Class-switch recombination (CSR) is essential for humoral immunity. However, the regulation of CSR is not completely understood. Here we demonstrate that phosphatidylinositol 3-kinase (PI3K) actively suppressed the onset and frequency of CSR in primary B cells. Consistently, mice lacking the lipid phosphatase, PTEN, in B cells exhibited a hyper-IgM condition due to impaired CSR, which could be restored in vitro by specific inhibition of PI3Kdelta. Inhibition of CSR by PI3K was partially dependent on the transcription factor, BLIMP1, linking plasma cell commitment and cessation of CSR. PI3K-dependent activation of the serine-threonine kinase, Akt, suppressed CSR, in part, through the inactivation of the Forkhead Box family (Foxo) of transcription factors. Reduced PI3K signaling enhanced the expression of AID (activation-induced cytidine deaminase) and accelerated CSR. However, ectopic expression of AID could not fully overcome inhibition of CSR by PI3K, suggesting that PI3K regulates both the expression and function of AID.

Expression of B cell-activating factor (BAFF), a critical B cell survival factor, is elevated in autoimmune and lymphoproliferative disorders. Mice overproducing BAFF develop systemic lupus erythematosus (SLE)-like disease and exhibit B cell activation of classical and alternative NF-kappaB-signaling pathways. We used a genetic approach and found that both NF-kappaB-signaling pathways contributed to disease development but act through distinct mechanisms. Whereas BAFF enhanced long-term B cell survival primarily through the alternative, but not the classical, NF-kappaB pathway, it promoted immunoglobulin class switching and generation of pathogenic antibodies through the classical pathway. Activation of the alternative NF-kappaB pathway resulted in integrin upregulation, thereby retaining autoreactive B cells in the splenic marginal zone, a compartment that contributes to their survival. Thus, both classical and alternative NF-kappaB signaling are important for development of lupus-like disease associated with BAFF overproduction. The same mechanisms may be involved in the pathogenesis of human SLE.

Mice lacking activity of the kinase MEKK1 ('Map3k1(deltaKD)' mice) have defective activation of the kinase Jnk and increased production of T helper type 2 cytokines after T cell receptor ligation. Here we show that Map3k1(deltaKD) mice had defective germinal center formation and diminished production of antibodies recognizing thymus-dependent antigens. Those defects were B cell intrinsic, as MEKK1 was necessary for CD40-mediated activation of the kinases Jnk and p38 and transcription factor c-Jun, as well as for expression of cyclin D2 and activation-induced deaminase. MEKK1 was recruited to CD40 and adaptor molecule TRAF2 after CD40 ligation, and Map3k1(deltaKD) B cells were hypoproliferative after CD40 stimulation. Our data emphasize that MEKK1 is an essential component of signaling cascades needed for thymus-dependent antigen-induced B cell proliferation and antibody production.

Mice lacking both PTEN and SHIP phosphatases develop spontaneous B cell lymphoma.

Phosphatidylinositol-3-kinase (PI3K) is activated downstream of numerous receptors and catalyzes the conversion of membrane phosphatidylinositol-(4,5)-bisphosphate (PI 4,5 P 2) to phosphatidylinositol-(3,4,5)-trisphosphate (PIP 3). PIP 3 acts as a second messenger, recruiting to the plasma membrane pleckstrin homology domain-containing adaptors and kinases such as PDK1, Akt, PLC-, Tec family kinases, and DOK, which then further modulate downstream signaling (Cully et al., 2006). Subsequent activation or inactivation of cytosolic and nuclear targets, including SGK, mTOR, PP2A, FOXO, and cyclins D and E, mediates diverse cellular responses such as survival, proliferation, migration, adhesion, and differentiation (Cully et al., 2006). In B cells, attenuated PI3K signaling impairs B cell survival and selection, leading to diminished numbers of peritoneal B-1 cells, splenic marginal zone (MZ) B cells, and germinal center (GC) B cells, as well as a general reduction in mature recirculating B cells (Donahue and Fruman, 2004). The action of PI3K is antagonized by two lipid phosphatases: the 3-inositol phosphatase, phosphatase and tensin homologue (PTEN), and the 5-inositol phosphatase Src homology 2 (SH2) domain-containing inositol phosphatase (SHIP).