Functional polymorphisms in the D1-D2 domain killer immunoglobulin-like receptors

The killer immunoglobulin-like receptor (KIR) family contains fifteen members of structurally-similar, highly-polymorphic genes. The KIR molecules can be found on the surface of NK cells, where they regulate NK cell activity against malignant and virally-infected cells. We investigated whether polymorphisms in the KIR genes result in functional differences between different receptor variants since these functional differences could contribute to the varied immune responses seen among individuals. Phenotypic differences among the KIR2DL2/3 alleles were initially examined, leading to further examination of the surface expression phenotype of the KIR2DL2*004 allele by flow cytometry and confocal fluorescence microscopy. The receptor encoded by the KIR2DL2*004 allele was not found to be expressed on the cell surface and mutagenesis of the receptor indicated that this effect is due to an arginine-to-threonine polymorphism at amino acid position 41 in the KIR D1 extracellular domain. R41 is conserved among all other KIR variants and molecular modeling analysis suggested that R41 participates in a critical protein folding motif by forming intramolecular interactions with residues on adjacent beta strands. This folding motif is somewhat conserved in the KIR D2 domain, where R141 appears to form critical interactions with nearby residues. All alleles of KIR2DS3 encode for a threonine in place of R141, suggesting that the KlR2DS3 surface expression phenotype may be similar to that found for the KIR2DL2*004 allelic product. Upon further investigation, flow cytometry analysis, confocal fluorescence microscopy, and surface-biotinylation experiments demonstrated that KlR2DS3 is expressed at the cell surface at greatly decreased levels. However, no single amino acid polymorphism appears to be responsible for this phenotype and the polymorphism at position 141 does not appear to contribute significantly. These data suggest that KIR polymorphisms may be more important than originally thought and could significantly modulate the immune response among different individuals.