STAT3 dual role in sex-biased and differentiation-gated interferon gamma memory
The innate immune system possesses a unique form of memory, known as trained immunity. An initial stimulus induces cellular reprogramming for a non-specific amplified response to a subsequent challenge. This process can be achieved through gene priming, where exposure to a specific cue sensitizes future gene expression. Interferon-gamma (IFN{gamma}) memory is a specialized form of priming, where both the initial and recall signals are mediated by the same IFN{gamma} cytokine. The mechanisms governing IFN{gamma} memory in humans remain poorly understood. We investigated the mechanisms governing IFN{gamma} memory and characterized it in human primary macrophages. We discovered a dual role for the STAT3 transcription factor in IFN{gamma} memory. It acts as a repressor of memory establishment by indirectly inhibiting STAT1 Ser727 phosphorylation, while also being required for memory recall. Furthermore, we demonstrated that IFN{gamma} memory is sex-biased, with males exhibiting a more pronounced effect characterized by reduced variability in gene expression upon re-exposure to IFN{gamma}. This defines IFN{gamma} recall as a function of transcriptional precision. We also identified a novel family of chemokine genes that exhibit IFN{gamma} concentration dependent memory effects. Additionally, we established a model system to study this process and found that memory is critically dependent on the cellular phenotype. Specifically, our results show that for some genes, monocyte-to-macrophage differentiation is necessary, while for others, this differentiation step is sufficient for recall of the previous response. Our findings redefine the IFN{gamma} memory program, establishing it as a sex-biased phenomenon governed by a novel regulatory network involving differentiation and STAT3 function. These insights provide critical context for understanding immune heterogeneity and are essential for the rational design of personalized therapies that harness trained immunity.