Regulation of gene expression by TPL-2 in innate immunity

The MAP 3-kinase TPL-2 is critical for TLR activation of ERK1/2 MAP kinases in myeloid cells during innate immune responses. TPL-2 is required for inflammatory immune responses to bacteria, fungi and viruses, mediating it effects by regulating the expression of key cytokines. My project’s objective was to investigate the transcription factors that control gene expression downstream of TPL-2. Global gene expression analysis of cultured macrophages by RNA sequencing confirmed that TPL-2 controls the mRNA abundance of numerous cytokines and transcription factors. Bioinformatic analysis of the most rapidly induced genes, together with analysis of published phosphoproteome data, identified that the TCF and ERF transcription factor families are regulated by TPL-2 activation of ERK1/2. Following TLR4 stimulation, TPL-2 induced the phosphorylation of ELK1, ETV3 and ERF via ERK1/2 activation. Additionally, the expression of known TCF target genes was dependent on TPL-2 signalling. FOS was identified as a target of the TPL-2 – ERK1/2 signalling pathway, where the expression and phosphorylation of FOS was shown to have dependence of TPL-2 kinase activity following TLR activation. Furthermore, RNA sequencing analysis of TCF-deficient (Elk1/4-/-) and FOS-deficient macrophages demonstrated that, directly and indirectly, TCFs mediate half and FOS mediates a third of the transcriptional output of TPL2 signalling, including immune response related genes and genes encoding transcription factors. Comparing TCF- and FOS-regulated genes suggested that TCFs and FOS co-ordinately regulate around one quarter of TPL-2’s output via a TPL-2-ERK1/2-TCF-FOS pathway. TPL-2 kinase activity, TCF and FOS expression were found to inhibit mRNA expression of the key cytokine IFN-β in macrophages following TLR stimulation. TPL-2 suppression of IFN-β production is important for efficient innate immune responses to Listeria monocytogenes and Mycobacterium tuberculosis. Results in this thesis suggest that activation of TCFs and FOS by the TPL-2–ERK1/2 MAP kinase pathway has an important role in controlling innate immunity