Electro-immunoassay for properdin. A comparison with the radioimmunoassay

A sandwich ELISA for properdin in clinical specimens

We have developed a symmetrical sandwich ELISA for measuring human properdin (P) in serum by using the globulin fraction from a commercial antiserum as the capture antibody adsorbed on the plastic. The detecting reagent was a glutaraldehyde conjugate of this Ig fraction with alkaline phosphatase. Two types of inhibition were observed in this study. First, inhibition was observed when greater than 2.5 micrograms/ml of the globulin fraction was used to coat the plates. A second type of inhibition was observed for serum dilutions less than 1/400; it was independent of the concentration of capture Ab and did not occur when purified P was assayed. The data generated with this assay could be fitted in log-log mode by a quadratic equation. The coefficient of the linear term in this equation was found to be the same for serum and purified P, within the limits of experimental error. The results for different samples run on the same plate were expressed in terms of the relative concentration of each sample required to produce an OD405 = 0.2. A sample of pooled normal human serum was run on each plate as a reference; it was assigned a titer of 100 ELISA units/ml (EU/ml). The titers of the unknown samples were expressed in terms of EU/ml by reference to this standard. For purified P, the assay could readily detect 10 ng/ml. By comparing purified P with our reference serum pool, we found that 1 EU equals 0.57 microgram. Day-to-day variation for a group of nine normal sera showed a mean difference of -0.85 EU/ml, SD 5.85 EU/ml. The mean titer for these normal sera was 78.9 EU/ml, SD 15.7 EU/ml. In three recovery experiments in which purified P was mixed with pooled normal serum, the recoveries ranged from 96 to 117%. We conclude that the sandwich ELISA constitutes an adequate immunochemical assay for human P in serum specimens.

A radial hemolysis method in agarose for the functional assay of properdin

A simple one-step radial hemolytic assay for properdin has been devised. In this assay, the test material is introduced into a well in an agarose plate containing optimal concentrations of normal human serum immunochemically depleted of properdin (RP), EGTA, magnesium ions and unsensitized guinea pig erythrocytes. Following radial diffusion, the area of the hemolytic zones resulting from the activation of the alternative complement pathway and bystander lysis of guinea pig erythrocytes was directly proportional to the concentration of properdin in the test material. The assay is specific reproducible and sensitive and the correlation with the radioimmunoassay for properdin is very good. The assay has been used to measure properdin activity in animal sera.

Babesia rodhaini: requirement of complement for penetration of human erythrocytes

A system has been developed in vitro in which human red cells, in the presence of fresh human (or rat) serum, are parasitized by the hemosporidian protozoan Babesia rodhaini. The ability of B. rodhaini to penetrate red cells depends on factors of the alternative complement pathway (properdin and factor B) as well as ionic magnesium and the third (C3) and the fifth (C5) components of complement. These data indicate a novel mechanism by which a parasite is able to utilize the complement system. The data are in accord with and further amplify earlier observations that demonstrated a requirement for complement in the development of babesial infection in rats.

Pathways of complement activation in chronic discoid lupus: serologic and immunofluorescence studies

To assess the relative roles of the classic and alternative pathways of complement activation in chronic discoid lupus erythematosus (CDLE), serum levels of properdin, C3, and C4, and deposition of these proteins in the dermal-epidermal junction (DEJ) of 20 CDLE patients were compared to the findings in patients with clinically active and inactive SLE. Properdin was demonstrated in the DEJ of 10 of 14 (71%) histologically typical skin lesions from patients with CDLE, usually in association with deposits of immunoglobulin, C3, and C4. Properdin levels in CDLE patients were significantly increased (137 +/- 34%) (P less than 0.05) when compared to normal controls (101 +/- 18%) or to patients with clinically active SLE (89 +/- 32%). C3, C4, DNA-binding, and antinuclear antibody tests in CDLE were indistinguishable from those in normals, but significantly different from patients with active SLE (P less than 0.05). The complement profiles of patients with clinically inactive SLE resembled those of CDLE patients more closely than those of active SLE patients.