NF-κB subunits RelA and c-Rel selectively control CD4+ T cell function in multiple sclerosis and cancer.

The outcome of cancer and autoimmunity is often dictated by the effector functions of CD4+ conventional T cells (Tconv). Although activation of the NF-κB signaling pathway has long been implicated in Tconv biology, the cell-autonomous roles of the separate NF-κB transcription-factor subunits are unknown. Here, we dissected the contributions of the canonical NF-κB subunits RelA and c-Rel to Tconv function. RelA, rather than c-Rel, regulated Tconv activation and cytokine production at steady-state and was required for polarization toward the TH17 lineage in vitro. Accordingly, RelA-deficient mice were fully protected against neuroinflammation in a model of multiple sclerosis due to defective transition to a pathogenic TH17 gene-expression program. Conversely, Tconv-restricted ablation of c-Rel impaired their function in the microenvironment of transplanted tumors, resulting in enhanced cancer burden. Moreover, Tconv required c-Rel for the response to PD-1-blockade therapy. Our data reveal distinct roles for canonical NF-κB subunits in different disease contexts, paving the way for subunit-targeted immunotherapies.

The NF-κB RelA transcription factor is not required for CD8+ T-cell function in acute viral infection and cancer.

CD8+ T cells are critical mediators of pathogen clearance and anti-tumor immunity. Although signaling pathways leading to the activation of NF-κB transcription factors have crucial functions in the regulation of immune responses, the CD8+ T cell-autonomous roles of the different NF-κB subunits, are still unresolved. Here, we investigated the function of the ubiquitously expressed transcription factor RelA in CD8+ T-cell biology using a novel mouse model and gene-edited human cells. We found that CD8+ T cell-specific ablation of RelA markedly altered the transcriptome of ex vivo stimulated cells, but maintained the proliferative capacity of both mouse and human cells. In contrast, in vivo experiments showed that RelA deficiency did not affect the CD8+ T-cell response to acute viral infection or transplanted tumors. Our data suggest that in CD8+ T cells, RelA is dispensable for their protective activity in pathological contexts.

An MYC-Driven Vicious Circuit Is a Targetable Achilles' Heel in Lymphoma.

SUMMARY: In this issue of Blood Cancer Discovery, Nakanishi et al. uncover a critical role for the elevated activity of the translation initiation factor eIF5A in the malignant growth of MYC-driven lymphoma. eIF5A is posttranslationally modified by hypusination through MYC oncoprotein-mediated hyperactivation of the polyamine-hypusine circuit, which may represent a promising therapeutic target because an enzyme of this circuit that is required for hypusinating eIF5A proved to be essential for lymphoma development. See related article by Nakanishi et al., p. 294 (4).

Cytokine receptor IL27RA is an NF-κB-responsive gene involved in CD38 upregulation in multiple myeloma.

Multiple myeloma (MM) shows constitutive activation of canonical and noncanonical nuclear factor κB (NF-κB) signaling via genetic mutations or tumor microenvironment (TME) stimulations. A subset of MM cell lines showed dependency for cell growth and survival on the canonical NF-κB transcription factor RELA alone, suggesting a critical role for a RELA-mediated biological program in MM pathogenesis. Here, we determined the RELA-dependent transcriptional program in MM cell lines and found the expression of the cell surface molecules interleukin-27 receptor-α (IL-27Rα) and the adhesion molecule JAM2 to be responsive to RELA at the messenger RNA and protein levels. IL-27Rα and JAM2 were expressed on primary MM cells at higher levels than on healthy long-lived plasma cells (PCs) in the bone marrow. IL-27 activated STAT1, and to a lesser extent STAT3, in MM cell lines and in PCs generated from memory B cells in an IL-21-dependent in vitro PC differentiation assay. Concomitant activity of IL-21 and IL-27 enhanced differentiation into PCs and increased the cell-surface expression of the known STAT target gene CD38. In accordance, a subset of MM cell lines and primary MM cells cultured with IL-27 upregulated CD38 cell-surface expression, a finding with potential implications for enhancing the efficacy of CD38-directed monoclonal antibody therapies by increasing CD38 expression on tumor cells. The elevated expression of IL-27Rα and JAM2 on MM cells compared with that on healthy PCs may be exploited for the development of targeted therapeutic strategies that modulate the interaction of MM cells with the TME.

NF-κB Mutations in Germinal Center B-Cell Lymphomas: Relation to NF-κB Function in Normal B Cells.

Most B cell lymphomas arise from the oncogenic transformation of B cells that have undergone the germinal center (GC) reaction of the T cell-dependent immune response, where high-affinity memory B cells and plasma cells are generated. The high proliferation of GC B cells coupled with occasional errors in the DNA-modifying processes of somatic hypermutation and class switch recombination put the cell at a risk to obtain transforming genetic aberrations, which may activate proto-oncogenes or inactivate tumour suppressor genes. Several subtypes of GC lymphomas harbor genetic mutations leading to constitutive, aberrant activation of the nuclear factor-κB (NF-κB) signaling pathway. In normal B cells, NF-κB has crucial biological roles in development and physiology. GC lymphomas highjack these activities to promote tumour-cell growth and survival. It has become increasingly clear that the separate canonical and non-canonical routes of the NF-κB pathway and the five downstream NF-κB transcription factors have distinct functions in the successive stages of GC B-cell development. These findings may have direct implications for understanding how aberrant NF-κB activation promotes the genesis of various GC lymphomas corresponding to the developmentally distinct GC B-cell subsets. The knowledge arising from these studies may be explored for the development of precision medicine approaches aimed at more effective treatments of the corresponding tumours with specific NF-κB inhibitors, thus reducing systemic toxicity. We here provide an overview on the patterns of genetic NF-κB mutations encountered in the various GC lymphomas and discuss the consequences of aberrant NF-κB activation in those malignancies as related to the biology of NF-κB in their putative normal cellular counterparts.

TIFAB accelerates MLL-AF9-Induced acute myeloid leukemia through upregulation of HOXA9.

We previously showed stabilization of NIK-induced activation of NF-κB non-canonical signaling suppresses MLL-AF9-induced AML. In the current study, we demonstrate that deletion of NF-κB non-canonical RelB prevents the inhibitory effect of NIK stabilization in MLL-AF9 AML. Mechanistically, RelB suppresses its direct target, TIFAB, which is upregulated in human AML and correlates negatively with the survival of AML patients. Forced expression of TIFAB reverses NIK-induced impaired AML development through downregulation of RelB and upregulation of HOXA9. Consistent with upregulation of HOXA9, gene set enrichment analysis shows that forced expression of TIFAB blocks myeloid cell development, upregulates leukemia stem cell signature and induces similar gene expression patterns to those of HOXA9-MEIS1 and HOXA9-NUP98, and upregulates oxidative phosphorylation. Accordingly, forced expression of HOXA9 also largely releases the inhibitory impact of NIK stabilization via downregulation of RelB and upregulation of RelA. Our data suggest that NIK/RelB suppresses MLL-AF9-induced AML mainly through downregulation of TIFAB/HOXA9.

A T cell-intrinsic function for NF-κB RelB in experimental autoimmune encephalomyelitis.

NF-kappaB (NF-κB) is a family of transcription factors with pleiotropic functions in immune responses. The alternative NF-κB pathway that leads to the activation of RelB and NF-κB2, was previously associated with the activation and function of T cells, though the exact contribution of these NF-κB subunits remains unclear. Here, using mice carrying conditional ablation of RelB in T cells, we evaluated its role in the development of conventional CD4+ T (Tconv) cells and their function in autoimmune diseases. RelB was largely dispensable for Tconv cell homeostasis, activation and proliferation, and for their polarization toward different flavors of Thelper cells in vitro. Moreover, ablation of RelB had no impact on the capacity of Tconv cells to induce autoimmune colitis. Conversely, clinical severity of experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis (MS) was significantly reduced in mice with RelB-deficient T cells. This was associated with impaired expression of granulocyte-macrophage colony-stimulating factor (GM-CSF) specifically in the central nervous system. Our data reveal a discrete role for RelB in the pathogenic function of Tconv cells during EAE, and highlight this transcription factor as a putative therapeutic target in MS.

Conditional Knockout Mouse Models to Study the Roles of Individual NF-κB Transcription Factors in Lymphocytes.

The NF-κB signal transduction pathway has crucial functions in cell growth, survival, and the development of lymphocytes and other immune cells. Upon activation of the pathway, five distinct NF-κB transcription factor subunits that occur as homodimers or heterodimers comprise the downstream mediators that transcribe NF-κB target genes. A major quest in NF-κB research is to understand the biology of the separate subunits. However, determining the functions of the individual subunits using constitutional knockout mice is often hampered by the marked cell type and/or developmental stage-specific activation of the NF-κB pathway. To overcome these problems, we and others have generated loxP-flanked alleles of the genes encoding the different NF-κB subunits that upon crossing to suitable Cre-expressing mouse lines can be conditionally deleted in the desired cell type or developmental stage. We here describe the basic characteristics of conditional NF-κB subunit alleles rel (encoding c-REL), rela (RELA), relb (RELB), and nfkb2 (NF-κB2) generated in our laboratory that are available to the research community through a repository, and provide basic methods to study the consequences of tissue-specific ablation of NF-κB transcription factors in lymphocytes.

Monoallelic IRF5 deficiency in B cells prevents murine lupus.

Gain-of-function polymorphisms in the transcription factor IFN regulatory factor 5 (IRF5) are associated with an increased risk of developing systemic lupus erythematosus. However, the IRF5-expressing cell type(s) responsible for lupus pathogenesis in vivo is not known. We now show that monoallelic IRF5 deficiency in B cells markedly reduced disease in a murine lupus model. In contrast, similar reduction of IRF5 expression in macrophages, monocytes, and neutrophils did not reduce disease severity. B cell receptor and TLR7 signaling synergized to promote IRF5 phosphorylation and increase IRF5 protein expression, with these processes being independently regulated. This synergy increased B cell-intrinsic IL-6 and TNF-α production, both key requirements for germinal center (GC) responses, with IL-6 and TNF-α production in vitro and in vivo being substantially lower with loss of 1 allele of IRF5. Mechanistically, TLR7-dependent IRF5 nuclear translocation was reduced in B cells from IRF5-heterozygous mice. In addition, we show in multiple lupus models that IRF5 expression was dynamically regulated in vivo with increased expression in GC B cells compared with non-GC B cells and with further sequential increases during progression to plasmablasts and long-lived plasma cells. Overall, a critical threshold level of IRF5 in B cells was required to promote disease in murine lupus.

c-Rel employs multiple mechanisms to promote the thymic development and peripheral function of regulatory T cells in mice.

The NF-κB transcription factor c-Rel is a critical regulator of Treg ontogeny, controlling multiple points of the stepwise developmental pathway. Here, we found that the thymic Treg defect in c-Rel-deficient (cRel-/- ) mice is quantitative, not qualitative, based on analyses of TCR repertoire and TCR signaling strength. However, these parameters are altered in the thymic Treg-precursor population, which is also markedly diminished in cRel-/- mice. Moreover, c-Rel governs the transcriptional programme of both thymic and peripheral Tregs, controlling a core of genes involved with immune signaling, and separately in the periphery, cell cycle progression. Last, the immune suppressive function of peripheral cRel-/- tTregs is diminished in a lymphopenic model of T cell proliferation and is associated with decreased stability of Foxp3 expression. Collectively, we show that c-Rel is a transcriptional regulator that controls multiple aspects of Treg development, differentiation, and function via distinct mechanisms.

NF-κB inhibition in keratinocytes causes RIPK1-mediated necroptosis and skin inflammation.

Tumor necrosis factor receptor 1 (TNFR1) activates NF-κB-dependent pro-inflammatory gene expression, but also induces cell death by triggering apoptosis and necroptosis. Inhibition of inhibitor of NF-κB kinase (IKK)/NF-κB signaling in keratinocytes paradoxically unleashed spontaneous TNFR1-mediated skin inflammation in mice, but the underlying mechanisms remain poorly understood. Here, we show that TNFR1 causes skin inflammation in mice with epidermis-specific knockout of IKK2 by inducing receptor interacting protein kinase 1 (RIPK1)-dependent necroptosis, and to a lesser extent also apoptosis, of keratinocytes. Combined epidermis-specific ablation of the NF-κB subunits RelA and c-Rel also caused skin inflammation by inducing TNFR1-mediated keratinocyte necroptosis. Contrary to the currently established model that inhibition of NF-κB-dependent gene transcription causes RIPK1-independent cell death, keratinocyte necroptosis, and skin inflammation in mice with epidermis-specific RelA and c-Rel deficiency also depended on RIPK1 kinase activity. These results advance our understanding of the mechanisms regulating TNFR1-induced cell death and identify RIPK1-mediated necroptosis as a potent driver of skin inflammation.

Mouse Models in the Study of Mature B-Cell Malignancies.

Over the past two decades, genomic analyses of several B-cell lymphoma entities have identified a large number of genes that are recurrently mutated, suggesting that their aberrant function promotes lymphomagenesis. For many of those genes, the specific role in normal B-cell development is unknown; moreover, whether and how their deregulated activity contributes to lymphoma initiation and/or maintenance is often difficult to determine. Genetically engineered mouse models that faithfully mimic lymphoma-associated genetic alterations represent valuable tools for elucidating the pathogenic roles of candidate oncogenes and tumor suppressors in vivo, as well as for the preclinical testing of novel therapeutic principles in an intact microenvironment. Here we summarize what has been learned about the mechanisms of oncogenic transformation from accurately modeling the most common and well-characterized genetic alterations identified in mature B-cell malignancies. This information is expected to guide the design of improved molecular diagnostics and mechanism-based therapeutic approaches for these diseases.

c-Rel orchestrates energy-dependent epithelial and macrophage reprogramming in fibrosis.

Fibrosis is a common pathological feature of chronic disease. Deletion of the NF-κB subunit c-Rel limits fibrosis in multiple organs, although the mechanistic nature of this protection is unresolved. Using cell-specific gene-targeting manipulations in mice undergoing liver damage, we elucidate a critical role for c-Rel in controlling metabolic changes required for inflammatory and fibrogenic activities of hepatocytes and macrophages and identify Pfkfb3 as the key downstream metabolic mediator of this response. Independent deletions of Rel in hepatocytes or macrophages suppressed liver fibrosis induced by carbon tetrachloride, while combined deletion had an additive anti-fibrogenic effect. In transforming growth factor-ß1-induced hepatocytes, c-Rel regulates expression of a pro-fibrogenic secretome comprising inflammatory molecules and connective tissue growth factor, the latter promoting collagen secretion from HMs. Macrophages lacking c-Rel fail to polarize to M1 or M2 states, explaining reduced fibrosis in RelΔLysM mice. Pharmacological inhibition of c-Rel attenuated multi-organ fibrosis in both murine and human fibrosis. In conclusion, activation of c-Rel/Pfkfb3 in damaged tissue instigates a paracrine signalling network among epithelial, myeloid and mesenchymal cells to stimulate fibrogenesis. Targeting the c-Rel-Pfkfb3 axis has potential for therapeutic applications in fibrotic disease.

"B" aware: Memory lane access is restricted!

Understanding the molecular mechanisms that govern the differentiation of high-affinity germinal center (GC) B cells into memory B cells versus plasma cells is a major quest of adaptive immunity. In this issue, Toboso-Navasa et al. (https://doi.org/10.1084/jem.20191933) provide evidence that the MYC-MIZ1 transcriptional repressor complex restricts the differentiation of GC B cells into MBCs.

Computational Model Reveals a Stochastic Mechanism behind Germinal Center Clonal Bursts.

Germinal centers (GCs) are specialized compartments within the secondary lymphoid organs where B cells proliferate, differentiate, and mutate their antibody genes in response to the presence of foreign antigens. Through the GC lifespan, interclonal competition between B cells leads to increased affinity of the B cell receptors for antigens accompanied by a loss of clonal diversity, although the mechanisms underlying clonal dynamics are not completely understood. We present here a multi-scale quantitative model of the GC reaction that integrates an intracellular component, accounting for the genetic events that shape B cell differentiation, and an extracellular stochastic component, which accounts for the random cellular interactions within the GC. In addition, B cell receptors are represented as sequences of nucleotides that mature and diversify through somatic hypermutations. We exploit extensive experimental characterizations of the GC dynamics to parameterize our model, and visualize affinity maturation by means of evolutionary phylogenetic trees. Our explicit modeling of B cell maturation enables us to characterise the evolutionary processes and competition at the heart of the GC dynamics, and explains the emergence of clonal dominance as a result of initially small stochastic advantages in the affinity to antigen. Interestingly, a subset of the GC undergoes massive expansion of higher-affinity B cell variants (clonal bursts), leading to a loss of clonal diversity at a significantly faster rate than in GCs that do not exhibit clonal dominance. Our work contributes towards an in silico vaccine design, and has implications for the better understanding of the mechanisms underlying autoimmune disease and GC-derived lymphomas.