IL-12, but Not IFN-α, Promotes STAT4 Activation and Th1 Development in Murine CD4+ T Cells Expressing a Chimeric Murine/Human Stat2 Gene

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Abstract
Humans and mice have evolved distinct pathways for Th1 cell development. Although IL-12 promotes CD4+ Th1 development in both murine and human T cells, IFN-αβ drives Th1 development only in human cells. This IFN-αβ-dependent pathway is not conserved in the mouse species due in part to a specific mutation within murine Stat2. Restoration of this pathway in murine T cells would provide the opportunity to more closely model specific human disease states that rely on CD4+ T cell responses to IFN-αβ. To this end, the C terminus of murine Stat2, harboring the mutation, was replaced with the corresponding human Stat2 sequence by a knockin targeting strategy within murine embryonic stem cells. Chimeric m/h Stat2 knockin mice were healthy, bred normally, and exhibited a normal lymphoid compartment. Furthermore, the murine/human STAT2 protein was expressed in murine CD4+ T cells and was activated by murine IFN-α signaling. However, the murine/human STAT2 protein was insufficient to restore full IFN-α-driven Th1 development as defined by IFN-γ expression. Furthermore, IL-12, but not IFN-α, promoted acute IFN-γ secretion in collaboration with IL-18 stimulation in both CD4+ and CD8+ T cells. The inability of T cells to commit to Th1 development correlated with the lack of STAT4 phosphorylation in response to IFN-α. This finding suggests that, although the C terminus of human STAT2 is required for STAT4 recruitment and activation by the human type I IFNAR (IFN-αβR), it is not sufficient to restore this process through the murine IFNAR complex.