Is complement a target for therapy in renal disease?

Serum concentrations of the third component of complement in healthy dogs and dogs with protein-losing nephropathy

OBJECTIVE

To develop a method for determining the concentration of the third component of complement (C3) in canine serum, to establish a reference range for C3 in healthy dogs, and to evaluate dogs with protein-losing nephropathy (PLN) to determine whether PLN is associated with decreased serum C3 concentrations.

ANIMALS

30 healthy dogs and 49 dogs with PLN.

PROCEDURES

Serum samples were obtained from healthy dogs at the time of examination, whereas serum samples were obtained from dogs with PLN at the time of diagnosis. All samples were frozen at -70 degrees C until analyzed. Serum C3 concentrations were determined by use of a sandwich ELISA. Concentrations were expressed as the number of dilutions in which C3 could be detected.

RESULTS

C3 was detectable in healthy control dogs (range, 1,920,000 to 15,400,000 dilutions; median, 9,600,000 dilutions). This represented a range of four 2-fold serum dilutions. In addition, C3 was detectable in dogs with PLN (range, 1,460,000 to 30,070,000 dilutions; median, 7,680,000 dilutions), which represented a range of six 2-fold serum dilutions. There was no significant difference in C3 concentrations between the 2 groups.

CONCLUSIONS AND CLINICAL RELEVANCE

C3 is a critical part of the immune defense system that has not been extensively examined in veterinary medicine. An ELISA was developed for measuring C3 concentrations, and a reference range for healthy dogs was established. Significant decreases in C3 concentrations were not detected in any dog with PLN. Additional studies will be required to definitively determine the importance of serum C3 concentrations in PLN.

Binding of Tamm-Horsfall protein to complement 1q and complement 1, including influence of hydrogen-ion concentration

The goal of this study was to further characterize the interaction between an abundant urinary glycoprotein, Tamm-Horsfall protein, and complement 1q to determine the robustness of this reaction under different environmental conditions (particularly pH) and to begin to determine the specificity of this reaction. The influence of pH coupled with ionic strength was evaluated with an ELISA that demonstrated immobilized Tamm-Horsfall protein bound complement 1q strongly with a KD in the nmol/L range from pH 9 to pH 5.5. Increasing the ionic strength from 10 mmol/L sodium chloride (NaCl) to 154 mmol/L NaCl decreased the affinity of Tamm-Horsfall protein for complement 1q slightly (2-7-fold) at pH 9 to pH 6.5. A resonant mirror biosensor was also utilized to evaluate the binding of Tamm-Horsfall protein to complement 1q at different pH values (pH 8.2-5.8). These studies indicated that, compared to at pH 8.2, Tamm-Horsfall protein bound complement 1q at pH 5.8 with an almost two-fold higher affinity (pH 8.2, KD = 5.1 nmol/L vs at pH 5.8, KD = 2.8 nmol/L) due to a faster association rate (pH 8.2 kass = 1.6 x 106 L/mol per s vs pH 5.8 kass = 2.9 x 106 L/mol per s). Surprisingly, the capacity of Tamm-Horsfall protein for complement 1q decreased significantly at pH 5.8, suggesting that a site for complement 1q binding to Tamm-Horsfall protein may be lost at the acidic pH. Biosensor studies also showed that Tamm-Horsfall protein bound the entire complement 1 complex with binding affinities and association rates similar to those obtained for complement 1q individually. This suggested that Tamm-Horsfall protein bound complement 1q at a site other than the region of its collagenous tail where C1r2 and C1s2 bind. By western blot analysis, it was demonstrated that Tamm-Horsfall protein bound preferentially to the C chain of complement 1q.

The complement system in renal diseases

The complement system has long been recognized as having a role in immune glomerular disease. This review provides an update on this association, some strategies for the clinical testing of complement in disease, and a brief commentary on current research directions. Evidence of complement activation in glomerulonephritis comes from characteristic patterns of a decrease in the serum concentrations of specific components, some of which are virtually diagnostic of certain nephritides. These patterns are often accompanied by the presence of complement components in the glomeruli and the detection of complement breakdown products in the circulation. In certain diseases, circulating complement-activating substances can be detected. Although there are over 20 complement proteins, clinical analysis is most often directed at C3 and C4, with occasional measurement of B and C5. Recently, a variety of mechanisms for complement-induced injury has been recognized. These mechanisms go far beyond simple passive lysis of erythrocytes, the earliest functional effect of complement studied. The role of such mechanisms in renal disease is just beginning to be studied. Local synthesis of complement components in the kidney may play a role both in host defense and in the promotion of interstitial inflammation and scarring. Such mechanisms will likely be defined more precisely with the availability of animals with specific complement deficiencies. Ultimately, an understanding of the role of complement in renal disease may permit specific targeted inhibition of one or more complement functions as a form of therapy.

Complement components as uremic toxins and their potential role as mediators of microinflammation

Cardiovascular disease is the major cause of death in end-stage renal disease (ESRD) patients. There is growing evidence that atherogenesis is an inflammatory rather than a purely degenerative process leading to a state of microinflammation. This raises the issue of whether treatment modalities of ESRD contribute to the microinflammatory state. One potential candidate in this context is the complement system. Here we consider three potential pathways linking complement activation to progression of atherosclerosis: (1) complement activation on artificial surfaces depends on their physicochemical characteristics, the effect of which is amplified because of the accumulation of complement factor D; (2) the exposure of ESRD patients to endotoxin creates a microinflammatory state, and this may amplify complement-induced damage; exposure to endotoxin may result from frequent infections because of the impairment of host-defense mechanisms or from transfer of bacterial contaminants across dialysis membranes into the blood stream; and (3) direct transduction of proinflammatory signals from blood-material interactions to the vascular system. We conclude that the complement system is an important candidate system in the genesis of microinflammation and accelerated atherogenesis in ESRD. We advance the hypothesis that the generation of proinflammatory signals, in which the complement system appears to be involved--both through systemic and local activation--plays a role in the development of late complications of uremia, including coronary heart disease. This hypothesis provides a rationale to maximize the biocompatibility of the dialysis procedure, that is, selection of nonactivating materials, use of ultrapure dialysis fluid, and--still theoretical--high-flux dialysis to remove factor D.

Urine levels of CD46 (membrane cofactor protein) are increased in patients with glomerular diseases

Soluble membrane cofactor protein (MCP, CD46) has not been detected by conventional ELISA in human urine. Here, we established a highly sensitive assay method for determination of urinary MCP (uMCP) using monoclonal antibody-coated paramagnetic beads. This method enabled us to detect less than 0.05 ng/ml of purified membrane and recombinant soluble MCP, a sensitivity 10-fold higher than that of conventional ELISA. In normal subjects, the levels of uMCP were <0. 05 ng/ml. The levels of uMCP were elevated in patients with IgA nephropathy and more prominently in patients with rapidly progressive glomerulonephritis. The levels of uMCP were correlated significantly with those of serum MCP (sMCP) and N-acetyl-beta-glucosaminidase and nonsignificantly with those of beta(2)-microglobulin, total urine protein, or serum creatinine. The properties of uMCP were inconsistent with those of the reported sMCP, since uMCP showed three bands on SDS-PAGE/immunoblotting with molecular mass profiles different from those of sMCP. uMCP exhibited factor I cofactor activity for cleavage of C3b comparable to that of sMCP. The origin of uMCP, however, remains to be determined. These results, taken together with the parameter correlation profiles, suggested that uMCP is secreted or produced secondary to tubular or glomerular damage. The physiological role and clinical significance of uMCP are now within the scope of our investigation by establishment of this assay.