Interferon Regulatory Factor 8 Regulates Pathways for Antigen Presentation in Myeloid Cells and during Tuberculosis

Abstract

IRF8 (Interferon Regulatory Factor 8) plays an important role in defenses against intracellular pathogens, including several aspects of myeloid cells function. It is required for ontogeny and maturation of macrophages and dendritic cells, for activation of anti-microbial defenses, and for production of the Th1-polarizing cytokine interleukin-12 (IL-12) in response to interferon gamma (IFNγ) and protection against infection with Mycobacterium tuberculosis. The transcriptional programs and cellular pathways that are regulated by IRF8 in response to IFNγ and that are important for defenses against M. tuberculosis are poorly understood. These were investigated by transcript profiling and chromatin immunoprecipitation on microarrays (ChIP-chip). Studies in primary macrophages identified 368 genes that are regulated by IRF8 in response to IFNγ/CpG and that behave as stably segregating expression signatures (eQTLs) in F2 mice fixed for a wild-type or mutant allele at IRF8. A total of 319 IRF8 binding sites were identified on promoters genome-wide (ChIP-chip) in macrophages treated with IFNγ/CpG, defining a functional G/AGAAnTGAAA motif. An analysis of the genes bearing a functional IRF8 binding site, and showing regulation by IFNγ/CpG in macrophages and/or in M. tuberculosis-infected lungs, revealed a striking enrichment for the pathways of antigen processing and presentation, including multiple structural and enzymatic components of the Class I and Class II MHC (major histocompatibility complex) antigen presentation machinery. Also significantly enriched as IRF8 targets are the group of endomembrane- and phagosome-associated small GTPases of the IRG (immunity-related GTPases) and GBP (guanylate binding proteins) families. These results identify IRF8 as a key regulator of early response pathways in myeloid cells, including phagosome maturation, antigen processing, and antigen presentation by myeloid cells.

Essential role of the IRF8-KLF4 transcription factor cascade in murine monocyte differentiation

“…Indeed, IRF8 directly binds to and upregulates multiple genes encoding major histocompatibility complex class II molecules (H2-Aa, H2-DMb2, H2-K1, and H2-Q7) and class II invariant chain (CD74), some of which have been reported previously to be regulated by IRF8. 30 It is possible that transcriptional regulators other than KLF4 also play roles downstream of IRF8. For example, IRF8-induced transcription factor genes such as Egr1, Egr2, Maf, and MafB have also been implicated in monocyte differentiation.…”

“…Previous ChIP-on-chip (chromatin immunoprecipitation followed by microarray hybridization) studies of IRF8 have used differentiation-arrested monocytic cell lines or mouse lungs infected with pathogen but not cells undergoing monocytic differentiation. 29,30 It was recently revealed that distal enhancers, characterized by histone H3 lysine 4 monomethylation (H3K4me1), are critical for cell lineage specification. 31 However, the above-mentioned ChIP-on-chip studies examined the regions relatively proximal (27.5 kb to 12.5 kb) to the transcription start sites (TSSs) and did not examine epigenetic changes.…”

Regulation of myelopoiesis by the transcription factor IRF8

“…The KLF4-dependent IRF8 target genes are preferentially related to the basic functions of monocytes, such as inflammatory response, chemotaxis, and locomotion, while the KLF4-independent IRF8 target genes are significantly related to more advanced functions, especially antigen presentation. Indeed, IRF8 directly binds to and upregulates multiple genes that encode MHC II and the invariant chain (CD74) [5,26]. Taken together, the evidence indicates that IRF8 cooperates with PU.1 to promote the formation of enhancers to induce various monocyte-related genes, including Klf4, and that the IRF8-KLF4 transcription factor cascade is essential for monocyte development.…”

Sca-1+Lin−CD117− Mesenchymal Stem/Stromal Cells Induce the Generation of Novel IRF8-Controlled Regulatory Dendritic Cells through Notch–RBP-J Signaling

“…These results confirm that, functionally, sBM-DCs are an IRF8-controlled regDC population. Strikingly, IRF8 was also reported to play a critical role in the regulation of several proinflammatory genes (including IL-12) that function during macrophage activation (53)(54)(55). Whether there is any relationship in IRF8 function between sBM-DC generation and macrophage activation deserves further study.…”