Interleukin-12 and Interleukin-18 Synergy skews Immune responses away from immunoregulation and towards autoimmunity through enhanced cytotoxicity

Leeana D. Peters, Joseph W. Dean, Maigan A. Brusko, Daniel J. Perry, Wen-I Yeh, and Todd M. Brusko

Department of Pathology, Immunology, and Laboratory Medicine, University of Florida Diabetes Institute, Gainesville, FL, USA. 

IL-12 and IL-18 synergy results in the production of IFNg by T cells and natural killer (NK) cells, which is critical for effective T cell and NK cytotoxic function in response to infections. In excess, however, these cytokines can lead to immune dysregulation in the form of exacerbated T_H1 immunity and have been implicated in type 1 diabetes (T1D) pathogenesis by both genetic associations and elevated expression signatures in patients. We investigated the influence of these cytokines on phenotype and function of human natural killer (NK) cells, autoreactive CD8⁺ cytotoxic T lymphocytes (CTL), and CD4⁺ regulatory T cells (Tregs). We observed increased specific killing of human β-cells in vitro by glucose-6-phosphatase 2 (G6Pase 2)-specific CTL avatars upon exposure to IL-12 and IL-18. Upregulation of natural cytotoxicity receptors, co-stimulatory receptors, and the high affinity IL-2 receptor (CD25) in response to IL-12 and IL-18 stimulation allowed NK cells to compete for IL-2 upon co-culture with Tregs, resulting in Th1 skewing and loss of suppressive function. Peripheral blood phenotyping revealed patient NK cell costimulatory receptor balance was skewed toward activation while CD25 was expressed at lower levels per cell compared to their first-degree relatives and control subjects. Immuno-expression quantitative trait loci (eQTL) analyses revealed a genotype-phenotype association at the IL2RA locus which confirmed our in vitro work, with donors homozygous for the susceptible allele displaying higher CD25 expression per cell than donors with homozygous protective (p<0.01) and heterozygous genotypes (p0.0001). These results offer insight into a potential mechanism of innate regulation of adaptive immunity in T1D, which holds significant clinical implications for trials of low dose IL-2 and provides support for using a precision medicine approach in T1D therapies.