A comparative analysis of the C1-binding ability of fragments derived from complement-fixing and noncomplement-fixing IgM proteins

The purpose of this study was to examine the molecular parameters necessary for initiation of complement fixation by IgM proteins. To determine why some IgM molecules are capable of complement fixation while others are not, several different Waldenström IgM proteins were examined for their ability to fix total hemolytic complement in the CH(50) assay. Subsequently, the C1 fixing ability of a 56-residue fragment derived from the Cmu4 domain of each of these IgM molecules was studied with C1 fixation assay. One of the three Waldenström IgM proteins (Gr) used in the present study was found unable to consume complement in a CH(50) assay when tested at the same concentration as the two complement-consuming IgM molecules (Dau and Bus). However, when the 56-residue C(H)4 fragment from the Cmu4 domain of each IgM molecule was tested for C1-fixing ability, all three were found to bind C1. On the basis of these observations, it is proposed that a C1 binding site exists within the Cmu4 domain of both complement-fixing and noncomplement-fixing IgM molecules. Presumably, the latter molecules are unable to interact in their native state with C1 in the manner required for initiation of the classical complement pathway, possibly due to the configurational inaccessibility of the entire C1 binding site.

Free and total insulin levels in a patient with insulin-resistant diabetes mellitus and chronic lymphatic leukemia: effect of prednisone therapy

An insulin-resistant diabetic patient who also has chronic lymphocytic leukemia and very high plasma levels of free and total insulin along with high levels of insulin antibodies is described. In response to prednisone therapy, his insulin requirement decreased, but the total and free insulin concentrations increased as insulin antibody measured as the maximal insulin-binding capacity of plasma remained unchanged. In insulin resistance, persistent hyperglycemia, in spite of high levels of immunoreactive free insulin, presumably reflects peripheral tissue unresponsiveness to insulin. The beneficial effect of prednisone treatment in this patient is discussed, and it is postulated to be the result of either increased availability of free insulin or an increased responsiveness of the tissues to insulin or both.

Evidence that C1s participates in the alternative complement pathway

Purified C1s, subcomponent of C1, induced electrophoretic conversion of factor B and consumption of hemolytic C3 and C5 in sera genetically deficient in C2 or C4. Blocking of the hemolytic activity of C1s in C2-deficient serum by F(ab')2 anti-C1s resulted in inhibition of the alternative pathway, as indicated by the failure of zymosan or cobra venom factor to induce comsumption of C3 and C6. Zymosan also failed to activate the alternative pathway when C1s was absorbed from C1r-deficient serum using a solid immunoabsorbent. These data, showing that C1s participates in the alternative pathway under certain experimental conditions, suggest the interesing possibility that C1s is important in the activation sequence of both the classical and the alternative pathway more generally.

C1 fixation and classical complement pathway activation by a fragment of the Cmu4 domain of IgM

A 56 residue fragment derived from a Waldenströme IgM protein and consisting of 24 residues of the amino-terminal portion of the Cmu4 domain disulfide bonded to 32 residues of the carboxy-terminal region of the loop has been shown to fix active C1 (C1) in a C1-fixation assay. Cleavage of the disulfide bond within the CH4 fragment resulted in a marked decrease of C1-fixing ability, although the isolated A and B fragments did retain a limited ability to fix C1. Upon incubation with normal human serum the intact CH4 fragment and equal molar amounts of the isolated A and B peptides consumed C4 suggesting that the C1-activating determinant of IgM remains intact in these three fragments. Furthermore, on a molar basis the intact or the reduced CH4 fragment consumed C4 as effectively as each of its component chains suggesting that transient binding of C1 by the individual A and B peptide chains is sufficient to activate C1. On the basis of these observations it is proposed that a classical complement fixation function, i.e. C1 binding and activation, can be localized within a region of the IgM molecule corresponding to the Cmu4 domain.

Detection and quantitation in plasma and synovial fluid of a fragment of human C4 with alpha mobility generated during the activation of the complement system

A newly identified fragment of human C4 was detected, using a particular antiserum, in human serum after activation by heat-aggregated immunoglobulins, but not after activation of the complement alternative pathway. This fragment was shown to have a sedimentation velocity of approximately 2.5S, to be heat stable, and to exhibit alpha mobility in immunoelectrophoresis. This C4alpha mobility fragment was not generated in human C4 deficient serum but was generated in human C2 deficient serum after incubation with heat-aggregated immunoglobulin. After precipitation of native C4 and its higher molecular weight fragments from serum by polyethylene glycol, it was possible to quantitate the lower molecular weight C4 alpha mobility fragment by radial immunodiffusion. In kinetic experiments, it was shown that the C4alpha mobility fragment was generated after some delay when compared to the disappearance of C4 hemolytic activity. Quantitation of the C4alpha mobility fragment may be of further use in human diseases for the evaluation of the catabolism of C4: joint fluids of patients suffering from rheumatoid arthritis contained high levels of the C4alpha mobility fragment, and low concentrations were found in patients with degenerative joint disease.

The structural basis for binding of complement by immunoglobulin M

An insight into the structural features of human IgM that are responsible for its capacity to bind the first component of complement (C) has been obtained by examining the ability of IgM subfragments to bind active C1 (C1). The smallest two fragments found to bind C1 were the major CNBr fragment of the Fc portion of IgM and the C(H)4 fragment of the carboxy-terminal domain. The smallest fragment which fixes C1 has a disaggregated mol wt of 6,800, consists of 60 residues, and contains no carbohydrate. Structural considerations and sequence overlaps suggest that the amino-terminal side of the C(H)4 domain (24 amino acid residues) might be responsible for fixing C1.

Inactivation of C1 in rheumatoid synovial fluid, purfied C1 and C1 esterase, by gold compounds

The effects of gold compounds on the functional activities of rheumatoid synovial fluid and normal serum Cl, C4, and C2 were investigated in vitro. Commercial and purified gold sodium thiomalate in concentrations, as low as 1·25 μg/ml (expressed as elemental gold) inactivated native C[unk] and highly purified C[unk]s, whereas equivalent or higher concentrations of thiomalate had no effect. C[unk] inactivation was caused also by other gold compounds such as gold chloride and gold sodium thiosulphate. The C[unk] inactivation was not reversible following extensive dialysis. The partial protection of C4 and C2, the two natural substrates for C[unk], indirectly verified the C[unk] inactivation. This is the first study to show that gold compounds inactivate C[unk], one of the reactants in the pathogenesis and/or perpetuation of rheumatoid arthritis.

Specific antisera to Clr. Quantitation of Clr in normal and pathological human sera

Monospecific neutralizing and precipitating antisera to C1r, a subunit of the first complement component, were obtained. These antisera neutralized C1r activity in purified preparations and in macromolecular C1 and did not react with C1q or C1s. They formed one line of precipitation in the β-globulin region with normal human serum, C1^hu and C1r at various stages of purification. Using anti-C1r antiserum and a radial immunodiffusion technique, the concentration of C1r was determined in normal, SLE and RA sera. It was 101 μg/ml in normal sera and lower in sera of active SLE patients (69·7 μg/ml). No significant variations from normal were found in sera of SLE patients in periods of remission or in RA patients.

The autosomal recessive mode of inheritance of C1r deficiency in a large Puerto Rican family

All living members of three generations of a large family with C1 r deficiency were studied. The two index cases showed undetectable C1 r and partial deficiency of C1 s associated with cutaneous, renal and joint disease as described previously (Moncada et al., 1972). The quantitative C1 q, C1 r and C1 s studies on the parents and the normal siblings were consistent with an autosomal recessive mode of inheritance for the C1 r trait.

There was more corrleation of the C1 r levels with the C1 s levels than with the C1 q levels, compatible with a linkage of the synthesis or catabolism of C1 s with C1 r.

Insulin and IgG complexes. An immunologic bypass for complement activation

Because of prior immunohistochemical findings of insulin, gamma globulin and complement components in diabetic microangiopathy, the in vitro interaction of insulin and IgG was studied. When native insulin and purified IgG were reacted under mild reducing conditions, a precipitate formed which was highly reactive with serum complement. Neither insulin nor IgG alone, reduced under the same conditions, demonstrated significant complement fixing activity. Maximal complement fixing activity was obtained from such a mixture at 30° C. in approximately one hour, the time of appearance of a visible precipitate. The presence of both insulin and IgG in the complement fixing precipitate was demonstrated by Ouchterlony analysis and by use of radio-labeled insulin and IgG. The tracer experiments indicated a precipitate composition of approximately 80 per cent insulin and 20 per cent IgG by weight. The utilization of complement components was similar to that obtained with IgG altered by heat aggregation or reaction with antigen. Complement fixation was not affected by alleviation of the precipitate, indicating that free sulfhydryl groups are not necessary. This interaction of denatured insulin and IgG to form a complement fixing complex offers a possible explanation for the presence of these components in micro-angiopathic lesions and may be a contributing factor to the pathogenesis of diabetic microangiopathy. Although complement is activated by an immunoglobulin insulin complex, this mechanism would be considered an immune bypass since specific antibody to insulin is not required.

Studies on the first component of complement (C1) and the inhibitor of C1 esterase in rheumatoid synovial fluids

The specific activity of C1 (haemolytic units per unit of C1q or C1s protein) was two to three times lower in synovial fluid of patients with sero-positive rheumatoid arthritis (RA) than in non RA or sero-negative RA fluids. These fluids contain a high ratio of C1s inhibitor to C1s. Experiments with radiolabelled C1s established that the C1s inhibitor in the synovial fluids of these patients is functional as it binds radiolabelled C1s. At the high ratios observed, it is possible that a complex of C1s and C1s inhibitor forms in these synovial fluids and alters the activation of the complement system. The potential for excess C1s inhibitor to slow or diminish complement utilization was shown by adding purified C1s inhibitor to serum in order to simulate the conditions in joint fluid (i.e. a high ratio of C1s inhibitor to C1s). Subsequent whole complement, C1 and C4 activites after incubation with selected immune complexes were higher as compared to controls without added inhibitor.

Ultrastructure of the human complement component, Clq (negative staining-glutamine synthetase-biologically active Clq)

The human complement component, Clq, is a fragile molecule of delicate structure consisting of three distinct parts, a central subunit, connecting strands, and terminal subunits. Each terminal subunit is further subdivided into a large and a small subunit, and the central subunit appears to be divided into two equal parts. In the intact molecule, six connecting strands link six terminal subunits by their larger subdivision to the central subunit. The overall diameter of the molecule when viewed from the "top" is about 35 nm (350 A).

C1r, subunit of the first complement component: purification, properties, and assay based on its linking role

A method to obtain C1r, a subunit of the first complement component, in a highly purified state has been described for the first time. The stepwise method starts with a neutral euglobulin precipitation, after diethylaminoethyl- and carboxymethyl-cellulose chromatography and a final preparative polyacrylamide electrophoresis step. Such C1r preparations are devoid of C1q and C1s activities and show only one protein band on analytic polyacrylamide electrophoresis. Rabbits injected with this preparation produced antisera showing only one precipitation band. The stability of C1r activity was determined under different conditions, and C1r was found to be labile at 37 degrees C, pH 7-8 and low ionic strength. The electrophoretic mobility of purified C1r is that of a beta-globulin on disc acrylamide electrophoresis and on agarose electrophoresis at pH 8.6. Its molecular weight as estimated by sephadex chromatography is 168,100.A sensitive hemolytic assay based on the property of C1r to link C1s to C1q and thereby to generate macromolecular C[unk]1 is described. The number of C[unk]1 molecules generated is stoichiometrically related to the concentration of C1r for a fixed C1q and C1s concentration provided that the titration is carried out below the plateau zone. Macromolecular C1 can be separated from free C1s as the former is cell bound. This method of purification and assay should allow the development of monospecific antisera and further chemical study of C1r.

Sub-units of the first complement component in immunologic deficiency syndromes: independence of Cls and Clq

Normal concentrations of C1 esterase, C1s, have been found in nineteen individuals with a variety of immunologic deficiency syndromes. C1q levels are markedly low in patients with lymphopenic hypogammaglobulinaemia. The lack of correlation of serum concentrations of C1s with C1q in several patients suggests a separate mode of synthesis or catabolism for these C1 subunits. Furthermore, the extreme C1q deficiency in lymphopenic hypogammaglobulinaemic patients may play a role in their serious prognosis; however the normal C1s concentrations would allow some complement mediated functions.

Deficiency of C1r in human serum. Effects on the structure and function of macromolecular C1

THE EXPERIMENTS PRESENTED HERE UTILIZE A HUMAN SERUM MARKEDLY DEFICIENT IN HEMOLYTIC COMPLEMENT ACTIVITY TO SHOW THAT: (a) The hemolytic deficiency is the result of a selective deficiency in hemolytic C1. (b) The relative absence of hemolytic C1 is due to a profound deficit in C1r function associated with less than normal C1s protein and hemolytic function and normal C1q protein concentration and function. This deficit in C1r in the face of normal C1q suggests that different cell types are responsible for the synthesis of each of these components. (c) Whatever the basis for the deficiency of C1r function, this defect results in an inadequate association of the remaining C1 subcomponents, C1q and C1s, even in the presence of calcium ions, thus suggesting that C1r has an important role in the assembly and/or maintenance of macromolecular C1.