The development of mature B lymphocytes requires the combined function of CD19 and the p110δ subunit of PI3K

Active PI3K abrogates central tolerance in high-avidity autoreactive B cells

High-avidity autoreactive B cells are typically removed by central tolerance mechanisms in the bone marrow. Greaves et al. demonstrate that B cell–intrinsic expression of active PI3Kα prevents central tolerance and effectively promotes differentiation and activation of high-avidity autoreactive B cells in the periphery.

Autoreactive B cells that bind self-antigen with high avidity in the bone marrow undergo mechanisms of central tolerance that prevent their entry into the peripheral B cell population. These mechanisms are breached in many autoimmune patients, increasing their risk of B cell–mediated autoimmune diseases. Resolving the molecular pathways that can break central B cell tolerance could therefore provide avenues to diminish autoimmunity. Here, we show that B cell–intrinsic expression of a constitutively active form of PI3K-P110α by high-avidity autoreactive B cells of mice completely abrogates central B cell tolerance and further promotes these cells to escape from the bone marrow, differentiate in peripheral tissue, and undergo activation in response to self-antigen. Upon stimulation with T cell help factors, these B cells secrete antibodies in vitro but remain unable to secrete autoantibodies in vivo. Overall, our data demonstrate that activation of the PI3K pathway leads high-avidity autoreactive B cells to breach central, but not late, stages of peripheral tolerance.

Sprague Dawley Rag2-Null Rats Created from Engineered Spermatogonial Stem Cells Are Immunodeficient and Permissive to Human Xenografts

The rat is the preferred model for toxicology studies, and it offers distinctive advantages over the mouse as a preclinical research model including larger sample size collection, lower rates of drug clearance, and relative ease of surgical manipulation. An immunodeficient rat would allow for larger tumor size development, prolonged dosing and drug efficacy studies, and preliminary toxicologic testing and pharmacokinetic/pharmacodynamic studies in the same model animal. Here, we created an immunodeficient rat with a functional deletion of the Recombination Activating Gene 2 (Rag2) gene, using genetically modified spermatogonial stem cells (SSC). We targeted the Rag2 gene in rat SSCs with TALENs and transplanted these Rag2-deficient SSCs into sterile recipients. Offspring were genotyped, and a founder with a 27 bp deletion mutation was identified and bred to homozygosity to produce the Sprague-Dawley Rag2 - Rag2 ^tm1Hera (SDR) knockout rat. We demonstrated that SDR rat lacks mature B and T cells. Furthermore, the SDR rat model was permissive to growth of human glioblastoma cell line subcutaneously resulting in successful growth of tumors. In addition, a human KRAS-mutant non-small cell lung cancer cell line (H358), a patient-derived high-grade serous ovarian cancer cell line (OV81), and a patient-derived recurrent endometrial cancer cell line (OV185) were transplanted subcutaneously to test the ability of the SDR rat to accommodate human xenografts from multiple tissue types. All human cancer cell lines showed efficient tumor uptake and growth kinetics indicating that the SDR rat is a viable host for a range of xenograft studies. Mol Cancer Ther; 17(11); 2481-9. ©2018 AACR.

Deficiencies in the CD19 complex

Signaling via the CD19-complex, consisting of CD19, CD81, CD21 and CD225, is critically important for B-cell development, differentiation and maturation. In this complex, each protein has its own distinct function. Over the past decade, 15 patients with antibody deficiency due to deficiencies in the CD19-complex have been described. These patients have deficiencies in different complex-members, all caused by either homozygous or compound heterozygous mutations. Although all patients had antibody deficiencies, the clinical phenotype was different per deficient protein. We aimed to provide an overview of what is known about the function of the different complex-members, knowledge from mouse-studies and to summarize the clinical phenotypes of the patients. Combining this knowledge together can explain why deficiencies in different members of the same complex, result in disease phenotypes that are alike, but not the same.

Genetic defects in PI3Kδ affect B-cell differentiation and maturation leading to hypogammaglobulineamia and recurrent infections

BACKGROUND

Mutations in PIK3CD and PIK3R1 cause activated PI3K-δ syndrome (APDS) by dysregulation of the PI3K-AKT pathway.

METHODS

We studied precursor and peripheral B-cell differentiation and apoptosis via flowcytometry. Furthermore, we performed AKT-phosphorylation assays and somatic hypermutations (SHM) and class switch recombination (CSR) analysis.

RESULTS

We identified 13 patients of whom 3 had new mutations in PIK3CD or PIK3R1. Patients had low total B-cell numbers with increased frequencies of transitional B cells and plasmablasts, while the precursor B-cell compartment in bone marrow was relatively normal. Basal AKT phosphorylation was increased in lymphocytes from APDS patients and natural effector B cells where most affected. PI3K mutations resulted in altered SHM and CSR and increased apoptosis.

CONCLUSIONS

The B-cell compartment in APDS patients is affected by the mutations in PI3K. There is reduced differentiation beyond the transitional stage, increased AKT phosphorylation and increased apoptosis. This B-cell phenotype contributes to the clinical phenotype.

Idelalisib Impacts Cell Growth through Inhibiting Translation-Regulatory Mechanisms in Mantle Cell Lymphoma

PURPOSE

PI3K is a critical node in the B-cell receptor pathway, which is responsible for survival and proliferation of B-cell malignancies. Idelalisib, a PI3Kδ-isoform-specific inhibitor, has been approved to treat B-cell malignancies. Although biological activity of the drug has been evaluated, molecular mechanisms and signaling pathway disruption leading to the biological effects of idelalisib are not yet well defined. Prior laboratory reports have identified transcription and translation as the primary events for attenuation of PI3Kα isoform. We hypothesized that PI3Kδ-isoform inhibition by idelalisib should also affect gene transcription and protein translation.

EXPERIMENTAL DESIGN

Using three mantle cell lymphoma cell lines and primary cells from patients, biological consequences such as apoptosis/cell-cycle analysis, as well as RNA/protein synthesis were evaluated. Proteomics analyses (RPPA and immunoblot assays) defined molecular events downstream of PI3K/AKT cassette.

RESULTS

Idelalisib treatment resulted in inhibition of protein synthesis, which correlated with reduction in cell size and cell growth. A moderate loss of viability without any change in cell-cycle profile was observed. Idelalisib treatment inhibited AKT activation, an immediate downstream PI3K effector, and also reduced phosphorylation levels of downstream AKT/mTOR pathway proteins such as PRAS40. In addition, idelalisib treatment impeded activation of the MAPK pathway, and MEK, ERK and p90RSK phosphorylation levels were reduced. Reduction in AKT, PDK1, and MEK phosphorylation correlated with protein synthesis inhibition.

CONCLUSIONS

Collectively, these results clarify the molecular mechanisms of actions and may provide biomarkers and targets for combination with idelalisib in B-cell malignancies. Clin Cancer Res; 23(1); 181-92. ©2016 AACR.

Immune consequences of kinase inhibitors in development, undergoing clinical trials and in current use in melanoma treatment

Metastatic malignant melanoma is a frequently fatal cancer. In recent years substantial therapeutic progress has occurred with the development of targeted kinase inhibitors and immunotherapeutics. Targeted therapies often result in rapid clinical benefit however responses are seldom durable. Immune therapies can result in durable disease control but responses may not be immediate. Optimal cancer therapy requires both rapid and durable cancer control and this can likely best be achieved by combining targeted therapies with immunotherapeutics. To achieve this, a detailed understanding of the immune consequences of the various kinase inhibitors, in development, clinical trial and currently used to treat melanoma is required.

The phosphoinositide 3-kinase signaling pathway in normal and malignant B cells: activation mechanisms, regulation and impact on cellular functions

The phosphoinositide 3-kinase (PI3K) pathway is a central signal transduction axis controlling normal B cell homeostasis and activation in humoral immunity. The p110δ PI3K catalytic subunit has emerged as a critical mediator of multiple B cell functions. The activity of this pathway is regulated at multiple levels, with inositol phosphatases PTEN and SHIP both playing critical roles. When deregulated, the PI3K pathway can contribute to B cell malignancies and autoantibody production. This review summarizes current knowledge on key mechanisms that activate and regulate the PI3K pathway and influence normal B cell functional responses including the development of B cell subsets, antigen presentation, immunoglobulin isotype switch, germinal center responses, and maintenance of B cell anergy. We also discuss PI3K pathway alterations reported in select B cell malignancies and highlight studies indicating the functional significance of this pathway in malignant B cell survival and growth within tissue microenvironments. Finally, we comment on early clinical trial results, which support PI3K inhibition as a promising treatment of chronic lymphocytic leukemia.