Molecular basis for potent B cell responses to antigen displayed on particles of viral size.

Multivalent viral epitopes induce rapid, robust and T cell-independent humoral immune responses, but the biochemical basis for such potency remains incompletely understood. We take advantage of a set of liposomes of viral size engineered to display affinity mutants of the model antigen (Ag) hen egg lysozyme. Particulate Ag induces potent 'all-or-none' B cell responses that are density dependent but affinity independent. Unlike soluble Ag, particulate Ag induces signal amplification downstream of the B cell receptor by selectively evading LYN-dependent inhibitory pathways and maximally activates NF-κB in a manner that mimics T cell help. Such signaling induces MYC expression and enables even low doses of particulate Ag to trigger robust B cell proliferation in vivo in the absence of adjuvant. We uncover a molecular basis for highly sensitive B cell responses to viral Ag display that is independent of encapsulated nucleic acids and is not merely accounted for by avidity and B cell receptor cross-linking.

NR4A nuclear receptors restrain B cell responses to antigen when second signals are absent or limiting.

Antigen stimulation (signal 1) triggers B cell proliferation and primes B cells to recruit, engage and respond to T cell help (signal 2). Failure to receive signal 2 within a defined time window results in B cell apoptosis, yet the mechanisms that enforce dependence on co-stimulation are incompletely understood. Nr4a1-3 encode a small family of orphan nuclear receptors that are rapidly induced by B cell antigen receptor stimulation. Here, we show that Nr4a1 and Nr4a3 play partially redundant roles to restrain B cell responses to antigen in the absence of co-stimulation and do so, in part, by repressing the expression of BATF and, consequently, MYC. The NR4A family also restrains B cell access to T cell help by repressing expression of the T cell chemokines CCL3 and CCL4, as well as CD86 and ICAM1. Such NR4A-mediated regulation plays a role specifically under conditions of competition for limiting T cell help.

An Integrated Signaling Threshold Initiates IgG Response toward Virus-like Immunogens.

Class-switched neutralizing Ab (nAb) production is rapidly induced upon many viral infections. However, due to the presence of multiple components in virions, the precise biochemical and biophysical signals from viral infections that initiate nAb responses remain inadequately defined. Using a reductionist system of synthetic virus-like structures, in this study, we show that a foreign protein on a virion-sized liposome can serve as a stand-alone danger signal to initiate class-switched nAb responses without T cell help or TLR but requires CD19. Introduction of internal nucleic acids (iNAs) obviates the need for CD19, lowers the epitope density (ED) required to elicit the Ab response, and transforms these structures into highly potent immunogens that rival conventional virus-like particles in their ability to elicit strong Ag-specific IgG. As early as day 5 after immunization, structures harboring iNAs and decorated with just a few molecules of surface Ag at doses as low as 100 ng induced all IgG subclasses of Ab in mice and reproduced the IgG2a/2c restriction that is long observed in live viral infections. These findings reveal a shared mechanism for the nAb response in mice. High ED is capable but not necessary for driving Ab secretion. Instead, even a few molecules of surface Ag, when combined with nucleic acids within these structures, can trigger strong IgG production. As a result, the signaling threshold for induction of IgG in individual B cells is set by dual signals originating from both ED on the surface and the presence of iNAs within viral particulate immunogens.

NR4A nuclear receptors in T and B lymphocytes: Gatekeepers of immune tolerance.

Random VDJ recombination early in T and B cell development enables the adaptive immune system to recognize a vast array of evolving pathogens via antigen receptors. However, the potential of such randomly generated TCRs and BCRs to recognize and respond to self-antigens requires layers of tolerance mechanisms to mitigate the risk of life-threatening autoimmunity. Since they were originally cloned more than three decades ago, the NR4A family of nuclear hormone receptors have been implicated in many critical aspects of immune tolerance, including negative selection of thymocytes, peripheral T cell tolerance, regulatory T cells (Treg), and most recently in peripheral B cell tolerance. In this review, we discuss important insights from many laboratories as well as our own group into the function and mechanisms by which this small class of primary response genes promotes self-tolerance and immune homeostasis to balance the need for host defense against the inherent risks posed by the adaptive immune system.

ATP-competitive partial antagonists of the IRE1α RNase segregate outputs of the UPR.

The unfolded protein response (UPR) homeostatically matches endoplasmic reticulum (ER) protein-folding capacity to cellular secretory needs. However, under high or chronic ER stress, the UPR triggers apoptosis. This cell fate dichotomy is promoted by differential activation of the ER transmembrane kinase/endoribonuclease (RNase) IRE1α. We previously found that the RNase of IRE1α can be either fully activated or inactivated by ATP-competitive kinase inhibitors. Here we developed kinase inhibitors, partial antagonists of IRE1α RNase (PAIRs), that partially antagonize the IRE1α RNase at full occupancy. Biochemical and structural studies show that PAIRs promote partial RNase antagonism by intermediately displacing the helix αC in the IRE1α kinase domain. In insulin-producing ß-cells, PAIRs permit adaptive splicing of Xbp1 mRNA while quelling destructive ER mRNA endonucleolytic decay and apoptosis. By preserving Xbp1 mRNA splicing, PAIRs allow B cells to differentiate into immunoglobulin-producing plasma cells. Thus, an intermediate RNase-inhibitory 'sweet spot', achieved by PAIR-bound IRE1α, captures a desirable conformation for drugging this master UPR sensor/effector.

Optimal Development of Mature B Cells Requires Recognition of Endogenous Antigens.

It has long been appreciated that highly autoreactive BCRs are actively removed from the developing B cell repertoire by Ag-dependent receptor editing and deletion. However, there is persistent debate about whether mild autoreactivity is simply tolerated or positively selected into the mature B cell repertoire as well as at what stage, to what extent, under what conditions, and into which compartments this occurs. In this study, we describe two minor, trackable populations of B cells in B1-8i Ig transgenic mice that express the VH186.2 H chain and recognize a common foreign Ag (the hapten 4-hydroxy-3-nitrophenylacetyl) but differ in L chain expression. We use the Nur77-eGFP reporter of BCR signaling to define their reactivity toward endogenous Ags. The less autoreactive of these two populations is strongly counterselected during the development of mature B1a, follicular, and marginal zone B cells. By genetically manipulating the strength of BCR signal transduction via the titration of surface CD45 expression, we demonstrate that this B cell population is not negatively selected but instead displays characteristics of impaired positive selection. We demonstrate that mild self-reactivity improves the developmental fitness of B cell clones in the context of a diverse population of B cells, and positive selection by endogenous Ags shapes the mature B cell repertoire.

Nur77 Links Chronic Antigen Stimulation to B Cell Tolerance by Restricting the Survival of Self-Reactive B Cells in the Periphery.

Nur77 (Nr4a1) belongs to a small family of orphan nuclear receptors that are rapidly induced by BCR stimulation, yet little is known about its function in B cells. We have previously characterized a reporter of Nr4a1 transcription, Nur77-eGFP, in which GFP expression faithfully detects Ag encounter by B cells in vitro and in vivo. In this study, we report that Nur77 expression correlates with the degree of self-reactivity, counterselection, and anergy among individual B cell clones from two distinct BCR transgenic mouse models but is dispensable for all of these tolerance mechanisms. However, we identify a role for Nur77 in restraining survival of self-reactive B cells in the periphery under conditions of competition for a limited supply of the survival factor BAFF. We find that Nur77 deficiency results in the progressive accumulation of self-reactive B cells in the mature repertoire with age and is sufficient to break B cell tolerance in VH3H9 H chain transgenic mice. We thus propose that Nur77 is upregulated in self-reactive B cells in response to chronic Ag stimulation and selectively restricts the survival of these cells, gradually pruning self-reactivity from the mature repertoire to impose a novel layer of peripheral B cell tolerance.

Synthetic Liposomal Mimics of Biological Viruses for the Study of Immune Responses to Infection and Vaccination.

Human viruses possess very complex supramolecular structures. Both icosahedral and enveloped viruses typically display an array of viral-encoded protein antigens at varied spatial densities on the viral particle surface. The viral nucleic acid genome, on the other hand, is encapsulated inside the viral particle. Although both the surface antigen and the interior nucleic acids could independently produce immunological responses, how B cells integrate these two types of signals and respond to a typical virus particle to initiate activation is not well understood at a molecular level. The study of these fundamental biological processes would benefit from the development of viral structural mimics that are well constructed to incorporate both quantitative and qualitative viral features for presentation to B cells. These novel tools would enable researchers to systematically dissect the underlying processes. Here we report the development of such particulate antigens based on liposomes engineered to display a model protein antigen, hen egg lysozyme (HEL). We developed methods to overexpress and purify various affinity mutants of HEL from E. coli. We conjugated the purified recombinant HEL proteins onto the surface of a virion-sized liposome in an orientation-specific manner at defined spatial densities and also encapsulated nucleic acid molecules into the interior of the liposome. Both the chemical conjugation of the HEL antigen on liposome surfaces and the encapsulation of nucleic acids were stable under physiologically relevant conditions. These liposomes elicited antigen-specific B-cell responses in vitro, which validate these supramolecular structures as a novel and effective approach to mimic and systematically isolate the role of essential viral features in directing the B-cell response to particulate antigens.

EPCAM mutation update: Variants associated with congenital tufting enteropathy and Lynch syndrome.

The epithelial cell adhesion molecule gene (EPCAM, previously known as TACSTD1 or TROP1) encodes a membrane-bound protein that is localized to the basolateral membrane of epithelial cells and is overexpressed in some tumors. Biallelic mutations in EPCAM cause congenital tufting enteropathy (CTE), which is a rare chronic diarrheal disorder presenting in infancy. Monoallelic deletions of the 3' end of EPCAM that silence the downstream gene, MSH2, cause a form of Lynch syndrome, which is a cancer predisposition syndrome associated with loss of DNA mismatch repair. Here, we report 13 novel EPCAM mutations from 17 CTE patients from two separate centers, review EPCAM mutations associated with CTE and Lynch syndrome, and structurally model pathogenic missense mutations. Statistical analyses indicate that the c.499dupC (previously reported as c.498insC) frameshift mutation was associated with more severe treatment regimens and greater mortality in CTE, whereas the c.556-14A>G and c.491+1G>A splice site mutations were not correlated with treatments or outcomes significantly different than random simulation. These findings suggest that genotype-phenotype correlations may be useful in contributing to management decisions of CTE patients. Depending on the type and nature of EPCAM mutation, one of two unrelated diseases may occur, CTE or Lynch syndrome.

IL-2 Modulates the TCR Signaling Threshold for CD8 but Not CD4 T Cell Proliferation on a Single-Cell Level.

Lymphocytes integrate Ag and cytokine receptor signals to make cell fate decisions. Using a specific reporter of TCR signaling that is insensitive to cytokine signaling, Nur77-eGFP, we identify a sharp, minimal threshold of cumulative TCR signaling required for proliferation in CD4 and CD8 T cells that is independent of both Ag concentration and affinity. Unexpectedly, IL-2 reduces this threshold in CD8 but not CD4 T cells, suggesting that integration of multiple mitogenic inputs may alter the minimal requirement for TCR signaling in CD8 T cells. Neither naive CD4 nor naive CD8 T cells are responsive to low doses of IL-2. We show that activated CD8 T cells become responsive to low doses of IL-2 more quickly than CD4 T cells, and propose that this relative delay in turn accounts for the differential effects of IL-2 on the minimal TCR signaling threshold for proliferation in these populations. In contrast to Nur77-eGFP, c-Myc protein expression integrates mitogenic signals downstream of both IL-2 and the TCR, yet marks an invariant minimal threshold of cumulative mitogenic stimulation required for cell division. Our work provides a conceptual framework for understanding the regulation of clonal expansion of CD8 T cells by subthreshold TCR signaling in the context of mitogenic IL-2 signals, thereby rendering CD8 T cells exquisitely dependent upon environmental cues. Conversely, CD4 T cell proliferation requires an invariant minimal intensity of TCR signaling that is not modulated by IL-2, thereby restricting responses to low-affinity or low-abundance self-antigens even in the context of an inflammatory milieu.

Cutting Edge: Targeting Epithelial ORMDL3 Increases, Rather than Reduces, Airway Responsiveness and Is Associated with Increased Sphingosine-1-Phosphate.

In this study, we used cre-lox techniques to generate mice selectively deficient in ORMDL3 in airway epithelium (Ormdl3Δ2-3/Δ2-3/CC10) to simulate an inhaled therapy that effectively inhibited ORMDL3 expression in the airway. In contrast to the anticipated reduction in airway hyperresponsiveness (AHR), OVA allergen-challenged Ormdl3Δ2-3/Δ2-3/CC10 mice had a significant increase in AHR compared with wild-type mice. Levels of airway inflammation, mucus, fibrosis, and airway smooth muscle were no different in Ormdl3Δ2-3/Δ2-3/CC10 and wild-type mice. However, levels of sphingosine-1-phosphate (S1P) were significantly increased in Ormdl3Δ2-3/Δ2-3/CC10 mice as well as in airway epithelial cells in which ORMDL3 was inhibited with small interfering RNA. Incubation of S1P with airway smooth muscle cells significantly increased contractility. Overall, Ormdl3Δ2-3/Δ2-3/CC10 mice exhibit increased allergen-induced AHR independent of inflammation and associated with increased S1P generation. These studies raise concerns for inhaled therapies that selectively and effectively inhibit ORMDL3 in airway epithelium in asthma.

Inflammasome-mediated disease animal models reveal roles for innate but not adaptive immunity.

NLRP3 nucleates the inflammasome, a protein complex responsible for cleavage of prointerleukin-1beta (IL-1beta) to its active form. Mutations in the NLRP3 gene cause the autoinflammatory disease spectrum cryopyrin-associated periodic syndromes (CAPS). The central role of IL-1beta in CAPS is supported by the response to IL-1-targeted therapy. We developed two Nlrp3 mutant knockin mouse strains to model CAPS to examine the role of other inflammatory mediators and adaptive immune responses in an innate immune-driven disease. These mice had systemic inflammation and poor growth, similar to some human CAPS patients, and demonstrated early mortality, primarily mediated by myeloid cells. Mating these mutant mice to various gene mutant backgrounds showed that the mouse disease phenotype required an intact inflammasome, was only partially dependent on IL-1beta, and was independent of T cells. These data suggest that CAPS are true inflammasome-mediated diseases and provide insight for more common inflammatory disorders.

An extracatalytic function of CD45 in B cells is mediated by CD22.

The receptor-like tyrosine phosphatase CD45 regulates antigen receptor signaling by dephosphorylating the C-terminal inhibitory tyrosine of the src family kinases. However, despite its abundance, the function of the large, alternatively spliced extracellular domain of CD45 has remained elusive. We used normally spliced CD45 transgenes either incorporating a phosphatase-inactivating point mutation or lacking the cytoplasmic domain to uncouple the enzymatic and noncatalytic functions of CD45 in lymphocytes. Although these transgenes did not alter T-cell signaling or development irrespective of endogenous CD45 expression, both partially rescued the phenotype of CD45-deficient B cells. We identify a noncatalytic role for CD45 in regulating tonic, but not antigen-mediated, B-cell antigen receptor (BCR) signaling through modulation of the function of the inhibitory coreceptor CD22. This finding has important implications for understanding how naïve B cells maintain tonic BCR signaling while restraining inappropriate antigen-dependent activation to preserve clonal "ignorance."

A sharp T-cell antigen receptor signaling threshold for T-cell proliferation.

T-cell antigen receptor (TCR) signaling is essential for activation, proliferation, and effector function of T cells. Modulation of both intensity and duration of TCR signaling can regulate these events. However, it remains unclear how individual T cells integrate such signals over time to make critical cell-fate decisions. We have previously developed an engineered mutant allele of the critical T-cell kinase zeta-chain-associated protein kinase 70 kDa (Zap70) that is catalytically inhibited by a small molecule inhibitor, thereby blocking TCR signaling specifically and efficiently. We have also characterized a fluorescent reporter Nur77-eGFP transgenic mouse line in which T cells up-regulate GFP uniquely in response to TCR stimulation. The combination of these technologies unmasked a sharp TCR signaling threshold for commitment to cell division both in vitro and in vivo. Further, we demonstrate that this threshold is independent of both the magnitude of the TCR stimulus and Interleukin 2. Similarly, we identify a temporal threshold of TCR signaling that is required for commitment to proliferation, after which T cells are able to proliferate in a Zap70 kinase-independent manner. Taken together, our studies reveal a sharp threshold for the magnitude and duration of TCR signaling required for commitment of T cells to proliferation. These results have important implications for understanding T-cell responses to infection and optimizing strategies for immunomodulatory drug delivery.

ORMDL3 transgenic mice have increased airway remodeling and airway responsiveness characteristic of asthma.

Orosomucoid-like (ORMDL)3 has been strongly linked with asthma in genetic association studies. Because allergen challenge induces lung ORMDL3 expression in wild-type mice, we have generated human ORMDL3 zona pellucida 3 Cre (hORMDL3(zp3-Cre)) mice that overexpress human ORMDL3 universally to investigate the role of ORMDL3 in regulating airway inflammation and remodeling. These hORMDL3(zp3-Cre) mice have significantly increased levels of airway remodeling, including increased airway smooth muscle, subepithelial fibrosis, and mucus. hORMDL3(zp3-Cre) mice had spontaneously increased airway responsiveness to methacholine compared to wild-type mice. This increased airway remodeling was associated with selective activation of the unfolded protein response pathway transcription factor ATF6 (but not Ire1 or PERK). The ATF6 target gene SERCA2b, implicated in airway remodeling in asthma, was strongly induced in the lungs of hORMDL3(zp3-Cre) mice. Additionally, increased levels of expression of genes associated with airway remodeling (TGF-ß1, ADAM8) were detected in airway epithelium of these mice. Increased levels of airway remodeling preceded increased levels of airway inflammation in hORMDL3(zp3-Cre) mice. hORMDL3(zp3-Cre) mice had increased levels of IgE, with no change in levels of IgG, IgM, and IgA. These studies provide evidence that ORMDL3 plays an important role in vivo in airway remodeling potentially through ATF6 target genes such as SERCA2b and/or through ATF6-independent genes (TGF-ß1, ADAM8).

Absence of cell-surface EpCAM in congenital tufting enteropathy.

Mutations in the epithelial cell adhesion molecule (EpCAM; CD326) gene are causal for congenital tufting enteropathy (CTE), a disease characterized by intestinal abnormalities resulting in lethal diarrhea in newborns. Why the different mutations all lead to the same disease is not clear. Here, we report that most mutations, including a novel intronic variant, will result in lack of EpCAM's transmembrane domain, whereas two mutations allow transmembrane localization. We find that these mutants are not routed to the plasma membrane, and that truncated mutants are secreted or degraded. Thus, all epcam mutations lead to loss of cell-surface EpCAM, resulting in CTE.

ORMDL3 is an inducible lung epithelial gene regulating metalloproteases, chemokines, OAS, and ATF6.

Orosomucoid like 3 (ORMDL3) has been strongly linked with asthma in genetic association studies, but its function in asthma is unknown. We demonstrate that in mice ORMDL3 is an allergen and cytokine (IL-4 or IL-13) inducible endoplasmic reticulum (ER) gene expressed predominantly in airway epithelial cells. Allergen challenge induces a 127-fold increase in ORMDL3 mRNA in bronchial epithelium in WT mice, with lesser 15-fold increases in ORMDL-2 and no changes in ORMDL-1. Studies of STAT-6-deficient mice demonstrated that ORMDL3 mRNA induction highly depends on STAT-6. Transfection of ORMDL3 in human bronchial epithelial cells in vitro induced expression of metalloproteases (MMP-9, ADAM-8), CC chemokines (CCL-20), CXC chemokines (IL-8, CXCL-10, CXCL-11), oligoadenylate synthetases (OAS) genes, and selectively activated activating transcription factor 6 (ATF6), an unfolded protein response (UPR) pathway transcription factor. siRNA knockdown of ATF-6α in lung epithelial cells inhibited expression of SERCA2b, which has been implicated in airway remodeling in asthma. In addition, transfection of ORMDL3 in lung epithelial cells activated ATF6α and induced SERCA2b. These studies provide evidence of the inducible nature of ORMDL3 ER expression in particular in bronchial epithelial cells and suggest an ER UPR pathway through which ORMDL3 may be linked to asthma.

Functional consequences of EpCam mutation in mice and men.

Congenital tufting enteropathy (CTE) is a severe diarrheal disease of infancy characterized by villous changes and epithelial tufts. We previously identified mutations in epithelial cell adhesion molecule (EpCAM) as the cause of CTE. We developed an in vivo mouse model of CTE based on EpCAM mutations found in patients with the aim to further elucidate the in vivo role of EpCAM and allow for a direct comparison to human CTE. Using Cre-LoxP recombination technology, we generated a construct lacking exon 4 in Epcam. Epcam(Δ4/Δ4) mice and CTE patient intestinal tissue integrity was analyzed by histology using both light immunohistochemistry and electron microscopy. Epcam(Δ4/Δ4) mice demonstrate neonatal lethality and growth retardation with pathological features, including epithelial tufts, enterocyte crowding, altered desmosomes, and intercellular gaps, similar to human CTE patients. Mutant EpCAM protein is present at low levels and is mislocalized in the intestine of Epcam(Δ4/Δ4) mice and CTE patients. Deletion of exon 4 was found to decrease expression of both EpCAM and claudin-7 causing a loss of colocalization, functionally disrupting the EpCAM/claudin-7 complex, a finding for the first time confirmed in CTE patients. Furthermore, compared with unaffected mice, mutation of Epcam leads to enhanced permeability and intestinal cell migration, uncovering underlying disease mechanisms.

Cutting edge: IL-6 is a marker of inflammation with no direct role in inflammasome-mediated mouse models.

IL-6 is a known downstream target of IL-1ß and is consistently increased in serum from patients with NLRP3 inflammasome-mediated conditions. Therefore, IL-6 could be a therapeutic target in the treatment of IL-1ß-provoked inflammation. IL-6 was increased in serum with accompanying neutrophilia in tissues of an inducible mouse model of Muckle-Wells syndrome. However, an IL-6-null background failed to provide phenotypic rescue and did not significantly impact inflammatory cytokine levels. In a second model of IL-1ß-driven inflammation, NLRP3 activation by monosodium urate crystals similarly increased IL-6. Consistent with our Muckle-Wells syndrome model, ablation of IL-6 did not impact an acute neutrophilic response in this in vivo evaluation of gouty arthritis. Taken together, our results indicate that IL-6 is a reliable marker of inflammation, with no direct role in inflammasome-mediated disease.

An integrated signaling threshold initiates IgG response towards virus-like immunogens.

Class-switched neutralizing antibody (nAb) production is rapidly induced upon many viral infections. However, due to the presence of multiple components in typical virions, the precise biochemical and biophysical signals from viral infections that initiate nAb responses remain inadequately defined. Using a reductionist system of synthetic virus-like structures (SVLS) containing minimal, highly purified biochemical components commonly found in enveloped viruses, here we show that a foreign protein on a virion-sized liposome can serve as a stand-alone danger signal to initiate class-switched nAb responses in the absence of cognate T cell help or Toll-like receptor signaling but requires CD19, the antigen (Ag) coreceptor on B cells. Introduction of internal nucleic acids (iNAs) obviates the need for CD19, lowers the epitope density (ED) required to elicit the Ab response and transforms these structures into highly potent immunogens that rival conventional virus-like particles in their ability to elicit strong Ag-specific IgG. As early as day 5 after immunization, structures harbouring iNAs and decorated with just a few molecules of surface Ag at doses as low as 100 ng induced all IgG subclasses of Ab known in mice and reproduced the IgG2a/2c restriction that has been long observed in live viral infections. These findings reveal a shared mechanism for nAb response upon viral infection. High ED is capable but not necessary for driving Ab secretion in vivo . Instead, even a few molecules of surface Ag, when combined with nucleic acids within these structures, can trigger strong antiviral IgG production. As a result, the signaling threshold for the induction of neutralizing IgG is set by dual signals originating from both ED on the surface and the presence of iNAs within viral particulate immunogens. One-sentence summary: Reconstitution of minimal viral signals necessary to initiate antiviral IgG.