Absence Of Complement Activation Research Articles

IL-1β Induces Human Endothelial Surface Expression of IL-15 by Relieving let-7c-3p Suppression of Protein Translation.

Expression of IL-15 on the surface of human graft endothelial cells (ECs) bound to the IL-15Rα subunit can increase the activation of CTLs, potentiating allograft rejection. Our previous work showed that surface expression of this protein complex could be induced by alloantibody-mediated complement activation through increased IL-1β synthesis, secretion, and autocrine/paracrine IL-1-mediated activation of NF-κB. In this article, we report that cultured human ECs express eight differently spliced IL-15 transcripts. Remarkably, IL-1β does not alter the expression level of any IL-15 transcript but induces surface expression independently of RNA polymerase II-mediated transcription while requiring new protein translation. Mechanistically, IL-1β causes an NF-κB-mediated reduction in the level of microRNA Let-7c-3p, thereby relieving a block of translation of IL-15 surface protein. Let7c-3p anti-miR can induce EC surface expression of IL-15/IL-15Rα in the absence of complement activation or of IL-1, enabling IL-15 transpresentation to boost CD8 T cell activation. Because of the complexity we have uncovered in IL-15 regulation, we recommend caution in interpreting increased total IL-15 mRNA or protein levels as a surrogate for transpresentation.

Molecular signature of postmortem lung tissue from COVID-19 patients suggests distinct trajectories driving mortality.

ABSTRACTTo elucidate the molecular mechanisms that manifest lung abnormalities during severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections, we performed whole-transcriptome sequencing of lung autopsies from 31 patients with severe COVID-19 and ten uninfected controls. Using metatranscriptomics, we identified the existence of two distinct molecular signatures of lethal COVID-19. The dominant ‘classical’ signature (n=23) showed upregulation of the unfolded protein response, steroid biosynthesis and complement activation, supported by massive metabolic reprogramming leading to characteristic lung damage. The rarer signature (n=8) that potentially represents ‘cytokine release syndrome’ (CRS) showed upregulation of cytokines such as IL1 and CCL19, but absence of complement activation. We found that a majority of patients cleared SARS-CoV-2 infection, but they suffered from acute dysbiosis with characteristic enrichment of opportunistic pathogens such as Staphylococcus cohnii in ‘classical’ patients and Pasteurella multocida in CRS patients. Our results suggest two distinct models of lung pathology in severe COVID-19 patients, which can be identified through complement activation, presence of specific cytokines and characteristic microbiome. These findings can be used to design personalized therapy using in silico identified drug molecules or in mitigating specific secondary infections.

Consumption of complement in a 26-year-old woman with severe thrombotic thrombocytopenia after ChAdOx1 nCov-19 vaccination

Extremely rare reactions characterized by thrombosis and thrombocytopenia have been described in subjects that received ChAdOx1 nCoV-19 vaccination 5–16 days earlier. Although patients with vaccine-induced thrombotic thrombocytopenia (VITT) have high levels of antibodies to platelet factor 4 (PF4)–polyanion complexes, the exact mechanism of the development of thrombosis is still unknown. Here we reported serum studies as well as proteomics and genomics analyses demonstrating a massive complement activation potentially linked to the presence of anti-PF4 antibodies in a patient with severe VITT. At admission, complement activity of the classical and lectin pathways were absent (0% for both) with normal levels of the alternative pathway (73%) in association with elevated levels of the complement activation marker sC5b-9 (630 ng/mL [n.v. 139–462 ng/mL]) and anti-PF4 IgG (1.918 OD [n.v. 0.136–0.300 OD]). The immunoblotting analysis of C2 showed the complete disappearance of its normal band at 110 kDa. Intravenous immunoglobulin treatment allowed to recover complement activity of the classical pathway (91%) and lectin pathway (115%), to reduce levels of sC5b-9 (135 ng/mL) and anti-PF4 IgG (0.681 OD) and to normalize the C2 pattern at immunoblotting. Proteomics and genomics analyses in addition to serum studies showed that the absence of complement activity during VITT was not linked to alterations of the C2 gene but rather to a strong complement activation leading to C2 consumption. Our data in a single patient suggest monitoring complement parameters in other VITT patients considering also the possibility to target complement activation with specific drugs.

Surprising lack of liposome-induced complement activation by artificial 1,3-diamidophospholipids in vitro

Cardio-vascular diseases are the main cause of death, emphasizing the need to improve patient treatment and survival. One therapeutic approach is a liposome-based drug carrier system specifically targeting constricted arteries. The recently discovered mechano-sensitive liposomes use hemodynamic shear-stress differences between healthy and constricted blood vessels as trigger for drug release. Liposomes are promising delivery containers but are being recognized as foreign by the immune system. Complement activation as essential factor of the recognition leads to adverse effects. Here, we tested complement activation by liposomes formulated from the artificial phospholipid Pad-PC-Pad in vitro. Surprisingly no complement activation was detected in human sera and porcine plasma. In in vivo experiments with three pigs, neither anaphylactic reactions nor other significant hemodynamic changes were observed even at comparably high liposome doses. The pilot study holds promise for an absence of complement-mediated adverse effects of Pad-PC-Pad liposomes in human. From the Clinical EditorA lot of research has been done on new treatment for cardiovascular diseases. Liposome-based carrier systems have also shown promises. In this article, the authors studied the potential risks of complement activation by liposomes in in-vivo experiments. The absence of complement activation by Pad-PC-Pad liposomes may indicate its use in humans.

How complement activation influences the development of chronic synovitis in a mouse model of rheumatoid arthritis

Objectives: Chronic synovitis is the main characteristic of rheumatoid arthritis (RA), defined by the proliferation of synovial lining cells, an important source of chemokines and cytokines associated with inflammation. The aim of this study was to examine the influence of complement functional activity on the development of chronic synovitis.Method: The experiments were conducted in zymosan-induced arthritis (ZIA) provoked by intra-articular injection of 180 μg zymosan. Mice were treated with 10 ng/g body weight (bw) cobra venom factor (CVF) on days −3 and −2 or with 10 ng/g bw CVF on days 7, 12, and 17 of ZIA. The percentage of neutrophils (CD11b+Ly6G+), macrophages (F4/80), and complement 5 anaphylatoxin receptor (C5aR)-positive cells in the synovial fluid (SF) were determined by flow cytometry and the expression of C5aR and C3aR in the synovium was detected immunohistochemically.Results: The induction of ZIA in the absence of complement activity strongly inhibited the development of synovitis. By contrast, complement activation during ZIA exacerbated chronic synovitis through an increase in macrophage infiltration, C5aR and C3aR expression in the joints, and C5aR expression on SF cells. Levels of C5a and soluble receptor activator of nuclear factor kappa B ligand (sRANKL) in the SF were elevated whereas neutrophil infiltration and levels of tumour necrosis factor (TNF)-α and interleukin (IL)-6 in the SF were unchanged.Conclusions: Our findings indicate an important role for functional complement activity in the maintenance of chronic synovitis in a model of RA. Antagonizing complement activation represents new possibilities for the amelioration of synovitis symptoms.

Presence of six different lesion types suggests diverse mechanisms of tissue injury in neuromyelitis optica

Neuromyelitis optica (NMO) is an autoimmune disease targeting aquaporin 4 (AQP4), localized mainly at the astrocytic foot processes. Loss of AQP4 and glial fibrillary acidic protein (GFAP) was reported, but the pathological significance of astrocytopathy is still controversial. Here we show that active lesions in NMO display a wide spectrum of pathology even within a single tissue block of an individual patient. We have distinguished six different lesion types. The first reflects complement deposition at the surface of astrocytes, associated with granulocyte infiltration and astrocyte necrosis and followed by demyelination, global tissue destruction and the formation of cystic, necrotic lesions (lesion type 2). Such destructive lesions lead to Wallerian degeneration in lesion-related tracts (lesion type 3). Around active NMO lesions AQP4 may selectively be lost in the absence of aquaporin 1 (AQP1) loss or other structural damage (lesion type 4). Another pattern is characterized by clasmatodendrosis of astrocytes, defined by cytoplasmic swelling and vacuolation, beading and dissolution of their processes and nuclear alterations resembling apoptosis, which was associated with internalization of AQP4 and AQP1 and astrocyte apoptosis in the absence of complement activation. Such lesions give rise to extensive astrocyte loss, which may occur in part in the absence of any other tissue injury, such as demyelination or axonal degeneration (lesion type 5). Finally, lesions with a variable degree of astrocyte clasmatodendrosis are found, which show plaque-like primary demyelination that is associated with oligodendrocyte apoptosis, but with preservation of axons (lesion type 6). In active multiple sclerosis (MS) lesions astrocytes reveal changes of reactive protoplasmatic or fibrillary gliosis. Only in a subset of lesions, in patients with aggressive disease, loss of AQP4 is observed in the initial stage of their formation, which is associated with retraction of astrocyte processes in the absence of complement deposition, granulocyte infiltration or loss of AQP1 or astrocytes. Our data underline the primary assault of astrocytes in NMO lesions, but also indicate that different mechanisms of tissue injury operate in parallel in the same patient and even within the same lesion.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-013-1116-7) contains supplementary material, which is available to authorized users.

C1q secreted in the tumour microenvironment promotes tumour growth in the absence of complement activation

C1q secreted in the tumour microenvironment promotes tumour growth in the absence of complement activation

Aquaporin-4 autoantibodies cause asymptomatic aquaporin-4 loss and activate astrocytes in mouse

BackgroundNeuromyelitis optica (NMO) is a central nervous system inflammatory demyelinating disorder. Up to 90% of patients are seropositive for aquaporin-4 autoantibodies (AQP4 Ab). The pathogenetic mechanisms underlying clinical onset and relapse of NMO are uncertain. ObjectiveStudy the pathogenicity of AQP4 Ab in the absence of complement activation. MethodsFemale C57BL/6N mice (human IgG cannot activate mouse complements) pretreated with complete Freund's adjuvant (CFA, day 0) and pertussis toxin (PTx, day 0 and day 2) were transferred with IgG isolated from serum of healthy subjects or NMO patients (AQP4 Ab-positive or negative) intraperitoneally (day 7–9). Mice were observed for signs of experimental autoimmune encephalomyelitis (EAE) by standard 6-grade EAE scores. Spinal cord was obtained at day 11 for immunohistochemistry. ResultsNone of the mice had clinical signs of encephalomyelitis, inflammatory cells infiltration or demyelination of spinal cord. CFA and PTx induce BBB breakdown evidenced by leakage of human IgG into cord parenchyma. Patchy areas of AQP4 loss were observed in spinal cord of mice transferred with IgG from AQP4 Ab-positive NMO patients but not in mice transferred with IgG from AQP4 Ab-negative NMO patients or healthy subjects; but there was no loss of glial fibrillary acidic protein immunoreactivity in all mice. Markedly increased proliferation of astrocytic processes suggestive of astrocytic activation was observed in mice transferred with IgG from AQP4 Ab-positive patients. ConclusionAQP4 Ab cause asymptomatic AQP4 loss and astrocytic activation but not myelitis, demyelination or astrocytic cytotoxicity in spinal cord of mouse in the absence of complement activation.

A Humanized Single-Chain Variable Fragment Antibody Against Beta3 Integrin in Escherichia coli

Patients with HIV-1 immune-related thrombocytopenia (HIV-1-ITP) have a unique antibody (Ab) against platelet GPIIIa49-66, which is capable of inducing oxidative platelet fragmentation in the absence of complement activation. By screening a human phage antibody library with the GPIIIa49-66 peptide as bait, we have developed several humanized phage Abs, which act similarly to the parental Ab. However, the presence of a stop codon in the heavy chain of the obtained phage clones limits their expression in soluble recombinant form. To circumvent this problem, we mutated the stop codon inside clone 11 that exhibits the highest binding activity to platelet GPIIIa49-66, resulting in a soluble scFv format (named A11) in Escherichia coli Rosseta. In in vitro binding assay, A11 exhibited similar binding specificity to parental Ab at various concentrations. Moreover, A11 is able to induce oxidative platelet fragmentation by preferentially binding to activated versus resting platelets. These findings provide a proof-of-principle for the development of a novel approach to inhibit arterial thrombosis by generating a selective scFv for the lysis of platelet-rich thrombi.

20-OR Multiple accelerated rejections in the absence of complement activation in a patient with endothelial cell reactive antibody

20-OR Multiple accelerated rejections in the absence of complement activation in a patient with endothelial cell reactive antibody

The level of mannan-binding protein regulates the binding of complement-derived opsonins to mannan and zymosan at low serum concentrations

When sera diluted to 5% in a buffer containing calcium and magnesium were incubated with mannan-coated ELISA plates, C4 fragments, properdin and factor B were bound to the plates as well as the expected opsonic C3 fragments, C3b and C3bi. The calcium-dependent lectin mannan-binding protein, which is structurally similar to C1q, was also shown to bind in this assay and analysis of sera from 179 healthy blood donors revealed that the binding levels of all these proteins were highly significantly correlated. Results obtained with a previously described C3b opsonic assay using zymosan also correlated with the mannan-binding levels. When the sera were diluted to 5% in the presence of Mg-EGTA there was no detectable binding of complement proteins to the mannan surface, confirming that no alternative pathway activation occurred at this serum concentration. When sera were diluted to 5% in a buffer containing EDTA in order to study immunoglobulin binding in the absence of complement activation, the levels of bound IgG1, IgG2, IgG3, IgA and IgM antibodies were found to be completely unrelated to the C3bi binding levels previously observed. The results suggest that in this experimental system using low concentrations of serum, mannan-binding protein initiates an antibody-independent mechanism of cleavage of the classical pathway component C4, which subsequently regulates the degree of cleavage of C3 and recruitment of alternative pathway proteins.

Anti-microbial activities of mannose-binding lectin

Mannose-binding lectin (MBL; also known as mannan-binding lectin) is involved in first-line defence by binding to bacteria, viruses, protozoa and helminths through a pattern-recognition mode of detection and then initiating a range of host responses. Currently, we have been unable to extrapolate from what we know of the biochemistry of MBL binding to predict accurately the interaction of MBL with individual micro-organisms; even subtle surface alterations have been shown to have an extensive impact on MBL-mediated recognition of pathogens. MBL has a major protective effect through activation of the complement system via MBL-associated serine proteases (MASPs). This can cause the lysis of Gram-negative bacteria and also opsonize a wide spectrum of potential pathogens for phagocytosis. MBL may also influence phagocytosis in the absence of complement activation through an interaction with one or more collectin receptors. This may also be the basis for a direct effect of the protein on inflammatory responses. MBL can alter the function of microbial structures, such as gp120 of HIV, to prevent infection. The protein may also interact with the components of other cascade systems such as the clotting system, which will have a role in microbial pathogenesis. An understanding of these basic mechanisms will be vital if we are to use purified or recombinant MBL in therapeutic applications.

Biocompatibility of Hemodialysis Membranes: Interrelations between Plasma Complement and Cytokine Levels

Hemodialysis (HD) membrane biocompatibility is defined as absence of complement activation. We have recently shown that circulating levels of interleukin (IL) 1 and IL-2 predict death and survival, respectively, of HD patients. Studies have assessed IL-1 in treatments with biocompatible and less biocompatible dialysis membranes, but no study has correlated circulating levels of all these immunoreactants. We assessed these immunoreactants, and temperature as an outcome, during HD in patients treated with different membranes. Twelve stable patients, receiving thrice-weekly chronic bicarbonate HD, were randomly dialyzed with three different types of membranes, composed of: Cuprophan, cuprammonium rayon modified cellulose, and Hemophan. Blood was drawn from the arterial line port before (Pre) and 15, 30, and 60 min during and after (Post) HD. Patients’ temperatures were measured before and after each treatment. The plasma concentrations of IL-1 and IL-2 and factors C3a and C5a were assessed by ELISA. There were no differences between baseline levels of any of the immunoreactants in patients treated with different dialyzers. C3a, C5a, and IL-1 levels increased significantly during HD treatments with all three different membranes. C3a, C5a, and IL-1 levels during Cuprophan and Hemophan treatments were significantly higher than the levels during modified cellulose treatment at 30 and 60 min and Post (p < 0.01). For all the immunoreactants, however, the Post levels were higher than the Pre levels. In contrast to IL-1, there were no differences in mean IL-2 levels during treatments when different membranes were compared. There were few correlations of plasma C3a and C5a levels with plasma IL-1 levels, but there was only one treatment time in one dialyzer group during which IL-2 and any of the other factors were correlated. Pre and Post temperature values and percent change in temperature were not correlated with any of the immunoreactants measured. These data show that C3a, C5a, and IL-1 responses are similar, but not identical, during treatments with different membranes. The response of circulating IL-2 levels to treatments is quite different from that of plasma C3a, C5a and IL-1 levels and suggests that these changes are not solely due to treatment factors. Treatment with modified cellulose membranes is associated with a different immunoreactive profile as compared with patients dialyzed using other cellulose membranes. We suggest that circulating IL-1 levels are good biocompatibility markers.

Alpha-galactosyl epitope-mediated activation of porcine aortic endothelial cells: type II activation.

Xenoreactive natural antibodies (XNAs) and complement mediate hyperacute rejection of discordant xenografts. Inhibition of complement alone results in some prolongation of graft survival, but delayed xenograft rejection still precludes long-term graft survival. In vitro data provide evidence for the direct proinflammatory activation of endothelial cells (ECs) by XNAs. These antibodies are primarily directed against galactose alpha(1-3)-galactose (alpha-gal), the major xenoantigen in the pig to primate xenotransplant model. Previous studies have shown EC activation by XNAs but failed to address the question of whether alpha-gal-specific ligands can induce EC activation. The aim of this study was to investigate whether agonist binding to the alpha-gal epitope by alpha-gal-specific lectins as compared with XNAs or elicited xenoreactive antibodies can directly elicit type II porcine aortic EC (PAEC) activation (i.e., activation that requires protein synthesis). The tetravalent, alpha-gal-binding Bandeiraea simplicifolia lectin I (BS-I), the wholly alpha-gal-specific BS-I isolectin B4, and elicited primate anti-pig xenoreactive antibodies (decomplemented cynomolgus monkey anti-porcine serum) induced E-selectin protein expression in PAECs. This induction was alpha-gal-specific, as preincubation with synthetic alpha-gal carbohydrate or adsorption of lectin or serum to rabbit, but not human, red blood cells removed the activating component. E-selectin expression, induced by BS-I, was inhibited in the presence of genistein, a tyrosine kinase inhibitor, and by mepacrine, an inhibitor of phospholipase A2. Human and primate XNAs lacked this activity when tested at relevant concentrations; however, stimulation of PAECs with affinity-purified human XNA (IgM and IgG) resulted in slightly increased interleukin-8 and P-selectin mRNA levels but had no apparent effects on E-selectin transcription. BS-I strongly induced E-selectin, P-selectin, intercellular adhesion molecule-1, and interleukin-8 mRNA in an NF-kappaB-dependent manner. Several agonists that specifically bind to alpha-gal can evoke type II EC activation. Hence, anti-Gal antibodies may contribute directly to xenograft rejection in the absence of complement activation.

Granulocyte activation during haemodialysis in the absence of complement activation: inhibition by calcium channel blockers

The effect of the calcium channel blockers nifedipine (9 and 18 micrograms kg-1 h-1), diltiazem (100 and 200 micrograms kg-1 h-1) and verapamil (19 micrograms kg-1 h-1) continuously infused during haemodialysis on granulocyte and complement activation was investigated. Plasma levels of lactoferrin, elastase in complex with alpha 1-proteinase inhibitor (E-alpha 1PI) and C3a were measured in patients dialysed with dialysers made of cuprophane, polymethylmethacrylate (PMMA) and polyacrylonitrile (PAN). Calcium channel blockers caused no change of blood pressure during haemodialysis in all patients. There was no effect of nifedipine, diltiazem or verapamil on plasma lactoferrin, E-alpha 1PI or C3a levels in patients dialysed with cuprophane. However, plasma lactoferrin and E-alpha 1PI values were significantly reduced by all calcium channel blockers in patients dialysed with PMMA, and also by nifedipine and verapamil in patients dialysed with PAN. Our data indicate that calcium channel blockers inhibit granulocyte activation occurring in dialysers with very little anaphylatoxin formation. These drugs, however, are ineffective in patients dialysed with cuprophane where complement activation takes place. Therefore, granulocyte activation during haemodialysis in the absence of complement activation seems to be mediated by calcium ions.

Intrarenal hemodynamic alterations induced by anti-GBM antibody

An isolated perfused kidney system (IPK) was used to study the direct intrarenal hemodynamic effects of binding of anti-glomerular-basement membrane (anti-GBM) antibody in the absence of all other circulating humoral and cellular inflammatory mediators. Control IPK's (perfused with Krebs-Henseleit buffered 5% albumin solution containing non-immune globulin) had a renal vascular resistance (RVR) mean +/- SEM 3.10 +/- 0.47 mm Hg/ml/min and a GFR mean +/- SEM 0.63 +/- 0.8 ml/min/g. Anti-GBM antibody administration raised RVR (4.83 +/- 0.52 mm Hg/ml/min, P less than 0.01) and lowered GFR (0.34 +/- 0.04 ml/min/g, P less than 0.01). Perfusate renin activity was higher after antibody administration (684 +/- 87 ng AI/ml/hr compared with control 308 +/- 42 ngAI/ml/hr, P less than 0.01). Treatment with Sar1Ala8All (3 X 10(-6) M) or captopril (10 mg/ml) attenuated antibody-induced vasoconstriction (RVR mm Hg/ml/min, Sara1Ala8All = 3.78 +/- 0.13 captopril = 3.26 +/- 0.12, both P less than 0.05 compared with anti-GBM alone). Both inhibitors of the renin-angiotensin system (RAS) also aggrevated the decline in GFR seen after antibody administration (GFR ml/min/g, Sara1Ala8All = 0.24 +/- 0.05, Captopril = 0.18 +/- 0.03, both P less than 0.05 compared with anti-GBM alone). These IPK studies demonstrate that anti-GBM antibody itself may directly induce intrarenal hemodynamic alterations in the absence of complement activation, neutrophil infiltration, neural influences or circulating vasoactive substances. The results from perfusate renin sampling and blockade of the RAS provide evidence that anti-GBM antibody deposition activates the intrarenal RAS and thereby induces significant hemodynamic alterations.

Eimeria tenella: quantitative in vitro and in vivo studies on the effects of mouse polyclonal and monoclonal antibodies on sporozoites.

Murine, polyclonal and monoclonal antibodies, raised against sporozoites of Eimeria tenella, were tested for their ability to neutralize sporozoite infectivity in vitro and in vivo. Neutralization was effected via three mechanisms. Firstly, sporozoites fixed complement, at low titres, and lysis occurred by the alternative pathway of complement activation. Secondly, in the absence of complement activity, the murine heat-inactivated, hyperimmune antiserum neutralized sporozoites at relatively low titres. At high titres, even though sporozoites were agglutinated, neither the heat-inactivated hyperimmune antiserum nor the monoclonal antibody neutralized sporozoites. Finally, in the presence of complement and specific antibodies, at titres which by themselves would not neutralize sporozoites, neutralization was effected due to lysis via the classical pathway of complement activation.

Early (anaphylactoid) antivenom reactions: absence of complement activation and immune complex formation in human patients: Warrell, D. A.et al. (Faculty of Tropical Medicine, Mahidol University, 420/6 Rajvithi Road, Bangkok 10400, Thailand)

Early (anaphylactoid) antivenom reactions: absence of complement activation and immune complex formation in human patients: Warrell, D. A.et al. (Faculty of Tropical Medicine, Mahidol University, 420/6 Rajvithi Road, Bangkok 10400, Thailand)

Effect of antibody concentration on opsonic requirements for phagocytosis in vitro of Streptococcus pneumoniae types 7 and 19.

We investigated the effect of the concentration of type-specific antibody to pneumococcal polysaccharide (PPS) on opsonic requirements for phagocytosis in vitro of Streptococcus pneumonia types 7 and 19. We measured the uptake by human neutrophils of radiolabeled S. pneumoniae opsonized with either complement-intact, complement-depleted (by heat inactivation), or alternative complement pathway-activated (by magnesium dichloride-ethylene glycol tetraacetic acid (MgEGTA) chelation) immune sera with varying concentrations of antibody from individuals immunized with polyvalent PPS vaccine. Increased opsonization was found with increasing concentrations of type-specific antibody in the sera. Higher concentrations of antibody were required to opsonize type 7 than type 19 bacteria, both in the presence and absence of complement activity. Type 19 bacteria were more efficiently opsonized via the alternative complement pathway than type 7. For both types, antibody and the alternative complement pathway provided most of the opsonic activity in sera with lower concentrations of type-specific antibody. At high antibody concentrations, effective opsonization occurred in heat-activated sera, indicating the requirement for complement could be overcome with sufficient amounts of antibody alone.

Changes in the haemolytic activity of serum complement during acute Babesia bovis infection in cattle.

The haemolytic activity of serum complement was measured in cattle during acute Babesia bovis infection. The level fell significantly on day 5 post infection (p.i.) and was not detectable by day 10 p.i. After clinical recovery between days 15 and 20 p.i. haemolytic activity was again detected in serum and gradually returned to the pre-infection level. The components of both classical and alternative pathways were involved in complement depletion and factors such as protease secretion by the parasites, antigen-antibody reactions, and the release of haemoglobin were implicated as causes of the decline. The absence of complement activity during the acute phase of the disease may interfere with immune mechanisms at a critical time for the host.