Esra Ozcan

Primary immune deficiencies with aberrant IgE production.

IgE antibodies play a central role in the pathogenesis of atopic diseases and in host immunity against parasitic infections. IgE has potent activities on mast cells and basophils. IgE class switching is a very tightly controlled process, and serum IgE levels are very low compared with other immunoglobulin isotypes. Transcription factors that activate or inhibit the IgE gene promoter, as well as T(H)1 and T(H)2 cytokines are important in the regulation of IgE levels. Hyper-IgE syndrome; Wiskott-Aldrich syndrome; immunodysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX); Omenn syndrome; and atypical complete DiGeorge syndrome are primary immune deficiencies that are associated with elevated serum IgE levels. Increased IgE levels in IPEX, Wiskott-Aldrich syndrome and Omenn syndrome are likely related to increased T(H)2 cytokine production caused by decreased a number or function of CD4(+)CD25(+)forkhead box protein P3(+) regulatory T cells. The link between signal transducer and activator of transcription 3 mutations and elevated serum IgE levels in hyper-IgE syndrome is unclear. Insight into IgE regulation provided by the study of primary immune deficiencies with elevated IgE has important implications for allergic diseases.

Phosphatase PP2A is requisite for the function of regulatory T cells

Homeostasis of the immune system depends on the proper function of regulatory T cells (Treg cells). Compromised suppressive activity of Treg cells leads to autoimmune disease and graft rejection and promotes anti-tumor immunity. Here we report a previously unrecognized requirement for the serine-threonine phosphatase PP2A in the function of Treg cells. Treg cells exhibited high PP2A activity, and Treg cell–specific ablation of the PP2A complex resulted in a severe, multi-organ, lymphoproliferative autoimmune disorder. Mass spectrometry revealed that PP2A associated with components of the mTOR metabolic-checkpoint kinase pathway and suppressed the activity of the mTORC1 complex. In the absence of PP2A, Treg cells altered their metabolic and cytokine profile and were unable to suppress effector immune responses. Therefore, PP2A is required for the function of Treg cells and the prevention of autoimmunity.

Transmembrane activator, calcium modulator, and cyclophilin ligand interactor drives plasma cell differentiation in LPS-activated B cells.

BACKGROUND Transmembrane activator, calcium modulator, and cyclophilin ligand interactor (TACI) expression on B cells is upregulated by Toll-like receptor (TLR) 4. OBJECTIVE We sought to examine whether TACI synergizes with TLR4 in driving immunoglobulin production by B cells and to examine the mechanism of this synergy. METHODS Purified mouse naive B cells were stimulated with the TACI ligand a proliferation-inducing ligand (APRIL) and with suboptimal concentrations of the TLR4 ligand LPS in the presence or absence of IL-4. Immunoglobulin secretion was measured by means of ELISA. Surface IgG1-positive B cells and CD138+ plasmacytoid cells were enumerated by means of FACS. Expression of gamma1 and epsilon germline transcripts, activation-induced cytidine deaminase, and gamma1 and epsilon mature transcripts was measured by means of RT-PCR. RESULTS APRIL synergized with LPS in driving B-cell proliferation and IgM, IgG1, IgG3, IgE, and IgA production. This was mediated by TACI because it was preserved in B-cell maturation antigen-/-, but not TACI-/-, B cells. APRIL and LPS synergized to promote isotype switching, as evidenced by increased expression of activation-induced cytidine deaminase and gamma1 and epsilon mature transcripts and generation of surface IgG1-positive cells. More importantly, APRIL and LPS strongly synergized to drive the plasma cell differentiation program, as evidenced by an increase in CD138+ cells and expression of B lymphocyte induced maturation protein-1 (Blimp-1), interferon regulatory factor-4 (IRF-4), and the spliced form of X-box binding protein-1 (XBP-1). TACI-/- mice had impaired IgM and IgG1 antibody responses to immunization, with a suboptimal dose of the type I T cell-independent antigen 2, 4, 6- Trinitrophenol (TNP)-LPS. CONCLUSIONS These observations suggest that TACI cooperates with TLR4 to drive B-cell differentiation and immunoglobulin production in vitro and in vivo.

The murine equivalent of the A181E TACI mutation associated with common variable immunodeficiency severely impairs B-cell function

TNFRSF13B, which encodes TACI (transmembrane activator and calcium-modulator and cyclophilin ligand interactor), is mutated in 10% of patients with common variable immune deficiency (CVID). One of the 2 most common TACI mutations in CVID, A181E, introduces a negative charge into the transmembrane domain. To define the consequence of the A181E mutation on TACI function, we studied the effect of its murine equivalent, mTACI A144E, on TACI signaling in transfected cells and on TACI function in transgenic mice. The mTACI A144E mutant, like its human TACI A181E counterpart, was expressed on the surface of 293T transfectants and was able to bind ligand, but exhibited impaired constitutive and ligand-induced NF kappaB signaling. In addition, constitutive and ligand-induced clustering of the intracellular domain was deficient for A144E as measured by fluorescence resonance energy transfer. Transgenic mice expressing the A144E mutant on TACI(-/-) background had low serum IgA levels and significantly impaired antibody responses to the type II T-independent antigen TNP-Ficoll. B cells from A144E transgenic mice were impaired in their capacity to proliferate and secrete IgG1 and IgA in response to TACI ligation. These results suggest that mTACI A144E mutation and its human counterpart, A181E, disrupt TACI signaling and impair TACI-dependent B-cell functions.

Toll-like receptor 9, transmembrane activator and calcium-modulating cyclophilin ligand interactor, and CD40 synergize in causing B-cell activation

BACKGROUND B cells receive activating signals from T cells through CD40, from microbial DNA through Toll-like receptor (TLR) 9, and from dendritic cells through transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI). TLR9 and CD40 ligation augment TACI-driven B-cell activation, but only the mechanism of synergy between CD40 and TACI has been explored. Synergy between CD40 and TLR9 in B-cell activation is controversial. OBJECTIVE We sought to examine the mechanisms by which TLR9 modulates CD40- and TACI-mediated activation of B cells and to determine whether all 3 receptors synergize to activate B cells. METHODS Naive murine B cells and human PBMCs were stimulated with combinations of anti-CD40, CpG, and a proliferation inducing ligand in the presence of IL-4. Proliferation was measured by means of tritiated thymidine incorporation. Immunoglobulin production was measured by means of ELISA. Class-switch recombination (CSR) was examined by measuring mRNA for germline transcripts, activation-induced cytidine deaminase (AICDA), and mature immunoglobulin transcripts. Plasma cell differentiation was examined by using syndecan-1/CD138 staining and mRNA expression of B lymphocyte-induced maturation protein 1 (Blimp-1). RESULTS TLR9 synergized with CD40 and TACI in driving CSR and inducing IgG(1) and IgE secretion by naive murine B cells and synergized with TACI in driving B-cell proliferation and plasma cell differentiation. All 3 receptors synergized together in driving murine B-cell proliferation, CSR, plasma cell differentiation, and IgG(1) and IgE secretion. TLR9 synergized with CD40 and TACI in driving IgG secretion in IL-4-stimulated human B cells. CONCLUSION Signals from TLR9, TACI, and CD40 are integrated to promote B-cell activation and differentiation.

Transmembrane activator and calcium-modulator and cyclophilin ligand interactor mutations in common variable immunodeficiency

Purpose of reviewTNFRSF13B, the gene which encodes transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), is mutated in nearly 10% of patients with common variable immune deficiency (CVID), an antibody deficiency syndrome characterized by loss of memory B cells and plasma cells. This review discusses the normal function of TACI and the role of TACI mutants in CVID. Recent findingsTACI activates isotype switching, mediates immunoglobulin production in response to type II T-independent antigens, and plays an inhibitory role in B cell homeostasis. Recent evidence indicates that TACI synergizes with CD40 and Toll-like receptors for immunoglobulin secretion and promotion of plasma cell differentiation. The two most common TACI mutants associated with CVID – C104R and A181E – are primarily found as heterozygous mutations suggesting that they either cause haploinsufficiency or exert a dominant negative effect. TACI mutations in CVID are associated with an increased susceptibility to autoimmunity and lymphoproliferation. SummaryTACI has a dual function in promoting B cell antibody responses and inhibiting B cell proliferation. The observation that TACI mutations are present in healthy participants suggests that modifier genes may play an important role in the development of CVID. The discovery of these genes will help understand the pathogenesis of this disease.

Cdc42 interacting protein 4 (CIP4) is essential for integrin-dependent T-cell trafficking

The F-BAR domain containing protein CIP4 (Cdc42 interacting protein 4) interacts with Cdc42 and WASP/N-WASP and is thought to participate in the assembly of filamentous actin. CIP4−/− mice had normal T- and B-lymphocyte development but impaired T cell-dependent antibody production, IgG antibody affinity maturation, and germinal center (GC) formation, despite an intact CD40L–CD40 axis. CIP4−/− mice also had impaired contact hypersensitivity (CHS) to haptens, and their T cells failed to adoptively transfer CHS. Ovalbumin-activated CD4+ effector T cells from CIP4−/−/OT-II mice migrated poorly to antigen-challenged skin. Activated CIP4−/− T cells exhibited impaired adhesion and polarization on immobilized VCAM-1 and ICAM-1 and defective arrest and transmigration across murine endothelial cell monolayers under shear flow conditions. These results demonstrate an important role for CIP4 in integrin-dependent T cell-dependent antibody responses and GC formation and in integrin-mediated recruitment of effector T cells to cutaneous sites of antigen-driven immune reactions.

TRIF Signaling Is Essential for TLR4-Driven IgE Class Switching

The TLR4 ligand LPS causes mouse B cells to undergo IgE and IgG1 isotype switching in the presence of IL-4. TLR4 activates two signaling pathways mediated by the adaptor molecules MyD88 and Toll/IL-IR domain-containing adapter-inducing IFN-β (TRIF)–related adaptor molecule (TRAM), which recruits TRIF. Following stimulation with LPS plus IL-4, Tram−/− and Trif−/− B cells completely failed to express Cε germline transcripts (GLT) and secrete IgE. In contrast, Myd88−/− B cells had normal expression of Cε GLT but reduced IgE secretion in response to LPS plus IL-4. Following LPS plus IL-4 stimulation, Cγ1 GLT expression was modestly reduced in Tram−/− and Trif−/− B cells, whereas Aicda expression and IgG1 secretion were reduced in Tram−/−, Trif−/−, and Myd88−/− B cells. B cells from all strains secreted normal amounts of IgE and IgG1 in response to anti-CD40 plus IL-4. Following stimulation with LPS plus IL-4, Trif−/− B cells failed to sustain NF-κB p65 nuclear translocation beyond 3 h and had reduced binding of p65 to the Iε promoter. Addition of the NF-κB inhibitor, JSH-23, to wild-type B cells 15 h after LPS plus IL-4 stimulation selectively blocked Cε GLT expression and IgE secretion but had little effect on Cγ1 GLT expression and IgG secretion. These results indicate that sustained activation of NF-κB driven by TRIF is essential for LPS plus IL-4–driven activation of the Cε locus and class switching to IgE.