The C3a receptor antagonist SB 290157 has agonist activity

Knowing the enemy: strategic targeting of complement to treat Alzheimer disease

The complement system protects against infection, positively responds to tissue damage, clears cell debris, directs and modulates the adaptive immune system, and functions in neuronal development, normal synapse elimination and intracellular metabolism. However, complement also has a role in aberrant synaptic pruning and neuroinflammation - processes that lead to a feedforward loop of inflammation, injury and neuronal death that can contribute to neurodegenerative and neurological disorders, including Alzheimer disease. This Review provides justification, largely from preclinical mouse models but also from correlates with human tissue and biomarkers, for targeting specific complement components for therapeutic intervention in Alzheimer disease. We discuss promising strategies to slow the progression of cognitive loss with minimal undesired effects. The diverse interactions and functions of complement system components can influence biological processes in the healthy and diseased brain; here, these functions are described as a prerequisite to selecting appropriate, safe and effective therapeutic targets for translation to the clinic.

Structural insights into small-molecule agonist recognition and activation of complement receptor C3aR

The complement system plays crucial roles in innate immunity and inflammatory responses. The anaphylatoxin C3a mediates pro-inflammatory and chemotactic functions through the G protein-coupled receptor C3aR. While the active structure of the C3a-C3aR-Gi complex has been determined, the inactive conformation and activation mechanism of C3aR remain elusive. Here we report the cryo-EM structure of ligand-free, G protein-free C3aR, providing insights into its inactive conformation. In addition, we determine the structures of C3aR in complex with the synthetic small-molecule agonist JR14a in two distinct conformational states: a G protein-free intermediate, and a fully active Gi-bound state. The structure of the active JR14a-bound C3aR reveals that JR14a engages in highly conserved interactions with C3aR, similar to the binding of the C-terminal pentapeptide of C3a, along with JR14a-specific interactions. Structural comparison of C3aR in the apo, intermediate, and fully active states provides novel insights into the conformational landscape and activation mechanism of C3aR and defines a molecular basis explaining its high basal activity. Our results may aid in the rational design of therapeutics targeting complement-related inflammatory disorders.

Microglia aggravate white matter injury via C3/C3aR pathway after experimental subarachnoid hemorrhage

The activation of glial cells is intimately associated with the pathophysiology of neuroinflammation and white matter injury (WMI) during both acute and chronic phases following subarachnoid hemorrhage (SAH). The complement C3a receptor (C3aR) has a dual role in modulating inflammation and contributes to neurodevelopment, neuroplasticity, and neurodegeneration. However, its impact on WMI in the context of SAH remains unclear. In this study, 175 male C57BL/6J mice underwent SAH through endovascular perforation. Oxyhemoglobin (oxy-Hb) was employed to simulate SAH in vitro. A suite of techniques, including immunohistochemistry, transcriptomic sequencing, and a range of molecular biotechnologies, were utilized to evaluate the activation of the C3-C3aR pathway on microglial polarization and WMI. Results revealed that post-SAH abnormal activation of microglia was accompanied by upregulation of complement C3 and C3aR. The inhibition of C3aR decreased abnormal microglial activation, attenuated neuroinflammation, and ameliorated WMI and cognitive deficits following SAH. RNA-Seq indicated that C3aR inhibition downregulated several immune and inflammatory pathways and mitigated cellular injury by reducing p53-induced death domain protein 1 (Pidd1) and Protein kinase RNA-like ER kinase (Perk) expression, two factors mainly function in sensing and responding to cellular stress and endoplasmic reticulum (ER) stress. The deleterious effects of the C3-C3aR axis in the context of SAH may be related to endoplasmic reticulum (ER) stress-dependent cellular injury and inflammasome formation. Agonists of Perk can exacerbate the cellular injury and neuroinflammation, which was attenuated by C3aR inhibition after SAH. Additionally, intranasal administration of C3a during the subacute phase of SAH was found to decrease astrocyte reactivity and alleviate cognitive deficits post-SAH. This research deepens our understanding of the complex pathophysiology of WMI following SAH and underscores the therapeutic potential of C3a treatment in promoting white matter repair and enhancing functional recovery prognosis. These insights pave the way for future clinical application of C3a-based therapies, promising significant benefits in the treatment of SAH and its related complications.

Complement C3aR signaling: Immune and metabolic modulation and its impact on Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia among the elderly population. Despite its widespread prevalence, our comprehension of the intricate mechanisms governing the pathogenesis of the disease remains incomplete, posing a challenge for the development of efficient therapies. Pathologically characterized by the presence of amyloid β plaques and neurofibrillary tau tangles, AD is also accompanied by the hyperactivation of glial cells and the immune system. The complement cascade, the evolutionarily conserved innate immune pathway, has emerged as a significant contributor to AD. This review focuses on one of the complement components, the C3a receptor (C3aR), covering its structure, ligand-receptor interaction, intracellular signaling and its functional consequences. Drawing insights from cellular and AD mouse model studies, we present the multifaceted role of complement C3aR signaling in AD and attempt to convey to the readers that C3aR acts as a crucial immune and metabolic modulator to influence AD pathogenesis. Building on this framework, the objective of this review is to inform future research endeavors and facilitate the development of therapeutic strategies for this challenging condition.

Beneficial versus Detrimental Effects of Complement-Microglial Interactions in Alzheimer's Disease

Research indicates that brain-region-specific synapse loss and dysfunction are early hallmarks and stronger neurobiological correlates of cognitive decline in Alzheimer's disease (AD) than amyloid plaque and neurofibrillary tangle counts or neuronal loss. Even though the precise mechanisms underlying increased synaptic pruning in AD are still unknown, it has been confirmed that dysregulation of the balance between complement activation and inhibition is a crucial driver of its pathology. The complement includes three distinct activation mechanisms, with the activation products C3a and C5a, potent inflammatory effectors, and a membrane attack complex (MAC) leading to cell lysis. Besides pro-inflammatory cytokines, the dysregulated complement proteins released by activated microglia bind to amyloid β at the synaptic regions and cause the microglia to engulf the synapses. Additionally, research indicating that microglia-removed synapses are not always degenerating and that suppression of synaptic engulfment can repair cognitive deficits points to an essential opportunity for intervention that can prevent the loss of intact synapses. In this study, we focus on the latest research on the role and mechanisms of complement-mediated microglial synaptic pruning at different stages of AD to find the right targets that could interfere with complement dysregulation and be relevant for therapeutic intervention at the early stages of the disease.

Functional profiling of the G protein-coupled receptor C3aR1 reveals ligand-mediated biased agonism

G protein-coupled receptors (GPCRs) are leading druggable targets for several medicines, but many GPCRs are still untapped for their therapeutic potential due to poor understanding of specific signaling properties. The complement C3a receptor 1 (C3aR1) has been extensively studied for its physiological role in C3a-mediated anaphylaxis/inflammation, and in TLQP-21-mediated lipolysis, but direct evidence for the functional relevance of the C3a and TLQP-21 ligands and signal transduction mechanisms are still limited. In addition, C3aR1 G protein coupling specificity is still unclear, and whether endogenous ligands, or drug-like compounds, show ligand-mediated biased agonism is unknown. Here, we demonstrate that C3aR1 couples preferentially to Gi/o/z proteins and can recruit β-arrestins to cause internalization. Furthermore, we showed that in comparison to C3a63-77, TLQP-21 exhibits a preference toward Gi/o-mediated signaling compared to β-arrestin recruitment and internalization. We also show that the purported antagonist SB290157 is a very potent C3aR1 agonist, where antagonism of ligand-stimulated C3aR1 calcium flux is caused by potent β-arrestin-mediated internalization. Finally, ligand-mediated signaling bias impacted cell function as demonstrated by the regulation of calcium influx, lipolysis in adipocytes, phagocytosis in microglia, and degranulation in mast cells. Overall, we characterize C3aR1 as a Gi/o/z-coupled receptor and demonstrate the functional relevance of ligand-mediated signaling bias in key cellular models. Due to C3aR1 and its endogenous ligands being implicated in inflammatory and metabolic diseases, these results are of relevance toward future C3aR1 drug discovery.

Cntnap4 partial deficiency exacerbates α-synuclein pathology through astrocyte-microglia C3-C3aR pathway

Parkinson's disease (PD) is the most common progressive neurodegenerative movement disorder, which is characterized by dopaminergic (DA) neuron death and the aggregation of neurotoxic α-synuclein. Cntnap4, a risk gene of autism, has been implicated to participate in PD pathogenesis. Here we showed Cntnap4 lacking exacerbates α-synuclein pathology, nigrostriatal DA neuron degeneration and motor impairment, induced by injection of adeno-associated viral vector (AAV)-mediated human α-synuclein overexpression (AAV-hα-Syn). This scenario was further validated in A53T α-synuclein transgenic mice injected with AAV-Cntnap4 shRNA. Mechanistically, α-synuclein derived from damaged DA neuron stimulates astrocytes to release complement C3, activating microglial C3a receptor (C3aR), which in turn triggers microglia to secrete complement C1q and pro-inflammatory cytokines. Thus, the astrocyte-microglia crosstalk further drives DA neuron death and motor dysfunction in PD. Furthermore, we showed that in vivo depletion of microglia and microglial targeted delivery of a novel C3aR antagonist (SB290157) rescue the aggravated α-synuclein pathology resulting from Cntnap4 lacking. Together, our results indicate that Cntnap4 plays a key role in α-synuclein pathogenesis by regulating glial crosstalk and may be a potential target for PD treatment.

Molecular Mechanisms of Neuroinflammation in Aging and Alzheimer's Disease Progression

Aging is the most prominent risk factor for late-onset Alzheimer's disease. Aging associates with a chronic inflammatory state both in the periphery and in the central nervous system, the evidence thereof and the mechanisms leading to chronic neuroinflammation being discussed. Nonetheless, neuroinflammation is significantly enhanced by the accumulation of amyloid beta and accelerates the progression of Alzheimer's disease through various pathways discussed in the present review. Decades of clinical trials targeting the 2 abnormal proteins in Alzheimer's disease, amyloid beta and tau, led to many failures. As such, targeting neuroinflammation via different strategies could prove a valuable therapeutic strategy, although much research is still needed to identify the appropriate time window. Active research focusing on identifying early biomarkers could help translating these novel strategies from bench to bedside.

The Complement Pathway: New Insights into Immunometabolic Signaling in Diabetic Kidney Disease

Significance: The metabolic disorder, diabetes mellitus, results in microvascular complications, including diabetic kidney disease (DKD), which is partly believe to involve disrupted energy generation in the kidney, leading to injury that is characterized by inflammation and fibrosis. An increasing body of evidence indicates that the innate immune complement system is involved in the pathogenesis of DKD; however, the precise mechanisms remain unclear. Recent Advances: Complement, traditionally thought of as the prime line of defense against microbial intrusion, has recently been recognized to regulate immunometabolism. Studies have shown that the complement activation products, Complement C5a and C3a, which are potent pro-inflammatory mediators, can mediate an array of metabolic responses in the kidney in the diabetic setting, including altered fuel utilization, disrupted mitochondrial respiratory function, and reactive oxygen species generation. In diabetes, the lectin pathway is activated via autoreactivity toward altered self-surfaces known as danger-associated molecular patterns, or via sensing altered carbohydrate and acetylation signatures. In addition, endogenous complement inhibitors can be glycated, whereas diet-derived glycated proteins can themselves promote complement activation, worsening DKD, and lending support for environmental influences as an additional avenue for propagating complement-induced inflammation and kidney injury. Critical Issues: Recent evidence indicates that conventional renoprotective agents used in DKD do not target the complement, leaving this web of inflammatory stimuli intact. Future Directions: Future studies should focus on the development of novel pharmacological agents that target the complement pathway to alleviate inflammation, oxidative stress, and kidney fibrosis, thereby reducing the burden of microvascular diseases in diabetes. Antioxid. Redox Signal. 37, 781-801.

The molecular identity of the TLQP-21 peptide receptor

The TLQP-21 neuropeptide has been implicated in functions as diverse as lipolysis, neurodegeneration and metabolism, thus suggesting an important role in several human diseases. Three binding targets have been proposed for TLQP-21: C3aR1, gC1qR and HSPA8. The aim of this review is to critically evaluate the molecular identity of the TLQP-21 receptor and the proposed multi-receptor mechanism of action. Several studies confirm a critical role for C3aR1 in TLQP-21 biological activity and a largely conserved mode of binding, receptor activation and signaling with C3a, its first-identified endogenous ligand. Conversely, data supporting a role of gC1qR and HSPA8 in TLQP-21 activity remain limited, with no signal transduction pathways being described. Overall, C3aR1 is the only receptor for which a necessary and sufficient role in TLQP-21 activity has been confirmed thus far. This conclusion calls into question the validity of a multi-receptor mechanism of action for TLQP-21 and should inform future studies.

Complement as a powerful "influencer" in the brain during development, adulthood and neurological disorders

The complement system was long considered as only a powerful effector arm of the immune system that, while critically protective, could lead to inflammation and cell death if overactivated, even in the central nervous system (CNS). However, in the past decade it has been recognized as playing critical roles in key physiological processes in the CNS, including neurogenesis and synaptic remodeling in the developing and adult brain. Inherent in these processes are the interactions with cells in the brain, and the cascade of interactions and functional consequences that ensue. As a result, investigations of therapeutic approaches for both suppressing excessive complement driven neurotoxicity and aberrant sculpting of neuronal circuits, require broad (and deep) knowledge of the functional activities of multiple components of this highly evolved and regulated system to avoid unintended negative consequences in the clinic. Advances in basic science are beginning to provide a roadmap for translation to therapeutics, with both small molecule and biologics. Here, we present examples of the critical roles of proper complement function in the development and sculpting of the nervous system, and in enabling rapid protection from infection and clearance of dying cells. Microglia are highlighted as important command centers that integrate signals from the complement system and other innate sensors that are programed to provide support and protection, but that direct detrimental responses to aberrant activation and/or regulation of the system. Finally, we present promising research areas that may lead to effective and precision strategies for complement targeted interventions to promote neurological health.

Anaphylatoxin receptor promiscuity for commonly used complement C5a peptide agonists

The complement system is an essential component of innate immunity. Its activation generates the effector cleavage proteins, anaphylatoxins C3a and C5a, that exert activity by interacting with three structurally related seven-transmembrane receptors. C3a activates C3aR, whilst C5a interacts with both C5aR1 and C5aR2 with equal potency. Of the three receptors, C5aR1 in particular is considered the most functionally potent inflammatory driver and has been the major target for pharmacological development. Multiple peptidic C5a agonists have been developed to target C5aR1, with the full agonists EP54 (YSFKPMPLaR) and EP67 (YSFKDMP(MeL)aR), and the partial agonist C028 (C5a^pep, NMe-FKPdChaChadR) being the most commonly utilised in research. Recent studies have indicated that small complement peptide ligands may lack selectivity amongst the three anaphylatoxin receptors, however this has not been uniformly confirmed for these commonly used C5a agonists. In the present study, we therefore characterised the pharmacological activity of EP54, EP67, and C5a^pep at human C5aR1, C5aR2 and C3aR, by conducting signalling assays in transfected cell lines, and in human primary macrophages. Our results revealed that none of the compounds tested were selective for human C5aR1. Both EP54 and EP67 were potent, full C3aR agonists, and EP54 and C5a^pep potently and partially activated human C5aR2. Therefore, we caution against the usage of these ligands, particularly EP54 and EP67, as C5a surrogates in C5a/ C5aR research.

Neuroinflammation in Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disease associated with human aging. Ten percent of individuals over 65 years have AD and its prevalence continues to rise with increasing age. There are currently no effective disease modifying treatments for AD, resulting in increasingly large socioeconomic and personal costs. Increasing age is associated with an increase in low-grade chronic inflammation (inflammaging) that may contribute to the neurodegenerative process in AD. Although the exact mechanisms remain unclear, aberrant elevation of reactive oxygen and nitrogen species (RONS) levels from several endogenous and exogenous processes in the brain may not only affect cell signaling, but also trigger cellular senescence, inflammation, and pyroptosis. Moreover, a compromised immune privilege of the brain that allows the infiltration of peripheral immune cells and infectious agents may play a role. Additionally, meta-inflammation as well as gut microbiota dysbiosis may drive the neuroinflammatory process. Considering that inflammatory/immune pathways are dysregulated in parallel with cognitive dysfunction in AD, elucidating the relationship between the central nervous system and the immune system may facilitate the development of a safe and effective therapy for AD. We discuss some current ideas on processes in inflammaging that appear to drive the neurodegenerative process in AD and summarize details on a few immunomodulatory strategies being developed to selectively target the detrimental aspects of neuroinflammation without affecting defense mechanisms against pathogens and tissue damage.

Structural insights from an in silico molecular docking simulation of complement component 3a receptor 1 with an antagonist

Complement component 3a receptor 1 (C3aR) is an anaphylatoxin receptor that mediates inflammatory processes. Although considerable effort has gone into discovering the antagonists and agonists of C3aR, structural insights are required to search for effective ligands and to elucidate their binding modes and the mechanism of activation and inactivation. No experimental structural data of C3aR have yet been reported. We investigated the binding mode of an antagonist of C3aR using a combination of homology modeling, ligand docking, molecular dynamics simulations, and binding free energy calculations. We produced a plausible binding model consistent with the reported experimental data. We believe that this model is appropriate for the identification of new C3aR antagonists, as it can distinguish between antagonists and decoy compounds.

The "C3aR Antagonist" SB290157 is a Partial C5aR2 Agonist

Innate immune complement activation generates the C3 and C5 protein cleavage products C3a and C5a, defined classically as anaphylatoxins. C3a activates C3aR, while C5a activates two receptors (C5aR1 and C5aR2) to exert their immunomodulatory activities. The non-peptide compound, SB290157, was originally reported in 2001 as the first C3aR antagonist. In 2005, the first report on the non-selective nature of SB290157 was published, where the compound exerted clear agonistic, not antagonistic, activity in variety of cells. Other studies also documented the non-selective activities of this drug in vivo. These findings severely hamper data interpretation regarding C3aR when using this compound. Unfortunately, given the dearth of C3aR inhibitors, SB290157 still remains widely used to explore C3aR biology (>70 publications to date). Given these issues, in the present study we aimed to further explore SB290157's pharmacological selectivity by screening the drug against three human anaphylatoxin receptors, C3aR, C5aR1 and C5aR2, using cell models. We identified that SB290157 exerts partial agonist activity at C5aR2 by mediating β-arrestin recruitment at higher compound doses. This translated to a functional outcome in both human and mouse primary macrophages, where SB290157 significantly dampened C5a-induced ERK signaling. We also confirmed that SB290157 acts as a potent agonist at human C3aR in transfected cells, but as an antagonist in primary human macrophages. Our results therefore provide even more caution against using SB290157 as a research tool to explore C3aR function. Given the reported immunomodulatory and anti-inflammatory activities of C5aR2 agonism, any function observed with SB290157 could be due to these off-target activities.

Exploiting the role of stereoelectronic effects to design the antagonists of the human complement C3a receptor

Stereoelectronic effects are crucial in governing the conformational behaviour of small molecules bearing heterocyclic rings adjacent to amides.

The Complement C3a and C3a Receptor Pathway in Kidney Diseases

The pathogenesis of some kidney diseases is closely associated with complement activation, where the C3a/C3a receptor (C3aR) might play a crucial role. C3a/C3aR has dual roles and may exert anti-inflammatory or pro-inflammatory effects depending on different cell types and diseases. In the kidneys, C3aR is primarily expressed on the tubular epithelium and less in glomerular podocytes. C3aR expression is enhanced and the levels of C3a in the plasma and urine are increased in kidney diseases of several types, and are associated with disease progression and severity. The C3a/C3aR pathway facilitates the progression of glomerular and tubulointerstitial diseases, while it has opposite effects on urinary tract infections. Clinical trials targeting C3a/C3aR in kidney diseases are lacking. Here, we reviewed the studies on the C3a/C3aR pathway in kidney disease, with the aim of understanding in-depth its controversial roles and its potential therapeutic value.

Pharmacological characterisation of small molecule C5aR1 inhibitors in human cells reveals biased activities for signalling and function

The complement fragment C5a is a core effector of complement activation. C5a, acting through its major receptor C5aR1, exerts powerful pro-inflammatory and immunomodulatory functions. Dysregulation of the C5a-C5aR1 axis has been implicated in numerous immune disorders, and the therapeutic inhibition of this axis is therefore imperative for the treatment of these diseases. A myriad of small-molecule C5aR1 inhibitors have been developed and independently characterised over the past two decades, however the pharmacological properties of these compounds has been difficult to directly compare due to the wide discrepancies in the model, read-out, ligand dose and instrumentation implemented across individual studies. Here, we performed a systematic characterisation of the most commonly reported and clinically advanced small-molecule C5aR1 inhibitors (peptidic: PMX53, PMX205 and JPE1375; non-peptide: W545011, NDT9513727, DF2593A and CCX168). Through signalling assays measuring C5aR1-mediated cAMP and ERK1/2 signalling, and β-arrestin 2 recruitment, this study highlighted the signalling-pathway dependence of the rank order of potencies of the C5aR1 inhibitors. Functional experiments performed in primary human macrophages demonstrated the high insurmountable antagonistic potencies for the peptidic inhibitors as compared to the non-peptide compounds. Finally, wash-out studies provided novel insights into the duration of inhibition of the C5aR1 inhibitors, and confirmed the long-lasting antagonistic properties of PMX53 and CCX168. Overall, this study revealed the potent and prolonged antagonistic activities of selected peptidic C5aR1 inhibitors and the unique pharmacological profile of CCX168, which thus represent ideal candidates to fulfil diverse C5aR1 research and clinical therapeutic needs.

C3a receptor antagonist therapy is protective with or without thrombolysis in murine thromboembolic stroke

BACKGROUND AND PURPOSE

Intravenous thrombolysis (IVT) after stroke enhances C3a generation, which may abrogate the benefits of reperfusion. The C3aR antagonist SB290157 is neuroprotective following transient but not permanent middle cerebral artery occlusion (MCAo). SB290157 remains untested in thromboembolic (TE) models, which better approximate human stroke and also facilitate testing in combination with IVT. We hypothesized SB290157 would confer neuroprotection in TE stroke with and without "late" IVT.

EXPERIMENTAL APPROACH

We used two different models of TE stroke to examine the efficacy of SB290157 alone and in combination with late IVT. We evaluated the benefit of SB290157 in attenuating post-ischaemic behavioural deficits, infarction, brain oedema and haemorrhage.

KEY RESULTS

Plasma C3a was elevated 6 hr after TE stroke alongside increased cerebrovascular C3aR expression, which was sustained to 4 weeks. Increased C3aR expression also was visualized in human ischaemic brain. In a photothrombotic (PT) stroke model, which exhibits rapid spontaneous reperfusion, SB290157 given at 1 hr post-PT significantly improved neurofunction and reduced infarction at 48 hr. In an embolic (eMCAo) model, SB290157 administered at 2 hr improved histological and functional outcomes. Conversely, late IVT administered 4.5 hr post-eMCAo was ineffective likely due to increased haemorrhage and brain oedema. However, SB290157 administered prior to late IVT ameliorated haemorrhage and oedema and improved outcomes.

CONCLUSIONS AND IMPLICATIONS

We conclude that SB290157 is safe and effective with and without late IVT following TE stroke. Therefore, C3a receptor antagonist therapy represents a promising candidate for clinical translation in stroke, particularly as an adjuvant to IVT.

VGF-derived peptide TLQP-21 modulates microglial function through C3aR1 signaling pathways and reduces neuropathology in 5xFAD mice

BACKGROUND

Multiomic studies by several groups in the NIH Accelerating Medicines Partnership for Alzheimer's Disease (AMP-AD) identified VGF as a major driver of Alzheimer's disease (AD), also finding that reduced VGF levels correlate with mean amyloid plaque density, Clinical Dementia Rating (CDR) and Braak scores. VGF-derived peptide TLQP-21 activates the complement C3a receptor-1 (C3aR1), predominantly expressed in the brain on microglia. However, it is unclear how mouse or human TLQP-21, which are not identical, modulate microglial function and/or AD progression.

METHODS

We performed phagocytic/migration assays and RNA sequencing on BV2 microglial cells and primary microglia isolated from wild-type or C3aR1-null mice following treatment with TLQP-21 or C3a super agonist (C3aSA). Effects of intracerebroventricular TLQP-21 delivery were evaluated in 5xFAD mice, a mouse amyloidosis model of AD. Finally, the human HMC3 microglial cell line was treated with human TLQP-21 to determine whether specific peptide functions are conserved from mouse to human.

RESULTS

We demonstrate that TLQP-21 increases motility and phagocytic capacity in murine BV2 microglial cells, and in primary wild-type but not in C3aR1-null murine microglia, which under basal conditions have impaired phagocytic function compared to wild-type. RNA sequencing of primary microglia revealed overlapping transcriptomic changes induced by treatment with TLQP-21 or C3a super agonist (C3aSA). There were no transcriptomic changes in C3aR1-null or wild-type microglia exposed to the mutant peptide TLQP-R21A, which does not activate C3aR1. Most of the C3aSA- and TLQP-21-induced differentially expressed genes were linked to cell migration and proliferation. Intracerebroventricular TLQP-21 administration for 28 days via implanted osmotic pump resulted in a reduction of amyloid plaques and associated dystrophic neurites and restored expression of subsets of Alzheimer-associated microglial genes. Finally, we found that human TLQP-21 activates human microglia in a fashion similar to activation of murine microglia by mouse TLQP-21.

CONCLUSIONS

These data provide molecular and functional evidence suggesting that mouse and human TLQP-21 modulate microglial function, with potential implications for the progression of AD-related neuropathology.

Potent Thiophene Antagonists of Human Complement C3a Receptor with Anti-Inflammatory Activity

Structure-activity relationships for a series of small-molecule thiophenes resulted in potent and selective antagonism of human Complement C3a receptor. The compounds are about 100-fold more potent than the most reported antagonist SB290157. A new compound JR14a was among the most potent of the new antagonists in vitro, assessed by (a) inhibition of intracellular calcium release (IC₅₀ 10 nM) induced in human monocyte-derived macrophages by 100 nM C3a, (b) inhibition of β-hexosaminidase secretion (IC₅₀ 8 nM) from human LAD2 mast cells degranulated by 100 nM C3a, and (c) selectivity for human C3aR over C5aR. JR14a was metabolically stable in rat plasma and in rat liver microsomes and efficacious in rats when given orally to suppress rat paw inflammation, macrophage and mast cell activation, and histopathology induced by intraplantar paw administration of a C3aR agonist. Potent C3aR antagonists are now available for interrogating C3a receptor activation and suppressing C3aR-mediated inflammation in mammalian physiology and disease.

Distinct roles of the anaphylatoxin receptors C3aR, C5aR1 and C5aR2 in experimental meningococcal infections

The complement system is pivotal in the defense against invasive disease caused by Neisseria meningitidis (Nme, meningococcus), particularly via the membrane attack complex. Complement activation liberates the anaphylatoxins C3a and C5a, which activate three distinct G-protein coupled receptors, C3aR, C5aR1 and C5aR2 (anaphylatoxin receptors, ATRs). We recently discovered that C5aR1 exacerbates the course of the disease, revealing a downside of complement in Nme sepsis. Here, we compared the roles of all three ATRs during mouse nasal colonization, intraperitoneal infection and human whole blood infection with Nme. Deficiency of complement or ATRs did not alter nasal colonization, but significantly affected invasive disease: Compared to WT mice, the disease was aggravated in C3ar-/- mice, whereas C5ar1-/- and C5ar2-/- mice showed increased resistance to meningococcal sepsis. Surprisingly, deletion of either of the ATRs resulted in lower cytokine/chemokine responses, irrespective of the different susceptibilities of the mice. This was similar in ex vivo human whole blood infection using ATR inhibitors. Neutrophil responses to Nme were reduced in C5ar1-/- mouse blood. Upon stimulation with C5a plus Nme, mouse macrophages displayed reduced phosphorylation of ERK1/2, when C5aR1 or C5aR2 were ablated or inhibited, suggesting that both C5a-receptors prime an initial macrophage response to Nme. Finally, in vivo blockade of C5aR1 alone (PMX205) or along with C5aR2 (A8Δ71-73) resulted in ameliorated disease, whereas neither antagonizing C3aR (SB290157) nor its activation with a "super-agonist" peptide (WWGKKYRASKLGLAR) demonstrated a benefit. Thus, C5aR1 and C5aR2 augment disease pathology and are interesting targets for treatment, whereas C3aR is protective in experimental meningococcal sepsis.

Intracortical Administration of the Complement C3 Receptor Antagonist Trifluoroacetate Modulates Microglia Reaction after Brain Injury

Worldwide, millions of individuals suffer an ischemic stroke each year, causing major disability, especially in the elderly, where stroke is the number one cause of disability. However, to date, no effective therapy exists that targets the functional recovery after stroke. After necrosis, neuroinflammation is a common feature of the acute stroke and a major obstacle to tissue restoration. In the lesioned area, the dying neurons release chemotactic signals, such as fractalkine/CX3CL1, which evoke “eat-me” signals that are recognized by microglia expressing complement C3a receptor (C3aR), resulting in phagocytosis of the dying but still viable neurons, known as secondary phagocytosis. Using a mouse model of stroke and two-photon microscopy, we aimed to attenuate poststroke phagocytosis of the dying but still viable neurons by using SB 290157, an antagonist of C3aR. We found that intracortical administration of SB 290157 reduced the number of inflammatory microglial cells expressing ED1 and Iba1 antigens at the lesion site. We could show, in vivo, that two days after a needle-induced cortical lesion there were less microglial cells present around the injury site, displaying less high-order branches and an increase in the lower order ones, suggesting an attenuated phagocytic phenotype in treated animals as compared with controls. We conclude that the C3aR antagonist, SB 290157, may be used in the future to limit the neuronal death by limiting secondary phagocytosis after stroke.

The Role of Complement C3a Receptor in Stroke

The complement system is a key regulator of the innate immune response against diseased tissue that functions across multiple organ systems. Dysregulation of complement contributes to the pathogenesis of a number of neurological diseases including stroke. The C3a anaphylatoxin, via its cognate C3a receptor (C3aR), mediates inflammation by promoting breakdown of the blood-brain barrier and the massive infiltration of leukocytes into ischemic brain in experimental stroke models. Studies utilizing complement deficient mice as well as pharmacologic C3aR antagonists have shown a reduction in tissue injury and mortality in murine stroke models. The development of tissue-specific C3aR knockout mice and more specific C3aR antagonists is warranted to facilitate our understanding of the role of the C3aR in brain ischemia with the ultimate goal of clinical translation of therapies targeting C3aR in stroke patients.

Complement C3a receptor modulates embryonic neural progenitor cell proliferation and cognitive performance

The complement system of innate immunity is emerging as a novel player in neurodevelopmental processes. The receptor for C3a, C3aR, shares a close evolutionary and functional relationship with C5a receptors. Whilst the C5a receptor, C5aR1, has been demonstrated to promote embryonic neural stem cell proliferation, little is known about the role of C3aR in this process. Here we show that C3aR is expressed in a similar manner to C5aR1 in mice, at the apical pole of the embryonic ventricular zone, though it has an opposing function. Using in utero delivery of C3aR agonist and antagonist compounds to the embryonic ventricle, we demonstrate that C3aR functions to decrease proliferation of apical neural progenitor cells (NPC). Intriguingly, C3aR^-/- animals also have altered NPC proliferation, but demonstrate an opposing phenotype to animals subjected to pharmacological blockade of C3aR. Finally, despite a grossly normal development of C3aR^-/- animals, cognitive behavioural testing of adult mice showed subtle deficits in recall memory. These data demonstrate that in addition to C5a, C3a also has a critical role in the normal development of the mammalian brain.

The role of the complement system in traumatic brain injury: a review

Traumatic brain injury (TBI) is an important cause of disability and mortality in the western world. While the initial injury sustained results in damage, it is the subsequent secondary cascade that is thought to be the significant determinant of subsequent outcomes. The changes associated with the secondary injury do not become irreversible until some time after the start of the cascade. This may present a window of opportunity for therapeutic interventions aiming to improve outcomes subsequent to TBI. A prominent contributor to the secondary injury is a multifaceted inflammatory reaction. The complement system plays a notable role in this inflammatory reaction; however, it has often been overlooked in the context of TBI secondary injury. The complement system has homeostatic functions in the uninjured central nervous system (CNS), playing a part in neurodevelopment as well as having protective functions in the fully developed CNS, including protection from infection and inflammation. In the context of CNS injury, it can have a number of deleterious effects, evidence for which primarily comes not only from animal models but also, to a lesser extent, from human post-mortem studies. In stark contrast to this, complement may also promote neurogenesis and plasticity subsequent to CNS injury. This review aims to explore the role of the complement system in TBI secondary injury, by examining evidence from both clinical and animal studies. We examine whether specific complement activation pathways play more prominent roles in TBI than others. We also explore the potential role of complement in post-TBI neuroprotection and CNS repair/regeneration. Finally, we highlight the therapeutic potential of targeting the complement system in the context of TBI and point out certain areas on which future research is needed.

Species-specific differences in regulation of macrophage inflammation by the C3a-C3a receptor axis

Complement is an important arm of the innate immune system. Recent studies have shown that products of complement pathway activation can interact directly with other innate immune signaling molecules, including TLRs and inflammasome family members, during some infectious and chronic inflammatory disorders. Activation of the complement system generates anaphylatoxins, such as C3a and C5a, which modulate inflammation. However, the biological effects of interactions between the anaphylatoxins with their receptors may vary across species. In this study, we demonstrate that human complement and rat complement differ in the way they modulate the inflammatory response to the human pathogen, Neisseria gonorrhoeae, as well as purified pathogen-associated ligands, such as LPS. While rat serum down-regulates MyD88-dependent pro-inflammatory cytokine responses in macrophages, human serum has no effect, or in some cases an enhancing effect. Further, the inhibitory effect of rat serum on otherwise pro-inflammatory stimuli is mediated by complement, specifically C3a-C3a receptor interactions, via an undefined signaling mechanism that down-regulates the transcription factor, NF-κB and NLRP3 inflammasome-mediated caspase-1 activation. This study highlights important functional differences between rodent and human complement that could explain some of the differences in immune responses between these two species. Understanding the crosstalk between complement and other arms of the innate immune system will facilitate the development of better anti-inflammatory therapeutics.

Reduced bone loss in a murine model of postmenopausal osteoporosis lacking complement component 3

The growing field of osteoimmunology seeks to unravel the complex interdependence of the skeletal and immune systems. Notably, we and others have demonstrated that complement signaling influences the differentiation of osteoblasts and osteoclasts, the two primary cell types responsible for maintaining bone homeostasis. However, the net effect of complement on bone homeostasis in vivo was unknown. Our published in vitro mechanistic work led us to hypothesize that absence of complement component 3 (C3), a central protein in the complement activation cascade, protects against bone loss in the ovariectomy-based model of postmenopausal osteoporosis. Indeed, we report here that, when compared to their C57BL/6J (WT) counterparts, ovariectomized C3 deficient mice experienced reduced bone loss at multiple sites and increased stiffness at the femoral neck, the latter potentially improving mechanical function. WT and B6;129S4-C3^tm1Crr /J (C3^-/- ) mice were either ovariectomized or sham-operated at 6 weeks of age and euthanized at 12 weeks. MicroCT on harvested bones revealed that the trabecular bone volume fraction in the metaphyses of both the proximal tibiae and distal femora of ovariectomized C3^-/- mice is significantly greater than that of their WT counterparts. Lumbar vertebrae showed significantly greater osteoid content and mineral apposition rates. Mechanical testing demonstrated significantly greater stiffness in the femoral necks of ovariectomized C3^-/- mice. These results demonstrate that C3 deficiency reduces bone loss at ovariectomy and may improve mechanical properties. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:118-128, 2018.

Many drugs for many targets: novel treatments for complement-mediated glomerular disease

There is a large body of experimental and clinical evidence that complement activation contributes to glomerular injury in multiple different diseases. However, the underlying mechanisms that trigger complement activation vary from disease to disease. Immune complexes activate the classical pathway of complement in many types of glomerulonephritis, whereas the alternative pathway and mannose-binding lectin pathways are directly activated in some diseases. Eculizumab is an inhibitory antibody to C5 that has been approved for the treatment of atypical hemolytic uremic syndrome, and case reports suggest that it is also effective in other types of glomerulonephritis. Furthermore, new complement-inhibitory drugs are being developed that target additional proteins within the complement cascade, raising the possibility of blocking the specific complement proteins involved in a given disease. This review examines the rationale for targeting different proteins within the complement cascade, the new anti-complement drugs currently in development and some of the challenges that investigators will face in bringing these drugs to the clinic.

Targeting Neuroinflammation to Treat Alzheimer's Disease

Over the past few decades, research on Alzheimer's disease (AD) has focused on pathomechanisms linked to two of the major pathological hallmarks of extracellular deposition of beta-amyloid peptides and intra-neuronal formation of neurofibrils. Recently, a third disease component, the neuroinflammatory reaction mediated by cerebral innate immune cells, has entered the spotlight, prompted by findings from genetic, pre-clinical, and clinical studies. Various proteins that arise during neurodegeneration, including beta-amyloid, tau, heat shock proteins, and chromogranin, among others, act as danger-associated molecular patterns, that-upon engagement of pattern recognition receptors-induce inflammatory signaling pathways and ultimately lead to the production and release of immune mediators. These may have beneficial effects but ultimately compromise neuronal function and cause cell death. The current review, assembled by participants of the Chiclana Summer School on Neuroinflammation 2016, provides an overview of our current understanding of AD-related immune processes. We describe the principal cellular and molecular players in inflammation as they pertain to AD, examine modifying factors, and discuss potential future therapeutic targets.

Therapeutic targeting of complement to modify disease course and improve outcomes in neurological conditions

The recognition that complement proteins are abundantly present and can have pathological roles in neurological conditions offers broad scope for therapeutic intervention. Accordingly, an increasing number of experimental investigations have explored the potential of harnessing the unique activation pathways, proteases, receptors, complexes, and natural inhibitors of complement, to mitigate pathology in acute neurotrauma and chronic neurodegenerative diseases. Here, we review mechanisms of complement activation in the central nervous system (CNS), and explore the effects of complement inhibition in cerebral ischemic-reperfusion injury, traumatic brain injury, spinal cord injury, Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease and Huntington's disease. We consider the challenges and opportunities arising from these studies. As complement therapies approach clinical translation, we provide perspectives on how promising complement-targeted therapeutics could become part of novel and effective future treatment options to improve outcomes in the initiation and progression stages of these debilitating CNS disorders.

Complement in Lupus Nephritis: New Perspectives

BACKGROUND

Systemic lupus erythematosus (SLE) is an autoimmune disorder caused by loss of tolerance to self-antigens, the production of autoantibodies and deposition of complement-fixing immune complexes (ICs) in injured tissues. SLE is characterized by a wide range of clinical manifestations and targeted organs, with lupus nephritis being one of the most serious complications. The complement system consists of three pathways and is tightly controlled by a set of regulatory proteins to prevent injudicious complement activation on host tissue. The involvement of the complement system in the pathogenesis of SLE is well accepted; yet, its exact role is still not clear.

SUMMARY

Complement plays dual roles in the pathogenesis of SLE. On the one hand, the complement system appears to have protective features in that hereditary homozygous deficiencies of classical pathway components, such as C1q and C4, are associated with an increased risk for SLE. On the other hand, IC-mediated activation of complement in affected tissues is clearly evident in both experimental and human SLE along with pathological features that are logical consequences of complement activation. Studies in genetically altered mice have shown that lack of complement inhibitors, such as complement factor H (CFH) or decay-accelerating factor (DAF) accelerates the development of experimental lupus nephritis, while treatment with recombinant protein inhibitors, such as Crry-Ig, CR2-Crry, CR2-DAF and CR2-CFH, ameliorates the disease development. Complement-targeted drugs, including soluble complement receptor 1 (TP10), C1 esterase inhibitor and a monoclonal anti-C5 antibody (eculizumab), have been shown to inhibit complement safely, and are now being investigated in a variety of clinical conditions.

KEY MESSAGES

SLE is an autoimmune disorder which targets multiple systems. Complement is centrally involved and plays dual roles in the pathogenesis of SLE. Studies from experimental lupus models and clinical trials support the use of complement-targeted therapy in the treatment of SLE.

Is the complement activation product C3a a proinflammatory molecule? Re-evaluating the evidence and the myth

The complement activation product C3a is often described as a proinflammatory mediator, alongside its downstream cousin, C5a. However, emerging studies show that C3a has several anti-inflammatory facets in vivo. For example, in the acute inflammatory response, C3a acts in direct opposition to C5a, through preventing the accumulation of neutrophils in inflamed tissues by independently regulating their mobilization. This acute, protective, and opposing activity of C3a to C5a is also illustrated in models of septicemia. In this article, we reinvestigate the discovery and original classification of C3a as a proinflammatory mediator and highlight the emerging studies demonstrating anti-inflammatory effects for C3a in the immune response. It is our hope that this review illuminates these apparently contradictory roles for C3a and challenges the general dogma surrounding C3a, which, historically, has ubiquitously been described as a proinflammatory mediator. In light of this, we urge investigators to use "inflammatory modulator" as the descriptor for C3a.

Tumor necrosis-initiated complement activation stimulates proliferation of medulloblastoma cells

OBJECTIVE AND DESIGN

We sought to determine the effect of necrosis-induced activation of the complement protein C3 in medulloblastoma.

MATERIALS/METHODS

Twelve medulloblastoma surgical specimens were evaluated for complement activation using immunohistochemistry, with H&E stains performed on adjacent tissue sections to determine the relationship of complement activation to necrotic tissue. Flow cytometry and Western blot were performed on three established medulloblastoma lines and one surgically-procured cell culture to determine expression of C3a receptor (C3aR) in medulloblastoma. In vitro proliferation of siRNA C3aR knockdown cells was compared to that of control siRNA cells with cell line Daoy.

RESULTS

Three surgical specimens were found to have necrosis on H&E sections. In each case, iC3b staining was identified on adjacent sections, limited to the necrotic region. In no case did necrosis occur without iC3b staining on adjacent sections. C3aR protein was demonstrated on both the three established cell lines and on the surgical culture. Proliferation assays of Daoy cells with siRNA knockdown vs. control siRNA revealed significantly reduced proliferation at 72 h (p = 0.001).

CONCLUSIONS

Necrosis is associated with complement activation in medulloblastoma. Medulloblastoma cells express C3aR, and siRNA-mediated knockdown of C3aR inhibits proliferation of these cells in vitro.

Differential effects of complement activation products c3a and c5a on cardiovascular function in hypertensive pregnant rats

Early-onset pre-eclampsia is characterized by decreased placental perfusion, new-onset hypertension, angiogenic imbalance, and endothelial dysfunction associated with excessive activation of the innate immune complement system. Although our previous studies demonstrated that inhibition of complement activation attenuates placental ischemia-induced hypertension using the rat reduced uterine perfusion pressure (RUPP) model, the important product(s) of complement activation has yet to be identified. We hypothesized that antagonism of receptors for complement activation products C3a and C5a would improve vascular function and attenuate RUPP hypertension. On gestational day (GD) 14, rats underwent sham surgery or vascular clip placement on ovarian arteries and abdominal aorta (RUPP). Rats were treated once daily with the C5a receptor antagonist (C5aRA), PMX51 (acetyl-F-[Orn-P-(D-Cha)-WR]), the C3a receptor antagonist (C3aRA), SB290157 (N(2)-[(2,2-diphenylethoxy)acetyl]-l-arginine), or vehicle from GD 14-18. Both the C3aRA and C5aRA attenuated placental ischemia-induced hypertension without affecting the decreased fetal weight or decreased concentration of free circulating vascular endothelial growth factor (VEGF) also present in this model. The C5aRA, but not the C3aRA, attenuated placental ischemia-induced increase in heart rate and impaired endothelial-dependent relaxation. The C3aRA abrogated the acute pressor response to C3a peptide injection, but it also unexpectedly attenuated the placental ischemia-induced increase in C3a, suggesting nonreceptor-mediated effects. Overall, these results indicate that both C3a and C5a are important products of complement activation that mediate the hypertension regardless of the reduction in free plasma VEGF. The mechanism by which C3a contributes to placental ischemia-induced hypertension appears to be distinct from that of C5a, and management of pregnancy-induced hypertension is likely to require a broad anti-inflammatory approach.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Anaphylatoxins in organ transplantation

C3a and C5a (also called anaphylatoxins) are inflammatory peptides generated during complement activation. They do not only play important roles in innate immunity through the initiation and regulation of inflammatory responses, but also significantly influence adaptive immune responses. Organ transplantation triggers an initial inflammatory response and subsequent to the specific immune response (also called the alloimmune response), both of which contribute to graft rejection. Emerging evidence suggests that anaphylatoxins, particularly C5a, are significantly involved in both inflammatory and alloimmune responses following organ transplantation, thus influencing graft outcome. This review will provide the information on our current understanding of the roles for anaphylatoxins in ischemia-reperfusion injury, graft rejection, and transplant tolerance, and the therapeutic potential of targeting anaphylatoxin receptors in organ transplantation.

The receptor for complement component C3a mediates protection from intestinal ischemia-reperfusion injuries by inhibiting neutrophil mobilization

C3a is a key complement activation fragment, yet its neutrophil-expressed receptor (C3aR) still has no clearly defined role. In this study, we used a neutrophil-dependent mouse model of intestinal ischemia-reperfusion (IR) injury to explore the role of C3aR in acute tissue injuries. C3aR deficiency worsened intestinal injury, which corresponded with increased numbers of tissue-infiltrating neutrophils. Circulating neutrophils were significantly increased in C3aR(-/-) mice after intestinal ischemia, and C3aR(-/-) mice also mobilized more circulating neutrophils after granulocyte colony-stimulating factor infusion compared with WT mice, indicating a specific role for C3aR in constraining neutrophil mobilization in response to intestinal injury. In support of this role, C3aR(-/-) mice reconstituted with WT bone marrow reversed IR pathology back to WT levels. Complement C5a receptor (C5aR) antagonism in C3aR(-/-) mice also rectified the worsened pathology after intestinal IR injury but had no effect on circulating neutrophils, highlighting the opposing roles of C3a and C5a in disease pathogenesis. Finally, we found that using a potent C3a agonist to activate C3aR in vivo reduced neutrophil mobilization and ameliorated intestinal IR pathology in WT, but not C3aR(-/-), mice. This study identifies a role for C3aR in regulating neutrophil mobilization after acute intestinal injury and highlights C3aR agonism as a potential treatment option for acute, neutrophil-driven pathologies.

Complement in immune and inflammatory disorders: therapeutic interventions

With the awareness that immune-inflammatory cross-talk is at the heart of many disorders, the desire for novel immunomodulatory strategies in the therapy of such diseases has grown dramatically. As a prime initiator and important modulator of immunological and inflammatory processes, the complement system has emerged as an attractive target for early and upstream intervention in inflammatory diseases and has moved into the spotlight of drug discovery. Although prevalent conditions such as age-related macular degeneration have attracted the most attention, the diverse array of complement-mediated pathologies, with distinct underlying mechanisms, demands a multifaceted arsenal of therapeutic strategies. Fortunately, efforts in recent years have not only introduced the first complement inhibitors to the clinic but also filled the pipelines with promising candidates. With a focus on immunomodulatory strategies, in this review we discuss complement-directed therapeutic concepts and highlight promising candidate molecules.

Progress and Trends in Complement Therapeutics

The past few years have proven to be a highly successful and exciting period for the field of complement-directed drug discovery and development. Driven by promising experiences with the first marketed complement drugs, increased knowledge about the involvement of complement in health and disease, and improvements in structural and analytical techniques as well as animal models of disease, the field has seen a surge in creative approaches to therapeutically intervene at various stages of the cascade. An impressive panel of compounds that show promise in clinical trials is meanwhile being lined up in the pipelines of both small biotechnology and big pharmaceutical companies. Yet with this new focus on complement-targeted therapeutics, important questions concerning target selection, point and length of intervention, safety, and drug delivery emerge. In view of the diversity of the clinical disorders involving abnormal complement activity or regulation, which include both acute and chronic diseases and affect a wide range of organs, diverse yet specifically tailored therapeutic approaches may be needed to shift complement back into balance. This chapter highlights the key changes in the field that shape our current perception of complement-targeted drugs and provides a brief overview of recent strategies and emerging trends. Selected examples of complement-related diseases and inhibitor classes are highlighted to illustrate the diversity and creativity in field.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.