PTEN-Regulated AID Transcription in Germinal Center B Cells Is Essential for the Class-Switch Recombination and IgG Antibody Responses

SUMO-specific protease 1 regulates germinal center B cell response through deSUMOylation of PAX5

Humoral immunity depends on the germinal center (GC) reaction where B cells are tightly controlled for class-switch recombination and somatic hypermutation and finally generated into plasma and memory B cells. However, how protein SUMOylation regulates the process of the GC reaction remains largely unknown. Here, we show that the expression of SUMO-specific protease 1 (SENP1) is up-regulated in GC B cells. Selective ablation of SENP1 in GC B cells results in impaired GC dark and light zone organization and reduced IgG1-switched GC B cells, leading to diminished production of class-switched antibodies with high-affinity in response to a TD antigen challenge. Mechanistically, SENP1 directly binds to Paired box protein 5 (PAX5) to mediate PAX5 deSUMOylation, sustaining PAX5 protein stability to promote the transcription of activation-induced cytidine deaminase. In summary, our study uncovers SUMOylation as an important posttranslational mechanism regulating GC B cell response.

The miR-17∼92 miRNAs promote plasma cell differentiation by suppressing SOCS3-mediated NIK degradation

The miR-17∼92 family microRNAs (miRNAs) play a key role in germinal center (GC) reaction through promoting T follicular helper (TFH) cell differentiation. It remains unclear whether they also have intrinsic functions in B cell differentiation and function. Here we show that mice with B cell-specific deletion of the miR-17∼92 family exhibit impaired GC reaction, plasma cell differentiation, and antibody production in response to protein antigen immunization and chronic viral infection. Employing CRISPR-mediated functional screening, we identify Socs3 as a key functional target of miR-17∼92 in regulating plasma cell differentiation. Mechanistically, SOCS3, whose expression is elevated in miR-17∼92 family-deficient B cells, interacts with NIK and promotes its ubiquitination and degradation, thereby impairing NF-κB signaling and plasma cell differentiation. This moderate increase in SOCS3 expression has little effect on IL-21-STAT3 signaling. Our study demonstrates differential sensitivity of two key signaling pathways to alterations in the protein level of an miRNA target gene.

Deficiency of β-arrestin2 exacerbates inflammatory arthritis by facilitating plasma cell formation

β-arrestin2 (β-arr2) is, a key protein that mediates desensitization and internalization of G protein-coupled receptors and participates in inflammatory and immune responses. Deficiency of β-arr2 has been found to exacerbate collagen antibody-induced arthritis (CAIA) through unclear mechanisms. In this study we tried to elucidate the molecular mechanisms underlying β-arr2 depletion-induced exacerbation of CAIA. CAIA was induced in β-arr2-/- and wild-type (WT) mice by injection of collagen antibodies and LPS. The mice were sacrificed on d 13 after the injection, spleen, thymus and left ankle joints were collected for analysis. Arthritis index (AI) was evaluated every day or every 2 days. We showed that β-arr2-/- mice with CAIA had a further increase in the percentage of plasma cells in spleen as compared with WT mice with CAIA, which was in accordance with elevated serum IgG1 and IgG2A expression and aggravating clinical performances, pathologic changes in joints and spleen, joint effusion, and joint blood flow. Both LPS stimulation of isolated B lymphocytes in vitro and TNP-LPS challenge in vivo led to significantly higher plasma cell formation and antibodies production in β-arr2-/- mice as compared with WT mice. LPS treatment induced membrane distribution of toll-like receptor 4 (TLR4) on B lymphocytes, accordingly promoted the nuclear translocation of NF-κB and the transcription of Blimp1. Immunofluorescence analysis confirmed that more TLR4 colocalized with β-arr2 in B lymphocytes in response to LPS stimulation. Depletion of β-arr2 restrained TLR4 on B lymphocyte membrane after LPS treatment and further enhanced downstream NF-κB signaling leading to additional increment in plasma cell formation. In summary, β-arr2 depletion exacerbates CAIA and further increases plasma cell differentiation and antibody production through inhibiting TLR4 endocytosis and aggravating NF-κB signaling.

The Role of PTEN in Innate and Adaptive Immunity

The lipid and protein phosphatase and tensin homolog (PTEN) controls the differentiation and activation of multiple immune cells. PTEN acts downstream from T- and B-cell receptors, costimulatory molecules, cytokine receptors, integrins, and also growth factor receptors. Loss of PTEN activity in human and mice is associated with cellular and humoral immune dysfunction, lymphoid hyperplasia, and autoimmunity. Although most patients with PTEN hamartoma tumor syndrome (PHTS) have no immunological symptoms, a subclinical immune dysfunction is present in many, and clinical immunodeficiency in few. Comparison of the immune phenotype caused by PTEN haploinsufficiency in PHTS, phosphoinositide 3-kinase (PI3K) gain-of-function in activated PI3K syndrome, and mice with conditional biallelic Pten deletion suggests a threshold model in which coordinated activity of several phosphatases control the PI3K signaling in a cell-type-specific manner. Emerging evidence highlights the role of PTEN in polygenic autoimmune disorders, infection, and the immunological response to cancer. Targeting the PI3K axis is an emerging therapeutic avenue.

Integrative genomic analysis identifies associations of molecular alterations to APOBEC and BRCA1/2 mutational signatures in breast cancer

BACKGROUND

The observed mutations in cancer are the result of ~30 mutational processes, which stamp particular mutational signatures (MS). Nevertheless, it is still not clear which genomic alterations correlate to several MS. Here, a method to analyze associations of genomic data with MS is presented and applied to The Cancer Genome Atlas breast cancer data revealing promising associations.

METHODS

The MS were discretized into clusters whose extremes were statistically associated with mutations, copy number, and gene expression data.

RESULTS

Known associations for apolipoprotein B editing complex (APOBEC) and for BRCA1 and BRCA2 support the proposal. For BRCA1/2, mutations in ARAP3, three focal deletions, and one amplification were detected. Around 50 mutated genes for the two APOBEC signatures were identified including three kinesins (KIF13A, KIF1B, KIF4A), three ubiquitins (USP45, UBR4, UBR1), and two demethylases (KDM5B, KDM5C) among other genes also connected to DNA damage pathways. The results suggest novel roles for other genes currently not involved in DNA repair. The altered expression program was very high for the BRCA1/2 signature, high for APOBEC signature 13 clearly associated to immune response, and low for APOBEC signature 2. The remaining signatures show scarce associations.

CONCLUSION

Specific genetic alterations can be associated with particular MS.

The Key Roles of PTEN in T-Cell Acute Lymphoblastic Leukemia Development, Progression, and Therapeutic Response

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive blood cancer that comprises 10–15% of pediatric and ~25% of adult ALL cases. Although the curative rates have significantly improved over the past 10 years, especially in pediatric patients, T-ALL remains a challenge from a therapeutic point of view, due to the high number of early relapses that are for the most part resistant to further treatment. Considerable advances in the understanding of the genes, signaling networks, and mechanisms that play crucial roles in the pathobiology of T-ALL have led to the identification of the key drivers of the disease, thereby paving the way for new therapeutic approaches. PTEN is critical to prevent the malignant transformation of T-cells. However, its expression and functions are altered in human T-ALL. PTEN is frequently deleted or mutated, while PTEN protein is often phosphorylated and functionally inactivated by casein kinase 2. Different murine knockout models recapitulating the development of T-ALL have demonstrated that PTEN abnormalities are at the hub of an intricate oncogenic network sustaining and driving leukemia development by activating several signaling cascades associated with drug-resistance and poor outcome. These aspects and their possible therapeutic implications are highlighted in this review.

Signaling control of antibody isotype switching

Class switch recombination (CSR) generates isotype-switched antibodies with distinct effector functions essential for mediating effective humoral immunity. CSR is catalyzed by activation-induced deaminase (AID) that initiates DNA lesions in the evolutionarily conserved switch (S) regions at the immunoglobulin heavy chain (Igh) locus. AID-initiated DNA lesions are subsequently converted into DNA double stranded breaks (DSBs) in the S regions of Igh locus, repaired by non-homologous end-joining to effect CSR in mammalian B lymphocytes. While molecular mechanisms of CSR are well characterized, it remains less well understood how upstream signaling pathways regulate AID expression and CSR. B lymphocytes express multiple receptors including the B cell antigen receptor (BCR) and co-receptors (e.g., CD40). These receptors may share common signaling pathways or may use distinct signaling elements to regulate CSR. Here, we discuss how signals emanating from different receptors positively or negatively regulate AID expression and CSR.

The mTORC1/4E-BP/eIF4E Axis Promotes Antibody Class Switching in B Lymphocytes

During an adaptive immune response, activated mature B cells give rise to Ab-secreting plasma cells to fight infection. B cells undergo Ab class switching to produce different classes of Abs with varying effector functions. The mammalian/mechanistic target of rapamycin (mTOR) signaling pathway is activated during this process, and disrupting mTOR complex 1 (mTORC1) in B cells impairs class switching by a poorly understood mechanism. In particular, it is unclear which mTORC1 downstream substrates control this process. In this study, we used an in vitro murine model in which the mTORC1 inhibitor rapamycin, when added after a B cell has committed to divide, suppresses class switching while preserving proliferation. Investigation of mTORC1 substrates revealed a role for eukaryotic translation initiation factor 4E (eIF4E) and eIF4E-binding proteins in class switching. Mechanistically, we show that genetic or pharmacological disruption of eIF4E binding to eIF4G reduced cap-dependent translation, which specifically affected the expression of activation-induced cytidine deaminase protein but not Aicda mRNA. This translational impairment decreased Ab class switching independently of proliferation. These results uncover a previously undescribed role for mTORC1 and the eIF4E-binding proteins/eIF4E axis in activation-induced cytidine deaminase protein expression and Ab class switching in mouse B cells, suggesting that cap-dependent translation regulates key steps in B cell differentiation.