Multiple sedimenting species of properdin in human serum and interaction of purified properdin with the third component of complement.

Abstract

Properdin was originally described by Pillemer and co-workers in 1954 as a unique serum protein participating in an alternative pathway of complement (C) activation (1). Over the next several years, the sedimentation behavior of partially purified preparations of properdin was examined in several laboratories and values of 27S, 18S, 10S, and 5-7S were variously reported (1-5). Finally, in 1968 Pensky and co-workers obtained a homogenous preparation of properdin which sedimented at 5.2S in the analytical ultracentrifuge (6). Although earlier discrepancies could be attributed at least in part to methodological limitations, the possibility could not be excluded that the differences might also have been on a more substantive basis, perhaps related to the degree of purification achieved. With the development in our laboratory in 1973 of a radioimmunoassay for the detection of nanogram quantities of properdin (7), sucrose density gradient ultracentrifugation experiments were performed to determine the sedimentation behavior of properdin antigen in normal human serum. Radiolabeled or unlabeled properdin purified to homogeneity according to the method of Pensky et al. (6) served as control. The results, presented in this paper, provide evidence for the existence of multiple sedimenting species of properdin antigen in serum and identify the third component of C (C3) as a constituent of at least one of the heavier sedimenting species.

Requirements for the solubilization of immune aggregates by complement. The role of the classical pathway.

“…However, it is conceivable that the added purified P could bind to the solubilized complexes during coprecipitation because P has a binding site for C3b (23,24). This could happen either by an exchange reaction between the added P and previously bound P, or by the capture of the added P by some free C3b sites on the complexes.…”

“…Although the nature of the P receptor on the immune aggregates is unknown, C3 moieties become associated with the complexes at the same time that the binding of P takes place; that is, in the first minutes after incubation with serum (5). Because C3b binds P, (23)(24)(25) it is likely that the C3 molecules on the aggregates are P receptors.…”

Clinical significance of serum properdin levels and properdin deposition in the dermal-epidermal junction in systemic lupus erythematosus.

“…Recent studies by Chapitis and Lepow (48) and Fearon and Austen (17) indicate that properdin binds to native C3, C3b, or C3c in free solution, or to red cells coated with complement (EAC 43B). Thus, the possible consumption or binding of properdin via C3 was investigated in our patients.…”

Binding of components of the properdin system to cultured human lymphoblastoid cells and B lymphocytes.

“…Conceivably then, the inability to detect factor B on cells incubated with NHS may be due to the decay and release of the bound factor B. Alternatively, as stated above, it may be that cells, in contrast to other activators, do not generate C3b and, therefore, factor B cannot bind to cells. In the case of P such generation of C3b, although potentiating its binding to cells, is not a prerequisite since activated P can combine with native C3 (15,39,42).…”