Toxoplasma replication is inhibited by MMV676477 without development of resistance

Protozoan parasites cause life-threatening infections in both humans and animals, including agriculturally significant livestock. Available treatments are typically narrow spectrum and are complicated by drug toxicity and the development of resistant parasites. Protozoan tubulin is an attractive target for the development of broad-spectrum antimitotic agents. The Medicines for Malaria Pathogen Box compound MMV676477 was previously shown to inhibit replication of kinetoplastid parasites, such as Leishmania amazonensis and Trypanosoma brucei, and the apicomplexan parasite Plasmodium falciparum by selectively stabilizing protozoan microtubules. In this report, we show that MMV676477 inhibits intracellular growth of the human apicomplexan pathogen Toxoplasma gondii with an EC50 value of ~50 nM. MMV676477 does not stabilize vertebrate microtubules or cause other toxic effects in human fibroblasts. The availability of tools for genetic studies makes Toxoplasma a useful model for studies of the cytoskeleton. We conducted a forward genetics screen for MMV676477 resistance, anticipating that missense mutations would delineate the binding site on protozoan tubulin. Unfortunately, we were unable to use genetics to dissect target interactions because no resistant parasites emerged. This outcome suggests that future drugs based on the MMV676477 scaffold would be less likely to be undermined by the emergence of drug resistance.

The Binding Properties of Antibodies to Z-DNA in the Sera of Normal Healthy Subjects

Antibodies to DNA are a diverse set of antibodies that bind sites on DNA, a polymeric macromolecule that displays various conformations. In a previous study, we showed that sera of normal healthy subjects (NHS) contain IgG antibodies to Z-DNA, a left-handed helix with a zig-zig backbone. Recent studies have demonstrated the presence of Z-DNA in bacterial biofilms, suggesting a source of this conformation to induce responses. To characterize further antibodies to Z-DNA, we used an ELISA assay with brominated poly(dGdC) as a source of Z-DNA and determined the isotype of these antibodies and their binding properties. Results of these studies indicate that NHS sera contain IgM and IgA as well as IgG anti-Z-DNA antibodies. As shown by the effects of ionic strength in association and dissociation assays, the anti-Z-DNA antibodies bind primarily by electrostatic interactions; this type of binding differs from that of induced anti-Z-DNA antibodies from immunized animals which bind by non-ionic interactions. Furthermore, urea caused dissociation of NHS anti-Z-DNA at molar concentrations much lower than those for the induced antibodies. These studies also showed IgA anti-Z-DNA antibodies in fecal water. Together, these studies demonstrate that antibodies to Z-DNA occur commonly in normal immunity and may arise as a response to Z-DNA of bacterial origin.

Expanding the Anti-Phl p 7 Antibody Toolkit: An Anti-Idiotype Nanobody Inhibitor

We have previously produced a toolkit of antibodies, comprising recombinant human antibodies of all but one of the human isotypes, directed against the polcalcin family antigen Phl p 7. In this work, we complete the toolkit of human antibody isotypes with the IgD version of the anti-Phl p 7 monoclonal antibody. We also raised a set of nanobodies against the IgD anti-Phl p 7 antibody and identify and characterize one paratope-specific nanobody. This nanobody also binds to the IgE isotype of this antibody, which shares the same idiotype, and orthosterically inhibits the interaction with Phl p 7. The 2.1 Å resolution X-ray crystal structure of the nanobody in complex with the IgD Fab is described.

IgG3 subclass antibodies recognize antigenically drifted influenza viruses and SARS-CoV-2 variants through efficient bivalent binding

The constant domains of antibodies are important for effector functions, but less is known about how they can affect binding and neutralization of viruses. Here, we evaluated a panel of human influenza virus monoclonal antibodies (mAbs) expressed as IgG1, IgG2, or IgG3. We found that many influenza virus-specific mAbs have altered binding and neutralization capacity depending on the IgG subclass encoded and that these differences result from unique bivalency capacities of the subclasses. Importantly, subclass differences in antibody binding and neutralization were greatest when the affinity for the target antigen was reduced through antigenic mismatch. We found that antibodies expressed as IgG3 bound and neutralized antigenically drifted influenza viruses more effectively. We obtained similar results using a panel of SARS-CoV-2-specific mAbs and the antigenically advanced B.1.351 and BA.1 strains of SARS-CoV-2. We found that a licensed therapeutic mAb retained neutralization breadth against SARS-CoV-2 variants when expressed as IgG3, but not IgG1. These data highlight that IgG subclasses are not only important for fine-tuning effector functionality but also for binding and neutralization of antigenically drifted viruses.

M2e-specific antibodies protect against influenza PR8 virus in an isotype and route dependent manner

The ectodomain of influenza matrix protein 2 (M2e) is a promising target for the development of universal prophylactic and therapeutic agents against influenza viruses of different subtypes. We constructed three M2e-specific monoclonal antibody variants, M2A1-1 (IgG1), M2A1-2a (IgG2a), M2A1-2b (IgG2b), which have the same Fab region targeting the M2e epitope but different isotypes, and compared their protective efficacy in influenza PR8-infected mice. We found that anti-M2e antibodies provided protection against influenza virus in a subtype-dependent manner, with the IgG2a variant providing significantly better protection with lower virus titers and milder lung injury than IgG1 and IgG2b isotypes. Additionally, we observed that the protective efficacy was dependent on the administration routes, with intranasal administration of antibody providing better protection than intraperitoneal administration. The timing of administration was also critical in determining the protective efficacy; while all the antibody isotypes provided protection when administered before influenza challenge, only IgG2a provided minimal protection when the antibodies were administered after virus challenge. These results provide valuable information for optimizing the therapeutics usage of M2e-based antibodies and furthering the development of M2e-based universal influenza vaccines.

Antibodies of the immunoglobulin a isotype to novel antigens in early axial spondyloarthritis

Introduction

There is an unmet need for biomarkers to identify patients with axial spondyloarthritis (axSpA). Increasing evidence suggest the presence of autoantibodies in a subset of axSpA patients. The aim of this study was to identify novel IgA antibodies in early axSpA patients and to determine their diagnostic potential in combination with previously determined IgG antibodies against UH (Hasselt University)-axSpA-IgG antigens.

Methods

An axSpA cDNA phage display library constructed from axSpA hip synovium, was used to screen for novel IgA antibodies in plasma from early axSpA patients. The presence of these antibodies against novel UH-axSpA-IgA antigens was determined in two independent axSpA cohorts, in healthy controls and in patients with chronic low back pain.

Results

We identified antibodies to 7 novel UH-axSpA-IgA antigens, of which 6 correspond to non-physiological peptides and 1 to the human histone deacetylase 3 (HDAC3) protein. IgA antibodies against 2 of these 7 novel UH-axSpA-IgA antigens and IgG antibodies against 2 of the previously identified antigens were significantly more present in early axSpA patients from the UH cohort (18/70, 25.7%) and the (Bio)SPAR cohort (26/164, 15.9%), compared to controls with chronic low back pain (2/66, 3%). Antibodies to this panel of 4 antigens were present in 21.1% (30/142) of patients with early axSpA from the UH and (Bio)SPAR cohorts. The positive likelihood ratio for confirming early axSpA using antibodies to these 4 UH-axSpA antigens was 7.0. So far, no clinical correlation between the novel identified IgA antibodies and inflammatory bowel disease could be identified.

Discussion

In conclusion, screening an axSpA cDNA phage display library for IgA reactivity resulted in the identification of 7 novel UH-axSpA-IgA antigens, of which 2 show promising biomarker potential for the diagnosis of a subset of axSpA patients, in combination with previously identified UH-axSpA-IgG antigens.

A Zika virus-specific IgM elicited in pregnancy exhibits ultrapotent neutralization

Congenital Zika virus (ZIKV) infection results in neurodevelopmental deficits in up to 14% of infants born to ZIKV-infected mothers. Neutralizing antibodies are a critical component of protective immunity. Here, we demonstrate that plasma IgM contributes to ZIKV immunity in pregnancy, mediating neutralization up to 3 months post-symptoms. From a ZIKV-infected pregnant woman, we isolated a pentameric ZIKV-specific IgM (DH1017.IgM) that exhibited ultrapotent ZIKV neutralization dependent on the IgM isotype. DH1017.IgM targets an envelope dimer epitope within domain II. The epitope arrangement on the virion is compatible with concurrent engagement of all ten antigen-binding sites of DH1017.IgM, a solution not available to IgG. DH1017.IgM protected mice against viremia upon lethal ZIKV challenge more efficiently than when expressed as an IgG. Our findings identify a role for antibodies of the IgM isotype in protection against ZIKV and posit DH1017.IgM as a safe and effective candidate immunotherapeutic, particularly during pregnancy.

Antibody class-switching as a strategy to improve HIV-1 neutralization

Broadly neutralizing antibodies (bNAbs), when administered through passive immunization, are protective against HIV-1 infection. Current HIV-1 vaccine strategies are aimed at guiding the immune system to make bNAbs by mimicking their development during infection. Somatic hypermutation of the variable region is known to be crucial for the development of bNAbs. More recently, however, studies have shown how class-switch recombination (CSR) resulting in the generation of different antibody isotypes may serve as an additional mechanism through which antibodies can gain neutralization breadth and potency. In this review, we discuss the importance of different antibody isotypes for HIV-1 neutralization breadth and potency and how this information can be leveraged to improve passive and active immunization against HIV-1.

SARS-CoV-2 Serosurveys: How Antigen, Isotype and Threshold Choices Affect the Outcome

BACKGROUND

Evaluating the performance of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) serological assays and clearly articulating the utility of selected antigens, isotypes, and thresholds is crucial to understanding the prevalence of infection within selected communities.

METHODS

This cross-sectional study, implemented in 2020, screened PCRconfirmed coronavirus disease 2019 patients (n 86), banked prepandemic and negative samples (n 96), healthcare workers and family members (n 552), and university employees (n 327) for antiSARS-CoV-2 receptor-binding domain, trimeric spike protein, and nucleocapsid protein immunoglobulin (Ig)G and IgA antibodies with a laboratory-developed enzyme-linked immunosorbent assay and tested how antigen, isotype and threshold choices affected the seroprevalence outcomes. The following threshold methods were evaluated: (i) mean 3 standard deviations of the negative controls; (ii) 100 specificity for each antigen-isotype combination; and (iii) the maximal Youden index.

RESULTS

We found vastly different seroprevalence estimates depending on selected antigens and isotypes and the applied threshold method, ranging from 0.0 to 85.4. Subsequently, we maximized specificity and reported a seroprevalence, based on more than one antigen, ranging from 9.3 to 25.9.

CONCLUSIONS

This study revealed the importance of evaluating serosurvey tools for antigen-, isotype-, and threshold-specific sensitivity and specificity, to interpret qualitative serosurvey outcomes reliably and consistently across studies.

Lineage tracing reveals B cell antibody class switching is stochastic, cell-autonomous, and tuneable

To optimize immunity to pathogens, B lymphocytes generate plasma cells with functionally diverse antibody isotypes. By lineage tracing single cells within differentiating B cell clones, we identified the heritability of discrete fate controlling mechanisms to inform a general mathematical model of B cell fate regulation. Founder cells highly influenced clonal plasma-cell fate, whereas class switch recombination (CSR) was variegated within clones. In turn, these CSR patterns resulted from independent all-or-none expression of both activation-induced cytidine deaminase (AID) and IgH germline transcription (GLT), with the latter being randomly re-expressed after each cell division. A stochastic model premised on these molecular transition rules accurately predicted antibody switching outcomes under varied conditions in vitro and during an immune response in vivo. Thus, the generation of functionally diverse antibody types follows rules of autonomous cellular programming that can be adapted and modeled for the rational control of antibody classes for potential therapeutic benefit.

IgA rheumatoid factor in rheumatoid arthritis

OBJECTIVES

Rheumatoid factor (RF) is a well-established marker for the diagnosis and classification of rheumatoid arthritis (RA). Most studies evaluated IgM RF or isotype-nonspecific total RF assays. We evaluated the added value of IgA RF in this context.

METHODS

An international sample cohort consisting of samples from 398 RA patients and 1073 controls was tested for IgA RF with 3 commercial assays. For all RA patients and 100 controls essential clinical and serological data for ACR/EULAR classification were available.

RESULTS

The sensitivity of IgA RF for diagnosing RA was lower than the sensitivity of IgM RF. Differences in numerical values between IgA RF assays were observed. With all assays, the highest IgA RF values were found in patients with primary Sjögren's syndrome. Double positivity for IgM RF and IgA RF had a higher specificity for RA than either IgM RF or IgA RF. The sensitivity of double positivity was lower than the sensitivity of either IgA RF or IgM RF. Single positivity for IgA RF was at least as prevalent in controls than in RA patients. Adding IgA RF to IgM RF and anti-citrullinated protein antibodies (ACPA) did not affect RA classification. However, combined positivity for IgA RF, IgM RF and IgG ACPA had a higher specificity and lower sensitivity for RA classification than positivity for either of the antibodies.

CONCLUSIONS

IgA RF showed a lower sensitivity than IgM RF. Combining IgA RF with IgM RF and ACPA did not improve sensitivity of RA classification. Combined positivity (IgA-RF/IgM-RF/ACPA) increased specificity.

Regulation of Tubulin Gene Expression: From Isotype Identity to Functional Specialization

Genomes of higher eukaryotes encode a large tubulin gene superfamily consisting of at least six α and six β-tubulin isotypes. While some α and β-tubulin isotypes are ubiquitously expressed, others are cell-type specific. The subset of α and β-tubulins that is expressed in a given cell type is defined transcriptionally. But the precise mechanisms of how cells choose which α and β isotypes to express and at what level remain poorly understood. Differential expression of tubulin isotypes is particularly prominent during development and in specialized cells, suggesting that some isotypes are better suited for certain cell type-specific functions. Recent studies begin to rationalize this phenomenon, uncovering important differences in tubulin isotype behavior and their impact on the biomechanical properties of the microtubule cytoskeleton. I summarize our understanding of the regulation of tubulin isotype expression, focusing on the role of these complex regulatory pathways in building a customized microtubule network best suited for cellular needs.

Isotype-specific plasma cells express divergent transcriptional programs

Antibodies are produced across multiple isotypes with distinct properties that coordinate initial antigen clearance and confer long-term antigen-specific immune protection. Here, we interrogate the molecular programs of isotype-specific murine plasma cells (PC) following helper T cell-dependent immunization and within established steady-state immunity. We developed a single-cell-indexed and targeted molecular strategy to dissect conserved and divergent components of the rapid effector phase of antigen-specific IgM+ versus inflammation-modulating programs dictated by type 1 IgG2a/b+ PC differentiation. During antibody affinity maturation, the germinal center (GC) cycle imparts separable programs for post-GC type 2 inhibitory IgG1+ and type 1 inflammatory IgG2a/b+ PC to direct long-term cellular function. In the steady state, two subsets of IgM+ and separate IgG2b+ PC programs clearly segregate from splenic type 3 IgA+ PC programs that emphasize mucosal barrier protection. These diverse isotype-specific molecular pathways of PC differentiation control complementary modules of antigen clearance and immune protection that could be selectively targeted for immunotherapeutic applications and vaccine design.

A computational method for predicting the aggregation propensity of IgG1 and IgG4(P) mAbs in common storage buffers

The propensity for some monoclonal antibodies (mAbs) to aggregate at physiological and manufacturing pH values can prevent their use as therapeutic molecules or delay time to market. Consequently, developability assessments are essential to select optimum candidates, or inform on mitigation strategies to avoid potential late-stage failures. These studies are typically performed in a range of buffer solutions because factors such as pH can dramatically alter the aggregation propensity of the test mAbs (up to 100-fold in extreme cases). A computational method capable of robustly predicting the aggregation propensity at the pH values of common storage buffers would have substantial value. Here, we describe a mAb aggregation prediction tool (MAPT) that builds on our previously published isotype-dependent, charge-based model of aggregation. We show that the addition of a homology model-derived hydrophobicity descriptor to our electrostatic aggregation model enabled the generation of a robust mAb developability indicator. To contextualize our aggregation scoring system, we analyzed 97 clinical-stage therapeutic mAbs. To further validate our approach, we focused on six mAbs (infliximab, tocilizumab, rituximab, CNTO607, MEDI1912 and MEDI1912_STT) which have been reported to cover a large range of aggregation propensities. The different aggregation propensities of the case study molecules at neutral and slightly acidic pH were correctly predicted, verifying the utility of our computational method.

IgA Plasma Cells Are Long-Lived Residents of Gut and Bone Marrow That Express Isotype- and Tissue-Specific Gene Expression Patterns

Antibody secreting plasma cells are made in response to a variety of pathogenic and commensal microbes. While all plasma cells express a core gene transcription program that allows them to secrete large quantities of immunoglobulin, unique transcriptional profiles are linked to plasma cells expressing different antibody isotypes. IgA expressing plasma cells are generally thought of as short-lived in mucosal tissues and they have been understudied in systemic sites like the bone marrow. We find that IgA+ plasma cells in both the small intestine lamina propria and the bone marrow are long-lived and transcriptionally related compared to IgG and IgM expressing bone marrow plasma cells. IgA+ plasma cells show signs of shared clonality between the gut and bone marrow, but they do not recirculate at a significant rate and are found within bone marrow plasma cells niches. These data suggest that systemic and mucosal IgA+ plasma cells are from a common source, but they do not migrate between tissues. However, comparison of the plasma cells from the small intestine lamina propria to the bone marrow demonstrate a tissue specific gene transcription program. Understanding how these tissue specific gene networks are regulated in plasma cells could lead to increased understanding of the induction of mucosal versus systemic antibody responses and improve vaccine design.

Characterising the Phenotypic Diversity of Antigen-Specific Memory B Cells Before and After Vaccination

The diversity of B cell subsets and their contribution to vaccine-induced immunity in humans are not well elucidated but hold important implications for rational vaccine design. Prior studies demonstrate that B cell subsets distinguished by immunoglobulin (Ig) isotype expression exhibit divergent activation-induced fates. Here, the antigen-specific B cell response to tetanus toxoid (TTd) booster vaccination was examined in healthy adults, using a dual-TTd tetramer staining flow cytometry protocol. Unsupervised analyses of the data revealed that prior to vaccination, IgM-expressing CD27⁺ B cells accounted for the majority of TTd-binding B cells. 7 days following vaccination, there was an acute expansion of TTd-binding plasmablasts (PB) predominantly expressing IgG, and a minority expressing IgA or IgM. Frequencies of all PB subsets returned to baseline at days 14 and 21. TTd-binding IgG⁺ and IgA⁺ memory B cells (MBC) exhibited a steady and delayed maximal expansion compared to PB, peaking in frequencies at day 14. In contrast, the number of TTd-binding IgM⁺IgD⁺CD27⁺ B cells and IgM-only CD27⁺ B cells remain unchanged following vaccination. To examine TTd-binding capacity of IgG⁺ MBC and IgM⁺IgD⁺CD27⁺ B cells, surface TTd-tetramer was normalised to expression of the B cell receptor-associated CD79b subunit. CD79b-normalised TTd binding increased in IgG⁺ MBC, but remained unchanged in IgM⁺IgD⁺CD27⁺ B cells, and correlated with the functional affinity index of plasma TTd-specific IgG antibodies, following vaccination. Finally, frequencies of activated (PD-1⁺ICOS⁺) circulating follicular helper T cells (cT_FH), particularly of the CXCR3^-CCR6^- cT_FH2 cell phenotype, at their peak expansion, strongly predicted antigen-binding capacity of IgG⁺ MBC. These data highlight the phenotypic and functional diversity of the B cell memory compartment, in their temporal kinetics, antigen-binding capacities and association with cT_FH cells, and are important parameters for consideration in assessing vaccine-induced immune responses.

Hinge influences in murine IgG binding to Cryptococcus neoformans capsule

Decades of studies on antibody structure led to the tenet that the V region binds antigens while the C region interacts with immune effectors. In some antibodies, however, the C region affects affinity and/or specificity for the antigen. One example is the 3E5 monoclonal murine IgG family, in which the mIgG3 isotype has different fine specificity to the Cryptococcus neoformans capsule polysaccharide than the other mIgG isotypes despite their identical variable sequences. Our group serendipitously found another pair of mIgG1/mIgG3 antibodies based on the 2H1 hybridoma to the C. neoformans capsule that recapitulated the differences observed with 3E5. In this work, we report the molecular basis of the constant domain effects on antigen binding using recombinant antibodies. As with 3E5, immunofluorescence experiments show a punctate pattern for 2H1-mIgG3 and an annular pattern for 2H1-mIgG1; these binding patterns have been associated with protective efficacy in murine cryptococcosis. Also as observed with 3E5, 2H1-mIgG3 bound on ELISA to both acetylated and non-acetylated capsular polysaccharide, whereas 2H1-mIgG1 only bound well to the acetylated form, consistent with differences in fine specificity. In engineering hybrid mIgG1/mIgG3 antibodies, we found that switching the 2H1-mIgG3 hinge for its mIgG1 counterpart changed the immunofluorescence pattern to annular, but a 2H1-mIgG1 antibody with an mIgG3 hinge still had an annular pattern. The hinge is thus necessary but not sufficient for these changes in binding to the antigen. This important role for the constant region in antigen binding could affect antibody biology and engineering.

Isotype selection for antibody-based cancer therapy

The clinical application of monoclonal antibodies (mAbs) has revolutionized the field of cancer therapy, as it has enabled the successful treatment of previously untreatable types of cancer. Different mechanisms play a role in the anti-tumour effect of mAbs. These include blocking of tumour-specific growth factor receptors or of immune modulatory molecules as well as complement and cell-mediated tumour cell lysis. Thus, for many mAbs, Fc-mediated effector functions critically contribute to the efficacy of treatment. As immunoglobulin (Ig) isotypes differ in their ability to bind to Fc receptors on immune cells as well as in their ability to activate complement, they differ in the immune responses they activate. Therefore, the choice of antibody isotype for therapeutic mAbs is dictated by its intended mechanism of action. Considering that clinical efficacy of many mAbs is currently achieved only in subsets of patients, optimal isotype selection and Fc optimization during antibody development may represent an important step towards improved patient outcome. Here, we discuss the current knowledge of the therapeutic effector functions of different isotypes and Fc-engineering strategies to improve mAbs application.

Effects of gp120 Inner Domain (ID2) Immunogen Doses on Elicitation of Anti-HIV-1 Functional Fc-Effector Response to C1/C2 (Cluster A) Epitopes in Mice

Fc-mediated effector functions of antibodies, including antibody-dependent cytotoxicity (ADCC), have been shown to contribute to vaccine-induced protection from HIV-1 infection, especially those directed against non-neutralizing, CD4 inducible (CD4i) epitopes within the gp120 constant 1 and 2 regions (C1/C2 or Cluster A epitopes). However, recent passive immunization studies have not been able to definitively confirm roles for these antibodies in HIV-1 prevention mostly due to the complications of cross-species Fc–FcR interactions and suboptimal dosing strategies. Here, we use our stabilized gp120 Inner domain (ID2) immunogen that displays the Cluster A epitopes within a minimal structural unit of HIV-1 Env to investigate an immunization protocol that induces a fine-tuned antibody repertoire capable of an effective Fc-effector response. This includes the generation of isotypes and the enhanced antibody specificity known to be vital for maximal Fc-effector activities, while minimizing the induction of isotypes know to be detrimental for these functions. Although our studies were done in in BALB/c mice we conclude that when optimally titrated for the species of interest, ID2 with GLA-SE adjuvant will elicit high titers of antibodies targeting the Cluster A region with potent Fc-mediated effector functions, making it a valuable immunogen candidate for testing an exclusive role of non-neutralizing antibody response in HIV-1 protection in vaccine settings.

The Role of IgG Subclass in Antibody-Mediated Protection against Carbapenem-Resistant Klebsiella pneumoniae

Carbapenem-resistant Klebsiella pneumoniae is an urgent public health threat that causes life-threatening infections in immunocompromised hosts. Its resistance to nearly all antibiotics necessitates novel strategies to treat it, including the use of monoclonal antibodies. Monoclonal antibodies are emerging as important adjuncts to traditional pharmaceuticals, and studying how they protect against specific bacteria such as Klebsiella pneumoniae is crucial to their development as effective therapies. Antibody subclass is often overlooked but is a major factor in how an antibody interacts with other mediators of immunity. This paper is the first to examine how the subclass of anticapsular monoclonal antibodies can affect efficacy against CR-Kp. Additionally, this work sheds light on the viability of monoclonal antibody therapy in neutropenic patients, who are most vulnerable to CR-Kp infection.

Monoclonal antibodies (MAbs) have the potential to assist in the battle against multidrug-resistant bacteria such as carbapenem-resistant Klebsiella pneumoniae (CR-Kp). However, the characteristics by which these antibodies (Abs) function, such as the role of antibody subclass, must be determined before such modalities can be carried from the bench to the bedside. We performed a subclass switch on anticapsular monoclonal murine IgG₃ (mIgG₃) hybridomas and identified and purified a murine IgG₁ (mIgG₁) hybridoma line through sib selection. We then compared the ability of the mIgG₁ and mIgG₃ antibodies to control CR-Kp sequence type 258 (ST258) infection both in vitro and in vivo. We found by enzyme-limited immunosorbent assay (ELISA) and flow cytometry that mIgG₃ has superior binding to the CR-Kp capsular polysaccharide (CPS) and superior agglutinating ability compared to mIgG₁. The mIgG₃ also, predictably, had better complement-mediated serum bactericidal activity than the mIgG₁ and also promoted neutrophil-mediated killing at concentrations lower than that of the mIgG₁. In contrast, the mIgG₁ had marginally better activity in improving macrophage-mediated phagocytosis. Comparing their activities in a pulmonary infection model with wild-type as well as neutropenic mice, both antibodies reduced organ burden in a nonlethal challenge, regardless of neutrophil status, with mIgG₁ having the highest overall burden reduction in both scenarios. However, at a lethal inoculum, both antibodies showed reduced efficacy in neutropenic mice, with mIgG₃ retaining the most activity. These findings suggest the viability of monoclonal Ab adjunctive therapy in neutropenic patients that cannot mount their own immune response, while also providing some insight into the relative contributions of immune mediators in CR-Kp protection.

Electrostatic interactions modulate the differential aggregation propensities of IgG1 and IgG4P antibodies and inform charged residue substitutions for improved developability

Native state aggregation is an important concern in the development of therapeutic antibodies. Enhanced knowledge of mAb native state aggregation mechanisms would permit sequence-based selection and design of therapeutic mAbs with improved developability. We investigated how electrostatic interactions affect the native state aggregation of seven human IgG1 and IgG4P mAb isotype pairs, each pair having identical variable domains that are different for each set of IgG1 and IgG4P constructs. Relative aggregation propensities were determined at pH 7.4, representing physiological conditions, and pH 5.0, representing commonly used storage conditions. Our work indicates that the net charge state of variable domains relative to the net charge state of the constant domains is predominantly responsible for the different native state aggregation behavior of IgG1 and IgG4P mAbs. This observation suggests that the global net charge of a multi domain protein is not a reliable predictor of aggregation propensity. Furthermore, we demonstrate a design strategy in the frameworks of variable domains to reduce the native state aggregation propensity of mAbs identified as being aggregation-prone. Importantly, substitution of specifically identified residues with alternative, human germline residues, to optimize Fv charge, resulted in decreased aggregation potential at pH 5.0 and 7.4, thus increasing developability.

Adaption of human antibody λ and κ light chain architectures to CDR repertoires

Monoclonal antibodies bind with high specificity to a wide range of diverse antigens, primarily mediated by their hypervariable complementarity determining regions (CDRs). The defined antigen binding loops are supported by the structurally conserved β-sandwich framework of the light chain (LC) and heavy chain (HC) variable regions. The LC genes are encoded by two separate loci, subdividing the entity of antibodies into kappa (LC_κ) and lambda (LC_λ) isotypes that exhibit distinct sequence and conformational preferences. In this work, a diverse set of techniques were employed including machine learning, force field analysis, statistical coupling analysis and mutual information analysis of a non-redundant antibody structure collection. Thereby, it was revealed how subtle changes between the structures of LC_κ and LC_λ isotypes increase the diversity of antibodies, extending the predetermined restrictions of the general antibody fold and expanding the diversity of antigen binding. Interestingly, it was found that the characteristic framework scaffolds of κ and λ are stabilized by diverse amino acid clusters that determine the interplay between the respective fold and the embedded CDR loops. In conclusion, this work reveals how antibodies use the remarkable plasticity of the beta-sandwich Ig fold to incorporate a large diversity of CDR loops.

Impact of IgA isoforms on their ability to activate dendritic cells and to prime T cells

Human IgA could be from different isotypes (IgA1/IgA2) and/or isoforms (monomeric, dimeric, or secretory). Monomeric IgA mainly IgA1 are considered as an anti-inflammatory isotype whereas dimeric/secretory IgA have clearly dual pro- and anti-inflammatory effects. Here, we show that IgA isotypes and isoforms display different binding abilities to FcαRI, Dectin-1, DC-SIGN, and CD71 on monocyte-derived dendritic cells (moDC). We describe that IgA regulate the expression of their own receptors and trigger modulation of moDC maturation. We also demonstrate that dimeric IgA2 and IgA1 induce different inflammatory responses leading to cytotoxic CD8⁺ T cells activation. moDC stimulation by dimeric IgA2 was followed by a strong pro-inflammatory effect. Our study highlights differences regarding IgA isotypes and isoforms in the context of DC conditioning. Further investigations are needed on the activation of adaptive immunity by IgA in the context of microbiota/IgA complexes during antibody-mediated immune selection.

Historical H1N1 Influenza Virus Imprinting Increases Vaccine Protection by Influencing the Activity and Sustained Production of Antibodies Elicited at Vaccination in Ferrets

Influenza virus imprinting is now understood to significantly influence the immune responses and clinical outcome of influenza virus infections that occur later in life. Due to the yearly cycling of influenza viruses, humans are imprinted with the circulating virus of their birth year and subsequently build a complex influenza virus immune history. Despite this knowledge, little is known about how the imprinting strain influences vaccine responses. To investigate the immune responses of the imprinted host to split-virion vaccination, we imprinted ferrets with a sublethal dose of the historical seasonal H1N1 strain A/USSR/90/1977. After a +60-day recovery period to build immune memory, ferrets were immunized and then challenged on Day 123. Antibody specificity and recall were investigated throughout the time course. At challenge, the imprinted vaccinated ferrets did not experience significant disease, while naïve-vaccinated ferrets had significant weight loss. Haemagglutination inhibition assays showed that imprinted ferrets had a more robust antibody response post vaccination and increased virus neutralization activity. Imprinted-vaccinated animals had increased virus-specific IgG antibodies compared to the other experimental groups, suggesting B-cell maturity and plasticity at vaccination. These results should be considered when designing the next generation of influenza vaccines.

Programming Isotype-Specific Plasma Cell Function

Helper T cell induced plasma cells (PCs) that secrete class-switched neutralizing antibody are paramount to effective immunity. Following class-switch recombination (CSR), antigen-activated B cells differentiate into extrafollicular PCs or mature in germinal centers (GCs) to produce high-affinity memory B cells and follicular PCs. Many studies focus on the core transcriptional programs that drive central PC functions of longevity and antibody secretion. However, it is becoming clear that these central programs are further subdivided across antibody isotype with separable transcriptional trajectories. Divergent functions emerge at CSR, persist through PC terminal differentiation and further assort memory PC function following antigen recall. Here, we emphasize recent work that assorts divergent isotype-specific PC function across four major modules of immune protection.

Complex Interplay between Epitope Specificity and Isotype Dictates the Biological Activity of Anti-human CD40 Antibodies

Anti-CD40 monoclonal antibodies (mAbs) that promote or inhibit receptor function hold promise as therapeutics for cancer and autoimmunity. Rules governing their diverse range of functions, however, are lacking. Here we determined characteristics of nine hCD40 mAbs engaging epitopes throughout the CD40 extracellular region expressed as varying isotypes. All mAb formats were strong agonists when hyper-crosslinked; however, only those binding the membrane-distal cysteine-rich domain 1 (CRD1) retained agonistic activity with physiological Fc gamma receptor crosslinking or as human immunoglobulin G2 isotype; agonistic activity decreased as epitopes drew closer to the membrane. In addition, all CRD2-4 binding mAbs blocked CD40 ligand interaction and were potent antagonists. Thus, the membrane distal CRD1 provides a region of choice for selecting CD40 agonists while CRD2-4 provides antagonistic epitopes.

Therapeutic Antibodies: What Have We Learnt from Targeting CD20 and Where Are We Going?

Therapeutic monoclonal antibodies (mAbs) have become one of the fastest growing classes of drugs in recent years and are approved for the treatment of a wide range of indications, from cancer to autoimmune disease. Perhaps the best studied target is the pan B-cell marker CD20. Indeed, the first mAb to receive approval by the Food and Drug Administration for use in cancer treatment was the CD20-targeting mAb rituximab (Rituxan®). Since its approval for relapsed/refractory non-Hodgkin's lymphoma in 1997, rituximab has been licensed for use in the treatment of numerous other B-cell malignancies, as well as autoimmune conditions, including rheumatoid arthritis. Despite having a significant impact on the treatment of these patients, the exact mechanisms of action of rituximab remain incompletely understood. Nevertheless, numerous second- and third-generation anti-CD20 mAbs have since been developed using various strategies to enhance specific effector functions thought to be key for efficacy. A plethora of knowledge has been gained during the development and testing of these mAbs, and this knowledge can now be applied to the design of novel mAbs directed to targets beyond CD20. As we enter the "post-rituximab" era, this review will focus on the lessons learned thus far through investigation of anti-CD20 mAb. Also discussed are current and future developments relating to enhanced effector function, such as the ability to form multimers on the target cell surface. These strategies have potential applications not only in oncology but also in the improved treatment of autoimmune disorders and infectious diseases. Finally, potential approaches to overcoming mechanisms of resistance to anti-CD20 therapy are discussed, chiefly involving the combination of anti-CD20 mAbs with various other agents to resensitize patients to treatment.

Isotype Determination of Antibodies

On identifying a new monoclonal antibody, or in characterizing antibodies in sera evoked by disease or immunization, it is particularly informative to determine the serological class of the antibodies. The serological class of the protein is determined by the structure of the antibody constant region. Several methods of class or isotype determination are outlined in this unit: sandwich ELISA, electrophoresis, and immunofixation, or use of a variety of commercially available kits. Different purification schemes and approaches for enzymatic fragmentation of the antibodies depend on the class or isotype of the antibody, so this information streamlines these processes. © 2017 by John Wiley & Sons, Inc.

Human isotype-dependent inhibitory antibody responses against Mycobacterium tuberculosis

Accumulating evidence from experimental animal models suggests that antibodies play a protective role against tuberculosis (TB). However, little is known about the antibodies generated upon Mycobacterium tuberculosis (MTB) exposure in humans. Here, we performed a molecular and functional characterization of the human B‐cell response to MTB by generating recombinant monoclonal antibodies from single isolated B cells of untreated adult patients with acute pulmonary TB and from MTB‐exposed healthcare workers. The data suggest that the acute plasmablast response to MTB originates from reactivated memory B cells and indicates a mucosal origin. Through functional analyses, we identified MTB inhibitory antibodies against mycobacterial antigens including virulence factors that play important roles in host cell infection. The inhibitory activity of anti‐MTB antibodies was directly linked to their isotype. Monoclonal as well as purified serum IgA antibodies showed MTB blocking activity independently of Fc alpha receptor expression, whereas IgG antibodies promoted the host cell infection. Together, the data provide molecular insights into the human antibody response to MTB and may thereby facilitate the design of protective vaccination strategies.

TRIM21: a cytosolic Fc receptor with broad antibody isotype specificity

Antibodies are key molecules in the fight against infections. Although previously thought to mediate protection solely in the extracellular environment, recent research has revealed that antibody-mediated protection extends to the cytosolic compartment of cells. This postentry viral defense mechanism requires binding of the antibody to a cytosolic Fc receptor named tripartite motif containing 21 (TRIM21). In contrast to other Fc receptors, TRIM21 shows remarkably broad isotype specificity as it does not only bind IgG but also IgM and IgA. When viral pathogens coated with these antibody isotypes enter the cytosol, TRIM21 is rapidly recruited and efficient neutralization occurs before the virus has had the time to replicate. In addition, inflammatory signaling is induced. As such, TRIM21 acts as a cytosolic sensor that engages antibodies that have failed to protect against infection in the extracellular environment. Here, we summarize our current understanding of how TRIM21 orchestrates humoral immunity in the cytosolic environment.

Ig Constant Region Effects on Variable Region Structure and Function

The adaptive humoral immune response is responsible for the generation of antimicrobial proteins known as immunoglobulin molecules or antibodies. Immunoglobulins provide a defense system against pathogenic microbes and toxins by targeting them for removal and/or destruction. Historically, antibodies have been thought to be composed of distinct structural domains known as the variable and constant regions that are responsible for antigen binding and mediating effector functions such as opsonization and complement activation, respectively. These domains were thought to be structurally and functionally independent. Recent work has revealed however, that in some families of antibodies, the two regions can influence each other. We will discuss the body of work that led to these observations, as well as the mechanisms that have been proposed to explain how these two different antibody regions may interact in the function of antigen binding.

Inhibition of allergen-dependent IgE activity by antibodies of the same specificity but different class

IgG4 purified from patients undergoing specific allergen immunotherapy inhibits the activities of the serum IgE in in vitro assays and is thought to reduce the symptoms of the disease. However, it is not known whether this is related to an intrinsic property of this subclass or only the allergen specificity. We tested the hypothesis that allergen specificity is the critical determinant for this activity using a panel of antibodies with identical specificity but different subclasses. The different antibodies were all able to inhibit the activity of IgE to the same extent. We demonstrate that specificity is the dominant factor determining the ability of an antibody to block allergen-dependent IgE activity.

Accumulation of secreted antibodies in plant cell cultures varies according to the isotype, host species and culture conditions

Nicotiana tabacum suspension cells have been widely used to produce monoclonal antibodies, but the yield of secreted antibodies is usually low probably because of proteolytic degradation. Most IgGs that have been expressed in suspension cells have been of the human IgG1 isotype. In this study, we examined whether other isotypes displayed the same sensitivity to proteolytic degradation and whether the choice of plant host species mattered. Human serum IgG displayed different degradation profiles when incubated in spent culture medium from N. tabacum, Nicotiana benthamiana or Arabidopsis thaliana suspension cells. Zymography showed that the protease profile was host species dependent. Three human isotypes, IgG1, IgG2 and IgG4, and a mouse IgG2a were provided with the same heavy- and light-chain variable regions from an anti-human IgM antibody and expressed in N. tabacum cv. BY-2 and A. thaliana cv. Col-0 cells. Although all tested isotypes were detected in the extracellular medium using SDS-PAGE and a functional ELISA, up to 10-fold differences in the level of intact antibody were found according to the isotype expressed, to the host species and to the culture conditions. In the best combination (BY-2 cells secreting human IgG1), we reported accumulation of more than 30 mg/L of intact antibody in culture medium. The possibility of using IgG constant regions as a scaffold to allow stable accumulation of antibodies with different variable regions was demonstrated for human IgG2 and mouse IgG2a.

FcγRΙΙB controls the potency of agonistic anti-TNFR mAbs

Isotype plays a crucial role in therapeutic monoclonal antibody (mAb) function, mediated in large part through differences in Fcγ receptor (FcγR) interaction. Monoclonal Abs such as rituximab and alemtuzumab, which bind target cells directly, are designed for efficient recruitment of immune effector cells through their activatory FcγR engagement to mediate maximal target cell killing. In this setting, binding to inhibitory FcγRIIB is thought to inhibit function, making mAbs with high activatory/inhibitory (A/I) FcγR binding ratios, such as mouse IgG2a and human IgG1, the first choice for this role. In contrast, exciting new data show that agonistic mAbs directed against the tumour necrosis factor receptor superfamily member CD40 require interaction with FcγRIIB for in vivo function. Such ligation activates antigen-presenting cells, promotes myeloid and CTL responses and potentially stimulates effective anti-cancer immunity. It appears that the role of FcγRIIB is to mediate mAb hyper-crosslinking to allow CD40 downstream intracellular signalling. Previous work has shown that mAbs directed against other TNFR family members, Fas and death receptor 5 and probably death receptor 4, also require FcγRIIB hyper-crosslinking to promote target cell apoptosis, suggesting a common mechanism of action. In mouse models, IgG1 is optimal for these agents as it binds to FcγRIIB with tenfold higher affinity than IgG2a and hence has a relatively low A:I FcγR binding ratio. In contrast, human IgG isotypes have a universally low affinity for FcγRIIB, but in the case of human IgG1, engineering the Fc to increase its affinity for FcγRIIB can potentially overcome this problem. Thus, modifying the A/I binding ratio of human IgG Fc can be used to optimise different types of therapeutic activity by enhancing cytotoxic or hyper-crosslinking function.

Variable region identical immunoglobulins differing in isotype express different paratopes

The finding that the antibody (Ab) constant (C) region can influence fine specificity suggests that isotype switching contributes to the generation of Ab diversity and idiotype restriction. Despite the centrality of this observation for diverse immunological effects such as vaccine responses, isotype-restricted antibody responses, and the origin of primary and secondary responses, the molecular mechanism(s) responsible for this phenomenon are not understood. In this study, we have taken a novel approach to the problem by probing the paratope with (15)N label peptide mimetics followed by NMR spectroscopy and fluorescence emission spectroscopy. Specifically, we have explored the hypothesis that the C region imposes conformational constraints on the variable (V) region to affect paratope structure in a V region identical IgG(1), IgG(2a), IgG(2b), and IgG(3) mAbs. The results reveal isotype-related differences in fluorescence emission spectroscopy and temperature-related differences in binding and cleavage of a peptide mimetic. We conclude that the C region can modify the V region structure to alter the Ab paratope, thus providing an explanation for how isotype can affect Ab specificity.

Monoclonal antibodies and toxins--a perspective on function and isotype

Antibody therapy remains the only effective treatment for toxin-mediated diseases. The development of hybridoma technology has allowed the isolation of monoclonal antibodies (mAbs) with high specificity and defined properties, and numerous mAbs have been purified and characterized for their protective efficacy against different toxins. This review summarizes the mAb studies for 6 toxins—Shiga toxin, pertussis toxin, anthrax toxin, ricin toxin, botulinum toxin, and Staphylococcal enterotoxin B (SEB)—and analyzes the prevalence of mAb functions and their isotypes. Here we show that most toxin-binding mAbs resulted from immunization are non-protective and that mAbs with potential therapeutic use are preferably characterized. Various common practices and caveats of protection studies are discussed, with the goal of providing insights for the design of future research on antibody-toxin interactions.

Multiple actin isotypes in plants: diverse genes for diverse roles?

Plant actins are encoded by a gene family. Despite the crucial significance of the actin cytoskeleton for plant structure and function, the importance of individual actin isotypes and their specific roles in various plant tissues or even single cells is rather poorly understood. This review summarizes our current knowledge about the plant actin gene family including its evolution, gene and protein structure, and the expression profiles and regulation. Based on this background information, we review mutant and complementation analyses in Arabidopsis to draw an emerging picture of overlapping and specific roles of plant actin isotypes. Finally, we examine hypotheses explaining the mechanisms of isotype-specific functions.

Anti-heat shock protein 70 autoantibody epitope changes and BD091 promotes atherosclerosis in rats

It has been previously reported that the plasma levels of autoantibodies against heat shock protein 70 (HSP70) are elevated in atherosclerosis. The aim of the present study was to elucidate whether anti-HSP70 antibodies are involved in the pathogenesis of atherosclerosis. To determine this, we chose rats as an atherosclerosis model. Titers of plasma anti-HSP70 autoantibody were determined by ELISA. After the intravenous administration of antibody into the tail, the damaged areas of aorta were stained with Evans Blue, atheromatous plaque were stained by Oil Red O, and then they were measured and quantified with AxioVision computer software. The number of macrophages ([Formula: see text]), smooth muscle cells (SMCs), and T cells were determined by immunocytochemistry. The level of anti-HSP70 IgG1 antibody was apparently increased in the AS group at the tenth week, and one hybridoma of HSP70 antibody (BD091, IgG1, recognizing C-terminal) had the same binding epitope as plasma anti-HSP70 autoantibodies. After intravenous administration, the lesion area of aorta with BD091 was significantly larger than those of IgG(mouse) and SPA-810. Moreover, injection of BD091 resulted in significant endothelium damage, followed by a greater accumulation of [Formula: see text], T cells, and SMCs in lesions than in the control. In conclusion, BD091 reaction with HSP70 expressed on arterial endothelial cells inducing endothelium damage triggers the inflammatory response in the vessel wall that accelerates atherosclerosis in rats. BD091 shares the same binding epitope with HSP70 autoantibodies. These data indicated that a specific epitope of anti-HSP70 autoantibody participated in the pathogenesis of atherosclerosis.

The intestinal chemokine thymus-expressed chemokine (CCL25) attracts IgA antibody-secreting cells

Immunoglobulin A (IgA) provides protection against pathogens at mucosal surfaces. Chemotactic responses have been hypothesized to target IgA plasma cells involved in mucosal immune responses. We show here that thymus-expressed chemokine (TECK, CCL25) is a potent and selective chemoattractant for IgA antibody-secreting cells (ASC), efficiently recruiting IgA-producing cells from spleen, Peyer's patches, and mesenteric lymph node. Cells secreting IgA antibody in response to rotavirus, an intestinal pathogen, also respond well. In contrast, IgG- and IgM-ASC respond poorly. Epithelial cells in the small intestines, a principal site of IgA-ASC localization and IgA production in the body, highly and selectively express TECK. The migration of IgA-ASC to the intestinal epithelial cell chemokine TECK may help target IgA-producing cells to the gut wall, thus helping define and segregate the intestinal immune response.

Modulation of endogenous beta-tubulin isotype expression as a result of human beta(III)cDNA transfection into prostate carcinoma cells

Increases of individual beta tubulin isotypes in antimicrotubule drug resistant cell lines have been reported by several laboratories. We have previously described elevations in beta(III)and beta(IVa)isotypes in estramustine and paclitaxel resistant human prostate carcinoma cells. To investigate further the function of beta tubulin isotypes in antimicrotubule drug response, human prostate carcinoma cells that normally have very low to undetectable levels of beta(III)were stably transfected with beta(III)cDNA in pZeoSV system. An 18 bp haemagglutinin (HA) epitope tag was added at the 3' end prior to cloning into the vector. Cells were transfected with pZeoSV or pZeoSV-beta(III)plasmids and selected in the presence of Zeocin. Immunofluorescent staining of the transfectant cells have shown significant expression and incorporation of HA-tagged beta(III)tubulin into cellular microtubules. Quantitation of Western blots revealed the HA-tagged beta(III)levels to be approximately 7-fold higher than the vector control cells. RT-PCR analysis confirmed the increase at the transcript level and also revealed a collateral increase of beta(II)and beta(IVb)transcripts. Cell viability assays indicated that sensitivity of beta(III)transfected cells to various antimicrotubule agents was similar to vector transfected cells: IC50 values for estramustine, paclitaxel, colchicine and vinblastine were 4 microM, 4 nM, 22 nM and 2 nM, respectively for both cell lines. Thus, overexpression of beta(III)isotype in human prostate carcinoma cells by stable transfection failed to confer antimicrotubule drug resistance to these cells. Counterregulatory increases of endogenous beta(II)and beta(IVb)tubulin isotypes in these beta(III)transfected cells may be a compensatory mechanism used by the cells to overcome the effects of elevated beta(III)levels on the cellular microtubules. These results highlight the difficulty in isolating the contribution of single tubulin isotypes in drug response studies.

Complex pattern of Th1 and Th2 activation with a preferential increase of autoreactive Th1 cells in BALB/c mice with proteoglycan (aggrecan)-induced arthritis

The central role of CD4+ T cells and the balance between T helper (Th) subpopulations in the pathogenesis of autoimmune diseases have been extensively studied. Proteoglycan (aggrecan)-induced arthritis (PGIA) is a murine model for rheumatoid arthritis (RA), which is characterized by a Th1 dominance at the onset of the disease. In addition to CD4+ T cells, antigen-presenting B cells and autoantibodies seem to play an important role in the development and regulation of PGIA. To identify proteoglycan-specific CD4+ T cell subsets and Th1- and Th2-supported antibody isotypes during the progression of PGIA, spleen cells of proteoglycan-immunized BALB/c mice were harvested at different times of immunization, and at different stages of the disease, and their cytokine production and antigen-specific antibody isotype profiles were determined by enzyme-linked immunospot (ELISPOT) assays. Both Th1 and Th2 cytokine-producing cells, with the predominance of IL-4/IL-5-secreting cells, were detected during the prearthritic stage, and a shift toward a Th1 dominance was observed at the time of onset of arthritis. Tissue homogenates of acutely inflamed joints contained significantly higher levels of interferon-gamma than IL-4. The prearthritic period and both the acute and chronic phases of joint inflammation were characterized by IgG1 dominance in the sera and this correlated with the number of IgG1-secreting B cells in the spleen. However, the ratio of autoreactive IgG1/IgG2a-secreting cells decreased in arthritic animals. These results indicate the activation and possible regulatory roles of both Th1 and Th2 subsets in the autoimmune process, with the necessity of a relative increase of autoreactive Th1 cells for the induction of joint inflammation.