Nephritic Factors: An Overview of Classification, Diagnostic Tools and Clinical Associations

Abstract

Nephritic factors comprise a heterogeneous group of autoantibodies against neoepitopes generated in the C3 and C5 convertases of the complement system, causing its dysregulation. Classification of these autoantibodies can be clustered according to their stabilization of different convertases either from the classical or alternative pathway. The first nephritic factor described with the capacity to stabilize C3 convertase of the alternative pathway was C3 nephritic factor (C3NeF). Another nephritic factor has been characterized by the ability to stabilize C5 convertase of the alternative pathway (C5NeF). In addition, there are autoantibodies against assembled C3/C5 convertase of the classical and lectin pathways (C4NeF). These autoantibodies have been mainly associated with kidney diseases, like C3 glomerulopathy and immune complex-associated-membranoproliferative glomerulonephritis. Other clinical situations where these autoantibodies have been observed include infections and autoimmune disorders such as systemic lupus erythematosus and acquired partial lipodystrophy. C3 hypocomplementemia is a common finding in all patients with nephritic factors. The methods to measure nephritic factors are not standardized, technically complex, and lack of an appropriate quality control. This review will be focused in the description of the mechanism of action of the three known nephritic factors (C3NeF, C4NeF, and C5NeF), and their association with human diseases. Moreover, we present an overview regarding the diagnostic tools for its detection, and the main therapeutic approach for the patients with nephritic factors.

Keywords: C3 glomerulopathy; complement system; eculizumab; lipodystrophy; nephritic factor.

Figure 1

The complement system. The complement system can be activated by three pathways. The classical (CP) and lectin (LP) pathways involve recognition of target-bound antibody or pathogen-specific carbohydrates by C1q or MBL, respectively. In the alternative pathway (AP), continuous, low-level activation of C3 by spontaneous hydrolysis of the internal C3 thioester, or C3 cleavage by plasma proteases, generates C3(H₂O) or C3b. Activation by any of the three pathways leads to the generation of C3 convertase complexes (C4b2a in the CP/LP and C3bBb in the AP) that cleave C3 into C3a and C3b. Additionally, the AP C3 convertase can bind properdin (P), a positive regulator that stabilizes the enzyme, extending its half-life more than 10-fold. The C3b generated can, in turn, forms more AP C3 convertase, allowing amplification of complement activation. The binding of a new C3b molecule to the C3 convertases creates the C5 convertases (C4b2aC3b or C3bBbC3b), which cleave C5 into C5a and C5b. C5b then initiates the terminal complement pathway, which eventually leads to the formation of the membrane attack complex (C5b-9) and lysis of the target cells. Complement activation is controlled at various levels by different soluble and membrane regulatory proteins (indicated within boxes).

Figure 2

Mechanism of action of nephritic factors. (A) C3NeF binds to a neoepitope on the C3 convertase of the alternative pathway (C3bBb), preventing its spontaneous and FH-mediated decay, and increasing its half-life and C3 consumption. (B) C5NeF binds to a neoepitope on the C5 convertase of the alternative pathway in the presence of properdin (P) (C3bBbC3bP), preventing its spontaneous and regulator-mediated decay, and increasing its half-life and C5 consumption. (C) C4NeF binds to a neoepitope on the C3/C5 convertases of the classical/lectin pathway (C4b2a/C4b2aC3b), preventing its spontaneous and regulator-mediated decay, and increasing its half-life and C3 consumption. Soluble and membrane complement regulators are indicated within boxes.

Figure 3

Diagnostic tools for the detection of nephritic factors (NFs). The functional activity of C3NeF can be determined through quantifying complement activation products (mostly C3 fragments) by two-dimensional immunoelectrophoresis, immunofixation electrophoresis or western blotting (A). However, the main tools for the detection of NFs activity are the hemolytic assays, which measure the lysis of sheep (SE)/rabbit erythrocytes (RE) (B). In these assays, purified complement components are added to perform convertases on the surface SE sensitized with anti-sheep antibodies (EA). Later, purified FB, FD and P are added to form specifically alternative pathway convertase. Other assays are developed using whole serum (patient's serum and normal human serum (NHS), in ratio 1: 1) to generate convertases on the SE or RE surface. Considering that RE could be lysed by human serum, terminal pathway needs to be blocked using C5 blockers (OmCI or eculizumab). Another option to measure NF activity is through the quantification of anaphylatoxins, C3a and C5a (C). There are binding assays to detect C3NeF based on enzyme-linked immunosorbent assays (ELISA) that detect immunoglobulins bound to the preformed convertase complex (CO-Ig). Other versions of the ELISA-based binding assay are the so-called C3NeF stabilization assays, which use a polyclonal antibody to detect plate-bound Bb fragment generated either in the absence (COS assay) or in the presence (COS-P assay) of P.