NF-κB signaling in inflammation

The transcription factor NF-κB regulates multiple aspects of innate and adaptive immune functions and serves as a pivotal mediator of inflammatory responses.NF-κB induces the expression of various pro-inflammatory genes,including those encoding cytokines and chemokines,and also participates in inflammasome regulation.In addition,NF-κB plays a critical role in regulating the survival,activation and differentiation of innate immune cells and inflammatory T cells.Consequently,deregulated NF-κB activation contributes to the pathogenic processes of various inflammatory diseases.In this review,we will discuss the activation and function of NF-κB in association with inflammatory diseases and highlight the development of therapeutic strategies based on NF-κB inhibition.

New insight into the oncogenic mechanism of the retroviral oncoprotein Tax

Human T cell leukemia virus type 1(HTLV-1),an etio-logical factor that causes adult T cell leukemia and lym-phoma(ATL),infects over 20 million people worldwide.About 1 million of HTLV-1-infected patients develop ATL,a highly aggressive non-Hodgkin's lymphoma without an effective therapy.The pX region of the HTLV-1 viral genome encodes an oncogenic protein,Tax,which plays a central role in transforming CD4+ T lymphocytes by deregulating oncogenic signaling pathways and promoting cell cycle progression.Expression of Tax following viral entry is critical for promoting survival and proliferation of human T cells and is required for initiation of oncogenesis.Tax exhibits diverse functions in host cells,and this oncoprotein primarily targets IκB kinase complex in the cytoplasm,resulting in persistent activation of NF-κB and upregulation of its responsive gene expressions that are crucial for T cell survival and cell cycle progression.We here review recent advances for the pathological roles of Tax in modulating IκB kinase activity.We also discuss our recent observation that Tax connects the IκB kinase complex to autophagy pathways.Understanding Tax-mediated pathogenesis will provide insights into development of new therapeutics in controlling HTLV-1-associated diseases.

Peli： a family of signal-responsive E3 ubiquitin ligases mediating TLR signaling and T-cell tolerance

E3 ubiquitin ligases play a crucial role in regulating immune receptor signaling and modulating immune homeostasis and activation. One emerging family of such E3s is the Pelle-interacting （Peli） proteins, characterized by the presence of a cryptic forkhead-associated domain involved in substrate binding and an atypical RING domain mediating formation of both lysine （K） 63- and K48-1inked polyubiquitin chains. A well-recognized function of Peli family members is participation in the signal transduction mediated by Toll-like receptors （TLRs） and IL-1 receptor. Recent gene targeting studies have provided important insights into the in vivofunctions of Pelil in the regulation of TLR signaling and inflammation. These studies have also extended the biological functions of Pelil to the regulation of T-cell tolerance. Consistent with its immunoregulatory functions, Peli I responds to different immune stimuli for its gene expression and catalytic activation. In this review, we discuss the recent progress, as well as the historical perspectives in the regulation and biological functions of Peli.

Deubiquitinases as pivotal regulators of T cell functions

T cells efficiently respond to foreign antigens to mediate immune responses against infections but are tolerant to self-tissues. Defect in T cell activation is associated with severe immune deficiencies, whereas aberrant T cell activation contributes to the pathogenesis of diverse autoimmune and inflammatory diseases. An emerging mechanism that regulates T cell activation and tolerance is ubiquitination, a reversible process of protein modification that is counter-regulated by ubiquitinating enzymes and deubiquitinases （DUBs）. DUBs are isopeptidases that cleave polyubiquitin chains and remove ubiquitin from target proteins, thereby controlling the magnitude and duration of ubiquitin signaling. It is now well recognized that DUBs are crucial regulators of T cell responses and serve as potential therapeutic targets for manipulating immune responses in the treatment of immunological disorders and cancer. This review will discuss the recent progresses regarding the functions of DUBs in T cells.

TRAF2 regulates T cell immunity by maintaining a Tpl2-ERK survival signaling axis in effector and memory CD8 T cells

Generation and maintenance of antigen-specific effector and memory T cells are central events in immune responses against infections.We show that TNF receptor-associated factor 2(TRAF2)maintains a survival signaling axis in effector and memory CD8 T cells required for immune responses against infections.This signaling axis involves activation of Tpl2 and its downstream kinase ERK by NF-κB-inducing kinase(NIK)and degradation of the proapoptotic factor Bim.NIK mediates Tpl2 activation by stimulating the phosphorylation and degradation of the Tpl2 inhibitor p105.Interestingly,while NIK is required for Tpl2-ERK signaling under normal conditions,uncontrolled NIK activation due to loss of its negative regulator,TRAF2,causes constitutive degradation of p105 and Tpl2,leading to severe defects in ERK activation and effector/memory CD8 T cell survival.Thus,TRAF2 controls a previously unappreciated signaling axis mediating effector/memory CD8 T cell survival and protective immunity.

Survival and maintenance of regulatory T cells require the kinase TAK1

Regulatory T （Treg） cells play a central role in regulating peripheral immune tolerance and preventing autoimmunity. Despite the extensive studies on the development of Treg cells, the molecular mechanisms that maintain the population of committed Treg cells remain poorly understood. We show here that Treg-conditional ablation of the kinase TAK1 reduced the number of Treg cells in the peripheral lymphoid organs, causing abnormal activation of conventional T cells and autoimmune symptoms. Using an inducible gene knockout approach, we further demonstrate that TAK1 is crucial for the survival of Treg cells. Expression of a constitutively active IKB kinase partially restored the level of Treg cells in the TAKITreg-KO mice. These results suggest a crucial role for TAK1 for maintaining the survival of committed Treg cells under physiololzical conditions.

Lymphatic endothelial cells regulate B-cell homing to lymph nodes via a NIK-dependent mechanism

B cells home to the lymph nodes(LNs)via high endothelial venules(HEVs)under the guidance of chemokines,particularly CXCL13.However,as CXCL13 is not directly made in HEVs,the molecular mechanism mediating B-cell homing to LNs has remained unclear.We show here that nuclear factor(NF)-κB-inducing kinase(NIK),a kinase mediating activation of the noncanonical NF-κB pathway,functions in lymphatic endothelial cells(LECs)to regulate B-cell homing to LNs.LEC-conditional deletion of NIK in mice did not affect the integrity or global function of lymphatic vessels but caused a severe reduction in the frequency of B cells in LNs.The LEC-specific NIK deficiency did not affect the survival of B cells or the frequency of B cells in the spleen.B-cell adoptive transfer studies revealed that the LEC-specific NIK deletion impairs the ability of LNs to recruit B cells.We further show that NIK mediates expression of the chemokines CXCL13 and CCL19 in LECs.Although CCL19 is also expressed in blood endothelial cells(BECs),CXCL13 is not produced in BECs.These results suggest that NIK regulates naive B-cell homing to LNs via mediating production of the B-cell homing chemokine CXCL13 in LECs.

Regulation of antiviral innate immunity by deubiquitinase CYLD

An antiviral innate immune response involves induction of type I interferons(IFNs)and their subsequent autocrine and paracrine actions,but the underlying regulatory mechanisms are incompletely understood.Here we report that CYLD,a deubiquitinase that specifically digests lysine 63-linked ubiquitin chains,is required for antiviral host defense.Loss of CYLD renders mice considerably more susceptible to infection by vesicular stomatitis virus(VSV).Consistently,CYLD-deficient dendritic cells are more sensitive to VSV infection.This functional defect was not due to lack of type I IFN production but rather because of attenuated IFN receptor signaling.In the absence of CYLD,IFN-b is ineffective in the induction of antiviral genes and protection of cells from viral infection.These findings establish CYLD as a novel regulator of antiviral innate immunity and suggest a role for CYLD in regulating IFN receptor signaling.

TBK1 as a regulator of autoimmunity and antitumor immunity

TANK-binding kinase 1(TBK1)and its homolog IκB kinase(IKK)epsilon(IKKε,also known as IKKi)are serine/threonine kinases that are well known for their functions to mediate type I interferon(IFN-I)induction in antiviral innate immune responses.1 Various microbial components,or pathogen associated molecular patterns(PAMPs),activate TBK1 and IKKεvia stimulating pattern-recognition receptors(PRRs)in innate immune cells.Upon activation,TBK1 or IKKε(hereafter called TBK1/IKKε)phosphorylates the transcription factors IRF3 and IRF7 to trigger their dimerization and nuclear translocation,leading to induction of IFN-I gene expression.1,2 In this innate immune signaling pathway,TBK1 and IKKεdisplay functional redundancies,although TBK1 seems to be more crucial than IKKε.3,4 It is increasingly clear that the function of TBK1 and IKKεis beyond the regulation of antiviral innate immunity.

Targeting ubiquitin signaling for cancer immunotherapy

Cancer immunotherapy has become an attractive approach of cancer treatment with tremendous success in treating various advanced malignancies.The development and clinical application of immune checkpoint inhibitors represent one of the most extraordinary accomplishments in cancer immunotherapy.In addition,considerable progress is being made in understanding the mechanism of antitumor immunity and characterizing novel targets for developing additional therapeutic approaches.One active area of investigation is protein ubiquitination,a post-translational mechanism of protein modifcation that regulates the function of diverse immune cells in antitumor immunity.Accumulating studies suggest that E3 ubiquitin ligases and deubiquitinases form a family of potential targets to be exploited for enhancing antitumor immunity in cancer immunotherapy.