Autocatalytic activation of the proenzyme form of the C1s subunit of the first component of complement

Antagonism of the complement component C4 by flavivirus nonstructural protein NS1

The complement system plays an essential protective role in the initial defense against many microorganisms. Flavivirus NS1 is a secreted nonstructural glycoprotein that accumulates in blood, is displayed on the surface of infected cells, and has been hypothesized to have immune evasion functions. Herein, we demonstrate that dengue virus (DENV), West Nile virus (WNV), and yellow fever virus (YFV) NS1 attenuate classical and lectin pathway activation by directly interacting with C4. Binding of NS1 to C4 reduced C4b deposition and C3 convertase (C4b2a) activity. Although NS1 bound C4b, it lacked intrinsic cofactor activity to degrade C4b, and did not block C3 convertase formation or accelerate decay of the C3 and C5 convertases. Instead, NS1 enhanced C4 cleavage by recruiting and activating the complement-specific protease C1s. By binding C1s and C4 in a complex, NS1 promotes efficient degradation of C4 to C4b. Through this mechanism, NS1 protects DENV from complement-dependent neutralization in solution. These studies define a novel immune evasion mechanism for restricting complement control of microbial infection.

Proenzymic C1s associated with catalytic amounts of C1r. Study of the activation process

1. Proenzymic C1s isolated from human plasma by euglobulin precipitation and DEAE-cellulose chromatography is associated with trace amounts of C1r (0.5--1% on a molar basis). Incubation for 2 h at 37 degrees C leads to the proteolytic activation of C1s. The proteolysis is characterized by the sigmoidal appearance of C1s esterase activity and of the typical heavy (57 000-dalton) and light (28 000-dalton) fragments of C1s on sodium dodecyl sulphate-polyacrylamide gel electrophoresis. 2. The C1s activation process observed is markedly temperature and concentration dependent, and the rate of activation is decreased by calcium and high ionic strength (I = 0.9). Diisopropyl phosphorofluoridate, benzamidine, polyanethol sulfonate and pentosane polysulphate inhibit the activation, which is also sensitive to C1-inactivator and anti-C1r IgC. From the kinetic experiments and from the inhibition characteristics, the activation of C1s can be attributed to the presence of C1r, which appears to undergo activation and then to activate secondarily C1s.

Purification of proenzymic and activated human C1s free ofC1r. Effect of calcium and ionic strength on activated C1s

1. A rapid method for the purification of the proenzymic and activated forms of C1s is presented. In the case of proenzymic C1s, di-isopropyl phosphorofluoridate (0.5--5 mM) is added at all stages of the purification procedure, which includes euglobulin precipation followed by DEAE-cellulose chromatography and affinity chromatography on anti-C1r IgG-Sepharose 6B. The final step completely removes contaminant traces of C1r and/or C1r, ensuring that the final preparation of C1s is stable in the proenzyme form and suitable for activation studies. 2. The apparent molecular weight of C1s and C1s determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis is 85 000 +/- 2000. Reduction followed by alkylation of C1s gives two fragments of apparent molecular weights 57 000 and 28 000. Results of N-terminal amino acid determination and labelling with di-iso[3H]propyl phosphorofluoridate are consistent with previous reports. 3. The influence of calcium and ionic strength on the structure and activity of C1s has been investigated. Calcium leads to a shift of the sedimentation coefficient from 4.3 to 5.6 S, whereas variation in ionic strength has no effect on this parameter. The thermal inactivation curve is profoundly modified both by calcium and ionic strength. In contrast, the esterase activity is only slightly influenced as judged from the absence of gross modification of Km and V.

The unactivated form of the first component of human complement, C1

The first component of complement, C1, was isolated unactivated from human serum by repeated additions of di-isopropyl phosphorofluoridate during isolation. The unactivated subcomponents were also isolated, and evidence is given that the three subcomponents C1q, C1r and C1s account wholly for the activity of component C1 in serum. No evidence could be found for a fourth subcomponent, C1t. The approximate molar proportions of the subcomponents in serum are C1q/C1r/C1s = 1:2:2. Optimum activity by haemolytic assay was found at approximate molar proportions C1q/C1r/C1s of 1:4:4. No activity was found when subcomponents were assayed singly or in pairs, except for subcomponents C1q and C1s, which in molar ratio 1:4 gave 15-20% of the activity of the mixture C1q + C1r + C1s. The proteolytic activity of the isolated subcomponent C1s varied according to the method of activation used. Subcomponents C1q + C1r + C1s and C1q + C1s in the presence of antibody-antigen aggregates were activated and inactivated simultaneously, showing a peak of activity and subsequent loss of activity. Both reactions are probably due to proteolysis, and analysis of the peptide bonds split will be necessary to distinguish these two phenomena.