C3 receptors on macrophages

HylS', a fragment of truncated hyaluronidase of Streptococcus suis, contributes to immune evasion by interaction with host complement factor C3b

Pathogenic bacteria have evolved many strategies to evade surveillance and attack by complements. Streptococcus suis is an important zoonotic pathogen that infects humans and pigs. Hyaluronidase (HylA) has been reported to be a potential virulence factor of S. suis. However, in this study, it was discovered that the genomic region encoding HylA of the virulent S. suis strain SC19 and other ST1 strains was truncated into four fragments when aligned with a strain containing intact HylA and possessing hyaluronidase activity. As a result, SC19 had no hyaluronidase activity, but one truncated HylA fragment, designated as HylS,' directly interacted with complement C3b, as confirmed by western ligand blotting, pull-down, and ELISA assays. The deposition of C3b and membrane attack complex (MAC) formation on the surface of a HylS'-deleted mutant (ΔhylS') was significantly increased compared to wild-type SC19. In human sera and whole blood, ΔhylS' survival was significantly reduced compared to that in SC19. The resistance of ΔhylS' to macrophages and human polymorphonuclear neutrophil PMNs also decreased. In a mouse infection model, ΔhylS' showed reduced lethality and lower bacterial load in the organs compared to that of SC19. We conclude that the truncated hyaluronidase HylS' fragment contributes to complement evasion and the pathogenesis of S. suis.

Regulation of innate and adaptive immunity using herbal medicine: benefits for the COVID-19 vaccination

Vaccination is a major achievement that has become an effective prevention strategy against infectious diseases and active control of emerging pathogens worldwide. In response to the coronavirus disease 2019 (COVID-19) pandemic, several diverse vaccines against severe acute respiratory syndrome coronavirus 2 have been developed and deployed for use in a large number of individuals, and have been reported to protect against symptomatic COVID-19 cases and deaths. However, the application of vaccines has a series of limitations, including protective failure for variants of concern, unavailability of individuals due to immune deficiency, and the disappearance of immune protection for increasing infections in vaccinated individuals. These aspects raise the question of how to modulate the immune system that contributes to the COVID-19 vaccine protective effects. Herbal medicines are widely used for their immune regulatory abilities in clinics. More attractively, herbal medicines have been well accepted for their positive role in the COVID-19 prevention and suppression through regulation of the immune system. This review presents a brief overview of the strategy of COVID-19 vaccination and the response of the immune system to vaccines, the regulatory effects and mechanisms of herbal medicine in immune-related macrophages, natural killer cells, dendritic cells, and lymphocytes T and B cells, and how they help vaccines work. Later in the article, the potential role and application of herbal medicines in the most recent COVID-19 vaccination are discussed. This article provides new insights into herbal medicines as promising alternative supplements that may benefit from COVID-19 vaccination.

Graphical abstract

http://links.lww.com/AHM/A31.

Cytokine Signalling at the Microglial Penta-Partite Synapse

Microglial cell processes form part of a subset of synaptic contacts that have been dubbed microglial tetra-partite or quad-partite synapses. Since tetrapartite may also refer to the presence of extracellular matrix components, we propose the more precise term microglial penta-partite synapse for synapses that show a microglial cell process in close physical proximity to neuronal and astrocytic synaptic constituents. Microglial cells are now recognised as key players in central nervous system (CNS) synaptic changes. When synaptic plasticity involving microglial penta-partite synapses occurs, microglia may utilise their cytokine arsenal to facilitate the generation of new synapses, eliminate those that are not needed anymore, or modify the molecular and structural properties of the remaining synaptic contacts. In addition, microglia–synapse contacts may develop de novo under pathological conditions. Microglial penta-partite synapses have received comparatively little attention as unique sites in the CNS where microglial cells, cytokines and other factors they release have a direct influence on the connections between neurons and their function. It concerns our understanding of the penta-partite synapse where the confusion created by the term “neuroinflammation” is most counterproductive. The mere presence of activated microglia or the release of their cytokines may occur independent of inflammation, and penta-partite synapses are not usually active in a neuroimmunological sense. Clarification of these details is the main purpose of this review, specifically highlighting the relationship between microglia, synapses, and the cytokines that can be released by microglial cells in health and disease.

Pattern recognition pathways leading to a Th2 cytokine bias in allergic bronchopulmonary aspergillosis patients

BACKGROUND

Allergic bronchopulmonary aspergillosis (ABPA) is characterised by an exaggerated Th2 response to Aspergillus fumigatus, but the immunological pathways responsible for this effect are unknown.

OBJECTIVE

The aim of this study was to decipher the pattern recognition receptors (PRRs) and cytokines involved in the Aspergillus-specific Th2 response and to study Aspergillus-induced responses in healthy controls and ABPA patients.

METHODS

Peripheral blood mononuclear cells (PBMCs) were stimulated with heat-killed Aspergillus conidia, various other pathogens, or PRR ligands. PRRs and cytokine pathways were blocked with PRR-blocking reagents, anti-TNF (Etanercept or Adalimumab), IL-1Ra (Anakinra) or IFNγ (IFN-gamma). ELISA and FACS were used to analyse cytokine responses.

RESULTS

Aspergillus was the only pathogen that stimulated the Th2 cytokines IL-5 and IL-13, while Gram-negative bacteria, Gram-positive bacteria, Candida albicans, chitin, β-glucan or Toll-like receptor (TLR) ligands did not. Depletion of CD4(+) cells abolished IL-13 production. Blocking complement receptor 3 (CR3) significantly reduced IL-5 and IL-13, while blocking TLR2, TLR4 or dectin-1 had no effect. ABPA patients displayed increased Aspergillus-induced IL-5 and IL-13 and decreased IFNγ production compared with healthy controls. All biological agents tested showed the capability to inhibit Th2 responses, but also decreased Aspergillus-induced IFNγ.

CONCLUSIONS AND CLINICAL RELEVANCE

Aspergillus conidia are unique in triggering Th2 responses in human PBMCs, through a CR3-dependent pathway. ABPA patients display a significantly increased Aspergillus-induced Th2/Th1 ratio that can be modulated by biologicals. These data provide a rationale to explore IFNγ therapy in ABPA as a corticosteroid-sparing treatment option, by dampening Th2 responses and supplementing the IFNγ deficiency at the same time.

Innate immunity against Leishmania infections

Leishmaniasis is a major health problem that affects more than 300 million people throughout the world. The morbidity associated with the disease causes serious economic burden in Leishmania endemic regions. Despite the morbidity and economic burden associated with Leishmaniasis, this disease rarely gets noticed and is still categorized under neglected tropical diseases. The lack of research combined with the ability of Leishmania to evade immune recognition has rendered our efforts to design therapeutic treatments or vaccines challenging. Herein, we review the literature on Leishmania from innate immune perspective and discuss potential problems as well as solutions and future directions that could aid in identifying novel therapeutic targets to eliminate this parasite.

The class A macrophage scavenger receptor type I (SR-AI) recognizes complement iC3b and mediates NF-κB activation

The macrophage scavenger receptor SR-AI binds to host tissue debris to perform clearance and it binds to bacteria for phagocytosis. In addition, SR-AI modulates macrophage activation through cell signaling. However, investigation of SR-AI signaling on macrophages is complicated due to its promiscuous ligand specificity that overlaps with other macrophage receptors. Therefore, we expressed SR-AI on HEK 293T cells to investigate its ligand binding and signaling. On 293Tcells, SR-AI could respond to E. coli DH5α, leading to NF-κB activation and IL-8 production. However, this requires E. coli DH5α to be sensitized by fresh serum that is treated with heat-inactivation or complement C3 depletion. Anti-C3 antibody inhibits the binding of SR-AI to serum-sensitized DH5α and blocks DH5α stimulation of SR-AI signaling. Further analysis showed that SR-AI can directly bind to purified iC3b but not C3 or C3b. By mutagenesis, The SRCR domain of SR-AI was found to be essential in SR-AI binding to serum-sensitized DH5α. These results revealed a novel property of SR-AI as a complement receptor for iC3b-opsonized bacteria that can elicit cell signaling.

The role of macrophages in Wallerian degeneration

The present review focuses on macrophage properties in Wallerian degeneration. The identification of hematogenous phagocytes, the involvement of cell surface receptors and soluble factors, the state of activation during myelin removal and the signals and factors leading to macrophage recruitment into degenerating peripheral nerves after nerve transection are reviewed. The main effector cells in Wallerian degeneration are hematogenous phagocytes. Resident macrophages and Schwann cells play a minor role in myelin removal. The macrophage complement receptor type 3 is the main surface receptor involved in myelin recognition and uptake. The signals leading to macrophage recruitment are heterogenous and not yet defined in detail. Degenerating myelin and axons are suggested to participate. The relevance of these findings for immune-mediated demyelination are discussed since the definition of the role of macrophages might lead to a better understanding of the pathogenesis of demyelination.

The regulatory roles of C1q

C1q binds to immune complexes to elicit complement-dependent microbial killing and enhance phagocytosis. Besides this classical role, C1q also opsonizes apoptotic cells for clearance by phagocytes. C1q deficiency increases susceptibility to microbial infections and is also associated with elevated autoimmunity as characterized by increased apoptotic bodies in tissues. Most complement proteins are of liver origin, but C1q is predominantly synthesized by peripheral tissue macrophages and dendritic cells. Besides being found in the blood, C1q has also been found deposited in extracellular tissues around these cells. In vitro, immobilized C1q inhibits monocyte, macrophage and T-cell production of inflammatory cytokines. It also regulates T-cell activation. Therefore, mounting evidence suggest a major regulatory role for C1q in inflammation and autoimmunity.

Spatiotemporal quantification of recruit and resident macrophages after crush nerve injury utilizing immunohistochemistry

The purpose of this study was to investigate quantitatively the temporal and spatial regulation and the morphological changes of the recruit and resident macrophages in the sciatic nerve during Wallerian degeneration and the following regeneration using immunohistochemistry. Sciatic nerves in Sprague-Dawley (SD) rats were examined after nerve crush. The rats were anesthetized with 100 mg of ketamine and 20 mg of xylazine in a dose of 1 ml/kg by intraperitoneal injection. Anti-ED-1 antibody was used to detect phagocytic macrophage and anti-OX-6 antibody was used to detect MHC class II cells. Few ED-1-immunopositive cells were seen within the normal sciatic nerve. After crush injury the number and the size of ED-1-immunopositive cells started to increase in all the segments distal to the crush site 3 days after injury and the number and size reached its peak on day 14 when the population of macrophage was 150 times higher in all the segments compared to controls. However, the number of ED-1-immunopositive cells and the size of the cells remains significantly high even after day 56 when functional recovery and axonal regeneration were complete. OX-6-immunopositive cells were observed within the control sciatic nerves. The number decreases significantly 3 days after injury in all the segments distal to the crush site but showed no significant difference thereafter. There were also no significant differences in the cell areas. ED-1-immunopositive phagocytic macrophages show significant differences temporally in both the cell number and the size even after axonal regeneration.

The role of TNF-alpha during Wallerian degeneration

The role of TNF-alpha in the course of Wallerian degeneration of the sciatic nerve was studied in control and TNF-alpha deficient mice. In control animals, the characteristic phenomena of Wallerian degeneration such as axon and myelin degeneration as well as macrophage recruitment with subsequent myelin removal were observed. In TNF-alpha deficient mice, in contrast, macrophage recruitment into the degenerating nerves was impaired resulting in a delayed myelin removal. However, the myelin phagocytic capacity of macrophages was not affected as it could be demonstrated by a similar myelin load of control and TNF-alpha deficient macrophages. These data indicate that the main function of TNF-alpha during Wallerian degeneration is the induction of macrophage recruitment from the periphery without affecting myelin damage or phagocytosis.

Amino acid differences in the N-terminus of C(H)2 influence the relative abilities of IgG2 and IgG3 to activate complement

The four human IgG isotypes are highly conserved in amino acid sequence, but show differential ability to activate complement (C'): IgG3 and IgG1 are very active, IgG2 is active under certain conditions, and IgG4 is inactive. Although the second constant domain [C(H)2] is critical for C' activation, the individual amino acids that confer isotype-specific activity have not been identified. We have generated a series of mutants between IgG2 and IgG3, resulting in the exchange of the four N-terminal and six C-terminal polymorphic residues within C(H)2. Mutants containing the N-terminus of the C(H)2 of IgG3 were as effective as wildtype IgG3 in C1q binding, C1 activation and terminal complex (MAC) formation, but had reduced ability to effect C'-mediated lysis. IgG2 and mutants containing the N-terminal portion of the C(H)2 of IgG2 were reduced compared to IgG3 in activating C1, binding C1q and inducing assembly of the MAC, and were inactive in mediating lysis of target cells. Thus, the amino acid sequence differences in the N-terminus of C(H)2 play a critical role in determining the relative abilities of IgG2 and IgG3 to bind C1q and activate the C' cascade although additional residues of C(H)2 must be involved in mediating optimal target cells lysis. The sequence of the N-terminus of C(H)2 was less critical in determining C4 and C3 binding. Characterization of domain exchange mutants suggests that intermediate steps may be partly dependent on domains other than C(H)2. IgGs that do not direct target cell lysis nevertheless activate intermediate steps in the pathway, which may contribute to immune complex-associated disorders.

Role of Salmonella typhimurium Mn-superoxide dismutase (SodA) in protection against early killing by J774 macrophages

The Salmonella typhimurium gene for Mn-cofactored superoxide dismutase (sodA) was cloned by complementation of an Escherichia coli sodA sodB mutant for growth on minimal medium. Sequence analysis revealed an open reading frame of 618 bp encoding a polypeptide with 97% identity to E. coli SodA. A S. typhimurium sodA mutant was created by allelic exchange and tested for the ability to survive in the murine macrophage-like cell line J774. Growth of bacteria under iron-limiting conditions, inactivation of the Fur repressor, or expression of sodA from a plasmid resulted in increased resistance to early killing by J774 cells, which was abolished in the sodA mutant. These results suggest that resistance to the early oxygen-dependent microbicidal mechanisms of phagocytes involves the SodA gene product. The S. typhimurium sodA mutant was not significantly attenuated in mice, however, which suggests that resistance to early oxygen-dependent microbicidal mechanisms in vivo may play only a minor role in Salmonella pathogenesis.

Induction of major histocompatibility complex class II antigen on amoeboid microglial cells in early postnatal rats following intraperitoneal injections of lipopolysaccharide or interferon-gamma

The induction of major histocompatibility complex (MHC) class II antigen on amoeboid microglia cells (AMC) by lipopolysaccharide (LPS) and interferon-gamma (INF-gamma) in early postnatal rat brain was studied by immunohistochemistry. In newborn rats given successive intraperitoneal injections of LPS or INF-gamma and killed at the age of 7 days, MHC class II antigen expressing AMC were consistently present in specific areas throughout the entire brain, notably in the subcortical white matter and circumventricular region. It is concluded from this study that the induction of MHC class II antigen on AMC by LPS or INF-gamma is a widespread phenomenon in the developing brain. Since MHC class II antigen is essential for the initiation of immune response, it is suggested that besides their phagocytic nature, AMC may also be involved in immunological processes in the developing brain.

Differential modulation by glucocorticoids of alternative complement protein secretion in cells of the monocyte/macrophage lineage

The effect of the synthetic glucocorticoid dexamethasone (DXM) on the secretion by human monocytes of alternative complement proteins C3, factor B and factor H was investigated. Results indicated that DXM modulates this secretion in a direction which would be consistent with its anti-inflammatory properties. DXM, at therapeutic concentrations, had a suppressive effect on C3 and factor B secretion and a stimulatory effect on factor H secretion by monocytes. This differential modulation on C3, factor B and factor H secretion was similar in mature macrophages. Together with previous studies showing that DXM had a suppressive effect on C3 and factor B secretion and a stimulatory effect on factor H secretion by human endothelial cells, our results indicate that DXM appears to have the general property of regulating local production of complement components so as to control complement activation.

Cell surface interactions between Trypanosoma congolense and macrophages during phagocytosis in vitro

Trypanosoma congolense bloodstream forms preincubated with a high titer of anti-variant surface antigen (VSG)-specific antibody, a low amount of anti-VSG plus complement-active mouse serum (MS), MS alone, and trypsin were cocultivated with mouse peritoneal macrophages in vitro. Immunofluorescence as well as transmission and scanning electron microscopy revealed that upon attachment to the macrophages' surface, trypanosomes opsonized with anti-VSG/MS formed opsonized filopodia, which were rapidly internalized by the phagocytes. Although these cells attached as frequently as anti-VSG or trypsin-pretreated parasites, the rate of phagocytosis of anti-VSG/MS pretreated trypanosomes was reduced significantly. Trypanosomes pretreated with high antibody titers alone were lysed on the surface of the macrophages before phagocytosis was completed. Parasites opsonized with complement alone adhered only occasionally and were rarely phagocytosed. Trypsin-treated trypanosomes, which served as positive control cells, rapidly attached and remained intact until ingulfment by the macrophages was completed. Untreated control parasites did not attach to the macrophages and were not phagocytosed. Cocultivation of macrophages with anti-VSG/MS-opsonized trypanosomes caused internalization of the flagellum by membrane fusion. Filopodia formation by T. congolense is thus correlated with a marked reduction in phagocytosis even in the presence of only a sublytic antibody titer.

The role of complement in myelin phagocytosis during PNS wallerian degeneration

Myelin removal in nerves undergoing wallerian degeneration mainly depends on invading, non-resident macrophages. The present study clarifies the role of serum complement components in this process in vitro and in vivo. Macrophages cocultured with degenerating nerves in vitro were unable to invade these nerves in the presence of C3-deficient serum. Application of C3-deficient serum subsequent to cellular invasion abolished the myelin phagocytic capacity of the invaded macrophages. This indicates that opsonization of myelin by complement components is necessary in myelin ingestion via macrophage receptors. In vivo, a monoclonal antibody to the macrophage complement receptor type 3 (CR3) significantly reduced myelin phagocytosis. Immunohistochemistry with anti-C3 antibodies showed a marked reaction in degenerating nerves. Immunoelectron microscopy localized C3 particles at the degenerating myelin sheaths. Haematogenous cells, invading the degenerating nerves, also showed a strong reaction for C3 in their cytoplasm. These results indicate that complement components play a critical role both in macrophage invasion of degenerating nerves and in the ingestion of myelin by these cells.

Phagocytosis of agarose beads by receptors for C3b (CR1) and iC3b (CR3) on alveolar macrophages from patients with sarcoidosis

Alveolar macrophages (AM) from sarcoidosis patients exhibit no detectable defect in their potential to synthesize the functional alternative and terminal pathway of complement. They also synthesize more C9 than AM from healthy controls. Various authors have suggested that sarcoid AM have decreased phagocytic ability. In the present work we studied whether there was any difference in C3 receptor-mediated phagocytosis of serum-treated and native agarose beads by AM recovered from patients with active sarcoidosis compared with controls. AM from seven patients with active sarcoidosis and seven healthy controls were cultured under serum-free conditions for 2, 12, 24, and 48 h. We found a significantly increased CR1 and CR3 receptor-mediated phagocytosis of native agarose beads by AM from the seven patients. CR1 and CR3 were also detected on AM directly recovered from bronchoalveolar lavage fluid using fluorescein-conjugated monoclonal anti-receptor antibodies. The percentage of AM expressing CR appeared to be increased in sarcoidosis. The reason for the enhanced phagocytosis of agarose beads by the sarcoid AM is probably the result of both increased synthesis and receptors of complement. Altered complement production and complement receptors may be important for the pathogenesis of this granulomatous disorder.

Immunocytochemical localization of CR3 complement receptors with OX-42 in amoeboid microglia in postnatal rats

The present study described the labelling of amoeboid microglial cells in the postnatal rat brain with OX-42, an antibody that recognizes type 3 complement receptors CR3 in mononuclear phagocytes. Of the diverse morphological forms of amoeboid microglia present in the corpus callosum in early postnatal (2-5 days) rats, cells with a round regular outline, or showing short stout processes, were the most intensely stained. When traced from the main cell colony into the borderline zone with the cortex, the immunoreactivity of amoeboid microglia that assumed a ramified form was drastically reduced. Examination of materials from the late postnatal (8-12 days) age group showed that the majority of the OX-42 positive cells in the corpus callosum became oval, elongated and ramified. Immunoelectron microscopy confirmed the above observations, and also showed that the immunoreactivity in the round amoeboid microglia was localized in their plasma membrane, surface projections and invaginations, as well as in some of the subsurface vacuoles. The immunoreactivity was reduced in the oval cells, and diminished in the elongated or ramified form. It is proposed that the presence of CR3 membrane receptors in amoeboid microglial cells is related to their active role in endocytosis. These, however, diminish with the growth of the brain.

The leukocyte cell surface receptor(s) for the iC3b product of complement

CR3 is probably the major adhesion molecule on monocytes and neutrophils. Its function as a phagocytic receptor for iC3b-coated particles has been well characterized. CR3 also has binding affinity for other ligands, including those that compete with iC3b such as fibrinogen, factor X, and beta-glucan, and those that do not such as bacterial LPS. CR3 binding to endothelial cells probably plays an important role in the extravascular migration of monocytes and neutrophils, but the ligand that it recognizes on endothelial cells has not been identified. Structurally CR3 belongs to the integrin family, and it shares a common subunit with p150,95 and LFA-1. The expression of these three membrane antigens appear to be limited to leukocytes, and they are sometimes referred to collectively as the leukocyte integrins. All three antigens have a common binding affinity for bacterial LPS. p150,95 also has affinity for iC3b, but p150,95/iC3b-dependent cellular responses has not been demonstrated. Its status as a complement receptor therefore awaits further experimental support.

Activation of C3 and binding to Aspergillus fumigatus conidia and hyphae

Complement activation by Aspergillus fumigatus may play a crucial role in stimulating binding and killing of this organism by phagocytes. We examined the amount and type of C3 deposited on resting conidia, swollen conidia, and hyphae of A. fumigatus after incubation in pooled human serum. All three life forms of A. fumigatus were potent activators of the complement cascade, with deposition on the organisms of similar amounts of C3 per unit of surface area. The rate of deposition was slowest for resting conidia, although maximal deposition was still achieved within 40 min. The roles of the alternative and classical pathways were assessed by use of serum chelated with magnesium EGTA [magnesium ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] and with an alternative pathway reconstituted from the six purified alternative-pathway proteins. Complement activation by resting conidia was mediated by the alternative pathway. In contrast, there was a progressive dependence on the classical pathway as the fungal particles matured into swollen conidia and then hyphae. Treatment with hydroxylamine, which disrupts ester linkages, removed 89 to 95% of the C3 bound to all three forms of A. fumigatus. This released C3 contained a mixture of C3b and iC3b, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. These data demonstrate that although all three forms of A. fumigatus are potent activators of the complement system, the transition from resting conidia to swollen conidia to hyphae results in progressive changes in the manner in which the fungal particles interact with the complement system. The lack of participation of the classical pathway in complement activation by resting conidia may have important implications regarding their ability to effectively stimulate phagocytes.