Innate lymphocytes can play both protective and pathogenic roles in chronic inflammatory disorders. Recently, killer-cell immunoglobulin-like receptors (KIRs) and its cognitive ligands - human leukocyte antigen (HLA) class I molecules - were identified as genetic risk factors for Crohn's Disease (CD), a common inflammation symptom. Natural killer (NK) cells, the major KIR-expressing cell type, can be educated through KIR-HLA ligation. To uncover the cellular mechanism that determines CD susceptibility, we utilize a novel single-cell functional proteomics microchip and other highly-multiplexed assays (Ma, C. et al. Nature Medicine, 2011, 17, 738–743). We show that, in genetically pertinent individuals, natural killer (NK) cells are functionally reprogrammed to modulate the activation threshold of CD4+ T cells, a major cellular mediator of chronic inflammation.
Genetic study of 455 CD patients bearing the AA haplotype identifies that the HLA-C1/C1 allotype, ligand of the KIR2DL3 receptor, is significantly enriched (p<0.0001). Moreover, when evaluating the secretion of 20 cytokines from single purified NK cells that are retrieved from the peripheral blood, we observe that NK cells expressing KIR2DL3 were strongly polarized to robustly produce a myriad of pro-inflammatory cytokines and chemoattractants in copious amounts. Comparing to those from other subjects, NK cells from HLA-C1/C1 subjects produce significantly increased level (p<0.05) of 11 soluble mediators, including TNF, INF-g, ILs, and CCLs. Furthermore, among all NK cells within the HLA-C1/C1 subjects, NK cells expressing KIR2DL3 receptors are the most potent to produce cytokines (2-log higher) and exhibit the highest polyfunctionality. These observations are also confirmed by intracellular staining and ELISA assay of NK cell culture media.
Most importantly, the KIR-educated NK cells can strongly augment the activation and proliferation of CD4+ T cells. As shown in autologous NK and CD4+ T cell co-culture assay, CD4+ T cells proliferate aggressively in the presence of NK cells in a dose-dependent fashion (R2=0.99). NK surface costimulatory molecules blockage and NK-CD4+ T cells transwell-separation experiments indicate that this augmentation is not contact-dependent. On the other hand, NK cytokine depletion and ELISA essay of the co-culture media confirmed that soluble factors, such as ILs, IFN-g and TNF, activate CD4+ T cells.
KIR2DL3 signaling-mediated education licenses NK cells the capacity to secrete large amounts of pro-inflammatory cytokines and chemokines, which in turn lowers activation threshold of CD4+ T cells and increases susceptibility to chronic inflammation disorders. Our study establishes, for the first time, an immunologic cellular mechanism that explains the KIR genetics-based susceptibility to CD and other chronic inflammatory syndromes.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.