Direct enhancement of viral neutralising antibody potency by the complement system: a largely forgotten phenomenon

Neutralisation assays are commonly used to assess vaccine-induced and naturally acquired immune responses; identify correlates of protection; and inform important decisions on the screening, development, and use of therapeutic antibodies. Neutralisation assays are useful tools that provide the gold standard for measuring the potency of neutralising antibodies, but they are not without limitations. Common methods such as the heat-inactivation of plasma samples prior to neutralisation assays, or the use of anticoagulants such as EDTA for blood collection, can inactivate the complement system. Even in non-heat-inactivated samples, the levels of complement activity can vary between samples. This can significantly impact the conclusions regarding neutralising antibody potency. Restoration of the complement system in these samples can be achieved using an exogenous source of plasma with preserved complement activity or with purified complement proteins. This can significantly enhance the neutralisation titres for some antibodies depending on characteristics such as antibody isotype and the epitope they bind, enable neutralisation with otherwise non-neutralising antibodies, and demonstrate a better relationship between in vitro and in vivo findings. In this review, we discuss the evidence for complement-mediated enhancement of antibody neutralisation against a range of viruses, explore the potential mechanisms which underpin this enhancement, highlight current gaps in the literature, and provide a brief summary of considerations for adopting this approach in future research applications.

IgM N-glycosylation correlates with COVID-19 severity and rate of complement deposition

The glycosylation of IgG plays a critical role during human severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, activating immune cells and inducing cytokine production. However, the role of IgM N-glycosylation has not been studied during human acute viral infection. The analysis of IgM N-glycosylation from healthy controls and hospitalized coronavirus disease 2019 (COVID-19) patients reveals increased high-mannose and sialylation that correlates with COVID-19 severity. These trends are confirmed within SARS-CoV-2-specific immunoglobulin N-glycan profiles. Moreover, the degree of total IgM mannosylation and sialylation correlate significantly with markers of disease severity. We link the changes of IgM N-glycosylation with the expression of Golgi glycosyltransferases. Lastly, we observe antigen-specific IgM antibody-dependent complement deposition is elevated in severe COVID-19 patients and modulated by exoglycosidase digestion. Taken together, this work links the IgM N-glycosylation with COVID-19 severity and highlights the need to understand IgM glycosylation and downstream immune function during human disease.

Plasma levels of mannan-binding lectin-associated serine proteases are increased in type 1 diabetes patients with insulin resistance

Activation of the lectin pathway of the complement system, as demonstrated by elevated levels of mannan-binding lectin proteins (MBL), contributes to vascular pathology in type 1 diabetes (T1D). Vascular complications are greatest in T1D individuals with concomitant insulin resistance (IR), however, whether IR amplifies activiation of the lectin pathway in T1D is unknown. We pooled pretreatment data from two RCTs and performed a cross-sectional analysis on 46 T1D individuals. We employed estimated glucose disposal rate (eGDR), a validated IR surrogate with cut-points of: <5.1, 5.1-8.7, and > 8.7 mg/kg/min to determine IR status, with lower eGDR values conferring higher degrees of IR. Plasma levels of MBL-associated proteases (MASP-1, MASP-2, and MASP-3) and their regulatory protein MAp44 were compared among eGDR classifications. In a subset of 14 individuals, we assessed change in MASPs and MAp44 following improvement in IR. We found that MASP-1, MASP-2, MASP-3, and MAp44 levels increased in a stepwise fashion across eGDR thresholds with elevated MASPs and MAp44 levels conferring greater degrees of IR. In a subset of 14 patients, improvement in IR was associated with significant reductions in MASPs, but not MAp44, levels. In conclusion, IR in T1D amplifies levels of MASP-1/2/3 and their regulator MAp44, and improvement of IR normalizes MASP-1/2/3 levels. Given that elevated levels of these proteins contribute to vascular pathology, amplification of the lectin pathway of the complement system may offer mechanistic insight into the relationship between IR and vascular complications in T1D.

The Importance of Complement-Mediated Immune Signaling in Alzheimer's Disease Pathogenesis

As an essential component of our innate immune system, the complement system is responsible for our defense against pathogens. The complement cascade has complex roles in the central nervous system (CNS), most of what we know about it stems from its role in brain development. However, in recent years, numerous reports have implicated the classical complement cascade in both brain development and decline. More specifically, complement dysfunction has been implicated in neurodegenerative disorders, such as Alzheimer's disease (AD), which is the most common form of dementia. Synapse loss is one of the main pathological hallmarks of AD and correlates with memory impairment. Throughout the course of AD progression, synapses are tagged with complement proteins and are consequently removed by microglia that express complement receptors. Notably, astrocytes are also capable of secreting signals that induce the expression of complement proteins in the CNS. Both astrocytes and microglia are implicated in neuroinflammation, another hallmark of AD pathogenesis. In this review, we provide an overview of previously known and newly established roles for the complement cascade in the CNS and we explore how complement interactions with microglia, astrocytes, and other risk factors such as TREM2 and ApoE4 modulate the processes of neurodegeneration in both amyloid and tau models of AD.

Endogenous complement-activating IgM is not required for primary antibody responses but promotes plasma cell differentiation and secondary antibody responses to a large particulate antigen in mice

Lack of complement factor C1q of the classical pathway results in severely impaired primary antibody responses. This is a paradox because antibodies, especially IgM, are the most efficient activators of the classical pathway and very little specific IgM will be present at priming. A possible explanation would be that natural IgM, binding with low affinity to the antigen, may suffice to activate complement. In support of this, mice lacking secretory IgM have an impaired antibody response, which can be rescued by transfer of non-immune IgM. Moreover, passive administration of specific IgM together with antigen enhances the antibody response in a complement-dependent fashion. To test the idea, we have used a knock-in mouse strain (Cμ13) carrying a point mutation in the IgM heavy chain, rendering the IgM unable to activate complement. Mutant mice backcrossed to BALB/c or C57BL/6 background were primed and boosted with a low dose of sheep red blood cells. Confirming earlier data, no impairment in early, primary IgM- or IgG-responses were seen in either of the Cμ13 strains. However, in one of the mutant strains, late primary IgG responses were impaired. A more pronounced effect was observed after boost, when the IgG response, the number of germinal center B cells and antibody secreting cells as well as the opsonization of antigen were impaired in mutant mice. We conclude that complement activation by natural IgM cannot explain the role of C1q in primary antibody responses, but that endogenous, specific, wildtype IgM generated after immunization feedback-enhances the response to a booster dose of antigen. Importantly, this mechanism can only partially explain the role of complement in the generation of antibody responses because the IgG response was much lower in C3- or complement receptor 1 and 2-deficient mice than in Cμ13 mice.

Microbial evasion of the complement system: a continuous and evolving story

The complement system is a fundamental part of the innate immune system that plays a key role in the battle of the human body against invading pathogens. Through its three pathways, represented by the classical, alternative, and lectin pathways, the complement system forms a tightly regulated network of soluble proteins, membrane-expressed receptors, and regulators with versatile protective and killing mechanisms. However, ingenious pathogens have developed strategies over the years to protect themselves from this complex part of the immune system. This review briefly discusses the sequence of the complement activation pathways. Then, we present a comprehensive updated overview of how the major four pathogenic groups, namely, bacteria, viruses, fungi, and parasites, control, modulate, and block the complement attacks at different steps of the complement cascade. We shed more light on the ability of those pathogens to deploy more than one mechanism to tackle the complement system in their path to establish infection within the human host.

Complement activation drives antibody-mediated transfusion-related acute lung injury via macrophage trafficking and formation of NETs

ABSTRACT

Transfusion-related acute lung injury (TRALI) is one of the leading causes of transfusion-related fatalities and, to date, is without available therapies. Here, we investigated the role of the complement system in TRALI. Murine anti-major histocompatibility complex class I antibodies were used in TRALI mouse models, in combination with analyses of plasma samples from patients with TRALI. We found that in vitro complement activation was related to in vivo antibody-mediated TRALI induction, which was correlated with increased macrophage trafficking from the lungs to the blood in a fragment crystallizable region (Fc)-dependent manner and that this was dependent on C5. Human immunoglobulin G 1 variants of the murine TRALI-inducing antibody 34-1-2S, either unable to activate complement and/or bind to Fcγ receptors (FcγRs), revealed an essential role for the complement system, but not for FcγRs, in the onset of 34-1-2S-mediated TRALI in mice. In addition, we found high levels of complement activation in the plasma of patients with TRALI (n = 53), which correlated with elevated neutrophil extracellular trap (NET) markers. In vitro we found that NETs could be formed in a murine, 2-hit model, mimicking TRALI with lipopolysaccharide and C5a stimulation. Collectively, this reveals a critical role of Fc-mediated complement activation in TRALI, with a direct relation to macrophage trafficking from the lungs to the blood and an association with NET formation, suggesting that targeting the complement system may be an attractive therapeutic approach for combating TRALI.

Malaria sporozoites evade host complement attack

Complement is the first line of the host innate immune response against bacterial and viral infections; however, its role in the development of the malaria liver stage remains undefined. We found that sporozoite infection by either a mosquito bite or intravenous injection activated systemic complement, but neither depletion of C3 nor knockout of C3 had a significant effect on malaria liver stage development. Incubation of mouse serum with trypsin-treated sporozoites, but not naive sporozoites, led to the deposition of a membrane attack complex (MAC) on the surface of sporozoites and greatly reduced the number of exo-erythrocytic forms (EEF). Further studies have shown that the recruitment of complement H factor (CFH) may be associated with the prevention of MAC deposition on the surface of naïve sporozoites. Our data strongly suggest that sporozoites can escape complement attacks and provide us with a novel strategy to prevent malaria infection.

Complement

The role of complement in human autoimmune, inflammatory, and infectious diseases is reviewed, focusing on clinical applicability. A typical case is presented in which serum testing for C3 and C4 is performed to help assess a syndrome with a broad differential diagnosis. The review includes a discussion of complement deficiency states, consumption of complement by diseases characterized by immune-complex formation and deposition, usefulness and interpretation of laboratory tests for complement, and development of drugs targeting specific components of the complement pathway for a growing number of indications.

Detection of Intracellular Complement Activation by Nanoparticles in Human T Lymphocytes

The complement system is complex and includes two main components: the systemic or plasma complement and the so-called intracellular complement or complosome. The complement proteins expressed by the liver and secreted into blood plasma compose the plasma complement system, whereas complement proteins expressed by and functioning inside the cell represent the intracellular complement. The complement system plays an essential role in host defense; however, complement activation may lead to pathologies when uncontrolled. When such undesirable activation of the plasma complement occurs in response to a drug product, it leads to immediate-type hypersensitivity reactions independent of immunoglobulin E. These reactions are often called complement activation-related pseudoallergy (CARPA). In addition to the blood plasma, the complement protein C3 is found in many cells, including lymphocytes, monocytes, endothelial, and even cancer cells. The activation of the intracellular complement generates split products, which are exported from the cell onto the membrane. Since the activation of the intracellular complement in T lymphocytes was found to correlate with autoimmune disorders, and growing evidence is available for the involvement of T lymphocytes in the development of drug-induced hypersensitivity reactions, understanding the ability of nanomaterials to activate intracellular complement may aid in establishing a long-term safety profile for these materials. This chapter describes a flow cytometry-based protocol for detecting intracellular complement activation by engineered nanomaterials.

Complement-pentraxins synergy: Navigating the immune battlefield and beyond

The complement is a crucial immune defense system that triggers rapid immune responses and offers efficient protection against foreign invaders and unwanted host elements, acting as a sentinel. Activation of the complement system occurs upon the recognition of pathogenic microorganisms or altered self-cells by pattern-recognition molecules (PRMs) such as C1q, collectins, ficolins, and pentraxins. Recent accumulating evidence shows that pentraxins establish a cooperative network with different classes of effector PRMs, resulting in synergistic effects in complement activation. This review describes the complex interaction of pentraxins with the complement system and the implications of this cooperative network for effective host defense during pathogen invasion.

The Role of the Complement System in the Pathogenesis of Infectious Forms of Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is an acute disease and the most common cause of childhood acute renal failure. HUS is characterized by a triad of symptoms: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. In most of the cases, HUS occurs as a result of infection caused by Shiga toxin-producing microbes: hemorrhagic Escherichia coli and Shigella dysenteriae type 1. They account for up to 90% of all cases of HUS. The remaining 10% of cases grouped under the general term atypical HUS represent a heterogeneous group of diseases with similar clinical signs. Emerging evidence suggests that in addition to E. coli and S. dysenteriae type 1, a variety of bacterial and viral infections can cause the development of HUS. In particular, infectious diseases act as the main cause of aHUS recurrence. The pathogenesis of most cases of atypical HUS is based on congenital or acquired defects of complement system. This review presents summarized data from recent studies, suggesting that complement dysregulation is a key pathogenetic factor in various types of infection-induced HUS. Separate links in the complement system are considered, the damage of which during bacterial and viral infections can lead to complement hyperactivation following by microvascular endothelial injury and development of acute renal failure.

Anaphylatoxin signaling activates macrophages to control intracellular Rickettsia proliferation

IMPORTANCE

Pathogenic Rickettsia species are extremely dangerous bacteria that grow within the cytoplasm of host mammalian cells. In most cases, these bacteria are able to overpower the host cell and grow within the protected environment of the cytoplasm. However, a dramatic conflict occurs when Rickettsia encounter innate immune cells; the bacteria can "win" by taking over the host, or the bacteria can "lose" if the host cell efficiently fights the infection. This manuscript examines how the immune complement system is able to detect the presence of Rickettsia and alert nearby cells. Byproducts of complement activation called anaphylatoxins are signals that "activate" innate immune cells to mount an aggressive defensive strategy. This study enhances our collective understanding of the innate immune reaction to intracellular bacteria and will contribute to future efforts at controlling these dangerous infections.

Medical consult: aHUS, TTP? How to distinguish and what to do

Immune thrombotic thrombocytopenic purpura (iTTP) caused by an autoantibody-mediated deficiency of ADAMTS13 and atypical hemolytic syndrome (aHUS) caused by alternative complement dysregulation are the most common primary thrombotic microangiopathies (TMAs). The evaluation of a patient with TMA is a medical emergency since it is critical to quickly distinguish iTTP and aHUS from other causes of TMA. Untreated iTTP is rapidly fatal, and delays in initiating complement inhibition in aHUS increase the risk of irreversible renal failure. An ADAMTS13 activity level of less than 10% is diagnostic of iTTP in the appropriate clinical setting. In settings where rapid-turnaround ADAMTS13 testing is not available, clinical features and clinical prediction tools are useful to identify patients who should receive emergent plasma exchange. We present an evidence-based approach to the initial (first 24 hours) diagnosis and management of iTTP and review the clinical and laboratory features that can be used to identify patients with aHUS who will benefit from early C5 blockade. We also discuss the potential use of complement blockade to improve outcomes in selected patients with secondary TMA.

Complement levels during the first trimester predict disease flare and adverse pregnancy outcomes in systemic lupus erythematosus: A network meta-analysis on 532 pregnancies

BACKGROUND

Complement levels have been proposed as candidate biomarkers of disease activity and obstetric risk in systemic lupus erythematosus (SLE) pregnancies, but their reliability has been questioned due to the physiologic fluctuations of complement during gestation. Thus, this network meta-analysis aimed at assessing the clinical significance of complement fluctuations in lupus pregnant women.

METHODS

Corresponding authors of 19 studies meeting inclusion criteria were invited to contribute with additional data including C3 and C4 levels [before pregnancy, at conception, in every trimester (T) and 3 months after delivery]; data were pooled together in a network meta-analysis.

RESULTS

A total of 532 lupus women from four studies were included in the analysis. In SLE women, C3 and C4 increased progressively during gestation: levels remained stable during T1 and peaked in T2 to decrease in T3. Patients with previous lupus nephritis (LN) and those who experienced flares during pregnancy had significantly lower mean levels of C3 and C4 at all timepoints. The lowest levels of complement were observed, particularly during T1, in patients with LN and gestational flare. Both reduction and the lack of increase of C3 and C4 levels at T1 versus conception were associated with gestational flares, particularly in LN patients. Pregnancies with flare had a statistically significant higher rate of maternal and fetal complications(60% versus 50.3%; p = 0.03).

CONCLUSIONS

Low complement levels, particularly in T1, were associated with a higher frequency of gestational flare. Either reduction or smaller increase of C3 and/or C4 levels, even within normal range, might predict flares especially in early gestation.

Pentraxins in invertebrates and vertebrates: From structure, function and evolution to clinical applications

The immune system is divided into two broad categories, consisting of innate and adaptive immunity. As recognition and effector factors of innate immunity and regulators of adaptive immune responses, lectins are considered to be important defense chemicals against microbial pathogens, cell trafficking, immune regulation, and prevention of autoimmunity. Pentraxins, important members of animal lectins, play a significant role in protecting the body from pathogen infection and regulating inflammatory reactions. They can recognize and bind to a variety of ligands, including carbohydrates, lipids, proteins, nucleic acids and their complexes, and protect the host from pathogen invasion by activating the complement cascade and Fcγ receptor pathways. Based on the primary structure of the subunit, pentraxins are divided into short and long pentraxins. The short pentraxins are comprised of C-reactive protein (CRP) and serum amyloid P (SAP), and the most important member of the long pentraxins is pentraxin 3 (PTX3). The CRP and SAP exist in both vertebrates and invertebrates, while the PTX3 may be present only in vertebrates. The major ligands and functions of CRP, SAP and PTX3 and three activation pathways involved in the complement system are summarized in this review. Their different characteristics in various animals including humans, and their evolutionary trees are analyzed. The clinical applications of CRP, SAP and PTX3 in human are reviewed. Some questions that remain to be understood are also highlighted.

Characteristics of Complement Protein Deposition in Proliferative Glomerulonephritis with Monoclonal Immunoglobulin Deposition

BACKGROUND

Hypocomplementemia and complement co-deposition with monoclonal immunoglobulins in glomeruli are not rare in proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID). Deposition of monoclonal immunoglobulins in glomeruli has been suggested to activate complement and cause kidney injury. However, the profiles of complement activation in PGNMID and their clinical and pathologic significance need to be clarified.

METHODS

Forty-six patients with PGNMID were enrolled. Proteomic analysis of glomeruli using laser microdissection and mass spectrometry was performed for ten patients with PGNMID to determine the composition of glomerular deposits. Kidney deposition of complement components was detected by immunohistochemistry and immunofluorescence. Urinary and plasma levels of complement components were measured by an enzyme-linked immunosorbent assay. Group differences were assessed using t tests or Mann-Whitney U tests depending on the distribution. Correlation analysis was performed using Spearman rank correlation or Pearson correlation.

RESULTS

Laser microdissection and mass spectrometry-based proteomic analysis showed that complement components were the most enriched proteins deposited in the glomeruli of patients with PGNMID. Glomerular deposition of C3c, C4d, and C5b-9 was detected in most patients. Levels of urinary and plasma C3a, C5a, soluble C5b-9, C4d, Bb, and C1q as well as urinary mannose-binding lectin were significantly higher in patients with PGNMID compared with healthy controls. The intensity of C3c and C4d deposition in glomeruli correlated with serum creatinine and the percentage of crescents, respectively. Furthermore, levels of urinary complement components correlated positively with serum creatinine, urinary protein excretion, percentage of crescents, and global glomerulosclerosis in kidney biopsies, whereas plasma levels of most complement components did not show a significant correlation with clinicopathologic parameters. In multivariable analysis, a higher level of urinary C4d was identified as an independent risk factor of kidney failure.

CONCLUSIONS

The complement system was found to be overactivated in PGNMID, and levels of urinary complements correlated with disease severity. A higher level of urinary C4d was identified as an independent risk factor of kidney failure.

Outer membrane protein OMP76 of Riemerella anatipestifer contributes to complement evasion and virulence by binding to duck complement factor vitronectin

Riemerella anatipestifer is an important bacterial pathogen in poultry. Pathogenic bacteria recruit host complement factors to resist the bactericidal effect of serum complement. Vitronectin (Vn) is a complementary regulatory protein that inhibits the formation of the membrane attack complex (MAC). Microbes use outer membrane proteins (OMPs) to hijack Vn for complement evasion. However, the mechanism by which R. anatipestifer achieves evasion is unclear. This study aimed to characterise OMPs of R. anatipestifer which interact with duck Vn (dVn) during complement evasion. Far-western assays and comparison of wild-type and mutant strains that were treated with dVn and duck serum demonstrated particularly strong binding of OMP76 to dVn. These data were confirmed with Escherichia coli strains expressing and not expressing OMP76. Combining tertiary structure analysis and homology modelling, truncated and knocked-out fragments of OMP76 showed that a cluster of critical amino acids in an extracellular loop of OMP76 mediate the interaction with dVn. Moreover, binding of dVn to R. anatipestifer inhibited MAC deposition on the bacterial surface thereby enhancing survival in duck serum. Virulence of the mutant strain ΔOMP76 was attenuated significantly relative to the wild-type strain. Furthermore, adhesion and invasion abilities of ΔOMP76 decreased, and histopathological changes showed that ΔOMP76 was less virulent in ducklings. Thus, OMP76 is a key virulence factor of R. anatipestifer. The identification of OMP76-mediated evasion of complement by recruitment of dVn contributes significantly to the understanding of the molecular mechanism by which R. anatipestifer escapes host innate immunity and provides a new target for the development of subunit vaccines.

Complement biomarkers in the antiphospholipid syndrome - Approaches to quantification and implications for clinical management

Complement is a major driver of antiphospholipid syndrome (APS) and a promising therapeutic target in refractory and catastrophic APS. Complement testing in APS is largely limited to research settings, and reliable, rapid-turnaround biomarkers are needed to predict those at risk for adverse clinical outcomes and most likely to benefit from complement inhibition. We review complement biomarkers and their association with thrombosis and obstetric outcomes, including: (i) complement proteins and activation fragments in the fluid phase; (ii) assays that evaluate complement on cell membranes (e.g. in vivo cell-bound complement fragments, hemolytic assays, and ex vivo 'functional' cell-based assays, and (iii) sequencing of complement genes. Current studies highlight the inconsistencies in testing both between studies and various aPL/APS subgroups, suggesting that either cell-based testing or multiplex panels employing a combination of biomarkers simultaneously may be most clinically relevant. Standardization of complement assays is needed to ensure reproducibility and establish clinically relevant applications.

The Yin Yang of Complement and Cancer

Cancer-related inflammation is a crucial component of the tumor microenvironment (TME). Complement activation occurs in cancer and supports the development of an inflammatory microenvironment. Complement has traditionally been considered a mechanism of immune resistance against cancer, and its activation is known to contribute to the cytolytic effects of antibody-based immunotherapeutic treatments. However, several studies have recently revealed that complement activation may exert protumoral functions by sustaining cancer-related inflammation and immunosuppression through different molecular mechanisms, targeting both the TME and cancer cells. These new discoveries have revealed that complement manipulation can be considered a new strategy for cancer therapies. Here we summarize our current understanding of the mechanisms by which the different elements of the complement system exert antitumor or protumor functions, both in preclinical studies and in human tumorigenesis. Complement components can serve as disease biomarkers for cancer stratification and prognosis and be exploited for tumor treatment.

PepO and CppA modulate Streptococcus sanguinis susceptibility to complement immunity and virulence

Streptococcus sanguinis is a ubiquitous commensal species of the oral cavity commonly involved as an opportunistic pathogen in cardiovascular infections. In this study, we investigated the functions of endopeptidase O (PepO) and a C3-degrading protease (CppA) in the systemic virulence of S. sanguinis. Isogenic mutants of pepO and cppA obtained in strain SK36 showed increased susceptibility to C3b deposition and to opsonophagocytosis by human polymorphonuclear neutrophils (PMN). These mutants differ, however, in their profiles of binding to serum amyloid P component (SAP) and C1q, whereas both showed reduced interaction with C4b-binding protein (C4BP) and/or factor H (FH) regulators as compared to SK36. The two mutants showed defects in ex vivo persistence in human blood, serum-mediated invasion of HCAEC endothelial cells, and virulence in a Galleria mellonella infection model. The transcriptional activities of pepO and cppA, assessed by RT-qPCR in nine wild-type strains, further indicated strain-specific profiles of pepO/cppA expression. Moreover, non-conserved amino acid substitutions were detected among the strains, mostly in CppA. Phylogenetic comparisons with homologues of streptococcal species of the oral and oropharyngeal sites suggested that S. sanguinis PepO and CppA have independent ancestralities. Thus, this study showed that PepO and CppA are complement evasion proteins expressed by S. sanguinis in a strain-specific manner, which are required for multiple functions associated with cardiovascular virulence.

Genetic, clinical, and pathological study of patients with severe hypertension-associated renal microangiopathy

BACKGROUND

Severe hypertension may be a prominent manifestation of complement-mediated thrombotic microangiopathy. Furthermore, patients with severe hypertension-associated thrombotic microangiopathy may present with concurrent hematologic abnormalities that mimic complement-mediated thrombotic microangiopathy. Whether or not severe hypertension-associated thrombotic microangiopathy is associated with genetic susceptibility in complement- and/or coagulation-pathway genes remains unclear, and there is thus a need to identify clinicopathological clues to distinguish between these entities.

METHODS

Forty-five patients with concomitant severe hypertension and thrombotic microangiopathy on kidney biopsy were identified retrospectively. Whole-exome sequencing was performed to identify rare variants in 29 complement- and coagulation-cascade genes. Clinicopathological features were compared between patients with severe hypertension-associated thrombotic microangiopathy and complement-mediated thrombotic microangiopathy with severe hypertension.

RESULTS

Three patients with pathogenic variants diagnostic of complement-mediated thrombotic microangiopathy and two with anti-factor H antibody positivity were diagnosed with complement-mediated thrombotic microangiopathy with severe hypertension. Among the 40 patients with severe hypertension-associated thrombotic microangiopathy, 53 rare variants of uncertain significance were found in the analyzed genes in 34 (34/40, 85%) patients, of whom 12 patients harbored two or more variants. Compared with complement-mediated thrombotic microangiopathy patients with severe hypertension, patients with severe hypertension-associated thrombotic microangiopathy were more likely to have left ventricular wall thickening (p < 0.001), less-severe acute glomerular thrombotic microangiopathy lesions including mesangiolysis and subendothelial space widening (both p < 0.001), and less arteriolar thrombosis formation (p < 0.001).

CONCLUSIONS

Rare genetic variants involving complement and coagulation pathways can be found in patients with severe hypertension-associated thrombotic microangiopathy; their role needs further investigation. Cardiac remodeling and acute glomerular TMA lesions may help to differentiate between severe hypertension-associated thrombotic microangiopathy and complement-mediated thrombotic microangiopathy with severe hypertension.

A human immune system (HIS) mouse model that dissociates roles for mouse and human FcR+ cells during antibody-mediated immune responses

Human immune system (HIS) mice provide a model to study human immune responses in vivo. Currently available HIS mouse models may harbor mouse Fc Receptor (FcR)-expressing cells that exert potent effector functions following administration of human Ig. Previous studies showed that the ablation of the murine FcR gamma chain (FcR-γ) results in loss of antibody-dependent cellular cytotoxicity and antibody-dependent cellular phagocytosis in vivo. We created a new FcR-γ-deficient HIS mouse model to compare host (mouse) versus graft (human) effects underlying antibody-mediated immune responses in vivo. FcR-γ-deficient HIS recipients lack expression and function of mouse activating FcRs and can be stably and robustly reconstituted with human immune cells. By screening blood B-cell depletion by rituximab Ig variants, we found that human FcγRs-mediated IgG1 effects, whereas mouse activating FcγRs were dominant in IgG4 effects. Complement played a role as an IgG1 variant (IgG1 K322A) lacking complement binding activity was largely ineffective. Finally, we provide evidence that FcγRIIIA on human NK cells could mediate complement-independent B-cell depletion by IgG1 K322A. We anticipate that our FcR-γ-deficient HIS model will help clarify mechanisms of action of exogenous administered human antibodies in vivo.