p110δ Phosphoinositide 3-Kinase Represses IgE Switch by Potentiating BCL6 Expression

Abstract

PI3Ks are key signaling enzymes required for triggering many immunological functions. In B lymphocytes, PI3K signaling is required for Ag-induced proliferation and robust production of most Ab isotypes. Paradoxically, PI3K was found to have a negatively regulatory function regarding Ab class switch recombination, and blockade of PI3K can strongly potentiate IgE switch. In this article, we explore the mechanisms of this unexpected negative regulatory function of PI3K regarding IgE. We demonstrate that p110δ PI3K selectively regulates IgE switch in a B cell-intrinsic manner by controlling germline transcription of the IgE promoter (εGLT). Although p110δ can regulate transcription of activation-induced cytidine deaminase via Akt, repression of εGLT and IgE switch is not dependent on Akt signaling. Inhibition of p110δ, but not Akt, leads to reduced expression of transcriptional repressor B cell lymphoma 6 (BCL6) and concomitant upregulation of εGLT and other BCL6-target genes. p110δ inhibitor treatment strikingly alters the balance between BCL6 and IRF4 (a transcription factor that antagonizes BCL6), leading to increased IRF4 and decreased BCL6 expression levels in germinal center B cells. Ectopic expression of BCL6 can partially overcome the elevated εGLTs and potentiated IgE switching in p110δ-inhibited B cells. To our knowledge, these results provide the first evidence that p110δ PI3K signaling regulates BCL6 expression and indicate that PI3K promotes the germinal center B cell program and selectively represses IgE switch by maintaining sufficient levels of BCL6.

mTOR kinase inhibitors promote antibody class switching via mTORC2 inhibition

“…2G), the mutator protein required for initiation of CSR (13). As reported previously (14,15), inhibiting PI3K strongly induced switching to IgE with lesser effects of AKT inhibition (Fig. S3).…”

Novel Phosphoinositide 3-Kinaseδ,γInhibitor: Potent Anti-Inflammatory Effects and Joint Protection in Models of Rheumatoid Arthritis

“…Genetic or pharmaceutical blockade of PI3K p110d augments class switch recombination from IgG1 to IgE despite the decrease in autoreactive IgG2a antibodies (Zhang et al, 2008(Zhang et al, , 2012. Oral treatment with another PI3Kd inhibitor, IC87114 (Sadhu et al, 2003), demonstrated a similar effect, implicating the d isoform activity rather than g in our studies (Zhang et al, 2008).…”

Imbalanced PTEN and PI3K Signaling Impairs Class Switch Recombination

“…Since surface staining cannot reliably evaluate the level of IgE CSR induced by anti-CD40/IL-4, we adopted a method by pre-treating cells with acidic buffer then employing an intracellular staining approach to detect IgE CSR (36-38). After 3 days of stimulation, about 11% of wt B6 B cells displayed positive staining for intracellular IgE whereas isotype control-Ig staining was negligible (Figure 2C).…”

“…Since PI3K pathway may play a critical role in regulating IgE CSR (16, 17, 19, 38, 48), we employed the intracellular IgE staining to investigate whether constitutive activation of p110α affected IgE CSR. Our data demonstrated that hyperactivation of p110α significantly inhibited the level of IgE CSR evidenced by the reduction of IgE + and IgG1 + IgE + switched B cells (Figure 4D).…”

“…Activated B cells were examined with flow cytometry to detect the percentage of IgG1 + or IgE + class-switched B cells. Intracellular IgE was detected using methods based on previous studies (36-38). Briefly, single cell suspension was treated for 1 min with acid buffer (0.085M NaCl, 0.005M KCl, 0.01M EDTA, and 0.05M NaAcetate [pH 4]) to remove cytophilic IgE (defined as extrinsic, CD23-bound) (39, 40), then the samples were neutralized with 3 ml of cell culture medium and washed with 2% FBS 1×PBS buffer sequentially.…”