KIR2DS4

KIR2DS4 is an activating killer-cell immunoglobulin-like receptor on NK cells that recognizes HLA class I molecules and triggers pro-inflammatory effector functions including degranulation and cytokine secretion. The full-length receptor binds subsets of HLA-C alleles and HLA-A*11 through a proline-valine motif at positions 71–72 acquired by gene conversion from KIR3DL2, with crystal structure revealing displacement of contact loop L2 that reshapes ligand specificity; it also exhibits peptide selectivity for tryptophan at position 8 of HLA-C*05:01-presented peptides, enabling recognition of conserved bacterial RecA epitopes from multiple human pathogens (PMID:19858347, PMID:31138701). A common 22-bp deletion in exon 5 causes a frameshift yielding a secreted, non-functional soluble form (KIR1D) lacking transmembrane and cytoplasmic domains, while the membrane-bound form activates NK cells to secrete GM-CSF, XCL1, CCL1, IFN-γ, and MIP-1β and can induce trogocytosis of CCR7 from target cells (PMID:12445308, PMID:17321903, PMID:27815424, PMID:25961063). Transcription of KIR2DS4 is regulated by CpG methylation within a three-CpG-site promoter island, with hypomethylation correlating with increased expression (PMID:22939905).

1. 2002 High

   Identification of a 22-bp exon 5 deletion established that the most common KIR2DS4 allele encodes a truncated, secreted protein (KIR1D) rather than a membrane-bound activating receptor, resolving why many individuals carrying KIR2DS4 lack functional surface expression.

   Evidence cDNA cloning and deletion-specific PCR across multiple individuals

   PMID:12445308

   Open questions at the time

   • Whether the soluble KIR1D form has any biological activity was not determined
   • Frequency of deletion versus full-length alleles across diverse populations was not fully catalogued
2. 2004 Medium

   Discovery that KIR2DS4 recognizes a non-MHC class I ligand on melanoma cells, with distinct residues mediating this interaction versus HLA-Cw4 binding, revealed dual-ligand recognition capacity.

   Evidence Binding assays and site-directed mutagenesis with melanoma lines and HLA-C-transfected cells

   PMID:15265913

   Open questions at the time

   • The non-MHC ligand was not molecularly identified
   • Not independently replicated in a second laboratory
   • Physiological relevance of non-MHC ligand recognition remains undefined
3. 2007 High

   Confirmation at the protein level that full-length KIR2DS4 is membrane-anchored and functional while the deletion variant is secreted solidified the functional dichotomy between allelic forms.

   Evidence In vitro and in vivo protein expression studies with PCR-SSOP typing

   PMID:17321903

   Open questions at the time

   • Whether soluble KIR1D competes with membrane KIR2DS4 for ligand was not tested
4. 2009 High

   Structural and mutagenesis work revealed that KIR2DS4 binds HLA-C subsets and HLA-A*11 through a proline-valine motif at positions 71–72 acquired from KIR3DL2 by gene conversion, with crystal structure showing displacement of loop L2 — explaining its unique and broader HLA specificity compared to other KIR2D receptors.

   Evidence X-ray crystallography, site-directed swap mutagenesis, NKL cell activation assays

   PMID:19858347

   Open questions at the time

   • How KIR2DS4 discriminates between A*1101 and A*1102 at the structural level was not resolved
   • Contribution of peptide cargo to binding was not addressed
5. 2012 Medium

   Demonstration that KIR2DS4 promoter CpG methylation inversely correlates with mRNA expression established an epigenetic mechanism governing receptor transcription, addressing how stochastic KIR expression patterns arise.

   Evidence Bisulfite sequencing of the three-CpG promoter island coupled with qPCR in post-transplant NK cells

   PMID:22939905

   Open questions at the time

   • Causal relationship between demethylation and transcription was not formally demonstrated
   • Transcription factor binding at demethylated CpGs was not identified
6. 2014 Medium

   Linking KIR2DS4 surface expression to polyfunctional NK activity (degranulation, IFN-γ, MIP-1β) in HIV-infected individuals provided direct evidence that the receptor drives a pro-inflammatory NK cell state in vivo.

   Evidence Ex vivo flow cytometry-based degranulation and intracellular cytokine assays stratified by KIR2DS4 expression

   PMID:24901871

   Open questions at the time

   • Whether KIR2DS4 directly recognized HIV-infected cells or responded to altered HLA expression was not determined
   • Single cohort study
7. 2015 Medium

   Discovery that KIR2DS4 engagement triggers trogocytosis of CCR7 from HLA-C+ target cells revealed a non-cytolytic effector mechanism, though this activation could be overridden by NKG2A inhibitory signaling.

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

   PMID:25961063

   Open questions at the time

   • Physiological consequence of CCR7 trogocytosis on NK cell migration was not tested
   • Limited to clonal NK populations
8. 2016 High

   Profiling of uterine NK cells showed KIR2DS4 triggering induces secretion of GM-CSF, XCL1, and CCL1 that promote trophoblast invasion, establishing a tissue-specific reproductive function for this activating receptor.

   Evidence Flow cytometry, 120-cytokine screen, and functional stimulation of uterine NK cells

   PMID:27815424

   Open questions at the time

   • Specific HLA ligand on trophoblasts engaging KIR2DS4 was not defined
   • In vivo validation of trophoblast invasion promotion not performed
9. 2019 High

   Identification of peptide selectivity for Trp at p8 of HLA-C*05:01-bound peptides, including conserved bacterial RecA epitopes, demonstrated that KIR2DS4 functions as a pathogen-sensing receptor with fine peptide discrimination — resolving the long-standing question of whether activating KIR can detect microbial antigens.

   Evidence Primary NK cell degranulation assays with defined peptide-HLA complexes; bacterial epitope screening

   PMID:31138701

   Open questions at the time

   • Whether KIR2DS4 peptide selectivity extends to viral epitopes was not tested
   • Structural basis for p8-Trp preference not determined crystallographically

• Key unresolved questions include the identity of the non-MHC ligand on melanoma, the structural basis for peptide selectivity, whether soluble KIR1D has immunomodulatory function, and how KIR2DS4 signaling integrates with other activating and inhibitory KIR in vivo.
• Non-MHC ligand remains molecularly unidentified
• No crystal structure of KIR2DS4 bound to peptide-HLA complex
• Functional role of soluble KIR1D untested