Complement is activated by IgG hexamers assembled at the cell surface

The Molecular Microscope Diagnostic System: Assessment of Rejection and Injury in Heart Transplant Biopsies

This review describes the development of the Molecular Microscope Diagnostic System (MMDx) for heart transplant endomyocardial biopsies (EMBs). MMDx-Heart uses microarrays to measure biopsy-based gene expression and ensembles of machine learning algorithms to interpret the results and compare each new biopsy to a large reference set of earlier biopsies. MMDx assesses T cell-mediated rejection (TCMR), antibody-mediated rejection (AMR), recent parenchymal injury, and atrophy-fibrosis, continually "learning" from new biopsies. Rejection-associated transcripts mapped in kidney transplants and experimental systems were used to identify TCMR, AMR, and recent injury-induced inflammation. Rejection and injury emerged as gradients of intensity, rather than binary classes. AMR was one-third donor-specific antibody (DSA)-negative, and many EMBs first considered to have no rejection displayed minor AMR-like changes, with increased probability of DSA positivity and subtle inflammation. Rejection-associated transcript-based algorithms now classify EMBs as "Normal," "Minor AMR changes," "AMR," "possible AMR," "TCMR," "possible TCMR," and "recent injury." Additionally, MMDx uses injury-associated transcript sets to assess the degree of parenchymal injury and atrophy-fibrosis in every biopsy and study the effect of rejection on the parenchyma. TCMR directly injures the parenchyma whereas AMR usually induces microcirculation stress but relatively little initial parenchymal damage, although slowly inducing parenchymal atrophy-fibrosis. Function (left ventricular ejection fraction) and short-term risk of failure are strongly determined by parenchymal injury. These discoveries can guide molecular diagnostic applications, either as a central MMDx system or adapted to other platforms. MMDx can also help calibrate noninvasive blood-based biomarkers to avoid unnecessary biopsies and monitor response to therapy.

Humoral Immune Responses to P. falciparum Circumsporozoite Protein (Pfcsp) Induced by the RTS, S Vaccine - Current Update

Malaria vaccines targeting the circumsporozoite protein (CSP) of the P. falciparum parasite have been overall relatively promising. RTS, S is a pre-erythrocytic recombinant protein-based malaria vaccine that targets CSP. RTS, S effectiveness shows some limited success regardless of its 58% efficacy for severe disease. P. falciparum circumsporozoite protein (Pfcsp) has stood to be the main candidate protein for most pre-erythrocytic stage vaccines. Studies on the structural and biophysical characteristics of antibodies specific to CSP (anti-CSP) are underway to achieve fine specificity with the CSP polymorphic regions. More recent studies have proposed the use of different kinds of monoclonal antibodies, the use of appropriate adjuvants, ideal vaccination dose and frequency, and improved targeting of particular epitopes for the robust production of functional antibodies and high complement-fixing activity as other potential methods for achieving long-lasting RTS, S. This review highlights recent findings regarding humoral immune responses to CSP elicited by RTS, S vaccine.

Factors affecting IgG4-mediated complement activation

Of the four human immunoglobulin G (IgG) subclasses, IgG4 is considered the least inflammatory, in part because it poorly activates the complement system. Regardless, in IgG4 related disease (IgG4-RD) and in autoimmune disorders with high levels of IgG4 autoantibodies, the presence of these antibodies has been linked to consumption and deposition of complement components. This apparent paradox suggests that conditions may exist, potentially reminiscent of in vivo deposits, that allow for complement activation by IgG4. Furthermore, it is currently unclear how variable glycosylation and Fab arm exchange may influence the ability of IgG4 to activate complement. Here, we used well-defined, glyco-engineered monoclonal preparations of IgG4 and determined their ability to activate complement in a controlled system. We show that IgG4 can activate complement only at high antigen and antibody concentrations, via the classical pathway. Moreover, elevated or reduced Fc galactosylation enhanced or diminished complement activation, respectively, with no apparent contribution from the lectin pathway. Fab glycans slightly reduced complement activation. Lastly, we show that bispecific, monovalent IgG4 resulting from Fab arm exchange is a less potent activator of complement than monospecific IgG4. Taken together, these results imply that involvement of IgG4-mediated complement activation in pathology is possible but unlikely.

Evidence of variable human Fcγ receptor-Fc affinities across differentially-complexed IgG

Antibody-mediated effector functions are widely considered to unfold according to an associative model of IgG-Fcγ receptor (FcγR) interactions. The associative model presupposes that Fc receptors cannot discriminate antigen-bound IgG from free IgG in solution and have equivalent affinities for each. Therefore, the clustering of Fcγ receptors (FcγR) in the cell membrane, cross-activation of intracellular signaling domains, and the formation of the immune synapse are all the result of avid interactions between the Fc region of IgG and FcγRs that collectively overcome the individually weak, transient interactions between binding partners. Antibody allostery, specifically conformational allostery, is a competing model in which antigen-bound antibody molecules undergo a physical rearrangement that causes them to stand out from the background of free IgG by virtue of greater FcγR affinity. Various evidence exists in support of this model of antibody allostery, but it remains controversial. We report observations from multiplexed, label-free kinetic experiments in which the affinity values of FcγR were characterized for covalently immobilized, captured, and antigen-bound IgG. Across the strategies tested, receptors had greater affinity for the antigen-bound mode of IgG presentation. This phenomenon was observed across multiple FcγRs and generalized to multiple antigens, antibody specificities, and subclasses. Furthermore, the thermodynamic signatures of FcγR binding to free or immune-complexed IgG in solution differed when measured by an orthogonal label-free method, but the failure to recapitulate the trend in overall affinity leaves open questions as to what additional factors may be at play.

Development of Effective Siglec-9 Antibodies Against Cancer

PURPOSE OF REVIEW

This study aims to review state-of-the-art advances in Siglec-9-directed antibodies and to highlight specific aspects of Siglec-9 antibodies that are suitable to mount anti-tumor immunity.

RECENT FINDINGS

Controversies surrounding studies on Siglec-9 antibodies can confound future studies. In this review, we have highlighted some controversies, explained the distinction between Siglec-9 agonistic and antagonistic (endocytic) antibodies, and discussed their suitability in sustaining anti-tumor immunity. Siglec-9 is an immune checkpoint target and an immunoinhibitory receptor that can engage either sialic acid ligands or agonistic antibodies. Through Siglec-9 sialic acid interactions, activated immunoreceptor tyrosine-based inhibitory signaling of the immune cells can lead to unfavorable immunosuppression. To overcome tumor-related immunosuppression, different types of Siglec-9 antibody blockade need to be developed. However, whether a Siglec-9-directed antibody is agonistic or antagonistic is probably affinity-dependent and not epitope-dependent. Additionally, unlike immune-modulatory antibodies such as agonistic antibodies (OX40, CD28, ICOS, and 4-1BB) or Fc-inert antibodies (PD1 and PD-L1) directed against cancer cells, the nature of antagonistic Siglec-9 antibodies is more suitable to enhance anti-tumor immunity and will be discussed.

Protein therapeutics and their lessons: Expect the unexpected when inhibiting the multi-protein cascade of the complement system

Over a century after the discovery of the complement system, the first complement therapeutic was approved for the treatment of paroxysmal nocturnal hemoglobinuria (PNH). It was a long-acting monoclonal antibody (aka 5G1-1, 5G1.1, h5G1.1, and now known as eculizumab) that targets C5, specifically preventing the generation of C5a, a potent anaphylatoxin, and C5b, the first step in the eventual formation of membrane attack complex. The enormous clinical and financial success of eculizumab across four diseases (PNH, atypical hemolytic uremic syndrome (aHUS), myasthenia gravis (MG), and anti-aquaporin-4 (AQP4) antibody-positive neuromyelitis optica spectrum disorder (NMOSD)) has fueled a surge in complement therapeutics, especially targeting diseases with an underlying complement pathophysiology for which anti-C5 therapy is ineffective. Intensive research has also uncovered challenges that arise from C5 blockade. For example, PNH patients can still face extravascular hemolysis or pharmacodynamic breakthrough of complement suppression during complement-amplifying conditions. These "side" effects of a stoichiometric inhibitor like eculizumab were unexpected and are incompatible with some of our accepted knowledge of the complement cascade. And they are not unique to C5 inhibition. Indeed, "exceptions" to the rules of complement biology abound and have led to unprecedented and surprising insights. In this review, we will describe initial, present and future aspects of protein inhibitors of the complement cascade, highlighting unexpected findings that are redefining some of the mechanistic foundations upon which the complement cascade is organized.

Antibody-Dependent Enhancement (ADE) and the role of complement system in disease pathogenesis

Antibody-dependent enhancement (ADE) has been associated with severe disease outcomes in several viral infections, including respiratory infections. In vitro and in vivo studies showed that antibody-response to SARS-CoV and MERS-CoV could exacerbate infection via ADE. Recently in SARS CoV-2, the in vitro studies and structural analysis shows a risk of disease severity via ADE. This phenomenon is partially attributed to non-neutralizing antibodies or antibodies at sub-neutralizing levels. These antibodies result in antigen-antibody complexes' deposition and propagation of a chronic inflammatory process that destroys affected tissues. Further, antigen-antibody complexes may enhance the internalization of the virus into cells through the Fc gamma receptor (FcγR) and lead to further virus replication. Thus, ADE occur via two mechanisms; 1. Antibody mediated replication and 2. Enhanced immune activation. Antibody-mediated effector functions are mainly driven by complement activation, and the first complement in the cascade is complement 1q (C1q) which binds to the virus-antibody complex. Reports say that deficiency in circulating plasma levels of C1q, an independent predictor of mortality in high-risk patients, including diabetes, is associated with severe viral infections. Complement mediated ADE is reported in several viral infections such as dengue, West Nile virus, measles, RSV, Human immunodeficiency virus (HIV), and Ebola virus. This review discusses ADE in viral infections and the in vitro evidence of ADE in coronaviruses. We outline the mechanisms of ADE, emphasizing the role of complements, especially C1q in the outcome of the enhanced disease.

A tick C1q protein alters infectivity of the Lyme disease agent by modulating interferon γ

In North America, the Lyme disease agent, Borrelia burgdorferi, is commonly transmitted by the black-legged tick, Ixodes scapularis. Tick saliva facilitates blood feeding and enhances pathogen survival and transmission. Here, we demonstrate that I. scapularis complement C1q-like protein 3 (IsC1ql3), a tick salivary protein, directly interacts with B. burgdorferi and is important during the initial stage of spirochetal infection of mice. Mice fed upon by B. burgdorferi-infected IsC1ql3-silenced ticks, or IsC1ql3-immunized mice fed upon by B. burgdorferi-infected ticks, have a lower spirochete burden during the early phase of infection compared with control animals. Mechanically, IsC1ql3 interacts with the globular C1q receptor present on the surface of CD4+ and CD8+ T cells, resulting in decreased production of interferon γ. IsC1ql3 is a C1q-domain-containing protein identified in arthropod vectors and has an important role in B. burgdorferi infectivity as the spirochete transitions from the tick to vertebrate host.

Serum Immunoglobulin G Is a Marker of Hidradenitis Suppurativa Disease Severity

Hidradenitis suppurativa (HS) is a chronic inflammatory condition of the skin that is brought about by autoinflammation and hyperkeratosis at the pilosebaceous unit. The clinical severity of HS can be measured using static (Hurley Severity Scoring (HSS)) and/or dynamic (International HS Severity Scoring System (IHS4)) severity scoring instruments. However, few clinically available serological parameters have been found to correlate with disease severity. In this study, we sought to investigate the role of serum immunoglobulin (Ig) G, M and A levels as biomarkers of disease severity and to compare them with other, more conventional inflammatory indices, such as the erythrocyte sedimentation rate, C-reactive protein, the neutrophil-lymphocyte ratio, the platelet-lymphocyte ratio and the systemic immune-inflammation index. In this cross-sectional study, patients were recruited from the only dermatology referral centre in Malta, Europe, and subjected to clinical examination and the assessment of inflammatory and immunologic parameters. Serum IgG, M and A levels were assessed using the Atellica^® NEPH 630 System (SIEMENS-Healthineers AF, Erlangen, Germany) nephelometric analyser. Serum IgG, M and A levels correlate with both dynamic and static HS severity scoring systems. Serum IgG behaves as a marker of severe HS disease as categorised by HSS and the IHS4. Our findings suggest that the serum IgG level can be used in the clinical setting as a biomarker of disease severity and, therefore, as an adjunct to clinical severity scoring.

C1q and the classical complement cascade in geographic atrophy secondary to age-related macular degeneration

Geographic atrophy (GA) secondary to age-related macular degeneration (AMD) is a retinal neurodegenerative disorder. Human genetic data support the complement system as a key component of pathogenesis in AMD, which has been further supported by pre-clinical and recent clinical studies. However, the involvement of the different complement pathways (classical, lectin, alternative), and thus the optimal complement inhibition target, has yet to be fully defined. There is evidence that C1q, the initiating molecule of the classical pathway, is a key driver of complement activity in AMD. C1q is expressed locally by infiltrating phagocytic cells and C1q-activating ligands are present at disease onset and continue to accumulate with disease progression. The accumulation of C1q on photoreceptor synapses with age and disease is consistent with its role in synapse elimination and neurodegeneration that has been observed in other neurodegenerative disorders. Furthermore, genetic deletion of C1q, local pharmacologic inhibition within the eye, or genetic deletion of downstream C4 prevents photoreceptor cell damage in mouse models. Hence, targeting the classical pathway in GA could provide a more specific therapeutic approach with potential for favorable efficacy and safety.

IgG subclass and Fc glycosylation shifts are linked to the transition from pre- to inflammatory autoimmune conditions

A crucial factor for the development of inflammatory autoimmune diseases is the occurrence of antibodies directed against self-tissues and structures, which leads to damage and inflammation. While little is known about the cause of the development of mis-directed, disease-specific T and B cells and resulting IgG autoantibody responses, there is increasing evidence that their induction can occur years before disease symptoms appear. However, a certain proportion of healthy individuals express specific IgG autoantibodies without disease symptoms and not all subjects who generate autoantibodies may develop disease symptoms. Thus, the development of inflammatory autoimmune diseases seems to involve two steps. Increasing evidence suggests that harmless self-directed T and B cell and resulting IgG autoantibody responses in the pre-autoimmune disease stage might switch to more inflammatory T and B cell and IgG autoantibody responses that trigger the inflammatory autoimmune disease stage. Here, we summarize findings on the transition from the pre-disease to the disease stage and vice versa, e.g. by pregnancy and treatment, with a focus on low-/anti-inflammatory versus pro-inflammatory IgG autoantibody responses, including IgG subclass and Fc glycosylation features. Characterization of biomarkers that identify the transition from the pre-disease to the disease stage might facilitate recognition of the ideal time point of treatment initiation and the development of therapeutic strategies for re-directing inflammatory autoimmune conditions.

Receptor clustering and pathogenic complement activation in myasthenia gravis depend on synergy between antibodies with multiple subunit specificities

Myasthenia gravis is an autoimmune disorder defined by muscle weakness and fatigability associated with antibodies against proteins of the neuromuscular junction (NMJ). The most common autoantibody target is the acetylcholine receptor (AChR). Three mechanisms have been postulated by which autoantibodies might interfere with neurotransmission: direct antagonism of the receptor, complement-mediated destruction of the postsynaptic membrane, and enhanced internalization of the receptor. It is very likely that more than one of these mechanisms act in parallel. Dissecting the mechanisms of autoantibody-mediated pathology requires patient-derived, monoclonal antibodies. Using membrane antigen capture activated cell sorting (MACACS), we isolated AChR-specific B cells from patients with myasthenia gravis, and produced six recombinant antibodies. All AChR-specific antibodies were hypermutated, including isotypes IgG1, IgG3, and IgG4, and recognized different subunits of the AChR. Despite clear binding, none of the individual antibodies showed significant antagonism of the AChR measured in an in vitro neuromuscular synapse model, or AChR-dependent complement activation, and they did not induce myasthenic signs in vivo. However, combinations of antibodies induced strong complement activation in vitro, and severe weakness in a passive transfer myasthenia gravis rat model, associated with NMJ destruction and complement activation in muscle. The strongest complement activation was mediated by combinations of antibodies targeting disparate subunits of the AChR, and such combinations also induced the formation of large clusters of AChR on the surface of live cells in vitro. We propose that synergy between antibodies of different epitope specificities is a fundamental feature of this disease, and possibly a general feature of complement-mediated autoimmune diseases. The importance of synergistic interaction between antibodies targeting different subunits of the receptor can explain the well-known discrepancy between serum anti-AChR titers and clinical severity, and has implications for therapeutic strategies currently under investigation.

Clinical utility of serial serum dilutions for HLA antibody interpretation

Luminex single antigen bead (SAB) testing has increased the sensitivity and specificity of accurately identifying HLA antibodies, in support of all organ transplantation. However, as described in manufacturers' recommendation, the output of the assay, using mean fluorescence intensity (MFI) units, is only semi-quantitative. Therefore, the ability to use MFI values to compare between different assays, to accurately guide clinical practice, or be used as an endpoint measure in clinical trials, is limited. To improve potential quantification, one must circumvent inherent limitations of SAB assays such as interference and saturation phenomena. In this review, we discuss how measurement of pre-transplant serum dilutions can be used to determine unacceptable antigens for wait-listing, determine the likelihood for successful HLA antibody reduction with desensitization, and compare degree of HLA (in)compatibility among various living donors. We also discuss how serum dilutions are optimal for measuring and comparing the efficacy of antibody depletion therapies for desensitization or antibody mediated rejection treatment post-transplant. Historically, one of the main criticisms for the use of serum dilutions and titer has been the potential labor and cost associated with additional testing. Here, we show how only one or two dilutions can add major value in most circumstances. In summary, the practical use of serum dilutions and titer determination are important methods that can be used before and after transplantation of all organs to quantify antibody accurately and reliably in routine practice and in clinical trials.

Deceiving and escaping complement - the evasive journey of the malaria parasite

During its life cycle, Plasmodium, the malaria parasite, is exposed to the human and mosquito complement systems. Early experiments demonstrated that activation of complement can pose a serious threat to parasites, but recent studies revealed complement-evasion mechanisms important for parasite survival. Blood-stage parasites and gametes recruit regulators to neutralize human complement activation, while ookinetes inhibit mosquito complement by disrupting epithelial nitration in response to midgut invasion. Here we provide an in-depth overview of the evasion mechanisms currently known and speculate on the existence of others not yet identified. Finally, we discuss how these mechanisms could provide novel targets for urgently needed malaria vaccines and therapeutics.

IMGT ® Nomenclature of Engineered IGHG Variants Involved in Antibody Effector Properties and Formats

The constant region of the immunoglobulin (IG) or antibody heavy gamma chain is frequently engineered to modify the effector properties of the therapeutic monoclonal antibodies. These variants are classified in regards to their effects on effector functions, antibody-dependent cytotoxicity (ADCC), antibody-dependent phagocytosis (ADCP), complement-dependent cytotoxicity (CDC) enhancement or reduction, B cell inhibition by the coengagement of antigen and FcγR on the same cell, on half-life increase, and/or on structure such as prevention of IgG4 half-IG exchange, hexamerisation, knobs-into-holes and the heteropairing H-H of bispecific antibodies, absence of disulfide bridge inter H-L, absence of glycosylation site, and site-specific drug attachment engineered cysteine. The IMGT engineered variant identifier is comprised of the species and gene name (and eventually allele), the letter 'v' followed by a number (assigned chronologically), and for each concerned domain (e.g, CH1, h, CH2 and CH3), the novel AA (single letter abbreviation) and IMGT position according to the IMGT unique numbering for the C-domain and between parentheses, the Eu numbering. IMGT engineered variants are described with detailed amino acid changes, visualized in motifs based on the IMGT numbering bridging genes, sequences, and structures for higher order description.

Targeting CD38 for acute leukemia

Acute leukemia (AL) is a hematological malignancy, and the prognosis of most AL patients hasn’t improved significantly, particularly for relapsed or refractory (R/R) AL. Therefore, new treatments for R/R adult acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) are urgently necessary. Novel developments have been made in AL treatment, including target and immune therapies. CD38 is one of the targets due to its high expression in many hematological malignancies, including multiple myeloma, ALL and a subset of AML. Consequently, targeting CD38 therapies, including CD38 monoclonal antibodies (mAbs), bispecific antibodies, and CAR-T cell therapy, exhibit promising efficacy in treating multiple myeloma without significant toxicity and are being explored in other hematological malignancies and nonhematological diseases. Herein, this review focuses on targeting CD38 therapies in ALL and AML, which demonstrate sound antileukemic effects in acute leukemia and are expected to become effective treatment methods.

Preclinical Characterization of Relatlimab, a Human LAG-3-Blocking Antibody, Alone or in Combination with Nivolumab

Novel therapeutic approaches combining immune-checkpoint inhibitors are needed to improve clinical outcomes for patients with cancer. Lymphocyte-activation gene 3 (LAG-3) is an immune-checkpoint molecule that inhibits T-cell activity and antitumor immune responses, acting through an independent mechanism from that of programmed death-1 (PD-1) and cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4). Here, we describe the development and preclinical characterization of relatlimab, a human antibody that binds to human LAG-3 with high affinity and specificity to block the interaction of LAG-3 with the ligands MHC II and fibrinogen-like protein-1, and to reverse LAG-3-mediated inhibition of T-cell function in vitro. Consistent with previous reports, in mouse models, the combined blockade of LAG-3 and PD-1 with surrogate antibodies resulted in enhanced antitumor activity greater than the individual blockade of either receptor. In toxicity studies in cynomolgus monkeys, relatlimab was generally well tolerated when combined with nivolumab. These results are consistent with findings from the RELATIVITY-047 phase II/III trial showing that relatlimab combined with nivolumab is a well-tolerated regimen that demonstrates superior progression-free survival compared with nivolumab monotherapy in patients with unresectable or metastatic melanoma.

IRF8 regulates efficacy of therapeutic anti-CD20 monoclonal antibodies

Anti-CD20 monoclonal antibodies such as Rituximab, Ofatumumab, and Obinutuzumab are widely used to treat lymphomas and autoimmune diseases. They act by depleting B cells, mainly through Fc-dependent effectors functions. Some patients develop resistance to treatment but the underlying mechanisms are poorly understood. Here, we performed a genome-wide CRISPR/Cas9 screen to identify genes regulating the efficacy of anti-CD20 antibodies. We used as a model the killing of RAJI B cells by Rituximab through complement-dependent-cytotoxicity (CDC). As expected, the screen identified MS4A1, encoding CD20, the target of Rituximab. Among other identified genes, the role of Interferon Regulatory Factor 8 (IRF8) was validated in two B-cell lines. IRF8 KO also decreased the efficacy of antibody-dependent cellular cytotoxicity and phagocytosis (ADCC and ADCP) induced by anti-CD20 antibodies. We further show that IRF8 is necessary for efficient CD20 transcription. Levels of IRF8 and CD20 RNA or proteins correlated in normal B cells and in hundreds of malignant B cells. Therefore, IRF8 regulates CD20 expression and controls the depleting capacity of anti-CD20 antibodies. Our results bring novel insights into the pathways underlying resistance to CD20-targeting immunotherapies.

Logic-gated antibody pairs that selectively act on cells co-expressing two antigens

The use of therapeutic monoclonal antibodies is constrained because single antigen targets often do not provide sufficient selectivity to distinguish diseased from healthy tissues. We present HexElect®, an approach to enhance the functional selectivity of therapeutic antibodies by making their activity dependent on clustering after binding to two different antigens expressed on the same target cell. lmmunoglobulin G (lgG)-mediated clustering of membrane receptors naturally occurs on cell surfaces to trigger complement- or cell-mediated effector functions or to initiate intracellular signaling. We engineer the Fc domains of two different lgG antibodies to suppress their individual homo-oligomerization while promoting their pairwise hetero-oligomerization after binding co-expressed antigens. We show that recruitment of complement component C1q to these hetero-oligomers leads to clustering-dependent activation of effector functions such as complement mediated killing of target cells or activation of cell surface receptors. HexElect allows selective antibody activity on target cells expressing unique, potentially unexplored combinations of surface antigens.

Toward Understanding How Staphylococcal Protein A Inhibits IgG-Mediated Phagocytosis

IgG molecules are crucial for the human immune response against bacterial infections. IgGs can trigger phagocytosis by innate immune cells, like neutrophils. To do so, IgGs should bind to the bacterial surface via their variable Fab regions and interact with Fcγ receptors and complement C1 via the constant Fc domain. C1 binding to IgG-labeled bacteria activates the complement cascade, which results in bacterial decoration with C3-derived molecules that are recognized by complement receptors on neutrophils. Next to FcγRs and complement receptors on the membrane, neutrophils also express the intracellular neonatal Fc receptor (FcRn). We previously reported that staphylococcal protein A (SpA), a key immune-evasion protein of Staphylococcus aureus, potently blocks IgG-mediated complement activation and killing of S. aureus by interfering with IgG hexamer formation. SpA is also known to block IgG-mediated phagocytosis in absence of complement, but the mechanism behind it remains unclear. In this study, we demonstrate that SpA blocks IgG-mediated phagocytosis and killing of S. aureus and that it inhibits the interaction of IgGs with FcγRs (FcγRIIa and FcγRIIIb, but not FcγRI) and FcRn. Furthermore, our data show that multiple SpA domains are needed to effectively block IgG1-mediated phagocytosis. This provides a rationale for the fact that SpA from S. aureus contains four to five repeats. Taken together, our study elucidates the molecular mechanism by which SpA blocks IgG-mediated phagocytosis and supports the idea that in addition to FcγRs, the intracellular FcRn is also prevented from binding IgG by SpA.

Antibody-mediated cell depletion therapies in multiple sclerosis

Development of disease-modifying therapies including monoclonal antibody (mAb)-based therapeutics for the treatment of multiple sclerosis (MS) has been extremely successful over the past decades. Most of the mAb-based therapies approved for MS deplete immune cell subsets and act through activation of cellular Fc-gamma receptors expressed by cytotoxic lymphocytes and phagocytes, resulting in antibody-dependent cellular cytotoxicity or by initiation of complement-mediated cytotoxicity. The therapeutic goal is to eliminate pathogenic immune cell components and to potentially foster the reconstitution of a new and healthy immune system. Ab-mediated immune cell depletion therapies include the CD52-targeting mAb alemtuzumab, CD20-specific therapeutics, and new Ab-based treatments which are currently being developed and tested in clinical trials. Here, we review recent developments in effector mechanisms and clinical applications of Ab-based cell depletion therapies, compare their immunological and clinical effects with the prototypic immune reconstitution treatment strategy, autologous hematopoietic stem cell transplantation, and discuss their potential to restore immunological tolerance and to achieve durable remission in people with MS.

Complement and myasthenia gravis

Myasthenia gravis is a neuromuscular disease associated with antibodies against components of the neuromuscular junction, most often against the acetylcholine receptor (AChR). Although several mechanisms have been postulated to explain how these autoantibodies can lead to the pathology of the disease, convincing evidence suggests that destruction of the receptor-bearing postsynaptic membrane by complement membrane attack complex is of central importance. In this review, evidence for the importance of complement, and possible relationships between autoantigen, autoantibodies, complement activation, and the destruction of the membrane are discussed. More recent insights from the results of the complement-inhibiting therapeutic antibody eculizumab are also described, and the mechanisms connecting antibody binding to complement activation are considered from a structural viewpoint.

Dual Fc optimization to increase the cytotoxic activity of a CD19-targeting antibody

Targeting CD19 represents a promising strategy for the therapy of B-cell malignancies. Although non-engineered CD19 antibodies are poorly effective in mediating complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC) or antibody-dependent cellular phagocytosis (ADCP), these effector functions can be enhanced by Fc-engineering. Here, we engineered a CD19 antibody with the aim to improve effector cell-mediated killing and CDC activity by exchanging selected amino acid residues in the Fc domain. Based on the clinically approved Fc-optimized antibody tafasitamab, which triggers enhanced ADCC and ADCP due to two amino acid exchanges in the Fc domain (S239D/I332E), we additionally added the E345K amino acid exchange to favor antibody hexamerization on the target cell surface resulting in improved CDC. The dual engineered CD19-DEK antibody bound CD19 and Fcγ receptors with similar characteristics as the parental CD19-DE antibody. Both antibodies were similarly efficient in mediating ADCC and ADCP but only the dual optimized antibody was able to trigger complement deposition on target cells and effective CDC. Our data provide evidence that from a technical perspective selected Fc-enhancing mutations can be combined (S239D/I332E and E345K) allowing the enhancement of ADCC, ADCP and CDC with isolated effector populations. Interestingly, under more physiological conditions when the complement system and FcR-positive effector cells are available as effector source, strong complement deposition negatively impacts FcR engagement. Both effector functions were simultaneously active only at selected antibody concentrations. Dual Fc-optimized antibodies may represent a strategy to further improve CD19-directed cancer immunotherapy. In general, our results can help in guiding optimal antibody engineering strategies to optimize antibodies’ effector functions.

Engineered DNA-encoded monoclonal antibodies targeting Plasmodium falciparum circumsporozoite protein confer single dose protection in a murine malaria challenge model

Novel approaches for malaria prophylaxis remain important. Synthetic DNA-encoded monoclonal antibodies (DMAbs) are a promising approach to generate rapid, direct in vivo host-generated mAbs with potential benefits in production simplicity and distribution coupled with genetic engineering. Here, we explore this approach in a malaria challenge model. We engineered germline-reverted DMAbs based on human mAb clones CIS43, 317, and L9 which target a junctional epitope, major repeat, and minor repeat of the Plasmodium falciparum circumsporozoite protein (CSP) respectively. DMAb variants were encoded into a plasmid vector backbone and their expression and binding profiles were characterized. We demonstrate long-term serological expression of DMAb constructs resulting in in vivo efficacy of CIS43 GL and 317 GL in a rigorous mosquito bite mouse challenge model. Additionally, we engineered an Fc modified variant of CIS43 and L9-based DMAbs to ablate binding to C1q to test the impact of complement-dependent Fc function on challenge outcomes. Complement knockout variant DMAbs demonstrated similar protection to that of WT Fc DMAbs supporting the notion that direct binding to the parasite is sufficient for the protection observed. Further investigation of DMAbs for malaria prophylaxis appears of importance.

Afucosylated IgG responses in humans - structural clues to the regulation of humoral immunity

Healthy immune responses require efficient protection without excessive inflammation. Recent discoveries on the degree of fucosylation of a human N-linked glycan at a conserved site in the immunoglobulin IgG-Fc domain might add an additional regulatory layer to adaptive humoral immunity. Specifically, afucosylation of IgG-Fc enhances the interaction of IgG with FcγRIII and thereby its activity. Although plasma IgG is generally fucosylated, afucosylated IgG is raised in responses to enveloped viruses and Plasmodium falciparum proteins expressed on infected erythrocytes, as well as during alloimmune responses. Moreover, while afucosylation can exacerbate some infectious diseases (e.g., COVID-19), it also correlates with traits of protective immunity against malaria and HIV-1. Herein we discuss the implications of IgG afucosylation for health and disease, as well as for vaccination.

Molecular diagnosis of ABMR with or without donor-specific antibody in kidney transplant biopsies: Differences in timing and intensity but similar mechanisms and outcomes

We studied the clinical, histologic, and molecular features distinguishing DSA-negative from DSA-positive molecularly defined antibody-mediated rejection (mABMR). We analyzed mABMR biopsies with available DSA assessments from the INTERCOMEX study: 148 DSA-negative versus 248 DSA-positive, compared with 864 no rejection (excluding TCMR and Mixed). DSA-positivity varied with mABMR stage: early-stage (EABMR) 56%; fully developed (FABMR) 70%; and late-stage (LABMR) 58%. DSA-negative patients with mABMR were usually sensitized, 60% being HLA antibody-positive. Compared with DSA-positive mABMR, DSA-negative mABMR was more often C4d-negative; earlier by 1.5 years (average 2.4 vs. 3.9 years); and had lower ABMR activity and earlier stage in molecular and histology features. However, the top ABMR-associated transcripts were identical in DSA-negative versus DSA-positive mABMR, for example, NK-associated (e.g., KLRD1 and GZMB) and IFNG-inducible (e.g., PLA1A). Genome-wide class comparison between DSA-negative and DSA-positive mABMR showed no significant differences in transcript expression except those related to lower intensity and earlier time of DSA-negative ABMR. Three-year graft loss in DSA-negative mABMR was the same as DSA-positive mABMR, even after adjusting for ABMR stage. Thus, compared with DSA-positive mABMR, DSA-negative mABMR is on average earlier, less active, and more often C4d-negative but has similar graft loss, and genome-wide analysis suggests that it involves the same mechanisms. SUMMARY SENTENCE: In 398 kidney transplant biopsies with molecular antibody-mediated rejection, the 150 DSA-negative cases are earlier, less intense, and mostly C4d-negative, but use identical molecular mechanisms and have the same risk of graft loss as the 248 DSA-positive cases.

Antibody Interfaces Revealed Through Structural Mining

Antibodies are fundamental effectors of humoral immunity, and have become a highly successful class of therapeutics. There is increasing evidence that antibodies utilize transient homotypic interactions to enhance function, and elucidation of such interactions can provide insights into their biology and new opportunities for their optimization as drugs. Yet the transitory nature of weak interactions makes them difficult to investigate. Capitalizing on their rich structural data and high conservation, we have characterized all the ways that antibody fragment antigen-binding (Fab) regions interact crystallographically. This approach led to the discovery of previously unrealized interfaces between antibodies. While diverse interactions exist, β-sheet dimers and variable-constant elbow dimers are recurrent motifs. Disulfide engineering enabled interactions to be trapped and investigated structurally and functionally, providing experimental validation of the interfaces and illustrating their potential for optimization. This work provides first insight into previously undiscovered oligomeric interactions between antibodies, and enables new opportunities for their biotherapeutic optimization.

Inositol 1,4,5-trisphosphate receptor type 1 autoantibody (ITPR1-IgG/anti-Sj)-associated autoimmune cerebellar ataxia, encephalitis and peripheral neuropathy: review of the literature

Background

In 2014, we first described novel autoantibodies to the inositol 1,4,5-trisphosphate receptor type 1 (ITPR1-IgG/anti-Sj) in patients with autoimmune cerebellar ataxia (ACA) in this journal. Here, we provide a review of the available literature on ITPR1-IgG/anti-Sj, covering clinical and paraclinical presentation, tumour association, serological findings, and immunopathogenesis.

Methods

Review of the peer-reviewed and PubMed-listed English language literature on ITPR1-IgG/anti-Sj. In addition, we provide an illustrative report on a new patient with ITPR1-IgG-associated encephalitis with cognitive decline and psychosis.

Results

So far, at least 31 patients with serum ITPR1-IgG/anti-Sj have been identified (clinical information available for 21). The most common manifestations were ACA, encephalopathy with seizures, myelopathy, and (radiculo)neuropathy, including autonomic neuropathy. In 45% of cases, an underlying tumour was present, making the condition a facultative paraneoplastic neurological disorder. The neurological syndrome preceded tumour diagnosis in all but one case. In most cases, immunotherapy had only moderate or no effect. The association of ITPR1-IgG/anti-Sj with manifestations other than ACA is corroborated by the case of a 48-year-old woman with high-titre ITPR1-IgG/anti-Sj antibodies and rapid cognitive decline, affecting memory, attention and executive function, and psychotic manifestations, including hallucinations, investigated here in detail. FDG-PET revealed right-temporal glucose hypermetabolism compatible with limbic encephalitis. Interestingly, ITPR1-IgG/anti-Sj mainly belonged to the IgG2 subclass in both serum and cerebrospinal fluid (CSF) in this and further patients, while it was predominantly IgG1 in other patients, including those with more severe outcome, and remained detectable over the entire course of disease. Immunotherapy with intravenous methylprednisolone, plasma exchange, and intravenous immunoglobulins, was repeatedly followed by partial or complete recovery. Long-term treatment with cyclophosphamide was paralleled by relative stabilization, although the patient noted clinical worsening at the end of each treatment cycle.

Conclusions

The spectrum of neurological manifestations associated with ITPR1 autoimmunity is broader than initially thought. Immunotherapy may be effective in some cases. Studies evaluating the frequency of ITPR1-IgG/anti-Sj in patients with cognitive decline and/or psychosis of unknown aetiology are warranted. Tumour screening is essential in patients presenting with ITPR1-IgG/anti-Sj.

Monoclonal antibodies effectively potentiate complement activation and phagocytosis of Staphylococcus epidermidis in neonatal human plasma

Central line associated bloodstream infections (CLABSI) with Staphylococcus epidermidis are a major cause of morbidity in neonates, who have an increased risk of infection because of their immature immune system. As especially preterm neonates suffer from antibody deficiency, clinical studies into preventive therapies have thus far focused on antibody supplementation with pooled intravenous immunoglobulins from healthy donors (IVIG) but with little success. Here we study the potential of monoclonal antibodies (mAbs) against S. epidermidis to induce phagocytic killing by human neutrophils. Nine different mAbs recognizing Staphylococcal surface components were cloned and expressed as human IgG1s. In binding assays, clones rF1, CR5133 and CR6453 showed the strongest binding to S. epidermidis ATCC14990 and CR5133 and CR6453 bound the majority of clinical isolates from neonatal sepsis (19 out of 20). To study the immune-activating potential of rF1, CR5133 and CR6453, bacteria were opsonized with mAbs in the presence or absence of complement. We observed that activation of the complement system is essential to induce efficient phagocytosis of S. epidermidis. Complement activation and phagocytic killing could be enhanced by Fc-mutations that improve IgG1 hexamerization on cellular surfaces. Finally, we studied the ability of the mAbs to activate complement in r-Hirudin neonatal plasma conditions. We show that classical pathway complement activity in plasma isolated from neonatal cord blood is comparable to adult levels. Furthermore, mAbs could greatly enhance phagocytosis of S. epidermidis in neonatal plasma. Altogether, our findings provide insights that are crucial for optimizing anti-S. epidermidis mAbs as prophylactic agents for neonatal CLABSI.

Fc engineered ACE2-Fc is a potent multifunctional agent targeting SARS-CoV2

Joining a function-enhanced Fc-portion of human IgG to the SARS-CoV-2 entry receptor ACE2 produces an antiviral decoy with strain transcending virus neutralizing activity. SARS-CoV-2 neutralization and Fc-effector functions of ACE2-Fc decoy proteins, formatted with or without the ACE2 collectrin domain, were optimized by Fc-modification. The different Fc-modifications resulted in distinct effects on neutralization and effector functions. H429Y, a point mutation outside the binding sites for FcγRs or complement caused non-covalent oligomerization of the ACE2-Fc decoy proteins, abrogated FcγR interaction and enhanced SARS-CoV-2 neutralization. Another Fc mutation, H429F did not improve virus neutralization but resulted in increased C5b-C9 fixation and transformed ACE2-Fc to a potent mediator of complement-dependent cytotoxicity (CDC) against SARS-CoV-2 spike (S) expressing cells. Furthermore, modification of the Fc-glycan enhanced cell activation via FcγRIIIa. These different immune profiles demonstrate the capacity of Fc-based agents to be engineered to optimize different mechanisms of protection for SARS-CoV-2 and potentially other viral pathogens.

Designing antibodies as therapeutics

Antibody therapeutics are a large and rapidly expanding drug class providing major health benefits. We provide a snapshot of current antibody therapeutics including their formats, common targets, therapeutic areas, and routes of administration. Our focus is on selected emerging directions in antibody design where progress may provide a broad benefit. These topics include enhancing antibodies for cancer, antibody delivery to organs such as the brain, gastrointestinal tract, and lungs, plus antibody developability challenges including immunogenicity risk assessment and mitigation and subcutaneous delivery. Machine learning has the potential, albeit as yet largely unrealized, for a transformative future impact on antibody discovery and engineering.

Proprotein Convertases and the Complement System

Proteins destined for secretion - after removal of the signal sequence - often undergo further proteolytic processing by proprotein convertases (PCs). Prohormones are typically processed in the regulated secretory pathway, while most plasma proteins travel though the constitutive pathway. The complement system is a major proteolytic cascade in the blood, serving as a first line of defense against microbes and also contributing to the immune homeostasis. Several complement components, namely C3, C4, C5 and factor I (FI), are multi-chain proteins that are apparently processed by PCs intracellularly. Cleavage occurs at consecutive basic residues and probably also involves the action of carboxypeptidases. The most likely candidate for the intracellular processing of complement proteins is furin, however, because of the overlapping specificities of basic amino acid residue-specific proprotein convertases, other PCs might be involved. To our surprise, we have recently discovered that processing of another complement protein, mannan-binding lectin-associated serine protease-3 (MASP-3) occurs in the blood by PCSK6 (PACE4). A similar mechanism had been described for the membrane protease corin, which is also activated extracellularly by PCSK6. In this review we intend to point out that the proper functioning of the complement system intimately depends on the action of proprotein convertases. In addition to the non-enzymatic components (C3, C4, C5), two constitutively active complement proteases are directly activated by PCs either intracellularly (FI), or extracellularly (MASP-3), moreover indirectly, through the constitutive activation of pro-factor D by MASP-3, the activity of the alternative pathway also depends on a PC present in the blood.

Avidity in antibody effector functions and biotherapeutic drug design

Antibodies are the cardinal effector molecules of the immune system and are being leveraged with enormous success as biotherapeutic drugs. A key part of the adaptive immune response is the production of an epitope-diverse, polyclonal antibody mixture that is capable of neutralizing invading pathogens or disease-causing molecules through binding interference and by mediating humoral and cellular effector functions. Avidity - the accumulated binding strength derived from the affinities of multiple individual non-covalent interactions - is fundamental to virtually all aspects of antibody biology, including antibody-antigen binding, clonal selection and effector functions. The manipulation of antibody avidity has since emerged as an important design principle for enhancing or engineering novel properties in antibody biotherapeutics. In this Review, we describe the multiple levels of avidity interactions that trigger the overall efficacy and control of functional responses in both natural antibody biology and their therapeutic applications. Within this framework, we comprehensively review therapeutic antibody mechanisms of action, with particular emphasis on engineered optimizations and platforms. Overall, we describe how affinity and avidity tuning of engineered antibody formats are enabling a new wave of differentiated antibody drugs with tailored properties and novel functions, promising improved treatment options for a wide variety of diseases.

Inside-Out of Complement in Cancer

The role of complement in cancer has received increasing attention over the last decade. Recent studies provide compelling evidence that complement accelerates cancer progression. Despite the pivotal role of complement in fighting microbes, complement seems to suppress antitumor immunity via regulation of host cell in the tumor microenvironment. Although most studies link complement in cancer to complement activation in the extracellular space, the discovery of intracellular activation of complement, raises the question: what is the relevance of this process for malignancy? Intracellular activation is pivotal for the survival of immune cells. Therefore, complement can be important for tumor cell survival and growth regardless of the role in immunosuppression. On the other hand, because intracellular complement (the complosome) is indispensable for activation of T cells, these functions will be essential for priming antitumor T cell responses. Here, we review functions of complement in cancer with the consideration of extra and intracellular pathways of complement activation and spatial distribution of complement proteins in tumors and periphery and provide our take on potential significance of complement as biomarker and target for cancer therapy.

At Critically Low Antigen Densities, IgM Hexamers Outcompete Both IgM Pentamers and IgG1 for Human Complement Deposition and Complement-Dependent Cytotoxicity

IgM is secreted as a pentameric polymer containing a peptide called the joining chain (J chain). However, integration of the J chain is not required for IgM assembly and in its absence IgM predominantly forms hexamers. The conformations of pentameric and hexameric IgM are remarkably similar with a hexagonal arrangement in solution. Despite these similarities, hexameric IgM has been reported to be a more potent complement activator than pentameric IgM, but reported relative potencies vary across different studies. Because of these discrepancies, we systematically investigated human IgM-mediated complement activation. We recombinantly generated pentameric and hexameric human IgM (IgM+J and IgM-J, respectively) mAbs and measured their ability to induce complement deposition and complement-dependent cytotoxicity when bound to several Ags at varying densities. At high Ag densities, hexameric and pentameric IgM activate complement to a similar extent as IgG1. However, at low densities, hexameric IgM outcompeted pentameric IgM and even more so IgG1. These differences became progressively more pronounced as antigenic density became critically low. Our findings highlight that the differential potency of hexameric and pentameric IgM for complement activation is profoundly dependent on the nature of its interactions with Ag. Furthermore, it underscores the importance of IgM in immunity because it is a more potent complement activator than IgG1 at low Ag densities.

IMM-BCP-01, a patient-derived anti-SARS-CoV-2 antibody cocktail, is active across variants of concern including Omicron BA.1 and BA.2

Monoclonal antibodies are an efficacious therapy against SARS-CoV-2. However, rapid viral mutagenesis, led to escape from most of these therapies, outlining the need for an antibody cocktail with a broad neutralizing potency. Using an unbiased interrogation of the memory B cell repertoire of convalescent COVID-19 patients, we identified human antibodies with broad antiviral activity in vitro and efficacy in vivo against all tested SARS-CoV-2 variants of concern, including Delta, Omicron BA.1 and BA.2. Here, we describe an antibody cocktail IMM-BCP-01, that consists of three patient-derived broadly neutralizing antibodies directed at non-overlapping surfaces on the SARS-CoV-2 spike protein. Two antibodies, IMM20184 and IMM20190, directly blocked Spike binding to the ACE2 receptor. Binding of the third antibody, IMM20253, to its cryptic epitope on the outer surface of RBD, altered the conformation of the Spike Trimer, promoting release of Spike monomers. These antibodies decreased Omicron SARS-CoV-2 infection in the lungs of Syrian golden hamsters in vivo, and potently induced antiviral effector response in vitro, including phagocytosis, ADCC, and complement pathway activation. Our pre-clinical data demonstrated that the three antibody cocktail IMM-BCP-01 could be a promising means for preventing or treating infection of SARS-CoV-2 variants of concern, including Omicron BA.1 and BA.2, in susceptible individuals.

Exploring complement-dependent cytotoxicity by rituximab isotypes in 2D and 3D-cultured B-cell lymphoma

BACKGROUND

The therapeutic IgG1 anti-CD20 antibody, rituximab (RTX), has greatly improved prognosis of many B-cell malignancies. Despite its success, resistance has been reported and detailed knowledge of RTX mechanisms are lacking. Complement-dependent cytotoxicity (CDC) is one important mode of action of RTX. The aim of this study was to systematically evaluate factors influencing complement-mediated tumor cell killing by RTX.

METHODS

Different RTX isotypes, IgG1, IgG3, IgA1 and IgA2 were evaluated and administered on four human CD20+ B-cell lymphoma cell lines, displaying diverse expression of CD20 and complement-regulatory protein CD59. Complement activation was assessed on lymphoma cells grown in 2 and 3-dimensional (3D) culture systems by trypan blue exclusion. CDC in 3D spheroids was additionally analyzed by Annexin V and propidium iodide staining by flow cytometry, and confocal imaging. Anti-CD59 antibody was used to evaluate influence of CD59 in RTX-mediated CDC responses. Statistical differences were determined by one-way ANOVA and Tukey post hoc test.

RESULTS

We found that 3 out of 4 lymphomas were sensitive to RTX-mediated CDC when cultured in 2D, while 2 out of 4 when grown in 3D. RTX-IgG3 had the greatest CDC potential, followed by clinical standard RTX-IgG1 and RTX-IgA2, whereas RTX-IgA1 displayed no complement activation. Although the pattern of different RTX isotypes to induce CDC were similar in the sensitive lymphomas, the degree of cell killing differed. A greater CDC activity was seen in lymphoma cells with a higher CD20/CD59 expression ratio. These lymphomas were also sensitive to RTX when grown in 3D spheroids, although the CDC activity was substantially reduced compared to 2D cultures. Analysis of RTX-treated spheroids demonstrated apoptosis and necrosis essentially in the outer cell-layers. Neutralization of CD59 overcame resistance to RTX-mediated CDC in 2D-cultured lymphoma cells, but not in spheroids.

CONCLUSIONS

The results demonstrate that CDC outcome in CD20+ B-cell lymphoma is synergistically influenced by choice of RTX isotype, antigen density, tumor structure, and degree of CD59 expression. Assessment of tumor signatures, such as CD20/CD59 ratio, can be advantageous to predict CDC efficiency of RTX in vivo and may help to develop rational mAbs to raise response rates in patients.

Biophysical Characterization of the Oligomeric States of Recombinant Immunoglobulins Type-M and Their C1q-Binding Kinetics by Biolayer Interferometry

Immunoglobulins type-M (IgMs) are one of the first antibody classes mobilized during immune responses against pathogens and tumor cells. Binding to specific target antigens enables the interaction with the C1 complex which strongly activates the classical complement pathway. This biological function is the basis for the huge therapeutic potential of IgMs. But, due to their high oligomeric complexity, in vitro production, biochemical characterization, and biophysical characterization are challenging. In this study, we present recombinant production of two IgM models (IgM617 and IgM012) in pentameric and hexameric states and the evaluation of their polymer distribution using different biophysical methods (analytical ultracentrifugation, size exclusion chromatography coupled to multi-angle laser light scattering, mass photometry, and transmission electron microscopy). Each IgM construct is defined by a specific expression and purification pattern with different sample quality. Nevertheless, both purified IgMs were able to activate complement in a C1q-dependent manner. More importantly, BioLayer Interferometry (BLI) was used for characterizing the kinetics of C1q binding to recombinant IgMs. We show that recombinant IgMs possess similar C1q-binding properties as IgMs purified from human plasma.

Antibody homotypic interactions are encoded by germline light chain complementarity determining region 2

Weak transient interactions are fundamental to immune responses, enabling avidity-driven triggers for pathogen neutralization and cellular regulation. In contrast to obligate binding interactions that can be directly investigated structurally, the low or transitory abundance of weak interactions make them difficult to identify and characterize. This study leverages receptor agonism systems that are sensitive to oligomerization to investigate transient homotypic interfaces between antibody Fab regions. Our results show that self-association determinants are encoded naturally by the antibody germline through light chain complementarity determining region 2 (CDRL2), and these determinants can be engineered into antibodies to enhance their therapeutic properties. Insights into avidity-driven interactions create opportunities for optimization, and accordingly this work expands the engineering toolbox for antibody-based drugs.

The utilization of avidity to drive and tune functional responses is fundamental to antibody biology and often underlies the mechanisms of action of monoclonal antibody drugs. There is increasing evidence that antibodies leverage homotypic interactions to enhance avidity, often through weak transient interfaces whereby self-association is coupled with target binding. Here, we comprehensively map the Fab–Fab interfaces of antibodies targeting DR5 and 4-1BB that utilize homotypic interaction to promote receptor activation and demonstrate that both antibodies have similar self-association determinants primarily encoded within a germline light chain complementarity determining region 2 (CDRL2). We further show that these determinants can be grafted onto antibodies of distinct target specificity to substantially enhance their activity. An expanded characterization of all unique germline CDRL2 sequences reveals additional self-association sequence determinants encoded in the human germline repertoire. Our results suggest that this phenomenon is unique to CDRL2, and is correlated with the less frequent antigen interaction and lower somatic hypermutation associated with this loop. This work reveals a previously unknown avidity mechanism in antibody native biology that can be exploited for the engineering of biotherapeutics.

Pathomechanisms in demyelination and astrocytopathy: autoantibodies to AQP4, MOG, GFAP, GRP78 and beyond

PURPOSE OF REVIEW

The purpose of this review is to highlight the recently emerging pathomechanisms of diseases associated with autoantibodies to AQP4, MOG, GFAP, GRP78 and further novel targets. We discuss novel biomarkers and therapeutic approaches.

RECENT FINDINGS

Although complement-mediated cytotoxicity (CDC) is regarded as the major effector mechanism for AQP4-IgG in neuromyelitis optica spectrum disorders (NMOSD), recent studies helped to understand the relevance of complement-independent effector mechanisms. For MOG-IgG mediated diseases the role of CDC is less clear. MOG-IgG may trigger a tightly controlled FcR and BTK-driven microglia proliferative response in MOG-antibody-associated diseases. Differences of antibody-mediated tissue damage may reflect differential response to therapy. In addition, antibodies to GFAP, GRP78 and further novel targets have been implicated in demyelination and astrocytopathy.

SUMMARY

Elucidating the whole spectrum of effector functions in diseases mediated by AQP4-IgG and MOG-IgG and understanding the role of additional novel autoantibodies involved in demyelination and astrocytopathy may guide further novel treatment decisions.

A novel FOXP3 mutation in a Chinese child with IPEX-associated membranous nephropathy

BACKGROUND

Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome is a monogenic immunodeficiency disease caused by forkhead box protein3 (FOXP3) mutation. The kidney is commonly involved in IPEX syndrome, but there were few studies focusing on renal involvement.

METHODS

Whole-exome sequencing was used to identify the novel FOXP3 mutation. We collected clinical manifestations, kidney pathology, and gene function of the proband. All the previously published studies with IPEX-associated renal involvement were reviewed.

RESULTS

We report a late-onset Chinese child with IPEX-associated membranous nephropathy (MN). Type 1 diabetes mellitus and nephrotic-range proteinuria are the main clinical manifestations. Whole-exome sequencing shows a novel c.766A > G mutation in the FOXP3 gene. The literature review indicates that renal manifestations include proteinuria, microscopic hematuria, and renal insufficiency. MN is the most common pathological type in children with IPEX, followed by tubulointerstitial nephritis, interstitial nephritis, minimal change nephrotic syndrome, and membranoproliferative glomerulonephritis.

CONCLUSION

In summary, we report a novel FOXP3 mutation (c.766A > G) with MN stage II in IPEX. In a literature review, MN is the most common pathological type in children with IPEX and proteinuria is the most prevalent clinical feature. IPEX should be considered in the differential diagnosis of MN patients with related endocrine diseases and immune disorders.

Characterization of the Estrogen Response Helps to Predict Prognosis and Identify Potential Therapeutic Targets in Cholangiocarcinoma

Cholangiocarcinoma (CCA) is an aggressive malignancy originating from the epithelium of the bile duct. The prognosis of patients is poor regardless of radical resection and chemoradiotherapy. The current classification and prognostic model of CCA are unable to satisfy the requirements for predicting the clinical outcome and exploring therapeutic targets. Estrogen signaling is involved in diverse cancer types, and it has long been established that CCA could be regulated by estrogen. In our study, estrogen response was identified to be significantly and stably correlated with poor prognosis in CCA. Employing several algorithms, CCA was classified into ES cluster A and B. ES cluster B was mainly composed of patients with fluke infection and overlapped with CCA cluster 1/2, and ES cluster A was mainly composed of patients without fluke infection and overlapped with CCA cluster 3/4. COMT and HSD17B1 were identified to be responsible for the differential estrogen response between ES clusters A and B, and the estrogen response may be correlated with the differentiation and cancer stemness of CCA at the single-cell level. Complement activation and the expression of C3 and C5, which are mainly expressed by CCA cells, were significantly downregulated in ES cluster B. An estrogen response risk score (ESRS) model was constructed to predict the prognosis of CCA, followed by a nomogram integrating ESRS and clinical features. Finally, altered pathways, applicable drugs and sensitivity to chemical drugs were analyzed specific to the estrogen response. In summary, our results provide insights into the role of the estrogen response in CCA progression as well as applicable drugs and potential therapeutic targets in estrogen metabolism, the complement system and ESRS-related pathways.

Hexamerization of Anti-SARS CoV IgG1 Antibodies Improves Neutralization Capacity

The SARS-CoV-2 pandemic and particularly the emerging variants have deepened the need for widely available therapeutic options. We have demonstrated that hexamer-enhancing mutations in the Fc region of anti-SARS-CoV IgG antibodies lead to a noticeable improvement in IC50 in both pseudo and live virus neutralization assay compared to parental molecules. We also show that hexamer-enhancing mutants improve C1q binding to target surface. To our knowledge, this is the first time this format has been explored for application in viral neutralization and the studies provide proof-of-concept for the use of hexamer-enhanced IgG1 molecules as potential anti-viral therapeutics.

Complement-Mediated Neutralisation Identified in Ebola Virus Disease Survivor Plasma: Implications for Protection and Pathogenesis

The 2013-2016 Ebola virus (EBOV) epidemic in West Africa was unprecedented in case numbers and fatalities, and sporadic outbreaks continue to arise. Antibodies to the EBOV glycoprotein (GP) are strongly associated with survival and their use in immunotherapy is often initially based on their performance in neutralisation assays. Other immune effector functions also contribute to EBOV protection but are more complex to measure. Their interactions with the complement system in particular are comparatively under-researched and commonly excluded from cellular immunoassays. Using EBOV convalescent plasma samples from the 2013-2016 epidemic, we investigated antibody and complement-mediated neutralisation and how these interactions can influence immunity in response to EBOV-GP and its secreted form (EBOV-sGP). We defined two cohorts: one with low-neutralising titres in relation to EBOV-GP IgG titres (LN cohort) and the other with a direct linear relationship between neutralisation and EBOV-GP IgG titres (N cohort). Using flow cytometry antibody-dependent complement deposition (ADCD) assays, we found that the LN cohort was equally efficient at mediating ADCD in response to the EBOV-GP but was significantly lower in response to the EBOV-sGP, compared to the N cohort. Using wild-type EBOV neutralisation assays with a cohort of the LN plasma, we observed a significant increase in neutralisation associated with the addition of pooled human plasma as a source of complement. Flow cytometry ADCD was also applied using the GP of the highly virulent Sudan virus (SUDV) of the Sudan ebolavirus species. There are no licensed vaccines or therapeutics against SUDV and it overlaps in endemicity with EBOV. We found that the LN plasma was significantly less efficient at cross-reacting and mediating ADCD. Overall, we found a differential response in ADCD between LN and N plasma in response to various Ebolavirus glycoproteins, and that these interactions could significantly improve EBOV neutralisation for selected LN plasma samples. Preservation of the complement system in immunoassays could augment our understanding of neutralisation and thus protection against infection.

Novel interaction of properdin and coagulation factor XI: Crosstalk between complement and coagulation

Background

Evidence of crosstalk between the complement and coagulation cascades exists, and dysregulation of either pathway can lead to serious thromboinflammatory events. Both the intrinsic pathway of coagulation and the alternative pathway of complement interact with anionic surfaces, such as glycosaminoglycans. Hitherto, there is no evidence for a direct interaction of properdin (factor P [FP]), the only known positive regulator of complement, with coagulation factor XI (FXI) or activated FXI (FXIa).

Objectives

The aim was to investigate crosstalk between FP and the intrinsic pathway and the potential downstream consequences.

Methods

Chromogenic assays were established to characterize autoactivation of FXI in the presence of dextran sulfate (DXS), enzyme kinetics of FXIa, and the downstream effects of FP on intrinsic pathway activity. Substrate specificity changes were investigated using SDS-PAGE and liquid chromatography-mass spectrometry (LC-MS). Surface plasmon resonance (SPR) was used to determine direct binding between FP and FXIa.

Results/Conclusions

We identified a novel interaction of FP with FXIa resulting in functional consequences. FP reduces activity of autoactivated FXIa toward S-2288. FXIa can cleave FP in the presence of DXS, demonstrated using SDS-PAGE, and confirmed by LC-MS. FXIa can cleave factor IX (FIX) and FP in the presence of DXS, determined by SDS-PAGE. DXS alone modulates FXIa activity, and this effect is further modulated by FP. We demonstrate that FXI and FXIa bind to FP with high affinity. Furthermore, FX activation downstream of FXIa cleavage of FIX is modulated by FP. These findings suggest a novel intercommunication between complement and coagulation pathways.

High-Resolution Native Mass Spectrometry

Native mass spectrometry (MS) involves the analysis and characterization of macromolecules, predominantly intact proteins and protein complexes, whereby as much as possible the native structural features of the analytes are retained. As such, native MS enables the study of secondary, tertiary, and even quaternary structure of proteins and other biomolecules. Native MS represents a relatively recent addition to the analytical toolbox of mass spectrometry and has over the past decade experienced immense growth, especially in enhancing sensitivity and resolving power but also in ease of use. With the advent of dedicated mass analyzers, sample preparation and separation approaches, targeted fragmentation techniques, and software solutions, the number of practitioners and novel applications has risen in both academia and industry. This review focuses on recent developments, particularly in high-resolution native MS, describing applications in the structural analysis of protein assemblies, proteoform profiling of─among others─biopharmaceuticals and plasma proteins, and quantitative and qualitative analysis of protein-ligand interactions, with the latter covering lipid, drug, and carbohydrate molecules, to name a few.

Novel treatment strategies for acetylcholine receptor antibody-positive myasthenia gravis and related disorders

The presence of autoantibodies directed against the muscle nicotinic acetylcholine receptor (AChR) is the most common cause of myasthenia gravis (MG). These antibodies damage the postsynaptic membrane of the neuromuscular junction and cause muscle weakness by depleting AChRs and thus impairing synaptic transmission. As one of the best-characterized antibody-mediated autoimmune diseases, AChR-MG has often served as a reference model for other autoimmune disorders. Classical pharmacological treatments, including broad-spectrum immunosuppressive drugs, are effective in many patients. However, complete remission cannot be achieved in all patients, and 10% of patients do not respond to currently used therapies. This may be attributed to production of autoantibodies by long-lived plasma cells which are resistant to conventional immunosuppressive drugs. Hence, novel therapies specifically targeting plasma cells might be a suitable therapeutic approach for selected patients. Additionally, in order to reduce side effects of broad-spectrum immunosuppression, targeted immunotherapies and symptomatic treatments will be required. This review presents established therapies as well as novel therapeutic approaches for MG and related conditions, with a focus on AChR-MG.

Immunoglobulin G is Increased in the Injured Spinal Cord in a Sex and Age Dependent Manner

There are limited studies examining age and sex as biological variables in the pathophysiology of spinal cord injury (SCI). The use of older animals and sex-balanced groups in SCI models is increasingly prioritized to better match clinical demographics. Including older animals in SCI studies is technically challenging, and outcomes are unpredictable with respect to biological and treatment responses. Incidental discoveries that are unrelated to the question under investigation often emerge while including age and sex as biological variables. When probing tissue homogenates on Western blots of 4- and 14-month-old (MO) mice, we identified a sex- and age-dependent increase in immunoglobulin G (IgG) within the spinal cords of older, 14-MO mice acutely after SCI, with females having more IgG compared with males. We further probed to determine whether differences in hemorrhage exist between sexes or ages by evaluating hemoglobin within spinal homogenates. Differences in hemoglobin between sexes and ages were not consistently observed. Because IgG was elevated in an age- and sex-dependent manner without of evidence of differences in hemorrhage, our findings point to potential pre-existing differences in IgG within mouse plasma in an age- and sex-dependent manner. This report has identified age- and sex-dependent differences in infiltrating IgG into the injured spinal cord environment that may affect injury and recovery processes. Our findings highlight that systemic contributions to SCI can be sex- and age-dependent and illustrate the value of reporting incidental discoveries.