Irf8-Regulated Genomic Responses Drive Pathological Inflammation during Cerebral Malaria

Interferon Regulatory Factor 8 (IRF8) is required for development, maturation and expression of anti-microbial defenses of myeloid cells. BXH2 mice harbor a severely hypomorphic allele at Irf8 (Irf8R294C) that causes susceptibility to infection with intracellular pathogens including Mycobacterium tuberculosis. We report that BXH2 are completely resistant to the development of cerebral malaria (ECM) following Plasmodium berghei ANKA infection. Comparative transcriptional profiling of brain RNA as well as chromatin immunoprecipitation and high-throughput sequencing (ChIP-seq) was used to identify IRF8-regulated genes whose expression is associated with pathological acute neuroinflammation. Genes increased by infection were strongly enriched for IRF8 binding sites, suggesting that IRF8 acts as a transcriptional activator in inflammatory programs. These lists were enriched for myeloid-specific pathways, including interferon responses, antigen presentation and Th1 polarizing cytokines. We show that inactivation of several of these downstream target genes (including the Irf8 transcription partner Irf1) confers protection against ECM. ECM-resistance in Irf8 and Irf1 mutants is associated with impaired myeloid and lymphoid cells function, including production of IL12p40 and IFNγ. We note strong overlap between genes bound and regulated by IRF8 during ECM and genes regulated in the lungs of M. tuberculosis infected mice. This IRF8-dependent network contains several genes recently identified as risk factors in acute and chronic human inflammatory conditions. We report a common core of IRF8-bound genes forming a critical inflammatory host-response network.干扰素调节因子8（Interferon Regulatory Factor 8，IRF8）是髓系细胞发育、成熟以及抗菌防御功能表达所必需的调控分子。BXH2小鼠在Irf8基因位点携带严重功能低下的等位基因（Irf8R294C），该突变会使小鼠对包括结核分枝杆菌（Mycobacterium tuberculosis）在内的多种胞内病原体感染易感。本研究发现，BXH2小鼠在感染伯氏疟原虫ANKA株（Plasmodium berghei ANKA）后，完全不会发生脑型疟疾（Cerebral Malaria，ECM）相关病变。 研究通过脑组织RNA比较转录组分析、染色质免疫共沉淀结合高通量测序（ChIP-seq）等技术，鉴定出表达水平与病理性急性神经炎症相关的IRF8调控基因。感染后表达上调的基因显著富集IRF8结合位点，提示IRF8在炎症调控程序中作为转录激活因子发挥功能。上述基因集显著富集髓系特异性通路，包括干扰素应答、抗原呈递以及Th1极化细胞因子相关通路。 本研究证实，靶向敲除其中数个下游靶基因（包括IRF8的转录协同因子Irf1），可使小鼠获得抗ECM的保护能力。Irf8与Irf1突变小鼠的ECM抵抗表型，与髓系细胞和淋巴细胞功能受损密切相关，具体体现为白细胞介素12p40（IL12p40）与γ干扰素（IFNγ）的产生水平显著降低。 研究还发现，在ECM进程中受IRF8结合并调控的基因，与结核分枝杆菌感染小鼠肺部组织中差异表达的基因存在高度重叠。这一IRF8依赖的调控网络包含数个近年被鉴定为人类急慢性炎症性疾病风险因子的基因。本研究鉴定出一组由IRF8结合基因组成的核心基因集，其构成了关键的炎症性宿主应答调控网络。