MyD88 primes macrophages for full-scale activation by interferon-gamma yet mediates few responses to Mycobacterium tuberculosis.
Macrophages are activated from a resting state by a combination of cytokines and microbial products. Microbes are often sensed through Toll-like receptors signaling through MyD88. We used large-scale microarrays in multiple replicate experiments followed by stringent statistical analysis to compare gene expression in wild-type (WT) and MyD88-/- macrophages. We confirmed key results by quantitative reverse transcription polymerase chain reaction, Western blot, and enzyme-linked immunosorbent assay. Surprisingly, many genes, such as inducible nitric oxide synthase, IRG-1, IP-10, MIG, RANTES, and interleukin 6 were induced by interferon (IFN)-gamma from 5- to 100-fold less extensively in MyD88-/- macrophages than in WT macrophages. Thus, widespread, full-scale activation of macrophages by IFN-gamma requires MyD88. Analysis of the mechanism revealed that MyD88 mediates a process of self-priming by which resting macrophages produce a low level of tumor necrosis factor. This and other factors lead to basal activation of nuclear factor kappaB, which synergizes with IFN-gamma for gene induction. In contrast, infection by live, virulent Mycobacterium tuberculosis (Mtb) activated macrophages largely through MyD88-independent pathways, and macrophages did not need MyD88 to kill Mtb in vitro. Thus, MyD88 plays a dynamic role in resting macrophages that supports IFN-gamma-dependent activation, whereas macrophages can respond to a complex microbial stimulus, the tubercle bacillus, chiefly by other routes.
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D000943:Antigens, Differentiation / Q000502:physiology*; D005786:Gene Expression Regulation / Q000187:drug effects; D007371:Interferon-gamma / Q000494:pharmacology*; D007375:Interleukin-1 / Q000096:biosynthesis; D008262:Macrophage Activation / Q000187:drug effects; D008264:Macrophages / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D009169:Mycobacterium tuberculosis / Q000502:physiology*; D053594:Myeloid Differentiation Factor 88; D016328:NF-kappa B / Q000502:physiology; D011956:Receptors, Cell Surface / Q000502:physiology; D011971:Receptors, Immunologic / Q000502:physiology*; D051193:Toll-Like Receptors; D015533:Transcriptional Activation; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology
D048868:Adaptor Proteins, Signal Transducing; D000943:Antigens, Differentiation; D007375:Interleukin-1; D008562:Membrane Glycoproteins; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D016328:NF-kappa B; D011956:Receptors, Cell Surface; D011971:Receptors, Immunologic; D051193:Toll-Like Receptors; D014409:Tumor Necrosis Factor-alpha; D007371:Interferon-gamma