A computational study for understanding the impact of p120-catenin on the cis-dimerization of cadherin

A prototype of cross-membrane signal transduction is that extracellular binding of cell surface receptors to their ligands induces intracellular signalling cascades. However, much less is known about the process in the opposite direction, called inside-out signalling. Recent studies show that it plays a more important role in regulating the functions of many cell surface receptors than we used to think. In particular, in cadherin-mediated cell adhesion, recent experiments indicate that intracellular binding of the scaffold protein p120-catenin (p120ctn) can promote extracellular clustering of cadherin and alter its adhesive function. The underlying mechanism, however, is not well understood. To explore possible mechanisms, we designed a new multiscale simulation procedure. Using all-atom molecular dynamics simulations, we found that the conformational dynamics of the cadherin extracellular region can be altered by the intracellular binding of p120ctn. More intriguingly, by integrating all-atom simulation results into coarse-grained random sampling, we showed that the altered conformational dynamics of cadherin caused by the binding of p120ctn can increase the probability of lateral interactions between cadherins on the cell surface. These results suggest that p120ctn could allosterically regulate the cis-dimerization of cadherin through two mechanisms. First, p120ctn controls the extracellular conformational dynamics of cadherin. Second, p120ctn oligomerization can further promote cadherin clustering. Therefore, our study provides a mechanistic foundation for the inside-out signalling in cadherin-mediated cell adhesion, while the computational framework can be generally applied to other cross-membrane signal transduction systems.

Improving Antibody Therapeutics by Manipulating the Fc Domain: Immunological and Structural Considerations

Interactions between the crystallizable fragment (Fc) domain of antibodies and a plethora of cellular Fc receptors (FcRs) or soluble proteins form a critical link between humoral and innate immunity. In particular, the immunoglobulin G Fc domain is critical for the clearance of target cells by processes that include (a) cytotoxicity, phagocytosis, or complement lysis; (b) modulation of inflammation; (c) antigen presentation; (d) antibody-mediated receptor clustering; and (e) cytokine release. More than 30 Fc-engineered antibodies aimed primarily at tailoring these effects for optimal therapeutic outcomes are in clinical evaluation or have already been approved. Nonetheless, our understanding of how FcR engagement impacts various immune cell phenotypes is still largely incomplete. Recent insights into FcR biology coupled with advances in Fc:FcR structural analysis, Fc engineering, and mouse models that recapitulate human biology are helping to fill in existing knowledge gaps. These advances will provide a blueprint on how to fine-tune the Fc domain to achieve optimal therapeutic efficacy. Expected final online publication date for the Annual Review of Biomedical Engineering, Volume 24 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Modified 5-aminolevulinic acid photodynamic therapy (M-PDT) inhibits cutaneous squamous cell carcinoma cell proliferation via targeting PP2A/PP5-mediated MAPK signaling pathway

BACKGROUND

We previously demonstrated that M-PDT is painless and effective in precancerous skin diseases treatment. However, whether M-PDT is effective in cSCC and the underlying inhibitory mechanism remains enigmatic.

OBJECTIVE

We aims to unveil the effect of M-PDT on cSCC cell proliferation and the regulatory effect of M-PDT on MAPK signaling.

METHODS

The proliferation and migration of cSCC cells were revealed by CCK8 assay, tumor sphere formation assay and scratch assay respectively. The expression of MAPKs was examined by western blot. The activity of PP2A and PP5 was regulated by inhibitor and recombinant adenoviruses.

RESULTS

Here, we show that M-PDT inhibits cSCC cell proliferation by activating p-JNK, p-p38 and inhibiting p-Erk1/2, as well as activation of PP2A and inactivation of PP5. Furthermore, pharmacological inhibition of PP2A conferred resistance to M-PDT's suppression on p-Erk1/2 and attenuated inhibitory effects of M-PDT on cell proliferation whereas overexpression of wild-type PP2A showed the contrary results. Pharmacological inhibition of PP5 potentiated M-PDT's elevation on p-JNK and strengthened inhibitory effects of M-PDT on cell proliferation whereas overexpression of wild-type PP5 exhibited the contrary results.

CONCLUSION

Our findings indicate that M-PDT inhibits cSCC cell proliferation via targeting PP2A/PP5-mediated MAPK signaling pathway.

Immunomodulation: Immunoglobulin Preparations Suppress Hyperinflammation in a COVID-19 Model via FcγRIIA and FcαRI

The rapid spread of SARS-CoV-2 has induced a global pandemic. Severe forms of COVID-19 are characterized by dysregulated immune response and "cytokine storm". The role of IgG and IgM antibodies in COVID-19 pathology is reasonably well studied, whereas IgA is neglected. To improve clinical outcome of patients, immune modulatory drugs appear to be beneficial. Such drugs include intravenous immunoglobulin preparations, which were successfully tested in severe COVID-19 patients. Here we established a versatile in vitro model to study inflammatory as well as anti-inflammatory processes by therapeutic human immunoglobulins. We dissect the inflammatory activation on neutrophil-like HL60 cells, using an immune complex consisting of latex beads coated with spike protein of SARS-CoV-2 and opsonized with specific immunoglobulins from convalescent plasma. Our data clarifies the role of Fc-receptor-dependent phagocytosis via IgA-FcαRI and IgG-FcγR for COVID-19 disease followed by cytokine release. We show that COVID-19 associated inflammation could be reduced by addition of human immunoglobulin preparations (IVIG and trimodulin), while trimodulin elicits stronger immune modulation by more powerful ITAMi signaling. Besides IgG, the IgA component of trimodulin in particular, is of functional relevance for immune modulation in this assay setup, highlighting the need to study IgA mediated immune response.

IgA and FcαRI: Versatile Players in Homeostasis, Infection, and Autoimmunity

Mucosal surfaces constitute the frontiers of the body and are the biggest barriers of our body for the outside world. Immunoglobulin A (IgA) is the most abundant antibody class present at these sites. It passively contributes to mucosal homeostasis via immune exclusion maintaining a tight balance between tolerating commensals and providing protection against pathogens. Once pathogens have succeeded in invading the epithelial barriers, IgA has an active role in host-pathogen defense by activating myeloid cells through divers receptors, including its Fc receptor, FcαRI (CD89). To evade elimination, several pathogens secrete proteins that interfere with either IgA neutralization or FcαRI-mediated immune responses, emphasizing the importance of IgA-FcαRI interactions in preventing infection. Depending on the IgA form, either anti- or pro-inflammatory responses can be induced. Moreover, the presence of excessive IgA immune complexes can result in continuous FcαRI-mediated activation of myeloid cells, potentially leading to severe tissue damage. On the one hand, enhancing pathogen-specific mucosal and systemic IgA by vaccination may increase protective immunity against infectious diseases. On the other hand, interfering with the IgA-FcαRI axis by monovalent targeting or blocking FcαRI may resolve IgA-induced inflammation and tissue damage. This review describes the multifaceted role of FcαRI as immune regulator between anti- and pro-inflammatory responses of IgA, and addresses potential novel therapeutic strategies that target FcαRI in disease.

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Polymorphisms in Fc Gamma Receptors and Susceptibility to Malaria in an Endemic Population

Repeated infections by Plasmodium falciparum result in a humoral response that could reduce disease symptoms and prevent the development of clinical malaria. The principal mechanism underlying this humoral response is that immunoglobulin G (IgG) binds directly to the parasites, thus causing their neutralization. However, the action of antibodies alone is not always sufficient to eliminate pathogens from an organism. One key element involved in the recognition of IgG that plays a crucial role in the destruction of the parasites responsible for spreading malaria is the family of Fc gamma receptors. These receptors are expressed on the surface of immune cells. Several polymorphisms have been detected in the genes encoding these receptors, associated with susceptibility or resistance to malaria in different populations. In this review, we describe identified polymorphisms within the family of Fc gamma receptors and the impact of these variations on the response of a host to infection as well as provide new perspectives for the design of an effective vaccine for malaria.

Inside-Out Control of Fc-Receptors

Receptors recognizing the Fc-part of immunoglobulins (FcR) are important in the engagement of phagocytes with opsonized micro-organisms, but they also play a major role in the pathogenesis of chronic inflammatory diseases. Different FcRs are specifically recognizing and binding the different classes of immunoglobulins, transmitting different signals into the cell. The function of IgG (FcγR's) and IgA (FcαR) recognizing receptors is controlled by cellular signals evoked by activation of heterologous receptors in a process generally referred to as inside-out control. This concept is clearly described for the regulation of integrin receptors. Inside-out control can be achieved at different levels by modulation of: (i) receptor affinity, (ii) receptor avidity/valency, (iii) interaction with signaling chains, (iv) interaction with other receptors and (v) localization in functionally different membrane domains. The inside-out control of FcRs is an interesting target for novel therapy by therapeutical antibodies as it can potentiate or decrease the functionality of the response to the antibodies depending on the mechanisms of the diseases they are applied for.

Antibody-Dependent Cellular Phagocytosis in Antiviral Immune Responses

Antiviral activities of antibodies may either be dependent only on interactions between the antibody and cognate antigen, as in binding and neutralization of an infectious virion, or instead may require interactions between antibody-antigen immune complexes and immunoproteins or Fc receptor expressing immune effector cells. These Fc receptor-dependent antibody functions provide a direct link between the innate and adaptive immune systems by combining the potent antiviral activity of innate effector cells with the diversity and specificity of the adaptive humoral response. The Fc receptor-dependent function of antibody-dependent cellular phagocytosis (ADCP) provides mechanisms for clearance of virus and virus-infected cells, as well as for stimulation of downstream adaptive immune responses by facilitating antigen presentation, or by stimulating the secretion of inflammatory mediators. In this review, we discuss the properties of Fc receptors, antibodies, and effector cells that influence ADCP. We also provide and interpret evidence from studies that support a potential role for ADCP in either inhibiting or enhancing viral infection. Finally, we describe current approaches used to measure antiviral ADCP and discuss considerations for the translation of studies performed in animal models. We propose that additional investigation into the role of ADCP in protective viral responses, the specific virus epitopes targeted by ADCP antibodies, and the types of phagocytes and Fc receptors involved in ADCP at sites of virus infection will provide insight into strategies to successfully leverage this important immune response for improved antiviral immunity through rational vaccine design.

FcαRI Dynamics Are Regulated by GSK-3 and PKCζ During Cytokine Mediated Inside-Out Signaling

IgA binding to FcαRI (CD89) is rapidly enhanced by cytokine induced inside-out signaling. Dephosphorylation of serine 263 in the intracellular tail of FcαRI by PP2A and PI3K activation are instrumental in this process. To further investigate these signaling pathways, we targeted downstream kinases of PI3K. Our experiments revealed that PI3K activates PKCζ, which subsequently inhibits GSK-3, a constitutively active kinase in resting cells and found here to be associated with FcαRI. We propose that GSK-3 maintains FcαRI in an inactive state at homeostatic conditions. Upon cytokine stimulation, GSK-3 is inactivated through a PI3K-PKCζ pathway, preventing the maintenance of phosphorylated inactive FcαRI. The concomitantly activated PP2A is then able to dephosphorylate and activate FcαRI. Moreover, FRAP and FLIP studies showed that FcαRI activation coincides with an increased mobile fraction of the receptor. This can enhance FcαRI valency and contribute to stronger avidity for IgA immune complexes. This tightly regulated inside-out signaling pathway allows leukocytes to respond rapidly and efficiently to their environment and could be exploited to enhance the efficacy of future IgA therapeutics.

Mechanisms of inside-out signaling of the high-affinity IgG receptor FcγRI

Fc receptors (FcRs) are an important bridge between the innate and adaptive immune system. Fc gamma receptor I (FcγRI; CD64), the high-affinity receptor for immunoglobulin G (IgG), plays roles in inflammation, autoimmune responses, and immunotherapy. Stimulation of myeloid cells with cytokines, such as tumor necrosis factor-α ( TNFα) and interferon-γ ( IFNγ), increases the binding of FcγRI to immune complexes (ICs), such as antibody-opsonized pathogens or tumor cells, through a process known as "inside-out" signaling. Using super-resolution imaging, we found that stimulation of cells with IL-3 also enhanced the clustering of FcγRI both before and after exposure to ICs. This increased clustering was dependent on an intact actin cytoskeleton. We found that chemical inhibition of the activity of the phosphatase PP1 reduced FcγRI inside-out signaling, although the phosphorylation of FcγRI itself was unaffected. Furthermore, the antibody-dependent cytotoxic activity of human neutrophils toward CD20-expressing tumor cells was increased after stimulation with TNFα and IFNγ. These results suggest that nanoscale reorganization of FcγRI, stimulated by cytokine-induced, inside-out signaling, enhances FcγRI cellular effector functions.

Rapamycin inhibits BAFF-stimulated cell proliferation and survival by suppressing mTOR-mediated PP2A-Erk1/2 signaling pathway in normal and neoplastic B-lymphoid cells

B-cell activating factor (BAFF) is involved in not only physiology of normal B cells, but also pathophysiology of aggressive B cells related to malignant and autoimmune diseases. Rapamycin, a lipophilic macrolide antibiotic, has recently shown to be effective in the treatment of human lupus erythematosus. However, how rapamycin inhibits BAFF-stimulated B-cell proliferation and survival has not been fully elucidated. Here, we show that rapamycin inhibited human soluble BAFF (hsBAFF)-induced cell proliferation and survival in normal and B-lymphoid (Raji and Daudi) cells by activation of PP2A and inactivation of Erk1/2. Pretreatment with PD98059, down-regulation of Erk1/2, expression of dominant negative MKK1, or overexpression of wild-type PP2A potentiated rapamycin's suppression of hsBAFF-activated Erk1/2 and B-cell proliferation/viability, whereas expression of constitutively active MKK1, inhibition of PP2A by okadaic acid, or expression of dominant negative PP2A attenuated the inhibitory effects of rapamycin. Furthermore, expression of a rapamycin-resistant and kinase-active mTOR (mTOR-T), but not a rapamycin-resistant and kinase-dead mTOR-T (mTOR-TE), conferred resistance to rapamycin's effects on PP2A, Erk1/2 and B-cell proliferation/viability, implying mTOR-dependent mechanism involved. The findings indicate that rapamycin inhibits BAFF-stimulated cell proliferation/survival by targeting mTOR-mediated PP2A-Erk1/2 signaling pathway in normal and neoplastic B-lymphoid cells. Our data highlight that rapamycin may be exploited for preventing excessive BAFF-induced aggressive B-cell malignancies and autoimmune diseases.

Unique ligand-binding property of the human IgM Fc receptor

The IgM Fc receptor (FcμR) is the newest FcR, and coligation of FcμR and Fas/CD95 on Jurkat cells with agonistic IgM anti-Fas mAb was shown to inhibit Fas-induced apoptosis. The ligand-binding activity of human FcμR was further examined. FcμR-mediated protection from apoptosis was partially blocked by addition of 10(4) molar excess of IgM or its soluble immune complexes, but it could be inhibited by addition of 10-fold excess of IgM anti-CD2 mAb. This suggests that FcμR binds more efficiently to the Fc portion of IgM reactive with plasma-membrane proteins than to the Fc portion of IgM in solution. The former interaction occurred in cis on the same cell surface, but not in trans between neighboring cells. This cis engagement of FcμR resulted in modulation of Ca(2+) mobilization via CD2 on Jurkat cells or BCRs on blood B cells upon cross-linkage with the corresponding IgM mAbs. Several functional changes were observed with FcμR mutants: 1) significant increase in IgM ligand binding in the cytoplasmic tail-deletion mutant, 2) enhanced cap formation in FcμR upon IgM binding at 4°C with a point mutation of the transmembrane His to Phe, and 3) less protective activity of FcμR in IgM anti-Fas mAb-mediated apoptosis assays with a point mutation of the membrane-proximal Tyr to Phe. These findings show the importance of the cis engagement of FcμR and its critical role in receptor function. Hence, FcμR on B, T, and NK cells may modulate the function of surface proteins recognized by natural or immune IgM Abs on the shared membrane cell surface.

T cell receptor signaling can directly enhance the avidity of CD28 ligand binding

T cell activation takes place in the context of a spatial and kinetic reorganization of cell surface proteins and signaling molecules at the contact site with an antigen presenting cell, termed the immunological synapse. Coordination of the activation, recruitment, and signaling from T cell receptor (TCR) in conjunction with adhesion and costimulatory receptors regulates both the initiation and duration of signaling that is required for T cell activation. The costimulatory receptor, CD28, is an essential signaling molecule that determines the quality and quantity of T cell immune responses. Although the functional consequences of CD28 engagement are well described, the molecular mechanisms that regulate CD28 function are largely unknown. Using a micropipet adhesion frequency assay, we show that TCR signaling enhances the direct binding between CD28 and its ligand, CD80. Although CD28 is expressed as a homodimer, soluble recombinant CD28 can only bind ligand monovalently. Our data suggest that the increase in CD28-CD28 binding is mediated through a change in CD28 valency. Molecular dynamic simulations and in vitro mutagenesis indicate that mutations at the base of the CD28 homodimer interface, distal to the ligand-binding site, can induce a change in the orientation of the dimer that allows for bivalent ligand binding. When expressed in T cells, this mutation allows for high avidity CD28–CD80 interactions without TCR signaling. Molecular dynamic simulations also suggest that wild type CD28 can stably adopt a bivalent conformation. These results support a model whereby inside-out signaling from the TCR can enhance CD28 ligand interactions by inducing a change in the CD28 dimer interface to allow for bivalent ligand binding and ultimately the transduction of CD28 costimulatory signals that are required for T cell activation.

BAFF activates Erk1/2 promoting cell proliferation and survival by Ca2+-CaMKII-dependent inhibition of PP2A in normal and neoplastic B-lymphoid cells

B-cell activating factor (BAFF) is involved in not only the physiology of normal B cells, but also the pathophysiology of aggressive B cells related to malignant and autoimmune diseases. However, how excessive BAFF promotes aggressive B-cell proliferation and survival is not well understood. Here we show that excessive human soluble BAFF (hsBAFF) enhanced cell proliferation and survival in normal and B-lymphoid (Raji) cells, which was associated with suppression of PP2A, resulting in activation of Erk1/2. This is supported by the findings that pretreatment with U0126 or PD98059, expression of dominant negative MKK1, or overexpression of PP2A prevented hsBAFF-induced activation of Erk1/2 and cell proliferation/viability in the cells. It appears that hsBAFF-mediated PP2A-Erk1/2 pathway and B-cell proliferation/viability was Ca(2+)-dependent, as pretreatment with BAPTA/AM, EGTA or 2-APB significantly attenuated these events. Furthermore, we found that inhibiting CaMKII with KN93 or silencing CaMKII also attenuated hsBAFF-mediated PP2A-Erk1/2 signaling and B-cell proliferation/viability. The results indicate that BAFF activates Erk1/2, in part through Ca(2+)-CaMKII-dependent inhibition of PP2A, increasing cell proliferation/viability in normal and neoplastic B-lymphoid cells. Our data suggest that inhibitors of CaMKII and Erk1/2, activator of PP2A or manipulation of intracellular Ca(2+) may be exploited for prevention of excessive BAFF-induced aggressive B-cell malignancies and autoimmune diseases.

Antibodies and their receptors: different potential roles in mucosal defense

Over recent years it has become increasingly apparent that mucosal antibodies are not only restricted to the IgM and IgA isotypes, but that also other isotypes and particularly IgG can be found in significant quantities at some mucosal surfaces, such as in the genital tract. Their role is more complex than traditionally believed with, among other things, the discovery of novel function of mucosal immunoglobulin receptors. A thorough knowledge in the source and function and mucosal immunoglobulins is particularly important in development of vaccines providing mucosal immunity, and also in the current climate of microbicide development, to combat major world health issues such as HIV. We present here a comprehensive review of human antibody mediated mucosal immunity.

Characterization of a mutated IgA2 antibody of the m(1) allotype against the epidermal growth factor receptor for the recruitment of monocytes and macrophages

IgA antibodies constitute an important part of the mucosal immune system, but their immunotherapeutic potential remains rather unexplored, in part due to biotechnological issues. For example, the IgA2m(1) allotype carries an unusual heavy and light chain pairing, which may confer production and stability concerns. Here, we report the generation and the biochemical and functional characterization of a P221R-mutated IgA2m(1) antibody against the epidermal growth factor receptor (EGFR). Compared with wild type, the mutated antibody demonstrated heavy chains covalently linked to light chains in monomeric as well as in joining (J)-chain containing dimeric IgA. Functional studies with wild type and mutated IgA2m(1) revealed similar binding to EGFR and direct effector functions such as EGFR down-modulation and growth inhibition. Furthermore, both IgA molecules triggered similar levels of indirect tumor cell killing such as antibody-dependent cell-mediated cytotoxicity (ADCC) by isolated monocytes, activated polymorphonuclear cells, and human whole blood. Interestingly, the dimeric IgA antibodies demonstrated higher efficiency in direct as well as in indirect effector mechanisms compared with their respective monomeric forms. Both wild type and mutated antibody triggered effective FcαRI-mediated tumor cell killing by macrophages already at low effector to target cell ratios. Interestingly, also polarized macrophages mediated significant IgA2-mediated ADCC. M2 macrophages, which have been described as promoting tumor growth and progression, may convert to ADCC-mediating effector cells in the presence of EGFR-directed antibodies. In conclusion, these results provide further insight into the immunotherapeutic potential of recombinant IgA antibodies for tumor immunotherapy and suggest macrophages as an additional effector cell population.

The human immunoglobulin A Fc receptor FcαRI: a multifaceted regulator of mucosal immunity

Immunoglobulin A (IgA) is commonly recognized as the most prevalent antibody (Ab) at mucosal sites with an important role in defense by shielding mucosal surfaces from invasion by pathogens. However, its potential to both actively dampen excessive immune responses or to initiate potent proinflammatory cellular processes is less well known. Interestingly, either functional outcome is mediated through interaction with the myeloid IgA Fc receptor FcαRI (CD89). Monomeric interaction of IgA with FcαRI triggers inhibitory signals that block activation via other receptors, whereas multimeric FcαRI crosslinking induces phagocytosis, reactive oxygen species production, antigen presentation, Ab-dependent cellular cytotoxicity, and cytokine release. Thus, FcαRI acts as a regulator between anti- and proinflammatory responses of IgA. As such, the biology of FcαRI, and its multifaceted role in immunity will be the focus of this review.

Serine phosphorylation of FcγRI cytoplasmic domain directs lipid raft localization and interaction with protein 4.1G

The high-affinity IgG receptor (CD64, FcγRI) has several special capacities, including the receptor-stimulated cleavage of the cell surface B cell-activating factor of the TNF superfamily (TNFSF13B). With the use of the yeast two-hybrid system, we and others have shown that FcγRI interacts with protein 4.1G (EPB41L2). Our mutational analyses identified two required 4.1G-interacting regions in the FcγRI CY and one FcγRI-interacting site in the C-terminus of protein 4.1G. Herein, we explore mechanism(s) that may regulate the interaction between protein 4.1G and FcγRI CY and influence FcγRI membrane mobility and function. We show that FcγRI CY interacts with protein 4.1G in vitro and that FcγRI coimmunoprecipitates protein 4.1G in freshly isolated human PBMC. With the use of immunostaining, we show that FcγRI colocalizes with protein 4.1G in unstimulated U937 cells, in which the FcγRI CY is constitutively serine-phosphorylated, but significant uncoupling occurs following FcγRI cross-linking, suggesting phosphoserine-regulated interaction. In vitro, protein 4.1G interacted preferentially with CK2-phosphorylated FcγRI CY, and compared with WT FcγRI, a nonphosphorylatable FcγRI mutant receptor was excluded from lipid rafts, suggesting a key role for protein 4.1G in targeting phosphorylated FcγRI to rafts. These data are consistent with a phosphoserine-dependent tethering role for protein 4.1G in maintaining FcγRI in lipid rafts and provide insight into the unique phosphoserine-based regulation of receptor signaling by FcγRI CY.

Immunoglobulin A: A next generation of therapeutic antibodies?

Although immunoglobulin (Ig) A is commonly recognized as the most prevalent antibody subclass at mucosal sites with an important role in mucosal defense, its potential as a therapeutic monoclonal antibody is less well known. However, IgA has multifaceted anti-, non-, and pro-inflammatory functions that can be exploited for different immunotherapeutical strategies, which will be the focus of this review.

Cytokine-induced immune complex binding to the high-affinity IgG receptor, FcγRI, in the presence of monomeric IgG

FcγRI is the sole high-affinity immunoglobulin G (IgG) receptor on leukocytes. Its role in immunity and the clearance of opsonized particles has been challenged, as the receptor function may well be hindered by serum IgG. Here, we document immune complex binding by FcγRI to be readily enhanced by cytokine stimulation, whereas binding of monomeric IgG only modestly increased. Enhanced immune complex binding was independent of FcγRI surface expression levels. FcγRI, saturated with prebound IgG, was found capable of effective immune complex binding upon cytokine stimulation. Cytokine-enhanced binding was observed across a variety of immune complexes, including huIgG3- or mIgG2a-opsonized red blood cells, rituximab- or ofatumumab-opsonized B-cell lymphoma, and cetuximab-opsonized glioblastoma cells. This study contributes to our understanding of how FcγRI can participate in the clearance of opsonized particles despite saturation by monomeric IgG.

A monomeric chicken IgY receptor binds IgY with 2:1 stoichiometry

IgY is the principal serum antibody in birds and reptiles, and an IgY-like molecule was the evolutionary precursor of both mammalian IgG and IgE. A receptor for IgY on chicken monocytes, chicken leukocyte receptor AB1 (CHIR-AB1), lies in the avian leukocyte receptor cluster rather than the classical Fc receptor cluster where the genes for mammalian IgE and IgG receptors are found. IgG and IgE receptors bind to the lower hinge region of their respective antibodies with 1:1 stoichiometry, whereas the myeloid receptor for IgA, FcalphaRI, and the IgG homeostasis receptor, FcRn, which are found in the mammalian leukocyte receptor cluster, bind with 2:1 stoichiometry between the heavy chain constant domains 2 and 3 of each heavy chain. In this paper, the extracellular domain of CHIR-AB1 was expressed in a soluble form and shown to be a monomer that binds to IgY-Fc with 2:1 stoichiometry. The two binding sites have similar affinities: K(a)(1) = 7.22 +/- 0.22 x 10(5) m(-1) and K(a)(2) = 3.63 +/- 1.03 x 10(6) m(-1) (comparable with the values reported for IgA binding to its receptor). The affinity constants for IgY and IgY-Fc binding to immobilized CHIR-AB1 are 9.07 +/- 0.07 x 10(7) and 6.11 +/- 0.02 x 10(8) m(-1), respectively, in agreement with values obtained for IgY binding to chicken monocyte cells and comparable with reported values for human IgA binding to neutrophils. Although the binding site for CHIR-AB1 on IgY is not known, the data reported here with a monomeric receptor binding to IgY at two sites with low affinity suggest an IgA-like interaction.