IFNγ-induced memory in human macrophages is sustained by the durability of cytokine signaling itself.

Macrophages, as key sentinel cells of the innate immune system, can retain memory of prior stimulus exposure. IFNγ plays a central role in maintaining trained immunity in vivo and can induce potent memory in macrophages. Such memory is associated with the formation of de novo enhancers that alter gene expression responses to subsequent stimuli. However, how such enhancers are maintained after cytokine exposure remains unclear. We report that the mechanism underlying durable IFNγ-induced enhancers is not cell intrinsic. IFNγ-treated macrophages continue to exhibit JAK/STAT signaling days after cytokine removal. Blocking IFNγ signaling with a JAK inhibitor or anti-IFNγ neutralizing antibodies after cytokine removal is sufficient to reverse IFNγ-induced enhancers and erase the potentiated state of the treated macrophages. Our findings suggest that epigenetic changes in macrophages do not inherently encode innate immune memory or a "potentiated" macrophage state, but in fact are themselves dependent on ongoing signaling from cytokines sequestered at the cell surface.