*3.2. TNF-*α

TNF-<sup>α</sup>, also known as TNF, was first identified as a tumoricidal protein that mediates endotoxin-induced hemorrhagic necrosis in sarcoma and other transplanted tumors in 1975 [95]. Later in 1984 human TNF was cloned [96].

#### 3.2.1. The Source of TNF-α

TNF is produced predominantly by activated macrophages and T lymphocytes as a plasma membrane bound 26 kDa precursor glycoprotein. TNF-α converting enzyme (TACE; also known as ADAM-17) mediates the cleavage in the extracellular domain of TNF-α precursor and releases a soluble 17 kDA form [97]. In addition to macrophages and T lymphocytes lineage, a wide range of cells can produce TNF-<sup>α</sup>, including mast cells, B lymphocytes, natural killer (NK) cells, neutrophils, endothelial cells, intestinal epithelial cells (IECs), smooth and cardiac muscle cells, fibroblasts, and osteoclasts [98,99]. TNF-α is not usually detectable in healthy individuals, but elevated serum and tissue levels are found in inflammatory conditions [100], and serum levels correlate with the severity of infections [101,102].

#### 3.2.2. The Function of TNF-α

TNF-α is a key regulator of inflammation and has been involved in many human diseases, including psoriasis, rheumatoid arthritis, and IBD [103]. Anti-TNF-α therapy is the best available therapeutic option to induce mucosal repair and clinical remission in IBD patients [104]. However, a recent report showed that TNF-α blockage may cause dysbiosis and increased Th17 cell population in the colon of healthy mice [104]. Another report demonstrated that TNF-α promotes colonic mucosal repair through induction of the platelet activating factor receptor (PAFR) via NF-B signaling in the intestinal epithelium. Increased PAFR expression leads to activation of epidermal growth factor receptor Src as well as increased Rac1 and FAK signaling to promote cellular migration and wound closure. Consistently, TNF-α neutralization ablates PAFR upregulation and impairs intestinal wound repair [105]. In addition, bone marrow-derived TNF-α binds to epithelial TNF receptors (TNFRs) and activates epithelial beta-catenin signaling, promotes intestinal stem cell proliferation and IEC expansion, and helps mucosal healing in chronic colitis patients [98]. This was shown as enhanced apoptosis, reduced IEC proliferation, and decreased Wnt signaling when stimulated with anti-CD3 mAb in TNF-deficient (Tnf −/−) mice [76]. TNFR2 was increased in the epithelial cells from IBD patients and disruption of TNFR2 in naïve CD8+ T cells increased the severity of colitis in Rag 2 −/− mice [106,107]. TNF-induced intestinal NF-κB activation is also crucial for prevention of local intestinal injury following ischemia–reperfusion [108].

## 3.2.3. The Regulation of TNF-α

At the transcriptional level, the TNF gene is induced in response to a diversity of specific stimuli including inflammation, infection, and stress [109]. Bacterial endotoxin specially activates TNF-α gene expression [110]. Analysis of human TNF-α promoter indicated that transcription factors such as Ets and c-Jun are involved in the transcriptional regulation of TNF-α [111]. Previously, we have also shown that HIF-2α is a positive regulator of TNF-α production in the intestinal epithelium [25].
