Cutting Edge: Tissue Antigen Expression Levels Fine-Tune T Cell Differentiation Decisions In Vivo

Immune homeostasis in peripheral tissues is, to a large degree, maintained by the differentiation and action of regulatory T cells (Treg) specific for tissue Ags. Using a novel mouse model, we have studied the differentiation of naive CD4⁺ T cells into Foxp3⁺ Treg in response to a cutaneous Ag (OVA). We found that expression of OVA resulted in fatal autoimmunity and in prevention of peripheral Treg generation. Inhibiting mTOR activity with rapamycin rescued the generation of Foxp3⁺ T cells. When we varied the level of Ag expression to modulate TCR signaling, we found that low Ag concentrations promoted the generation of Foxp3⁺ T cells, whereas high levels expanded effector T cells and caused severe autoimmunity. Our findings indicate that the expression level of tissue Ag is a key determinant of the balance between tissue-reactive effector and peripheral Foxp3⁺ T cells, which determines the choice between tolerance and autoimmunity.

HAX1 deletion impairs BCR internalization and leads to delayed BCR-mediated apoptosis

Deletion of HAX1 in mice causes a severe reduction in the numbers of lymphocytes in the bone marrow and in the spleen. Additionally, B220(+) B progenitor cells in the bone marrow are reduced, suggesting an important function of HAX1 in B cell development. HAX1 is thought to play a protective role in apoptotic processes; therefore, we investigated the role of HAX1 in bone marrow B progenitor cells and splenic B cells. We did not observe an effect on the survival of Hax1(-/-) bone marrow cells but detected enhanced survival of splenic Hax1(-/-) B cells upon in vitro starvation/growth-factor withdrawal. To explain this apparent inconsistency with previous reports of HAX1 function, we also studied the B cell receptor (BCR)-induced apoptosis of IgM-stimulated splenic naïve B cells and found that apoptosis decreased in these cells. We further found impaired internalization of the BCR from Hax1(-/-) splenic B cells after IgM crosslinking; this impaired internalization may result in decreased BCR signaling and, consequently, decreased BCR-mediated apoptosis. We measured HAX1 binding to the cytoplasmic domains of different Ig subtypes and identified KVKWI(V)F as the putative binding motif for HAX1 within the cytoplasmic domains. Because this motif can be found in almost all Ig subtypes, it is likely that HAX1 plays a general role in BCR-mediated internalization events and BCR-mediated apoptosis.

HAX1 deficiency: impact on lymphopoiesis and B-cell development

HAX1 was originally described as HS1-associated protein with a suggested function in receptor-mediated apoptotic and proliferative responses of lymphoid cells. Recent publications refer to a complex and multifunctional role of this protein. To investigate the in vivo function of HAX1 (HS1-associated protein X1) in B cells, we generated a Hax1-deficient mouse strain. Targeted deletion of Hax1 resulted in premature death around the age of 12 wk accompanied by a severe reduction of lymphocytes in spleen, thymus and bone marrow. In the bone marrow, all B-cell populations were lost comparably. In the spleen, B220(+) cells were reduced by almost 70%. However, as investigated by adoptive transfer experiments, this impairment is not exclusively B-cell intrinsic and we hypothesize that a HAX1-deficient environment cannot sufficiently provide the essential factors for proper lymphocyte development, trafficking and survival. Hax1(-/-) B cells show a significantly reduced expression of CXCR4, which might have an influence on the observed defects in B-cell development.

Limited humoral immunoglobulin E memory influences serum immunoglobulin E levels in blood

The switch of B cells expressing membrane-bound Igs, which serve as antigen receptors, to antibody-secreting plasmablasts and finally to non-dividing, long-lived plasma cells (PCs) lacking an antigen receptor, marks the terminal differentiation of a B cell. Antibody-secreting PCs represent the key cell type for the maintenance of a proactive humoral immunological memory. Although some populations of long-lived PCs persist in the spleen, most of them return to their 'place of birth' and travel to the bone marrow or invade inflamed tissues, where they survive up to several months in survival niches as resident, immobile cells. Existing data strongly support the notion that isotype-specific receptor signalling influences the migration behaviour of plasmablasts to the bone marrow. The recent observation in the murine system that the immigration of plasmablasts and the final differentiation to long-lived PCs in the bone marrow is dependent on the expressed B-cell isotype and the related expression of chemokine receptors leads to the conclusion that during a T-helper type 2 (Th2)-mediated immune response in wild type mice, IgE plasmablasts do not have the same chance to contribute to long-lived PC memory as IgG1 plasmablasts. The overall limited humoral IgE memory additionally restricts the quantity of IgE Igs in the serum.

Migration of antibody secreting cells towards CXCL12 depends on the isotype that forms the BCR

Truncation of the cytoplasmic tail of membrane-bound IgE in vivo results in lower serum IgE levels, decreased numbers of IgE-secreting plasma cells and the abrogation of specific secondary immune responses. Here we present mouse strain KN1 that expresses a chimeric epsilon-gamma1 BCR, consisting of the extracellular domains of the epsilon gene and the transmembrane and cytoplasmic domains of the gamma1 gene. Thus, differences in the IgE immune response of KN1 mice reflect the influence of the "gamma1-mediated signalling" of mIgE bearing B cells. KN1 mice show an increased serum IgE level, resulting from an elevated number of IgE-secreting cells. Although the primary IgE immune response in KN1 mice is inconspicuous, the secondary response is far more robust. Most strikingly, IgE-antibody secreting cells with "gamma1-signalling history" migrate more efficiently towards the chemokine CXCL12, which guides plasmablasts to plasma cell niches, than IgE-antibody secreting cells with WT "epsilon-signalling history". We conclude that IgE plasmablasts have an intrinsic, lower chance to contribute to the long-lived plasma cell pool than IgG1 plasmablasts.

Targeting the extracellular membrane-proximal domain of membrane-bound IgE by passive immunization blocks IgE synthesis in vivo

The classical allergic reaction starts seconds or minutes after Ag contact and is committed by Abs produced by a special subset of B lymphocytes. These Abs belong to the IgE subclass and are responsible for Type I hyperreactivity reactions. Treatment of allergic diseases with humanized anti-IgE Abs leads primarily to a decrease of serum IgE levels. As a consequence, the number of high-affinity IgE receptors on mast cells and basophils decreases, leading to a lower excitability of the effector cells. The biological mechanism behind anti-IgE therapy remains partly speculative; however, it is likely that these Abs also interact with membrane IgE (mIgE) on B cells and possibly interfere with IgE production. In the present work, we raised a mouse mAb directed exclusively against the extracellular membrane-proximal domain of mIgE. The interaction between the monoclonal anti-mIgE Ab and mIgE induces receptor-mediated apoptosis in vitro. Passive immunization experiments lead to a block of newly synthesized specific IgEs during a parallel application of recombinant Bet v1a, the major birch pollen allergen. The decrease of allergen-specific serum IgE might be related to tolerance-inducing mechanisms stopping mIgE-displaying B cells in their proliferation and differentiation.

HS1-associated protein X-1 interacts with membrane-bound IgE: impact on receptor-mediated internalization

Engagement of the BCR triggers signals that control affinity maturation, memory induction, differentiation, and various other physiological processes in B cells. In previous work, we showed that truncation of the cytoplasmic tail of membrane-bound Ig (mIg)E in vivo resulted in lower serum IgE levels, decreased numbers of IgE-secreting plasma cells, and the abrogation of specific secondary responses correlating with a defect in the selection of high-affinity Abs during the germinal center reaction. We concluded that the Ag receptor is necessary at all times during Ab responses not only for the maturation process, but also for the expansion of Ag-specific B cells. Based on these results, we asked whether the cytoplasmic tail of mIgE, or specific proteins binding the cytoplasmic tail in vivo commit a signal transduction accompanying the B cell along its differentiation process. In this study, we present the identification of HS1-associated protein X-1 as a novel protein interacting with the cytoplasmic tail of mIgE. ELISA, surface plasmon resonance analysis, and coimmunoprecipitation experiments confirmed the specific interaction in vitro. In functional assays, we clearly showed that HS1-associated protein X-1 expression levels influence the efficiency of BCR-mediated Ag internalization.

Inefficient processing of mRNA for the membraneform of IgE is a genetic mechanism to limit recruitment of IgE-secreting cells

Immunoglobulin E (IgE) is the key effector element in allergic diseases ranging from innocuous hay fever to life-threatening anaphylactic shock. Compared to other Ig classes, IgE serum levels are very low. In its membrane-bound form (mIgE), IgE behaves as a classical antigen receptor on B lymphocytes. Expression of mIgE is essential for subsequent recruitment of IgE-secreting cells. We show that in activated, mIgE-bearing B cells, mRNA for the membrane forms of both murine and human epsilon (epsilon) heavy chains (HC) are poorly expressed compared to mRNA for the secreted forms. In contrast, in mIgG-bearing B cells, mRNA for the membrane forms of murine gamma-1 (gamma1) and the corresponding human gamma4 HC are expressed at a much higher level than mRNA for the respective secreted forms. We show that these findings correlate with the presence of deviant polyadenylation signal hexamers in the 3' untranslated region (UTR) of both murine and human epsilon genes, causing inefficient processing of primary transcripts and thus poor expression of the proteins and poor recruitment of IgE-producing cells in the immune response. Thus, we have identified a genetic steering mechanism in the regulation of IgE synthesis that represents a further means to restrain potentially dangerous, high serum IgE levels.

Regulation of the IgE response at the molecular level: impact on the development of systemic anti IgE therapeutic strategies

The classical allergic reaction starts within seconds or minutes after antigen contact and is induced by antibodies produced by a special subset of B lymphocytes. These antibodies belong to the IgE subclass and are responsible for Type I hyper-reactivity reactions. IgE plays a minor role in healthy individuals. In allergic individuals, however, IgE antibodies trigger allergic responses through allergen-mediated cross-linking on effector cells followed by mediator release. The mechanisms inducing a switch to IgE production are not fully understood with the consequence that allergies are mainly treated with antisymptomatic drugs. To develop basic therapies, many questions concerning the very complex regulation of IgE expression have to be understood. Positive and negative regulators influence the synthesis of IgE. Experiments in our laboratory could show that not only regulatory molecules, but also the membrane bound IgE itself controls the quantity and quality of the IgE produced. This fact becomes more and more interesting, because the signals generated by the B-cell receptor may be important targets for interference in allergic patients, in whom the titer and the affinity of the IgE antibodies for the allergen are directly related to disease activity.

Construction of an sIgE:FLAG-mIgE:GFP reporter mouse strain

Like all other immunoglobulins, IgE can be secreted into the blood or expressed as a membrane receptor on the surface of B lymphocytes. Secreted immunoglobulins trace the antigen and contribute to its destruction. Membrane immunoglobulins accompany the B cell along its differentiation pathway, regulating processes like the induction and maintenance of immunological memory and differentiation of plasma cells. The regulation of the expression of IgE is very complex. A lot of positive and negative regulators influence the synthesis of IgE. In previous publications, we were able to show that the membrane IgE (mIgE) antigen receptor itself controls the quantity and quality of serum IgE produced. However, the knowledge about the regulatory function of the antigen receptor on these processes is at best limited. In the present paper, we present the construction of a reporter mouse strain, which will help us to follow an mIgE-bearing B cell during the immune response more precisely.

Phage display based cloning of proteins interacting with the cytoplasmic tail of membrane immunoglobulins

The reduced quantity and quality of serum immunoglobulins (sIgs) in mutant mice expressing truncated cytoplasmic tails of IgE and IgG1 indicate an active role for the cytoplasmic domains of mIgG1 and mIgE. We used phage display technology to identify candidate proteins able to interact with the cytoplasmic tail of mIgE. Using a murine cDNA B cell library displayed on the surface of phage as prey and the 28 amino acid long cytoplasmic tail of IgE as bait, we isolated phage encoding the murine hematopoietic progenitor kinase 1 (HPK1). Surface plasmon resonance analysis measurements confirmed affinity of HPK1 to the mIgE cytoplasmic tail and revealed association to other immunoglobulin isotypes as well. Immunoprecipitation experiments, using lysates from two B cell lines expressing nitrophenyl (NP) specific mIgE molecules showed co-precipitation of IgE and HPK1. The interaction of HPK1 with the cytoplasmic domains of membrane immunoglobulins indicate an active role of the tails as part of an isotype specific signal transduction, independent from the Igalpha/Igbeta heterodimers, and may represent a missing link to upstream regulatory elements of HPK1 activation.

Somatic diversity of the immunoglobulin repertoire is controlled in an isotype-specific manner

We have studied two aspects of the IgE immune response. First, we have compared the kinetics of the IgE response to the T cell-dependent antigen ph-Ox coupled to ovalbumin with that of the IgG1 response and we have assessed the quality of the IgE response. Second, we have studied the generation of somatic diversity, understood as the combined effect of somatic mutation and the selection of D(iversity) and J(oining) elements, in germinal center B cells at the molecular level, using the germ-line sequence of the prototype anti-ph-Ox heavy chain variable element V(H)Ox1 as reference. We evaluated sequences derived from mu-, gamma 1- and epsilon-variable elements and showed that somatic diversification was different for all isotypes studied. We further compared the IgE responses of wild-type mice with those of mice expressing a truncated cytoplasmic IgE tail (IgE(KVK Delta tail)). IgE(KVK Delta tail) mice showed a more diverse sequence pattern. We corroborated previous results suggesting that short CDR3 regions are indicative for high-affinity antibodies by measuring relative affinities of phage-expressed Fab fragments with prototype long and short CDR3 regions. Therefore, the composition of the antigen-receptor is responsible for the selection process and the expansion of antigen-specific cells, leading to an isotype-specific antibody repertoire.

The IgE antigen receptor: a key regulator for the production of IgE antibodies

Immunoglobulins in general form a substantial component of serum proteins, and play a role in homeostatic mechanisms, a first line of defense against pathogenic organisms and in immunological memory. In the secreted form, immunoglobulins represent the effector arm of the humoral immune system. However, immunoglobulins are not only secreted, but can also be expressed on the surface of a B lymphocyte (membrane immunoglobulin), and, in this physical state, most likely convey signals to steer the B cell along its differentiation pathway. A step forward in the understanding of the role of membrane immunoglobulins other than membrane IgM or IgD was achieved with two mouse lines with mutations in the epsilon heavy chain gene. In IgE(DeltaM1M2) mice serum IgE is reduced to less than 10% of normal mice, while IgE(KVKDeltatail) mice show a reduction of 50%, reflecting a serious impairment of the IgE-mediated immune response. We think that the cytoplasmic tail of IgE is involved in a signal transduction which leads to the expression of high quantities and qualities of secreted IgE immunoglobulins.