Identification of the genetic determinants for the frequency of regulatory T cells using system genetics

Abstract Regulatory T (Treg) cells are indispensable components of immune regulation contributing to the maintenance of respiratory tolerance and preventing autoimmunity and exaggerated immune responses. The frequency and absolute number of Treg cells in lymphoid and non-lymphoid tissues are tightly regulated through incompletely understood mechanisms. In this study, we sought to understand the pulmonary immune regulation through identification of the genetic determinants of the frequency and absolute number of CD4+FOXP3+T cells in mice using a genome-wide association study (GWAS) approach. We quantified the frequency and absolute number of CD4+FOXP3+T cells in the lungs and spleen of 102 inbred strains from the hybrid mouse diversity panel under steady state conditions and after exposure to house dust mite (HDM). Among strains of mice examined, the absolute number of pulmonary CD4+FOXP3+ T cells varied by 312-fold at the steady state and 210-fold after HDM exposure. A GWAS analysis results revealed several loci that contribute to the regulation of CD4+FOXP3+ T cells in the lungs and spleen of mice, including a locus on chromosome 5 harboring the homeodomain-only protein (Hopx) gene. Characterization of Hopx−/− mice validated the GWAS findings by demonstrating that the frequency and number of CD4+FOXP3+ T cells are higher in the lungs of Hopx−/− compared to WT mice at the steady state and after HDM exposure. We found that in contrast to the lungs, the number of CD4+FOXP3+ T cells is lower in the spleen of Hopx−/− compared to WT mice at the steady state. Our findings suggest that Hopx is a novel genetic determinant of the frequency and number of CD4+FOXP3+ T cells in mice with opposing directional effects in mucosal and secondary lymphoid organs.