Loss of Protein Tyrosine Phosphatase Nonreceptor Type 22 Regulates Interferon-γ-Induced Signaling in Human Monocytes

BACKGROUND & AIMS: A gain-of-function variation within the locus that encodes protein tyrosine phosphatase nonreceptor type (PTPN) 22 is associated with a reduced risk for Crohn's disease (CD), whereas a loss-of-function variant seems to promote autoimmune disorders. We investigated how loss of PTPN22 could contribute to chronic inflammation of the intestine. METHODS: Intestinal tissue samples from patients with or without inflammatory bowel disease (controls) were analyzed for levels of PTPN22 messenger RNA (mRNA) and protein. In human THP-1 monocytes, protein levels were analyzed by immunoblotting, mRNA levels by real-time polymerase chain reaction, and cytokine release by enzyme-linked immunosorbent assay. RESULTS: Intestinal tissue samples from patients with CD had reduced levels of PTPN22 mRNA and protein, compared with samples from controls. In human THP-1 monocytes, interferon-gamma (IFN-gamma) induced expression and activity of PTPN22. Loss of PTPN22 increased levels of p38-mitogen-activated protein kinase, but reduced phosphorylation of nuclear factor-kappa B subunits. Increased activity of suppressor of cytokine signaling-1 was accompanied by reduced phosphorylation of signal-transducer and activator of transcription protein 1 and signal-transducer and activator of transcription protein 3 in PTPN22-deficient cells incubated with cytokines. PTPN22 knockdown increased secretion of the inflammatory cytokines interleukin (IL)-6 and IL-17, but reduced expression or secretion of T-bet, intercellular adhesion molecule-1, nucleotide-binding oligomerization domain containing-2, monocyte chemoattractant protein-1, IL-2, and IL-12p40 in response to IFN-gamma. CONCLUSIONS: PTPN22 expression is reduced in intestinal tissues of patients with active CD. PTPN22 regulates intracellular signaling events and is induced by IFN-gamma in human monocytes. Knockdown of PTPN22 alters activation of inflammatory signal transducers, increasing secretion of T-helper 17-related inflammatory mediators. Genetic variants that reduce PTPN22 activity might contribute to the pathogenesis of CD by these mechanisms.