Functions and relevance of the terminal complement sequence

Malarial anemia: digestive vacuole of Plasmodium falciparum mediates complement deposition on bystander cells to provoke hemophagocytosis

The digestive vacuole (DV) of Plasmodium falciparum, which is released into the bloodstream upon rupture of each parasitized red blood cell (RBC), was recently discovered to activate the alternative complement pathway. In the present work, we show that C3- and C5-convertases assembling on the parasitic organelle are able to provoke deposition of activated C3 and C5b-9 on non-infected bystander erythrocytes. Direct contact of DVs with cells is mandatory for the effect, and bystander complement deposition occurs focally, possibly at the sites of contact. Complement opsonization promotes protracted erythrophagocytosis by human macrophages, an effect that is magnified when ring-stage infected RBCs with reduced CD55 and CD59, or paroxysmal nocturnal hemoglobinuria (PNH)-RBCs lacking these complement inhibitors are employed as targets. Bystander attack can also directly induce lysis of PNH-RBCs. Direct evidence for complement activation and bystander attack mediated by DVs was obtained through immunohistochemical analyses of brain paraffin sections from autopsies of patients who had died of cerebral malaria. C3d and the assembled C5b-9 complex could be detected in all sections, colocalizing with and often extending locally beyond massive accumulations of DVs that were identified under polarized light. This is the first demonstration that a complement-activating particle can mediate opsonization of bystander cells to promote their antibody-independent phagocytosis. The phenomenon may act in concert with other pathomechanisms to promote the development of anemia in patients with severe malaria.

Increased soluble C5b-9 in CSF of neuromyelitis optica

Neuromyelitis optica (NMO) and multiple sclerosis (MS) are two of the autoimmune inflammatory demyelinating diseases in the central nervous system. Complement is thought to have an important role in pathogenesis of these diseases, especially in NMO. However, the change of terminal complement complex (TCC, C5b-9) in patients with NMO is still unclear. Cerebrospinal fluid (CSF) C3a, C5a, sC5b-9 were measured by enzyme-linked immunosorbent assay in patients with NMO (n = 26), MS (n = 25) and other neurological disease (OND, n = 19). CSF levels of C5a in patients with NMO were higher than patients with OND (P = 0.006). Increased CSF sC5b-9 were found in the patients with NMO compared with patients with MS (P = 0.029) and OND (P = 0.0001). CSF sC5b-9 in patients with MS were also higher than patients with OND (P = 0.030). Patients with NMO revealed a trend to an increased disease disability with increased CSF sC5b-9 during relapse but not in MS (NMO: P = 0.006, MS: P = 0.097). CSF levels of sC5b-9 are increased in patients with NMO and reflect the activation of complement in NMO.

Diagnostic significance of membrane attack complex and vitronectin in childhood dermatomyositis

In the current study, whether particular histologic distributions of membrane attack complex and vitronectin are correlated to clinical characteristics of patients with childhood dermatomyositis was investigated. The specimens from 11 patients with childhood dermatomyositis were examined immunocytochemically and compared with those taken from 6 adult dermatomyositis and 5 childhood polymyositis patients. Four out of five membrane attack complex-positive specimens were derived from patients who were within 1 to 3 months from the onset of childhood dermatomyositis and were not treated with prednisone. There were two patients at early stages of childhood dermatomyositis without pathologic characteristics of the disease. However, marked deposits of membrane attack complex on capillaries were seen in one of them. The immunoreactivity of vitronectin was detected at locations similar to those of membrane attack complex. Our results suggest that the detection of membrane attack complex and vitronectin in muscle capillaries will help us diagnose childhood dermatomyositis at an early stage.

Complement activation and formation of the membrane attack complex on serogroup B Neisseria meningitidis in the presence or absence of serum bactericidal activity

Encapsulated meningococci are complement sensitive only in the presence of bactericidal antibodies by yet-unexplored mechanisms. The objective of this study was to investigate the involvement of major bacterial surface constituents on complement activation and membrane attack complex (MAC) formation on serogroup B meningococci in the presence or absence of antibody-dependent serum bactericidal activity (SBA). The strains used were the encapsulated H44/76, five of its variants differing in capsulation and expression of the class 1 porin (PorA), and its lipopolysaccharide (LPS)-deficient isogenic mutant (LPS(-)) pLAK33. Two normal sera, one with high SBA (SBA(+)) and one with no bactericidal activity (SBA(-)) against H44/76 as well as an a-gamma-globulinemic serum were used for sensibilization of the bacteria. C3b and iC3b deposition on H44/76, its unencapsulated variant v24, and pLAK33 was similar in SBA(+) and SBA(-) serum, and no difference was present between the strains. MAC deposition on H44/76 was higher in SBA(+) serum than in SBA(-) serum and the a-gamma-globulinemic serum. The amounts of C3b on H44/76, v24, and pLAK33 in the a-gamma-globulinemic serum were also not different, indicating immunoglobulin G (IgG)- and LPS-independent complement activation. H44/76 PorA(+) and its PorA(-) variant and the v24 PorA(+) and its PorA(-) variant incubated in SBA(-) serum induced comparable amounts of MAC, despite their different serum sensitivities. Complement formation on the surface of the bacteria occurred almost exclusively via the classical pathway, but the considerable amounts of Bb measured in the serum indicated alternative pathway activation in the fluid phase. We conclude that complement deposition on meningococci is, for the most part, independent of classical pathway IgG and is not influenced by the presence of PorA or LPS on the meningococcal surface. Addition of an anti-PorA chimeric antibody to the nonbactericidal normal serum, while promoting a dose-related bacterial lysis, did not influence the amounts of C3b, iC3b, and MAC formed on the bacterial surface. These findings support the hypothesis that proper MAC insertion rather than the quantity of MAC formed on the bacterial surface is of importance for efficient lysis of meningococci.

The terminal complement complex is generated in chronic leg ulcers in the absence of protectin (CD59)

Loss of membrane complement regulators accompanied by complement activation is suggested to be involved in the pathophysiological processes leading to tissue damage in myocardial ischaemia. In the present study we have investigated whether the same phenomenon may occur in ischaemic and/or venous hypertension leg ulcers. The deposition of complement, plasma complement regulators and expression of membrane regulators were detected by immunohistochemical methods, including immunofluorescence with antibodies against C3d, the terminal complement complex (TCC), vitronectin, clusterin, decay-accelerating factor (CD55) and protectin (CD59). Eleven frozen biopsies from ischaemic leg ulcers, 10 biopsies from venous hypertension leg ulcers, and 10 biopsies from normal skin were studied. In 9 of 11 ischaemic and in 5 of 10 venous hypertension leg ulcers, marked staining for TCC was found around the capillaries, most often at the ulcer margin. No TCC staining was found in normal skin. Staining for TCC was always accompanied by staining for clusterin and vitronectin and C3d. In normal skin, CD59 was found on the elastic fibers in the dermis, on the muscle coat, the Schwann sheath and acinar cells. Semiquantitative measurement of CD59 showed marked increased staining intensity in the endothelium in venous hypertension ulcers and diminished intensity in ischaemic ulcers compared to normal skin. No such difference could be observed for CD55. When TCC was positive in the capillary walls, weak or no staining for CD59 was found. A significantly higher ratio of TCC/CD59 was found in the ischaemic compared to venous ulcers (p = 0.018). This was due to a marked difference between the ulcer margins (p = 0.013). Localized areas in the venous ulcers had the same pattern as that seen in the ischaemic ulcers. Our results suggest that loss of CD59 may enhance deposition of TCC and that complement-dependent inflammation may be an important factor in the tissue-damaging processes seen in chronic leg ulcers.

Complement C6 deficiency protects against diet-induced atherosclerosis in rabbits

Low-density lipoprotein (LDL) can be transformed to an atherogenic moiety by nonoxidative, enzymatic degradation. Enzymatically degraded LDL induces macrophage foam cell formation, provokes release of cytokines, and also activates complement. To determine whether complement activation may contribute to atherogenesis, 6 pairs of homozygous C6-deficient rabbits and their non-C6-deficient heterozygous siblings were fed a cholesterol-rich diet for 14 weeks. Cholesterol levels and plasma lipoprotein profiles of the animals in the C6-competent and C6-deficient groups did not significantly differ, and the high density lipoprotein and LDL cholesterol ratios at the end of the experiment were 0.07+/-0.01 and 0.08+/-0.01 (SEM), respectively. However, differences in atherosclerotic plaque formation were discernible macroscopically, with extensive aortic lesions being visible in all C6-competent animals and absent in all C6-deficient animals. Aortas were sectioned from thorax to abdomen, and 10 sections were stained from each aorta. Quantification of atherosclerotic lesions and lumen stenosis with the use of computer-based morphometry documented a dramatic protective effect of C6 deficiency on the development of diet-induced atherosclerosis. We conclude that the terminal complement sequence is centrally involved in atherosclerotic lesion progression.

Characterization of soluble terminal complement complex assembled in C8beta-deficient plasma and serum

Sera genetically deficient in either the alpha-gamma or the beta-subunit of complement component C8 virtually lack haemolytic activity. We have studied the formation and the structural organization of the soluble terminal complement complex (TCC) assembled in these sera following activation with cobra venom factor (CVF). The TCC concentration in the activated C8alpha-gamma and C8beta-deficient samples was 0.2% and 4%, respectively, when compared with zymosan-activated normal serum. TCC was purified from the activated C8beta-deficient samples by affinity chromatography and analysed by immunoblotting and enzyme immunoassay. No C8beta was detected in one TCC preparation, while 7% of the normal level was present in the other. The level of the other terminal components, including that of C8alpha-gamma, was normal. The ability of C8alpha-gamma to promote the assembly of TCC in the presence of a limited amount of C8beta or in the apparent absence of this subunit was confirmed using purified components, by mixing C5b6 and either of the purified C8 subunits together with C7 and C9. These data show that soluble TCC can be formed in C8beta-deficient sera that contain little or no C8beta.

C-reactive protein frequently colocalizes with the terminal complement complex in the intima of early atherosclerotic lesions of human coronary arteries

There is increasing evidence that complement activation may play a role in atherogenesis. Complement proteins have been demonstrated to be present in early atherosclerotic lesions of animals and humans, and cholesterol-induced atherosclerotic lesion formation is reduced in complement-deficient animals. Potential complement activators in atherosclerotic lesions are now a subject matter of debate. C-reactive protein (CRP) is an acute-phase protein that is involved in inflammatory processes in numerous ways. It binds to lipoproteins and activates the complement system via the classic pathway. In this study we have investigated early atherosclerotic lesions of human coronary arteries by means of immunohistochemical staining. We demonstrate here that CRP deposits in the arterial wall in early atherosclerotic lesions with 2 predominant manifestations. First, there is a diffuse rather than a focal deposition in the deep fibroelastic layer and in the fibromuscular layer of the intima adjacent to the media. In this location, CRP frequently colocalizes with the terminal complement complex. Second, the majority of foam cells below the endothelium show positive staining for CRP. In this location, no colocalization with the terminal complement proteins can be observed. Our data suggest that CRP may promote atherosclerotic lesion formation by activating the complement system and being involved in foam cell formation.

MBP, anti-MBP and anti-PLP antibodies, and intrathecal complement activation in multiple sclerosis

Intrathecal immunoglobulin synthesis and activation of the complement cascade occurs in patients with multiple sclerosis (MS). The present study aimed at further studying the relation between intrathecal immunoglobulin synthesis and complement activation. We compared total intrathecal synthesis of IgA, IgG, and IgM, the number of cells secreting anti-myelin basic protein (MBP) and anti-proteolipid protein (PLP) antibodies of the IgG isotype and intrathecal activation of the complement cascade in patients with possible onset symptoms of MS (n = 18) or clinically definite MS (n = 30). Early activation of the complement cascade correlated with intrathecal synthesis of IgM. Intrathecal IgG, IgA and IgM synthesis also correlated weakly with the presence of cells secreting anti-MBP or anti-PLP autoantibodies. Full activation of the complement cascade did not correlate with any measures of intrathecal antibody synthesis. These findings suggest a complex relation between different immunoglobulin isotypes and complement activation which may have similarly complex roles in the pathogenesis of MS.

Intrathecal activation of the complement system and disability in multiple sclerosis

BACKGROUND

The pathogenesis of multiple sclerosis (MS) appears to involve autoimmune phenomena in the central nervous system. Activation of the complement system is suggested to be involved in the pathogenesis.

PATIENTS AND METHODS

Cerebrospinal fluid (CSF) and plasma samples from 65 patients with acute optic neuritis (ON) as a possible first symptom of MS (n=18), ON (n=16) or other attacks of clinically definite MS (n=15), and neurological control subjects (n=16) were studied. Activation of the initial part of the complement activation cascade was assessed by measuring activation of the C3 molecule; terminal activation of the complement cascade was assessed by measuring the terminal complement complex (TCC). Demyelination was estimated by the CSF concentration of myelin basic protein and neurological disability was assessed with the Kurtzke expanded disability status scale (EDSS) score.

RESULTS

Activation of the initial part of the complement activation cascade occurred in each of the three groups of patients with demyelinating disease, but was not correlated to demyelination or disability. Increased concentrations of TCC were detected in patients with attacks of MS other than ON. The CSF concentrations of TCC, myelin basic protein (MBP) and neurological disability correlated significantly. The strongest correlation was between neurological disability and the CSF concentration of TCC (r=0.55, P=0.003).

INTERPRETATION

Full activation of the complement cascade during attacks of MS may be restricted to patients with more advanced disease and is significantly correlated to the degree of neurological disability. This suggests that specific treatment with agents that inhibit complement activation may interfere with mechanisms involved in the pathogenesis of neurological disability in patients with MS.

Immunohistochemical colocalization of the terminal complex of human complement and smooth muscle cell alpha-actin in early atherosclerotic lesions

There is substantial evidence that activated components of the complement cascade are present in atherosclerotic lesions, and it was suggested some years ago that smooth muscle cells may be an important target of complement attack by the terminal components of the cascade, C5b-9, also called the membrane attack complex. Recent in vitro studies have shown that assembly of membrane attack complex on smooth muscle cells leads to the release of monocyte chemotactic protein-1, and, if this were to occur in vivo, then it could be responsible for the recruitment of monocytes into the lesion. In this study we have investigated the localization of C5b-9 in early atherosclerotic lesions of human coronary arteries, collected from autopsies, by immunohistochemical staining, C5b-9 was found to colocalize widely with smooth muscle cell alpha-actin, but not with intact macrophages, thus supporting the hypothesis that interaction of complement with smooth muscle cells may indeed be important in atherogenesis.

Processes in atherogenesis: complement activation

The complement system consists of a complex group of plasma proteins, which, on activation, lead to a cascade of interactions culminating in the production of a variety of pro-inflammatory molecules. The system also contains cellular receptors for complement fragments produced during activation and regulatory molecules. It is part of the innate immune system representing humoral defence, but in certain circumstances may itself contribute to disease. In the formation of atherosclerotic lesions, there are two outstanding cellular phenomena, monocyte recruitment, with subsequent development of lipid-filled foam cells and smooth muscle cell activation. Subendothelial deposition of low density lipoprotein appears to be an important stimulus in these events and substantial evidence suggests that complement activation may be a link between lipoprotein deposition and subsequent lesion development.

On the pathogenesis of atherosclerosis: enzymatic transformation of human low density lipoprotein to an atherogenic moiety

Combined treatment with trypsin, cholesterol esterase, and neuraminidase transforms LDL, but not HDL or VLDL, to particles with properties akin to those of lipid extracted from atherosclerotic lesions. Single or double enzyme modifications, or treatment with phospholipase C, or simple vortexing are ineffective. Triple enzyme treatment disrupts the ordered and uniform structure of LDL particles, and gives rise to the formation of inhomogeneous lipid droplets 10-200 nm in diameter with a pronounced net negative charge, but lacking significant amounts of oxidized lipid. Enzymatically modified LDL (E-LDL), but not oxidatively modified LDL (ox-LDL), is endowed with potent complement-activating capacity. As previously found for lipid isolated from atherosclerotic lesions, complement activation occurs to completion via the alternative pathway and is independent of antibody. E-LDL is rapidly taken up by human macrophages to an extent exceeding the uptake of acetylated LDL (ac-LDL) or oxidatively modified LDL. After 16 h, cholesteryl oleate ester formation induced by E-LDL (50 micrograms/ml cholesterol) was in the range of 6-10 nmol/mg protein compared with 3-6 nmol/mg induced by an equivalent amount of acetylated LDL. At this concentration, E-LDL was essentially devoid of direct cytotoxic effects. Competition experiments indicated that uptake of E-LDL was mediated in part by ox-LDL receptor(s). Thus, approximately 90% of 125I-ox-LDL degradation was inhibited by a 2-fold excess of unlabeled E-LDL. Uptake of 125I-LDL was not inhibited by E-LDL. We hypothesize that extracellular enzymatic modification may represent an important step linking subendothelial deposition of LDL to the initiation of atherosclerosis.

Soluble ligands of the alpha v beta 3 integrin mediate enhanced tyrosine phosphorylation of multiple proteins in adherent bovine pulmonary artery endothelial cells

Binding of substrate-bound extracellular matrix proteins to cell surface integrins results in a variety of cellular responses including adhesion, cytoskeletal reorganization, and gene expression. We have previously shown that addition of soluble SC5b-9, the complement-vitronectin complex, resulted in an RGD-dependent increase in lung venular hydraulic conductivity (Ishikawa, S., Tsukada, H., and Bhattacharya, J. (1993) J. Clin. Invest. 91, 103-109). To identify specific integrin(s) and signal transduction pathways that are responsive to soluble vitronectin-containing ligands, we exposed confluent bovine pulmonary artery cells to purified soluble human mono- or multimeric vitronectin, or SC5b-9, and determined the extent of endothelial cell protein tyrosine phosphorylation. Monomeric vitronectin (Vn) did not induce enhanced protein tyrosine phosphorylation. However, multimeric Vn and SC5b-9 elicited time- and concentration-dependent increases in tyrosine phosphorylation of numerous proteins. Antiserum against vitronectin, RGD peptides, and monoclonal and polyclonal antibodies against the alpha v beta 3 integrin blocked the vitronectin- or SC5b-9-induced enhanced accumulation of tyrosine phosphoproteins, while antibodies against beta 1 integrins and the alpha v beta 5 integrin did not. Clustering of the alpha v beta 3 integrin using monoclonal antibody LM609 caused a pattern of enhanced tyrosine phosphorylation similar to that caused by multimeric Vn and SC5b-9, suggesting that aggregation of alpha v beta 3 was critical for signaling. Among the proteins that underwent enhanced tyrosine phosphorylation in response to vitronectin were the cytoskeletal proteins paxillin, cortactin, and ezrin, as well as the SH2 domain-containing protein Shc, and p125FAK. We conclude that ligation of the alpha v beta 3 integrin by soluble ligands promotes enhanced phosphorylation of several proteins implicated in tyrosine kinase signaling and suggest that this pathway may be important in inflammatory states which are accompanied by accumulation of SC5b-9.

The kinetics and distribution of C9 and SC5b-9 in vivo: effects of complement activation

Many diseases associated with complement activation are characterized by tissue deposition of components of the terminal complement complex (TCC). The ninth component of complement (C9) plays an important role in the cytolytic effects, and may contribute to the non-lethal cell-regulating functions of the TCC. In this study we examined the behaviour of radiolabelled human C9 and its soluble complexed form SC5b-9 in vivo in order to determine the effects of complement activation on its turnover, distribution and molecular size. In normal rabbits the metabolic parameters of 125I-C9 (median and range) were: plasma half-life (t1/2) 25.9 (20.6-29.5) h, fractional catabolic rate (FCR) 5.7 (5.3-7.0)%/h, and extravascular/intravascular ratio (EV/IV) 0.7 (0.6-1.1). The distribution of radiolabelled C9 amongst body tissues was similar to that observed for rabbit serum albumin (RSA). Activation of the complement cascade with i.v. injection of cobra venom factor (CVF) resulted in rapid disappearance of C9 from the plasma and accumulation of protein-bound radiolabeled in the spleen (exceeding the plasma concentration) and the liver. RSA metabolism and distribution were unaffected by CVF. Fine performance liquid chromatography (FPLC) gel filtration of plasma samples suggested that monomeric C9 was the only major radiolabelled protein present during normal turnovers, whereas CVF administration was accompanied by the prompt appearance of a high mol. wt species consistent in size with SC5b-9. When injected directly, 125I-SC5b-9 disappeared rapidly from the plasma, falling by 50% in 0.7 (0.6-0.8) h, and less than 15% remaining after 4 h with accumulation of protein-bound label in the spleen and liver. These results demonstrate the complexity of C9 metabolism during complement activation.

Deposition of C3, the terminal complement complex and vitronectin in primary biliary cirrhosis and primary sclerosing cholangitis

Characteristics of primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC) are bile duct destruction and portal inflammation. Increased levels of circulating complement activation products are also present. This raises the possibility of involvement of complement-dependent cytotoxic mechanisms in the pathogenesis. Therefore, we investigated liver biopsy specimens from 21 patients with PBC, six patients with PSC and six controls for complement deposits by immunohistochemistry using polyclonal and monoclonal antibodies against C3d, the terminal complement complex (TCC) and vitronectin (S-protein). We found C3d, TCC and vitronectin deposits only in the portal tracts. C3d and TCC were present in the walls of the hepatic arteries and in the connective tissue stroma but never around the bile ducts. We found vitronectin deposits throughout the connective tissue, often independent of the TCC deposits. When vitronectin and TCC were co-localized, the staining patterns were inverse; that is, intense staining for TCC accompanied weak staining for vitronectin and vice versa. Occasionally complete dissociation between TCC and vitronectin staining was observed. Deposits of TCC and vitronectin showed a focal distribution leaving many portal tracts free of TCC. Our results question whether complement-dependent cytotoxic mechanisms take part in the bile duct destruction in PBC and PSC.

Quantitation of vitronectin in serum: evaluation of its usefulness in routine clinical practice

AIMS

To make a preliminary assessment of the clinical relevance of serum vitronectin concentrations in various disease groups, using a recently available commercial radial immunodiffusion kit.

METHODS

Serum vitronectin concentrations were measured in 80 control subjects and 144 patients with various diseases. The following characteristics were used to evaluate the test procedures: linearity of method, inter- and intrabatch precision, effect of storage, temperature and in vitro activation of the classical and alternative complement pathways on vitronectin concentrations.

RESULTS

Significantly reduced serum vitronectin concentrations were found in patients with liver disease, renal disease, and systemic lupus erythematosus (SLE) (normal C3 and C4 concentrations, when compared with normal subjects. This particular method was suitable for measuring vitronectin concentrations in serum samples provided they were stored at -20 degrees C.

CONCLUSIONS

The clinical value of measuring serum vitronectin seems to be limited, but a larger study may be justified to ascertain the clinical importance of reduced serum vitronectin concentrations in liver diseases, and the possible role of vitronectin in other disease processes.

Soluble complex of complement increases hydraulic conductivity in single microvessels of rat lung

We determined the effect of sera enriched with the soluble complex of complement (SC5b-9), on hydraulic conductivity (Lp) of single pulmonary venules (diameter 20-30 microns). Sera free of anticoagulants and blood cells were prepared from rat and human blood. Lp were determined by our split drop technique in isolated, blood-perfused lungs prepared from anesthetized rats (2% halothane; Sprague Dawley, 500 g; n = 73). Zymosan-activated (ZAS) and control sera were used for Lp determinations. In ZAS prepared from human serum, SC5b-9 concentration was > 300 micrograms/ml (control: < 1 microgram/ml) as determined by ELISA. At baseline, Lp averaged 3.4 +/- .4 x 10(-7) ml/(cm2.s.cm H2O), but it increased by 217 +/- 32% with undiluted ZAS (P < 0.05). The Lp increase correlated significantly with different ZAS dilutions for rat serum and with SC5b-9 concentration for human serum. Lp did not increase significantly with ZAS prepared from heat-treated sera, C6- and C8-deficient sera; or with ZAS in which SC5b-9 had been depleted by immunoprecipitation. The ZAS-induced increase of Lp was blocked completely by venular preinfusion with the arginine-glycine-aspartic acid (RGD) tripeptide (1 mg/ml, 10 min). We report for the first time that: (a) SC5b-9 increases lung endothelial Lp; and (b) the increase of Lp is attributable to an integrin-dependent mechanism.

Gene frequency and partial protein characterization of an allelic variant of mannan binding protein associated with low serum concentrations

Low plasma concentration of mannan binding protein (MBP) has been shown to be the basis for a common opsonic deficiency and suggested to be caused by a single nucleotide substitution at base 230 of exon 1 in the MBP gene. This substitution causes a replacement of glycine (codon GGC) with aspartic acid (codon GAC). Of 123 healthy Danish individuals investigated by polymerase chain reaction performed on exon 1, followed by restriction fragment length polymorphism or allospecific probing, 93 were homozygous (75.6%) for GGC, 28 heterozygous (22.8%), and two homozygous for GAC (1.6%). The gene frequency of the GAC allele was found to be 0.13. DNA sequencing of the cloned exon 1 from one GAC homozygous individual revealed no other substitution. The median MBP concentration in the group containing the GAC allele was 6.4 times lower than in the GGC homozygous group (195 and 1234 micrograms/l respectively). However, the range in plasma concentrations of MBP was wide and overlapping between the groups. MBP protein was detected in both the GAC homozygotes (9 and 387 micrograms/l). Furthermore, no difference in relative mass and biological activity (mannan binding) was found when sera containing the two forms of MBP were investigated. Accordingly, it can be concluded that the GAC allele is able to produce a functional MBP protein which may be detected in serum at low concentrations.