Phosphoinositide 3-kinases (PI3K) regulate immune activation via their roles in signal transduction of multiple classes of receptors. Here, we examined the effect of genetic inactivation of the hemopoietic cell-restricted PI3K isoform p110d on systemic cytokine and chemokine responses and allergic airway inflammation. We found that type 2 cytokine responses (IL-4, IL-5 and IL-13) are significantly decreased in p110d mutants, whereas type 1 cytokine responses (IFN-c and CXCL10) were robust. Elevated IFN-c production during the primary response to ovalbumin (OVA) was associated with reduced production of the regulatory cytokine IL-10. IFN-c and IL-10 production normalized after secondary OVA immunization; however, type 2 cytokine production was persistently reduced. Type 2 cytokine-dependent airway inflammation elicited by intranasal challenge with OVA was dramatically reduced, with reduced levels of eosinophil recruitment and mucus production observed in the lungs. Induction of respiratory hyper-responsiveness to inhaled methacholine, a hallmark of asthma, was markedly attenuated in p110d-inactivated mice. Adoptive transfer of OVA-primed splenocytes from normal but not p110d-inactivated mice could induce airway eosinophilia in naive, airway-challenged recipient mice. These data demonstrate a novel functional role for p110d signaling in induction of type 2 responses in vivo and may offer a new therapeutic target for Th2-mediated airway disease. IntroductionPhosphoinositide 3-kinases (PI3K) are key signaling enzymes regulating immune function [1,2]. PI3K are activated by stimulation through a variety of receptor types, including antigen receptors, costimulatory receptors and certain cytokine receptors. PI3K function by phosphorylating specific plasma membrane phospholipids to generate short-lived, membrane-integral lipid [3,4]. Effects of regulatory subunit deficiencies on T cells are less clear: deficiency of the p85a regulatory subunit for class 1A PI3K was reported to have no effect on T cell activation in vitro, while deficiency in the p85b regulatory subunit increased T cell proliferation in vitro [5]. One group found that p85-deficient mice have enhanced responses to Leishmania infection [6], but reduced immunity to nematode infection [7], suggesting that impaired class IA PI3K signaling leads to an immune dysregulation rather than a general immunodeficiency. However, the interpretation of these studies is complicated because regulatory subunits each affect the stability and activity of multiple catalytic subunits and may have adaptor functions in signaling independent of PI3K catalytic subunits.Recent work has begun to explore the specific roles of different PI3K catalytic subunit isoforms in immune function. Analysis of p110a and p110b deficiency has been hindered by the lethality of these mutations [8,9]. Mice deficient in the class IB subunit p110c are viable and have defects in T cell development and activation, as well as severe defects in neutrophil function, while B cell activation appears normal [1...

Resistance to Leishmania major and most intracellular pathogens is usually associated with a strong T cell-mediated immunity, particularly a CD4(+) Th1 response. Mice with an inactivating knock-in mutation in the p110delta isoform of PI3K (referred to as p110delta(D910A)) show severely impaired T cell responses. Because a strong T cell response is thought to mediate resistance to intracellular pathogens, we examined the outcome of L. major infection in p110delta(D910A) mice. Paradoxically, p110delta(D910A) mice on "resistant" and "susceptible" genetic backgrounds showed more robust resistance manifested as significantly reduced lesion size and accelerated parasite clearance. This enhanced resistance was associated with dramatically diminished immune responses, including impaired cell proliferation and effector cytokine (IFN-gamma and TNF) production. Interestingly, the ability of macrophages and dendritic cells from p110delta(D910A) mice to produce NO and destroy Leishmania parasites was similar to those of wild-type mice. We show that the enhanced resistance of p110delta(D910A) mice was due to impaired expansion and effector functions of regulatory T cells (Tregs). Adoptive transfer studies demonstrated that p110delta(D910A) mice lost their increased resistance when given enriched Tregs from wild-type mice. We suggest on the basis of these and further observations that the lack of this enzyme prominently affects Treg expansion and homing to infection sites, and that in the absence of Tregs, weak Th1 responses are capable of containing parasites and prevent pathology. We also suggest that temporary pharmacological inhibition of this enzyme may be a very effective form of treatment against cutaneous leishmaniasis.

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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.

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.