Receptor Cross-linking Research Articles

CD300f associates with IL-4 receptor α and amplifies IL-4–induced immune cell responses

IL-4 receptor (R) α, the common receptor chain for IL-4 and IL-13, is a critical component in IL-4- and IL-13-mediated signaling and subsequent effector functions such as those observed in type 2 inflammatory responses. Nonetheless, the existence of intrinsic pathways capable of amplifying IL-4Rα-induced responses remains unknown. In this study, we identified the myeloid-associated Ig receptor CD300f as an IL-4-induced molecule in macrophages. Subsequent analyses demonstrated that CD300f was colocalized and physically associated with IL-4Rα. Using Cd300f(-/-) cells and receptor cross-linking experiments, we established that CD300f amplified IL-4Rα-induced responses by augmenting IL-4/IL-13-induced signaling, mediator release, and priming. Consistently, IL-4- and aeroallergen-treated Cd300f(-/-) mice displayed decreased IgE production, chemokine expression, and inflammatory cell recruitment. Impaired responses in Cd300f(-/-) mice were not due to the inability to generate a proper Th2 response, because IL-4/IL-13 levels were markedly increased in allergen-challenged Cd300f(-/-) mice, a finding that is consistent with decreased cytokine consumption. Finally, CD300f expression was increased in monocytes and eosinophils obtained from allergic rhinitis patients. Collectively, our data highlight a previously unidentified role for CD300f in IL-4Rα-induced immune cell responses. These data provide new insights into the molecular mechanisms governing IL-4Rα-induced responses, and may provide new therapeutic tools to target IL-4 in allergy and asthma.

Polymeric biomaterials and nanomedicines

This overview intends to demonstrate the close relationship between the design of smart biomaterials and water-soluble polymer–drug conjugates. First, the discovery and systematic studies of hydrogels based on crosslinked poly(meth)acrylic acid esters and substituted amides is described. Then, the lessons learned for the design of water-soluble polymers as drug carriers are highlighted. The current state-of-the-art in water-soluble, mainly poly[N-(2-hydroxypropyl)methacylamide (HPMA), polymer–drug conjugates is shown including the design of backbone degradable HPMA copolymer carriers. In the second part, the modern design of hybrid hydrogels focuses on the self-assembly of hybrid copolymers composed from the synthetic part (backbone) and biorecognizable grafts (coiled-coil forming peptides or morpholino oligonucleotides) is shown. The research of self-assembling hydrogels inspired the invention and design of drug-free macromolecular therapeutics – a new paradigm in drug delivery where crosslinking of non-internalizating CD20 receptors results in apoptosis in vitro and in vivo. The latter is mediated by biorecognition of complementary motifs; no low molecular weight drug is needed.

Regulation and function of absent in melanoma 2 in human CD4 T cells (IRM7P.710)

Abstract Recognition of dsDNA by absent in melanoma 2 (AIM2) leads to inflammasome formation, activation and secretion of IL-1β and IL-18 in innate cells. While AIM2 inflammasome function has been well characterized in macrophages, it has not been explored in adaptive cells. Recently, we have shown that human CD4 T cells robustly upregulate expression of AIM2 following T cell activation. Interestingly, memory CD4 T cells have a higher resting level of expression of AIM2 and more rapid upregulation of AIM2 mRNA following activation. High-throughput targeted bisulfite sequencing and ChIPseq show that AIM2 expression in CD4 T cells is epigenetically regulated by DNA methylation and histone modifications. The AIM2 promoter exhibits both hypomethylated CpGs and increased H3K4me3 and H3K4me2 in memory CD4 T cells compared to naïve. Such epigenetic marks allow for both a higher basal level of AIM2 expression and stronger induction of AIM2 expression following T cell activation. Robust protein expression of AIM2 is also found following stimulation of these cells. While AIM2 is known to form inflammasomes and drive IL-1β and IL-18 expression in macrophages, CD4 T cells do not express either of these cytokines, suggesting a non-canonical role for AIM2 function in T cells. Our current studies are focused on determining the function of AIM2 in CD4 T cells, and preliminary work suggests that AIM2 plays a role in the regulation of apoptosis following T cell receptor crosslinking

Focus: Two-dimensional electron-electron double resonance and molecular motions: The challenge of higher frequencies.

The development, applications, and current challenges of the pulsed ESR technique of two-dimensional Electron-Electron Double Resonance (2D ELDOR) are described. This is a three-pulse technique akin to 2D Exchange Nuclear Magnetic Resonance, but involving electron spins, usually in the form of spin-probes or spin-labels. As a result, it required the extension to much higher frequencies, i.e., microwaves, and much faster time scales, with π/2 pulses in the 2-3 ns range. It has proven very useful for studying molecular dynamics in complex fluids, and spectral results can be explained by fitting theoretical models (also described) that provide a detailed analysis of the molecular dynamics and structure. We discuss concepts that also appear in other forms of 2D spectroscopy but emphasize the unique advantages and difficulties that are intrinsic to ESR. Advantages include the ability to tune the resonance frequency, in order to probe different motional ranges, while challenges include the high ratio of the detection dead time vs. the relaxation times. We review several important 2D ELDOR studies of molecular dynamics. (1) The results from a spin probe dissolved in a liquid crystal are followed throughout the isotropic → nematic → liquid-like smectic → solid-like smectic → crystalline phases as the temperature is reduced and are interpreted in terms of the slowly relaxing local structure model. Here, the labeled molecule is undergoing overall motion in the macroscopically aligned sample, as well as responding to local site fluctuations. (2) Several examples involving model phospholipid membranes are provided, including the dynamic structural characterization of the boundary lipid that coats a transmembrane peptide dimer. Additionally, subtle differences can be elicited for the phospholipid membrane phases: liquid disordered, liquid ordered, and gel, and the subtle effects upon the membrane, of antigen cross-linking of receptors on the surface of plasma membrane, vesicles can be observed. These 2D ELDOR experiments are performed as a function of mixing time, Tm, i.e., the time between the second and third π/2 pulses, which provides a third dimension. In fact, a fourth dimension may be added by varying the ESR frequency/magnetic field combination. Therefore, (3) it is shown how continuous-wave multifrequency ESR studies enable the decomposition of complex dynamics of, e.g., proteins by virtue of their respective time scales. These studies motivate our current efforts that are directed to extend 2D ELDOR to higher frequencies, 95 GHz in particular (from 9 and 17 GHz), in order to enable multi-frequency 2D ELDOR. This required the development of quasi-optical methods for performing the mm-wave experiments, which are summarized. We demonstrate state-of-the-art 95 GHz 2D ELDOR spectroscopy through its ability to resolve the two signals from a spin probe dissolved in both the lipid phase and the coexisting aqueous phase. As current 95 GHz experiments are restricted by limited spectral coverage of the π/2 pulse, as well as the very short T2 relaxation times of the electron spins, we discuss how these limitations are being addressed.

Galectin-3 Shapes Antitumor Immune Responses by Suppressing CD8+ T Cells via LAG-3 and Inhibiting Expansion of Plasmacytoid Dendritic Cells.

Galectin-3 is a 31-kDa lectin that modulates T-cell responses through several mechanisms, including apoptosis, T-cell receptor (TCR) cross-linking, and TCR downregulation. We found that patients with pancreatic ductal adenocarcinoma (PDA) who responded to a granulocyte-macrophage colony-stimulating factor-secreting allogeneic PDA vaccine developed neutralizing antibodies to galectin-3 after immunization. We show that galectin-3 binds activated antigen-committed CD8(+) T cells only in the tumor microenvironment. Galectin-3-deficient mice exhibit improved CD8(+) T-cell effector function and increased expression of several inflammatory genes. Galectin-3 binds to LAG-3, and LAG-3 expression is necessary for galectin-3-mediated suppression of CD8(+) T cells in vitro. Lastly, galectin-3-deficient mice have elevated levels of circulating plasmacytoid dendritic cells, which are superior to conventional dendritic cells in activating CD8(+) T cells. Thus, inhibiting galectin-3 in conjunction with CD8(+) T-cell-directed immunotherapies should enhance the tumor-specific immune response.

Roles of Ca(2+)/calmodulin-dependent protein kinase II in subcellular expression of striatal N-methyl-D-aspartate receptors in l-3, 4-dihydroxyphenylalanine-induced dyskinetic rats.

BackgroundThe role of N-Methyl-D-aspartate (NMDA) receptors is critical to the development of l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia (LID) in Parkinson’s disease (PD). Ca2+/calmodulin-dependent protein kinase II (CaMKII) is thought to regulate the expression and activation of NMDA receptors in LID, but the interaction between LID and CaMKII-modulated NMDA receptor activity is not clear so far.MethodsWe used 6-hydroxydopamine-lesioned rats to create PD rat model, and at least 21 days of l-DOPA was administrated followed with or without microinjection of CaMKII inhibitor KN-93 into the lesioned striatum of all the PD rats and sham rats. A surface receptor cross-linking assay was used to distinguish expression of striatal NMDA receptors in surface and intracellular compartments.Resultsl-DOPA treatment enhanced surface levels of GluN1 expression and reduced its intracellular expression, but did not change total levels of GluN1 protein in the lesioned striatum. In contrast, l-DOPA decreased GluN2A surface expression but increased its intracellular expression. l-DOPA increased GluN2B expression preferentially in the surface compartment. We also found that l-DOPA increased CaMKII autophosphorylation at T286 in striatal neurons. The inhibition of CaMKII by microinjecting CaMKII inhibitor KN-93 into the lesioned striatum largely reversed the l-DOPA-induced changes in three subunits. In addition, dyskinetic behaviors of animals were observed alleviated after treatment of KN-93.ConclusionOur research indicates that long-term l-DOPA administration activates CaMKII in striatal neurons. Activated CaMKII is involved at least in part in mediating l-DOPA-induced changes of NMDA receptors surface/intracellular expression.

Application of cultured human mast cells (CHMC) for the design and structure-activity relationship of IgE-mediated mast cell activation inhibitors.

Here we report the optimization of small molecule inhibitors of human mast cell degranulation via anti-IgE-mediated tryptase release following cross-linking and activation of IgE-loaded FcεR1 receptors. The compounds are selective upstream inhibitors of FcεR1-dependent human mast cell degranulation and proved to be devoid of activity in downstream ionomycin mediated degranulation. Structure-activity relationship (SAR) leading to compound 26 is outlined.

Zinc-binding metallothioneins are key modulators of IL-4 production by basophils

Zinc (Zn) is an essential nutrient, and Zn deficiency causes immunodeficiency and skin disorders. Basophils express FcɛRI on their surface and release multiple mediators after receptor cross-linking, including large amounts of IL-4. However, the mechanisms involved in the FcɛRI-mediated regulation of basophil IL-4 production are currently unclear. Here, we show that the Zn-binding metallothionein (MT) proteins are essential for the FcɛRI-induced basophil production of IL-4. Basophils from MT-I/II(−/−) mice produced significantly less FcɛRI-induced IL-4 than did wild-type basophils. The MTs were involved in maintaining intracellular Zn levels, thereby regulated the calcineurin activity and nuclear factor of activated T-cell (NFAT)-mediated IL-4 production. These results suggest that the MT-dependent control of Zn homeostasis is a novel mechanism for regulating basophil IL-4 production.

Generation of high-titre virus stocks using BrK.219, a B-cell line infected stably with recombinant Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is a gamma-2-lymphotropic human oncogenic herpesvirus associated with Kaposi's sarcoma (KS) and two B-cell lymphoproliferative diseases, primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD). KSHV establishes latency soon after infection in vivo and in vitro. Consequently, it is technically difficult to generate high-titre virus stocks required for infection experiments in tissue culture. Currently used methods of KSHV stock production involve induction of the lytic/productive cycle in PEL cell lines or in adherent cell lines harbouring recombinant KSHV genomes. In this study, the BJAB-derived B-cell line BrK.219, which is infected latently with a recombinant KSHV (rKSHV.219), is used to produce high-titre virus stocks. BrK.219 cells enter the lytic KSHV replication cycle upon cross-linking of B-cell receptors (BCRs) with anti-IgM antibodies without the need for additional, potentially toxic chemical inducers. High cell concentrations can be cultured and induced easily in spinner flasks, saving time and resources. The established protocol allows the generation of KSHV virus stocks with titres of up to 106IU/ml in unconcentrated culture supernatants, representing a 103–104-fold improvement compared to conventional methods.

Interleukin-33 exacerbates allergic bronchoconstriction in the mice via activation of mast cells.

Interleukin-33 (IL-33) is implicated as an epithelium-derived danger signal promoting Th2-dependent responses in asthma. We hypothesized that IL-33 might also have direct effects on mast cell-driven allergic airway obstruction. The effects of IL-33 on allergic responses in the airways of sensitized mice were assessed both in vivo and ex vivo, as well as on cultured mast cells in vitro. In vivo, the allergen-induced increase in resistance in the conducting airways was enhanced in mice pretreated with IL-33. Also, in the isolated airways, the allergen-induced contractions were increased in preparations from animals subjected to intranasal IL-33 pretreatment. These effects in vivo and ex vivo were blocked by the 5-HT2A receptor antagonist ketanserin and absent in mice without mast cells. Likewise, the IL-33-induced enhancement of the allergen response was absent in isolated airways from mice lacking the IL-33 receptor. Moreover, exposure to IL-33 increased secretion of serotonin from allergen-challenged isolated airways. In cultured mast cells, IL-33 enhanced the expression of tryptophan hydroxylase 1, serotonin synthesis, and storage, as well as the secretion of serotonin following IgE receptor cross-linking. These results demonstrate that IL-33 exacerbates allergic bronchoconstriction by increasing synthesis, storage, and secretion of serotonin from the mast cell. This mechanism has implications for the development of airway obstruction in asthma.

Interferon-α induces altered transitional B cell signaling and function in Systemic Lupus Erythematosus

Previous studies suggest that the B cells of patients with Systemic Lupus Erythematosus (SLE) are hyper-responsive to BCR crosslinking; however, it has been unclear whether this is the result of altered B cell signaling or differences in various B cell subpopulations in SLE patients as compared to healthy controls. Here we have developed a novel Phosflow technique that permits examination of cell signaling in distinct B cell subpopulations stratified based upon developmental stage and cell surface IgM levels, which we use to show that the naïve B cells of SLE patients are hyper-responsive to IgM receptor crosslinking, resulting in increased SYK phosphorylation. We further demonstrate that this hyper-responsiveness is most marked in the transitional B cell subset and that it is associated with altered function, resulting in decreased apoptosis and increased proliferation of these cells. Examination of repeated samples from the same patients revealed that the hyper-responsiveness fluctuated over time, suggesting that it may be mediated by pro-inflammatory factors rather than genetic variations between patients. In support of this concept, incubation of healthy control B cells with IFN-α or SLE plasma induced the hyper-responsive phenotype, which was blocked by anti-IFN-α antibody. Furthermore, no obvious correlation was seen between genetic variants that are proposed to alter BCR signaling and the increased SYK phosphorylation. The findings suggest that pro-inflammatory factors, in particular Type I IFNs, modulate B cell function in SLE in a way that could contribute to the breach of tolerance in this condition.

Damnacanthal inhibits IgE receptor-mediated activation of mast cells

Damnacanthal, an anthraquinone obtained from the noni fruit (Morinda citrifolia L.), has been described to possess anti-cancer and anti-inflammatory properties. Since mast cells are key players in various inflammatory conditions as well as in cancer, we considered the possibility that the biological actions of damnacanthal, at least partly, could be due to effects on mast cells. Many of the biological activities of mast cells are mediated by IgE receptor cross-linking, which results in degranulation with release of preformed granule mediators, as well as de novo synthesis and release of additional compounds. Here we show that damnacanthal has profound inhibitory activity on mast cell activation through this pathway. The release of the granule compounds beta-hexosaminidase and tryptase release was completely abrogated by damnacanthal at doses that were non-toxic to mast cells. In addition, damnacanthal inhibited activation-dependent pro-inflammatory gene induction, as well as cytokine/chemokine release in response to mast cell stimulation. The mechanism underlying damnacanthal inhibition was linked to impaired phosphorylation of Syk and Akt. Furthermore, damnacanthal inhibited mast cell activation in response to calcium ionophore A23187. Altogether, the data presented here demonstrate that damnacanthal inhibits mast cell activation induced by different stimuli and open a new window for the use of this compound as a mast cell stabilizer.

The major birch pollen allergen Bet v 1 induces different responses in dendritic cells of birch pollen allergic and healthy individuals.

Dendritic cells play a fundamental role in shaping the immune response to allergens. The events that lead to allergic sensitization or tolerance induction during the interaction of the major birch pollen allergen Bet v 1 and dendritic cells are not very well studied. Here, we analyzed the uptake of Bet v 1 and the cross-reactive celery allergen Api g 1 by immature monocyte-derived dendritic cells (iMoDCs) of allergic and normal donors. In addition, we characterized the allergen-triggered intracellular signaling and transcriptional events. Uptake kinetics, competitive binding, and internalization pathways of labeled allergens by iMoDCs were visualized by live-cell imaging. Surface-bound IgE was detected by immunofluorescence microscopy and flow cytometry. Allergen- and IgE-induced gene expression of early growth response genes and Th1 and Th2 related cytokines and chemokines were analyzed by real-time PCR. Phosporylation of signaling kinases was analyzed by Western blot. Internalization of Bet v 1 by iMoDCs of both donor groups, likely by receptor-mediated caveolar endocytosis, followed similar kinetics. Bet v 1 outcompeted Api g 1 in cell surface binding and uptake. MoDCs of allergic and healthy donors displayed surface-bound IgE and showed a pronounced upregulation of Th2 cytokine- and NFκB-dependent genes upon non-specific Fcε receptor cross-linking. In contrast to these IgE-mediated responses, Bet v 1-stimulation increased transcript levels of the Th2 cytokines IL-4 and IL-13 but not of NFκB-related genes in MoDCs of BP allergic donors. Cells of healthy donors were either unresponsive or showed elevated mRNA levels of Th1-promoting chemokines. Moreover, Bet v 1 was able to induce Erk1/2 and p38 MAPK activation in BP allergics but only a slight p38 activation in normal donors. In conclusion, our data indicate that Bet v 1 favors the activation of a Th2 program only in DCs of BP allergic individuals.

Protein A suppresses immune responses during Staphylococcus aureus bloodstream infection in guinea pigs.

ABSTRACT Staphylococcus aureus infection is not associated with the development of protective immunity, and disease relapses occur frequently. We hypothesize that protein A, a factor that binds immunoglobulin Fcγ and cross-links VH3 clan B cell receptors (IgM), is the staphylococcal determinant for host immune suppression. To test this, vertebrate IgM was examined for protein A cross-linking. High VH3 binding activity occurred with human and guinea immunoglobulin, whereas mouse and rabbit immunoglobulins displayed little and no binding, respectively. Establishing a guinea pig model of S. aureus bloodstream infection, we show that protein A functions as a virulence determinant and suppresses host B cell responses. Immunization with SpAKKAA, which cannot bind immunoglobulin, elicits neutralizing antibodies that enable guinea pigs to develop protective immunity.Importance Staphylococcus aureus is the leading cause of soft tissue and bloodstream infections; however, a vaccine with clinical efficacy is not available. Using mice to model staphylococcal infection, earlier work identified protective antigens; however, corresponding human clinical trials did not reach their endpoints. We show that B cell receptor (IgM) cross-linking by protein A is an important immune evasion strategy of S. aureus that can be monitored in a guinea pig model of bloodstream infection. Further, immunization with nontoxigenic protein A enables infected guinea pigs to elicit antibody responses that are protective against S. aureus. Thus, the guinea pig model may support preclinical development of staphylococcal vaccines.

Molecular and cellular basis for immunomodulation with monoclonal antibodies

Immunotherapy with immunostimulatroy monoclonal antibodies is based on the ability of antibodies to act as antagonists of inhibitory receptors or to act as agonists of costimulatory receptors. Interference of ligand to receptor binding underlies the first function while crosslinking of receptors to enforce signaling involves the second mode of action. Signalling receptors on lymphocytes and antigen presenting cells are embedded in the plasma membrane and are functionally linked to adaptors whose function is conducive to intracelluar signaling events. In the case of antibodies blocking CTLA-4 functions multiple mechanism of action take place: (i) efficient competition for costimulatory interactions of CD28 with the very same ligands (CD80 and CD86), (ii) functional expression on the surface of regulatory T cells that can be depleted by ADCC, (iii) prevention of the denudation by cooption of CD80 molecules from the surface of antigen presenting cells, a(iv) interference with putative negative signalling mediated by phosphatases. The restricted pattern of expression of CTLA-4 on Tregs and activated T cells is critical to understand its function and exploitation in immunotherapy. PD-1 upon interaction with PD-L1 or PD-L2 drives to the immune synapse the tyrosine phosphatases SHP-1 and SHP-2. These phosphatases, that are recruited to the cytoplasmic tail of PD-1 through its phosphorylated ITIM motive, dephosphorylate critical tyrosine residues in signaling adaptors activated by the concerted action of the TCR and CD28. Disruption of signalling in the immune synapse is prevented by blocking antibodies that avoid the guidance of PD-1 moieties to the synapse guided by PD-L1. Avoiding disruption of productive immune synapses is probably the main mechanism of action of anti-PD-1 and anti-PD-L1 blocking mAb. The costimulatory members of the TNFR family transduce costimulatory signals by recruitment of the adptors TRAF-1, -2, -5. Signal transduction enhances NF-κB and MAPK signaling routes. In this pathway K-63 polyubiquitination reactions are critical for early signaling and are regulated by specific deubiquitinases. Receptors signal depending on crosslinking by the agonist antibody. The pattern of expression of these receptors on the different subset resting or primed/activated T cells determines function. Overall precise understanding of the pattern of distributions of the receptors on immune cells and the signaling events that these receptors prevent or elicit will be ultimately important not only to understand the function but also for both therapeutic manipulation and biomarker finding.

Effect of Thanatophoric Dysplasia Type I Mutations on Fibroblast Growth Factor Receptor 3 Dimerization

Thanatophoric dysplasia type I (TDI) is a lethal human skeletal growth disorder with a prevalence of 1 in 20,000 to 1 in 50,000 births. TDI is known to arise due to five different mutations, all involving the substitution of an amino acid with a cysteine in fibroblast growth factor receptor 3 (FGFR3). Cysteine mutations in receptor tyrosine kinases have been previously proposed to induce receptor cross-linking in the unliganded state, thus emulating the effect of ligand and leading to receptor overactivation. Here, we characterize the effect of three TDI mutations, Arg248Cys, Ser249Cys, and Tyr373Cys, on FGFR3 dimerization in mammalian membranes, in the absence of ligand. We demonstrate that the mutations lead to modest stabilization and structural perturbations of the FGFR3 dimers. Based on available FGFR crystal structures, we argue that the effects of these mutations cannot emulate the effect of the ligand, thus challenging the current understanding of the molecular interactions that underlie TDI.

Cross-talk between human mast cells and bronchial epithelial cells in plasminogen activator inhibitor-1 production via transforming growth factor-β1.

Previous reports suggest that plasminogen activator inhibitor-1 (PAI-1) promotes airway remodeling and that human and mouse mast cells (MCs) are an important source of PAI-1. In the present study we investigated MC-epithelial cell (EC) interactions in the production of PAI-1. We stimulated the human MC line LAD2 with IgE-receptor cross-linking and collected the supernatants. We incubated the human bronchial EC line BEAS-2B with the LAD2 supernatants and measured the level of PAI-1. When the supernatants from IgE-stimulated LAD2 were added to BEAS-2B, there was a significant enhancement of PAI-1 production by BEAS-2B. When we treated the MC supernatants with a transforming growth factor (TGF)-β1 neutralizing antibody, the MC-derived induction of PAI-1 from BEAS-2B was completely abrogated. Although TGF-β1 mRNA was constitutively expressed in resting LAD2, it was not highly induced by IgE-mediated stimulation. Nonetheless, active TGF-β1 protein was significantly increased in LAD2 after IgE-mediated stimulation. Active TGF-β1 produced by primary cultured human MCs was significantly reduced in the presence of a chymase inhibitor, suggesting a role of MC chymase as an activator of latent TGF-β1. This study indicates that stimulation of human MCs by IgE receptor cross-linking triggers activation of TGF-β1, at least in part via chymase, which in turn induces the production of PAI-1 by bronchial ECs. Our data suggest that human MCs may play an important role in airway remodeling in asthma as a direct source of PAI-1 and by activating bronchial ECs to produce further PAI-1 via a TGF-β1-mediated activation pathway.

Blinking of Quantum Dot Probes in Single Membrane Molecule Rotation Measurements

Asymmetric quantum dots (QD) provide non-bleaching probes of the rotation of individual cell surface proteins. We have conducted imaging measurements of individual 2H3 cell Type I Fce receptor rotation on timescales down to 10 ms per frame. We have also used time-tagged single photon counting measurements of individual QD to examine μs timescales, though such rapid timescales are limited by QD emission rates. In both approaches we calculate the time-autocorrelation functions (TACF) for fluorescence polarization fluctuations. Decays of these fluctuations extend into the ms timescale, as implied by time-resolved phosphorescence anisotropy results. Effects of cell treatments such as receptor crosslinking suggest that this slow decay may be a property of the membrane itself, perhaps reflecting large-scale fluctuations of mesoscale membrane regions. However, depending on instrumental parameters used in data analysis, polarization fluctuation TACFs can contain a contribution from the intensity fluctuation TACF arising from QD blinking. Such QD blinking feed-through is extremely sensitive to these analysis parameters which effectively change slightly from one measurement to another. We discuss approaches based on the necessary statistical independence of polarization and intensity fluctuations to guarantee removal of a blinking-based component from rotation measurements. Supported by NSF grant MCB-1024668.

MAVS, cGAS, and endogenous retroviruses in T-independent B cell responses.

Multivalent molecules with repetitive structures including bacterial capsular polysaccharides and viral capsids elicit antibody responses through B cell receptor (BCR) crosslinking in the absence of T cell help. We report that immunization with these T cell-independent type 2 (TI-2) antigens causes up-regulation of endogenous retrovirus (ERV) RNAs in antigen-specific mouse B cells. These RNAs are detected via a mitochondrial antiviral signaling protein (MAVS)-dependent RNA sensing pathway or reverse-transcribed and detected via the cGAS-cGAMP-STING pathway, triggering a second, sustained wave of signaling that promotes specific immunoglobulin M production. Deficiency of both MAVS and cGAS, or treatment of MAVS-deficient mice with reverse transcriptase inhibitors, dramatically inhibits TI-2 antibody responses. These findings suggest that ERV and two innate sensing pathways that detect them are integral components of the TI-2 B cell signaling apparatus.

C3d adjuvant effects are mediated through the activation of C3d-specific autoreactive T cells.

Complement fragment C3d covalently attached to antigens enhances immune responses, particularly for antigens lacking T cell epitopes. Enhancement has been attributed to receptor cross-linking between complement receptor CR2 (CD21) and polysaccharide antigen to surface IgM on naïve B cells. Paradoxically, C3d has still been shown to increase immune responses in CD21 KO mice, suggesting that an auxiliary activation pathway exists. In prior studies, we demonstrated the CD21-independent C3d adjuvant effect might be due to T cell recognition of C3d T helper epitopes processed and presented by MHC class II on the B cell surface. C3d peptide sequences containing concentrated clusters of putative human C3 T cell epitopes were identified using the epitope-mapping algorithm, EpiMatrix. These peptide sequences were synthesized and shown in vitro to bind multiple HLA-DR alleles with high affinity, and induce IFNγ responses in healthy donor PBMCs. In the present studies, we establish further correlations between HLA binding and HLA-specific lymphocyte reactions with select epitope clusters. Additionally, we show that the T cell phenotype of C3d-specific reactive T cells is CD4+CD45RO+ memory T cells. Finally, mutation of a single T cell epitope residing within the P28 peptide segment of C3d resulted in significantly diminished adjuvant activity in BALB/c mice. Collectively, these studies support the hypothesis that the paradoxical enhancement of immune responses by C3d in the absence of CD21 is due to internalization and processing of C3d into peptides that activate autoreactive CD4+ T helper cells in the context of HLA class II.