Multiple mechanisms for TRAF3-mediated regulation of the T cell costimulatory receptor GITR

Abstract

Tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3) plays context-specific roles in multiple receptor-mediated signaling pathways in different cell types. Mice lacking TRAF3 in T cells display defective T-cell-mediated immune responses to immunization and infection and demonstrate defective early signaling via the TCR complex. However, the role of TRAF3 in the function of GITR/TNFRSF18, an important costimulatory member of the TNFR superfamily, is unclear. Here we investigated the impact of T cell TRAF3 status on both GITR expression and activation of specific kinases in the GITR signaling pathway in T cells. Our results indicate that TRAF3 negatively regulates GITR functions in several ways. First, expression of GITR protein was elevated in TRAF3-deficient T cells, resulting from both transcriptional and posttranslational regulation that led to greater GITR transcript levels, as well as enhanced GITR protein stability. TRAF3 associated with T cell GITR in a manner dependent upon GITR ligation. TRAF3 also inhibited several events of the GITR mediated early signaling cascade, in a manner independent of recruitment of phosphatases, a mechanism by which TRAF3 inhibits signaling through several other cytokine receptors. These results add new information to our understanding of GITR signaling and function in T cells, which is relevant to the potential use of GITR to enhance immune therapies.

TRAF3: Guardian of T lymphocyte functions

“…TRAF3 -/- T cells have higher GITR mRNA and protein than control T cells. In agreement with previous reports, this difference is not due to the increase in Treg cells in T- Traf3 -/- mice, as GITR levels are similar on TRAF3-deficient and sufficient Treg cells ( 13 , 23 ). Instead, the constitutive NF-κB2 activation present in all TRAF3-deficient cells is primarily responsible for upregulation of Gitr mRNA.…”

“…Instead, the constitutive NF-κB2 activation present in all TRAF3-deficient cells is primarily responsible for upregulation of Gitr mRNA. Interestingly, however, the increase in GITR protein is not due entirely to increased mRNA; there is also reduced GITR protein turnover in the absence of TRAF3 ( 23 ). GITR signaling causes a TRAF-C-dependent recruitment of TRAF3 to the GITR cytoplasmic domain, and activates canonical NF-κB, MAPK and mTOR-AKT-S6 kinase pathways ( 23 ).…”

“…Interestingly, however, the increase in GITR protein is not due entirely to increased mRNA; there is also reduced GITR protein turnover in the absence of TRAF3 ( 23 ). GITR signaling causes a TRAF-C-dependent recruitment of TRAF3 to the GITR cytoplasmic domain, and activates canonical NF-κB, MAPK and mTOR-AKT-S6 kinase pathways ( 23 ). Phosphorylation of IκBα, ERK, and S6 kinase are enhanced in TRAF3 -/- T cells, but the enhanced S6 kinase activation is ERK-dependent rather than AKT-dependent.…”

“…Phosphorylation of IκBα, ERK, and S6 kinase are enhanced in TRAF3 -/- T cells, but the enhanced S6 kinase activation is ERK-dependent rather than AKT-dependent. Interestingly, inhibition of Protein Tyrosine Phosphatase N2 (PTPN2) in GITR-stimulated TRAF3 -/- T cells reduces MAPK and canonical NF-κB activation to WT T cell levels, but TRAF3 deficiency does not affect the PTPN2-GITR association ( 23 ). This work establishes a role for TRAF3 in costimulatory receptor signaling beyond CD28, and reinforces the context-dependence of TRAF3 activity in T cells, even among subsets within this single cell type.…”

TRAF3 enhances type I interferon receptor signaling in T cells by modulating the phosphatase PTPN22

“…The data shown here support a role for TRAF3 in preventing the inappropriate association of PTPN2 and PTPN22 with the IFNAR complex. TRAF3 additionally impedes signaling by the T cell costimulatory TNFR superfamily member GITR, through an incompletely understood mechanism that does not implicate phosphatase recruitment ( 44 ). As discussed earlier, TRAF3 also limits T reg cell differentiation by regulating the strength of IL-2R signaling on pre-T reg cells ( 23 ).…”