Phosphorylation of Carma1, but not Bcl10, by Akt regulates TCR/CD28-mediated NF-κB induction and cytokine production

Phosphorylation-dependent substrate selectivity of protein kinase B (AKT1)

Protein kinase B (AKT1) is a central node in a signaling pathway that regulates cell survival. The diverse pathways regulated by AKT1 are communicated in the cell via the phosphorylation of perhaps more than 100 cellular substrates. AKT1 is itself activated by phosphorylation at Thr-308 and Ser-473. Despite the fact that these phosphorylation sites are biomarkers for cancers and tumor biology, their individual roles in shaping AKT1 substrate selectivity are unknown. We recently developed a method to produce AKT1 with programmed phosphorylation at either or both of its key regulatory sites. Here, we used both defined and randomized peptide libraries to map the substrate selectivity of site-specific, singly and doubly phosphorylated AKT1 variants. To globally quantitate AKT1 substrate preferences, we synthesized three AKT1 substrate peptide libraries: one based on 84 "known" substrates and two independent and larger oriented peptide array libraries (OPALs) of ∼10¹¹ peptides each. We found that each phospho-form of AKT1 has common and distinct substrate requirements. Compared with pAKT1^T308, the addition of Ser-473 phosphorylation increased AKT1 activities on some, but not all of its substrates. This is the first report that Ser-473 phosphorylation can positively or negatively regulate kinase activity in a substrate-dependent fashion. Bioinformatics analysis indicated that the OPAL-activity data effectively discriminate known AKT1 substrates from closely related kinase substrates. Our results also enabled predictions of novel AKT1 substrates that suggest new and expanded roles for AKT1 signaling in regulating cellular processes.

Ubiquitination and phosphorylation of the CARD11-BCL10-MALT1 signalosome in T cells

Antigen receptor-induced signaling plays an important role in inflammation and immunity. Formation of a CARD11-BCL10-MALT1 (CBM) signaling complex is a key event in T- and B cell receptor-induced gene expression by regulating NF-κB activation and mRNA stability. Deregulated CARD11, BCL10 or MALT1 expression or CBM signaling have been associated with immunodeficiency, autoimmunity and cancer, indicating that CBM formation and function have to be tightly regulated. Over the past years great progress has been made in deciphering the molecular mechanisms of assembly and disassembly of the CBM complex. In this context, several posttranslational modifications play an indispensable role in regulating CBM function and downstream signal transduction. In this review we summarize how the different CBM components as well as their interplay are regulated by protein ubiquitination and phosphorylation in the context of T cell receptor signaling.

Post-translational Modifications of the CARMA1-BCL10-MALT1 Complex in Lymphocytes and Activated B-Cell Like Subtype of Diffuse Large B-Cell Lymphoma

Piracy of the NF-κB transcription factors signaling pathway, to sustain its activity, is a mechanism often deployed in B-cell lymphoma to promote unlimited growth and survival. The aggressive activated B-cell like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) exploits a multi-protein complex of CARMA1, BCL10, and MALT1 (CBM complex), which normally conveys NF-κB signaling upon antigen receptors engagement. Once assembled, the CBM also unleashes MALT1 protease activity to finely tune the immune response. As a result, ABC DLBCL tumors develop a profound addiction to NF-κB and to MALT1 enzyme, leaving open a breach for therapeutics. However, the pleiotropic nature of NF-κB jeopardizes the success of its targeting and urges us to develop new strategies. In this review, we discuss how post-translational modifications, such as phosphorylation and ubiquitination of the CBM components, as well as, MALT1 proteolytic activity, shape the CBM activity in lymphocytes and ABC DLBCL, and may provide new avenues to restore vulnerability in lymphoma.

Mechanisms of Regulated and Dysregulated CARD11 Signaling in Adaptive Immunity and Disease

CARD11 functions as a key signaling scaffold that controls antigen-induced lymphocyte activation during the adaptive immune response. Somatic mutations in CARD11 are frequently found in Non-Hodgkin lymphoma, and at least three classes of germline CARD11 mutations have been described as the basis for primary immunodeficiency. In this review, we summarize our current understanding of how CARD11 signals, how its activity is regulated, and how mutations bypass normal regulation to cause disease.

Histone deacetylase (HDAC) inhibitor ACY241 enhances anti-tumor activities of antigen-specific central memory cytotoxic T lymphocytes against multiple myeloma and solid tumors

Histone deacetylases (HDAC) are therapeutic targets in multiple cancers. ACY241, an HDAC6 selective inhibitor, has shown anti-multiple myeloma (MM) activity in combination with immunomodulatory drugs and proteasome inhibitors. Here we show ACY241 significantly reduces the frequency of CD138+ MM cells, CD4+CD25+FoxP3+ regulatory T cells, and HLA-DRLow/-CD11b+CD33+ myeloid-derived suppressor cells; and decreases expression of PD1/PD-L1 on CD8+ T cells and of immune checkpoints in bone marrow cells from myeloma patients. ACY241 increased B7 (CD80, CD86) and MHC (Class I, Class II) expression on tumor and dendritic cells. We further evaluated the effect of ACY241 on antigen-specific cytotoxic T lymphocytes (CTL) generated with heteroclitic XBP1unspliced184-192 (YISPWILAV) and XBP1spliced367-375 (YLFPQLISV) peptides. ACY241 induces co-stimulatory (CD28, 41BB, CD40L, OX40) and activation (CD38) molecule expression in a dose- and time-dependent manner, and anti-tumor activities, evidenced by increased perforin/CD107a expression, IFN-γ/IL-2/TNF-α production, and antigen-specific central memory CTL. These effects of ACY241 on antigen-specific memory T cells were associated with activation of downstream AKT/mTOR/p65 pathways and upregulation of transcription regulators including Bcl-6, Eomes, HIF-1 and T-bet. These studies therefore demonstrate mechanisms whereby ACY241 augments immune response, providing the rationale for its use, alone and in combination, to restore host anti-tumor immunity and improve patient outcome.

H3K4me3 mediates the NF-κB p50 homodimer binding to the pdcd1 promoter to activate PD-1 transcription in T cells

PD-1 is a co-repressive receptor that curbs T cell activation and thereby serves as a protection mechanism against autoimmunity under physiological conditions. Under pathological conditions, tumor cells express PD-L1 as an adaptive resistant mechanism to suppress PD-1⁺ T cells to evade host immunosurveillance. PD-1 therefore is a key target in cancer immunotherapy. Despite the extensive studies of PD-1 expression regulation, the pdcd1 transcription machinery and regulatory mechanisms are still not fully understood. We report here that the NF-κB p50 homodimer is a transcription regulator of PD-1 in activated T cells. A putative κB sequence exists at the pdcd1 promoter. All five NF-κB Rel subunits are activated in activated T cells. However, only the p50 homodimer directly binds to the κB sequence at the pccd1 promoter in CD4⁺ and CD8⁺ T cells. Deficiency in p50 results in reduced PD-1 expression in both CD4⁺ and CD8⁺ T cells in vitro. Using an in vivo mixed bone marrow chimera mouse model, we show that p50 regulates PD-1 expression in a cell-intrinsic way and p50 deficiency leads to decreased PD-1 expression in both antigen-specific CD4⁺ and CD8⁺ T cells in vivo. The expression levels of H3K4me3-specific histone methyltransferase increased significantly, resulting in a significant increase in H3K4me3 deposition at the pdcd1 promoter in activated CD4⁺ and CD8⁺ T cells. Inhibition of H3K4me3 significantly decreased p50 binding to the pdcd1 promoter and PD-1 expression in a T cell line. Our findings determine that the p50-H3K4me3 axis regulates pdcd1 transcription activation in activated T cells.

Differential T-cell receptor signals for T helper cell programming

Upon encounter with their cognate antigen, naive CD4 T cells become activated and are induced to differentiate into several possible T helper (Th) cell subsets. This differentiation depends on a number of factors including antigen-presenting cells, cytokines and co-stimulatory molecules. The strength of the T-cell receptor (TCR) signal, related to the affinity of TCR for antigen and antigen dose, has emerged as a dominant factor in determining Th cell fate. Recent studies have revealed that TCR signals of high or low strength do not simply induce quantitatively different signals in the T cells, but rather qualitatively distinct pathways can be induced based on TCR signal strength. This review examines the recent literature in this area and highlights important new developments in our understanding of Th cell differentiation and TCR signal strength.

Role of IκB kinase β in regulating the remodeling of the CARMA1-Bcl10-MALT1 complex

The current work investigates the notion that inducible clustering of signaling mediators of the IKK pathway is important for platelet activation. Thus, while the CARMA1, Bcl10, and MALT1 (CBM) complex is essential for triggering IKK/NF-κB activation upon platelet stimulation, the signals that elicit its formation and downstream effector activation remain elusive. We demonstrate herein that IKKβ is involved in membrane fusion, and serves as a critical protein kinase required for initial formation and the regulation of the CARMA1/MALT1/Bcl10/CBM complex in platelets. We also show that IKKβ regulates these processes via modulation of phosphorylation of Bcl10 and IKKγ polyubiquitination. Collectively, our data demonstrate that IKKβ regulates membrane fusion and the remodeling of the CBM complex formation.

Lymphocyte signaling and activation by the CARMA1-BCL10-MALT1 signalosome

The CARMA1-BCL10-MALT1 (CBM) signalosome triggers canonical NF-κB signaling and lymphocyte activation upon antigen-receptor stimulation. Genetic studies in mice and the analysis of human immune pathologies unveiled a critical role of the CBM complex in adaptive immune responses. Great progress has been made in elucidating the fundamental mechanisms that dictate CBM assembly and disassembly. By bridging proximal antigen-receptor signaling to downstream signaling pathways, the CBM complex exerts a crucial scaffolding function. Moreover, the MALT1 subunit confers a unique proteolytic activity that is key for lymphocyte activation. Deregulated 'chronic' CBM signaling drives constitutive NF-κB signaling and MALT1 activation, which contribute to the development of autoimmune and inflammatory diseases as well as lymphomagenesis. Thus, the processes that govern CBM activation and function are promising targets for the treatment of immune disorders. Here, we summarize the current knowledge on the functions and mechanisms of CBM signaling in lymphocytes and how CBM deregulations contribute to aberrant signaling in malignant lymphomas.

TCR signaling to NF-κB and mTORC1: Expanding roles of the CARMA1 complex

Naïve T-cell activation requires signals from both the T-cell receptor (TCR) and the costimulatory molecule CD28. A central mediator of the TCR and CD28 signals is the scaffold protein CARMA1, which functions by forming a complex with partner proteins, Bcl10 and MALT1. A well-known function of the CARMA1 signaling complex is to mediate activation of IκB kinase (IKK) and its target transcription factor NF-κB, thereby promoting T-cell activation and survival. Recent evidence suggests that CARMA1 also mediates TCR/CD28-stimulated activation of the IKK-related kinase TBK1, which plays a role in regulating the homeostasis and migration of T cells. Moreover, the CARMA1 complex connects the TCR/CD28 signals to the activation of mTORC1, a metabolic kinase regulating various aspects of T-cell functions. This review will discuss the mechanism underlying the activation of the CARMA1-dependent signaling pathways and their roles in regulating T-cell functions.

Lenalidomide Polarizes Th1-specific Anti-tumor Immune Response and Expands XBP1 Antigen-Specific Central Memory CD3+CD8+ T cells against Various Solid Tumors

Introduction

Effective combination immunotherapeutic strategies may be required to enhance effector cells’ anti-tumor activities and improve clinical outcomes.

Methods

XBP1 antigen-specific cytotoxic T lymphocytes (XBP1-CTL) generated using immunogenic heteroclitic XBP1 US184-192 (YISPWILAV) and XBP1 SP367-375 (YLFPQLISV) peptides or various solid tumor cells over-expressing XBP1 target antigen were evaluated, either alone or in combination with lenalidomide, for phenotype and immune functional activity.

Results

Lenalidomide treatment of XBP1-CTL increased the proportion of CD45RO⁺ memory CD3+CD8+ T cells, but not the total CD3⁺CD8⁺ T cells. Lenalidomide upregulated critical T cell activation markers and costimulatory molecules (CD28, CD38, CD40L, CD69, ICOS), especially within the central memory CTL subset of XBP1-CTL, while decreasing TCRαβ and T cell checkpoint blockade (CTLA-4, PD-1). Lenalidomide increased the anti-tumor activities of XBP1-CTL memory subsets, which were associated with expression of Th1 transcriptional regulators (T-bet, Eomes) and Akt activation, thereby resulting in enhanced IFN-γ production, granzyme B upregulation and specific CD28/CD38-positive and CTLA-4/PD-1-negative cell proliferation.

Conclusions

These studies suggest the potential benefit of lenalidomide treatment to boost anti-tumor activities of XBP1-specific CTL against a variety of solid tumors and enhance response to an XBP1-directing cancer vaccine regime.