validation

   PMID:31138701

   Open questions at the time

   • Physiological relevance of bacterial epitope recognition in vivo not shown
   • Range of self versus microbial peptides recognized not fully mapped

• The intracellular signaling machinery and adaptor coupling that translate KIR2DS4 ligand engagement into NK activation remain uncharacterized in the available corpus.
• No signaling adaptor (e.g. coupling partner) identified in the timeline
• Function of the secreted deletion-variant protein unresolved
• Identity of the non-MHC melanoma ligand unknown