Selective alteration to CD4 T cell differentiation by heterozygous IRF4 ^L116R protects against neuroinflammation

The decision of naive T cells to differentiate into a specific Th cell subset after antigen encounter is a critical pivot point of the immune response and, when out of balance, can lead to autoimmunity, allergy or immunodeficiency. Th subset differentiation is determined by the expression of transcription factors including IRF4. We here describe a point mutation (IRF4 ^L116R ) in the DNA-binding domain of IRF4 leading to a dysregulation of CD4-T cell subsets and their functions. This point mutation does not alter overall protein expression. In sharp contrast to IRF4-null mice, neither the CD4/CD8 ratio nor T cell activation and memory is altered in naive mice carrying the point mutation. However, IRF4 ^L116R T cells are reduced in their capability to differentiate into Th1, Th17 and Treg cells in a dose-dependent manner, contrasting with the findings in IRF4 ^KO T cells. Particularly striking is the loss of Th1-differentiation in T cells from homozygous Irf4 ^L116R/L116R mice while T cells completely lacking IRF4 show no reduction in Th1 differentiation. Furthermore, despite maintained ability to generate Th17 cells, expression of the IRF4 ^L116R variant protects mice against disease development in the Th17 cell-dependent EAE mouse model of neuroinflammation. This contrasts with IRF4 knock-out mice, where expression of one wild-type allele of IRF4 is sufficient for full disease development. Together our results show that the L116R point mutation in IRF4 can selectively alter the differentiation and function of some CD4 T cell subsets and suggest that the L116R mutation is not a classical loss- or gain-of-function variant.