Summary The innate and adaptive immune responses that confer resistance to the intracellular pathogen Toxoplasma gondii critically depend on IL-12 production, which drives interferon-γ (IFN-γ) expression. Certain cytokines can activate STAT3 and limit IL-12 production to prevent infection-associated immune pathology but T.gondii also directly activates STAT3 to evade host immunity. We show that Suppressor of Cytokine Signaling molecule 3 (SOCS3), a target of STAT3 which limits signaling by the pleiotropic cytokine IL-6, is upregulated in response to infection but is dispensable for the immune-inhibitory effects of T. gondii. Unexpectedly, mice with targeted deletion of SOCS3 in macrophages and neutrophils have reduced IL-12 responses and succumb to toxoplasmosis. Anti–IL-6 administration or IL-12 treatment blocked disease susceptibility suggesting that in the absence of SOCS3, macrophages are hypersensitive to the anti-inflammatory properties of IL-6. Thus, SOCS3 has a critical role in suppressing IL-6 signals and promoting immune responses to control T. gondii infection.

T cell activation following antigen binding to the T cell receptor (TCR) involves the mobilization of intracellular Ca2؉ to activate the key transcription factors nuclear factor of activated T lymphocytes (NFAT) and NF-B. 2؉ -dependent checkpoint in TCR-induced NF-B signaling that has broad implications for the control of immune cell development and T cell functional specificity. Activation of T cells following antigen binding to the T cell antigen receptor (TCR)3 induces diverse lineage-and fate-specific proinflammatory and immune-modulatory responses. Central to these responses is the induction of quantitatively distinct intracellular Ca 2ϩ signals and their selective activation of the key transcription factors NFAT and NF-B (1-6). The mechanism by which Ca 2ϩ controls NFAT activation in lymphocytes is well established (7). In contrast, although Ca 2ϩ has been implicated in TCR-induced NF-B signaling (8 -10), how Ca 2ϩ regulates NF-B activity is largely unexplored and represents a significant gap in our understanding of transcriptional control of T cell development, activation, and functional specificity.In resting T cells, classical NF-B consists of heterodimers of p50/p65 or p50/c-Rel that are retained in the cytosol by members of the inhibitory family of IB proteins (11, 12). Following TCR engagement, IB kinase (IKK)-mediated phosphorylation triggers the ubiquitination and proteasomal degradation of IB␣, releasing p50/p65 and p50/c-Rel, which localize to the nucleus to initiate transcription of crucial immune-regulatory, proinflammatory, and proproliferative genes (13-30). Although TCR-mediated Ca 2ϩ mobilization has been implicated in proximal steps of NF-B activation (8 -10), the precise mechanisms and source of Ca 2ϩ that regulate nuclear localization and transcriptional activation of NF-B are poorly defined. It is well established that TCR signaling induces inositol 1,4,5-trisphosphate-mediated depletion of Ca 2ϩ from the endoplasmic reticulum (ER). A resulting Ca 2ϩ dissociation from the ER membrane protein stromal interaction molecule 1 (STIM1) triggers its oligomerization and relocalization to ER membrane domains juxtaposed to the plasma membrane (31-33), where STIM1 physically gates Orai (also known as Ca 2ϩ release-activated Ca 2ϩ ) channels, allowing extracellular Ca 2ϩ to enter the cell (34,35). However, it is not known whether Ca 2ϩ control of TCR-induced NF-B signaling requires STIM1-and Orai1-mediated Ca 2ϩ influx or whether the initial release of Ca 2ϩ from the ER is sufficient for classical NF-B activation.In this study, we sought to determine both the source and mechanism of Ca 2ϩ control of antigen receptor-induced NF-B activation in T cells. We show that influx of extracellular Ca 2ϩ via STIM1 and Orai is critical for TCR-but not TNFinduced IB␣ degradation and NF-B activation. Importantly, we also demonstrate that Ca 2ϩ -dependent, PKC␣-mediated phosphorylation of p65 critically regulates its nuclear localization and transcriptional activation following TCR engagement. Thus, our findings ...

Precise regulation of nuclear factor κB (NF-κB) signaling is crucial for normal immune responses, and defective NF-κB activity underlies a range of immunodeficiencies. NF-κB is activated through two signaling cascades: the classical and noncanonical pathways. The classical pathway requires inhibitor of κB kinase β (IKKβ) and NF-κB essential modulator (NEMO), and hypomorphic mutations in the gene encoding NEMO (ikbkg) lead to inherited immunodeficiencies, collectively termed NEMO-ID. Noncanonical NF-κB activation requires NF-κB–inducing kinase (NIK) and IKKα, but not NEMO. We found that noncanonical NF-κB was basally active in peripheral blood mononuclear cells from NEMO-ID patients, and that noncanonical NF-κB signaling was similarly enhanced in cell lines lacking functional NEMO. NIK, which normally undergoes constitutive degradation, was aberrantly present in resting NEMO-deficient cells, and regulation of its abundance was rescued by reconstitution with full-length NEMO, but not a mutant NEMO protein unable to physically associate with IKKα or IKKβ. Binding of NEMO to IKKα was not required for ligand-dependent stabilization of NIK or noncanonical NF-κB signaling. Rather, an intact and functional IKK complex was essential to suppress basal NIK activity in unstimulated cells. Despite interacting with IKKα and IKKβ to form an IKK complex, NEMO mutants associated with immunodeficiency failed to rescue classical NF-κB signaling or reverse the accumulation of NIK. Together, these findings identify a crucial role for classical NF-κB activity in the suppression of basal noncanonical NF-κB signaling.

The oncogenic fusion protein RET/PTC3 (RP3) that is expressed in papillary thyroid carcinoma (PTC) and thyroid epithelia in Hashimoto’s thyroiditis activates Nuclear Factor-kappa B (NF-κB) and induces pro-inflammatory gene expression; however, the mechanism of this activation is unknown. To address this, we expressed RP3 in murine embryonic fibroblasts (MEFs) lacking key classical and non-canonical NF-κB signaling components. In wild-type MEFs, RP3 upregulated CCL2, CXCL1, GM-CSF and TNF expression and activated classical but not non-canonical NF-κB. RP3 activated NF-κB in IKKβ−/− MEFs but not IKKα- or NEMO-deficient cells and activation was inhibited by a peptide that blocks NEMO binding to the IKKs. RP3 increased the levels of NF-κB-inducing kinase (NIK) and did not activate NF-κB in NIK-deficient MEFs. Notably, NIK stabilization was not accompanied by TRAF3 degradation demonstrating that RP3 disrupts normal basal NIK regulation. Dominant negative NIK blocked RP3-induced NF-κB activation and an RP3 signaling mutant (RP3Y588F) did not stabilize NIK. Finally, examination of PTC specimens revealed strong positive staining for NIK. We therefore conclude that RP3 activates classical NF-κB via NIK, NEMO and IKKα. Importantly, our findings reveal a novel mechanism for oncogene-induced NF-κB activation via stabilization of NIK.

Non-canonical NF-κB signaling is controlled by the precise regulation of NF-κB Inducing Kinase (NIK) stability. NIK is constitutively ubiquitylated by cellular inhibitor of apoptosis (cIAP) proteins 1 and 2, leading to its complete proteasomal degradation in resting cells. Following stimulation, cIAP-mediated ubiquitylation of NIK ceases and NIK is stabilized, allowing for Inhibitor of κB kinase (IKK)α activation and non-canonical NF-κB signaling. Non-canonical NF-κB signaling is terminated by feedback phosphorylation of NIK by IKKα that promotes NIK degradation; however, the mechanism of active NIK protein turnover remains unknown. To address this question, we established a strategy to precisely distinguish between basal degradation of newly synthesized endogenous NIK and induced active NIK in stimulated cells. Using this approach, we found that IKKα-mediated degradation of signal-induced activated NIK occurs through the proteasome. To determine whether cIAP1 or cIAP2 play a role in active NIK turnover, we utilized a Smac mimetic (GT13072), which promotes degradation of these E3 ubiquitin ligases. As expected, GT13072 stabilized NIK in resting cells. However, loss of the cIAPs did not inhibit proteasome-dependent turnover of signal-induced NIK showing that unlike the basal regulatory mechanism, active NIK turnover is independent of cIAP1 and cIAP2. Our results therefore establish that the negative feedback control of IKKα-mediated NIK turnover occurs via a novel proteasome-dependent and cIAP-independent mechanism.