An enzyme based assay for the measurement of complement mediated binding of immune complexes to red blood cells

Immune complex binding by immunocamouflaged [poly(ethylene glycol)-grafted] erythrocytes

Immune complexes (IC) are constantly formed at low levels in normal individuals. In humans, the red blood cell (RBC) complement receptor 1 (CR1) plays the dominant role in the IC binding and clearance. Over the last several years, we have investigated the potential utility of immunocamouflaged (methoxypoly(ethylene glycol) [mPEG] grafted) RBC to attenuate the risk of alloimmunization. Because the grafted polymer nonspecifically camouflages membrane proteins, its effects on CR1 detection and IC binding were assessed. The dose dependent (0-2.5 mM) effects of activated mPEG (CmPEG, 5 kDa; and BTCmPEG, 5 and 20 kDa) on CR1 detection and the binding of artificially generated IC [C3b coated alkaline phosphatase and antialkaline phosphatase complexes] to control and pegylated RBC was investigated by flow cytometry. In contrast to selected non-ABO blood group antigens, grafted mPEG did not effectively camouflage CR1. Surprisingly, however, even very low grafting concentrations of mPEG (>or=0.3 mM) resulted in a >or=95% loss in IC binding. Further reductions in grafting concentration (0.15 and 0.03 mM mPEG) still yielded decreased IC binding of approximately 60 and 40%, respectively. Importantly, unactivated mPEG had minimal effects on IC binding. These data demonstrate that even small amounts of grafted mPEG interfere with the multivalent CR1-IC interaction necessary for high affinity IC binding, hence large volume transfusions of mPEG-RBC may be contraindicated in patients with pre-existing IC disease. Whether this concern is of clinical significance in healthy humans is less clear due to dilutional effects and the presence of secondary clearance pathways.

A discrepancy between liquid phase and gel phase assays for evaluation of total complement activity and some possible explanations

This study was conducted to investigate the frequency and origin of discrepant assay results between two haemolytic assays which both measure activity of the classical pathway of complement (CH50) by haemolysis of sheep red blood cells (SRBCs). One is conducted in gel phase using undiluted sera and the other in liquid phase with sera in 1/100 dilution. The majority of discrepant readings are observed as low or absent haemolysis in the gel phase, with values within or above the normal range in the liquid phase. The incidence of discrepant assay readings was evaluated in 300 samples. Furthermore, 28 samples showing the most discrepant readings were investigated further for disturbing factors. Factors evaluated in the test sera were mannose-binding lectin, C-reactive protein (CRP), immune complexes, antibodies to SRBCs, rheumatoid factor and immunoglobulin A (IgA) and IgG anti-C1q antibodies. The results showed that discrepant readings are present in 10% of the 300 samples and false low gel assay readings account for 6.3%. The majority (68%) of the discrepant samples contained a heat-stable-inhibiting factor, and the main mediators found were elevated levels of IgA anti-C1q antibodies and antibodies to SRBCs. This could indicate a clinically relevant factor in the test sera but can also result from the difference in assay design.

Enhanced tryptophan degradation in systemic lupus erythematosus

In vitro and in vivo, tryptophan degradation was found to be associated with T cell functional loss and tolerance induction. In systemic lupus erythematosus (SLE) besides the Th2-type cytokine interleukin-10, Th1-type cytokines including interferon-gamma (IFN-gamma) are expressed especially during exacerbation of the disease. IFN-gamma stimulates the enzyme indoleamine (2,3)-dioxygenase (IDO) converting tryptophan to the metabolite kynurenine which in macrophages is subsequently degraded to other, partly neurotoxic compounds like quinolinic acid, and finally to nicrotinamides. We measured kynurenine and tryptophan concentrations in the sera of 55 SLE patients. In these patients, the concentrations of tryptophan (median, interquartile range: 53.9, 45.7-64.1 microM) were lower (p < 0.0001), and the kynurenine concentrations (2.45, 1.75-3.40 microM) were increased (p < 0.0005) compared to healthy blood donors (70.0, 63.8-80.6; 1.80, 1.45-2.27 microM, respectively). Also the kynurenine per tryptophan quotients (K/T), which allow to estimate IDO activity, were significantly higher in patients than in normals (0.043, 0.033-0.062 vs. 0.027, 0.021-0.030; p < 0.0001), indicating enhanced IDO-induced tryptophan degradation in SLE. There was no significant relationship between tryptophan, kynurenine and the SLEDAI, and also the correlation of K/T with SLEDAI was rather weak (rs = 0.243, p < 0.05). Higher K/T was found in patients presenting with serositis (p = 0.01), decrease of complement (c3, c4; p < 0.01) and blood count change (anemia, leucopenia, lymphopenia; p = 0.032) than in patients without such disease manifestations. The significant correlation found between K/T and neopterin (rs = 0.808, p < 0.001), a marker of immune activation, points to a role of immune activation to be responsible for tryptophan degradation in SLE patients.

Enzyme immunoassays for measuring complement-dependent prevention of immune precipitation (PIP) and solubilization of preformed antigen-antibody complexes (SOL)

We have developed simple and sensitive enzyme-based methods for evaluating the ability of serum complement to prevent immune complex precipitation (PIP) or to solubilize preformed immune complexes (SOL). Alkaline phosphatase, serving both as antigen and label, is added to goat IgG anti-alkaline phosphatase antibodies, with serum present throughout the assay (PIP), or added after immune complex formation (SOL). After incubation at 37 degrees C for 1 h followed by centrifugation, the enzyme activity of the supernatant, reflecting the amount of immune complexes in solution, is measured by colorimetry. Results are expressed with reference to a standard serum pool assigned 100 arbitrary units (AU). Intra- and inter-assay variabilities are within 10%. The normal ranges were 67-133 AU for PIP and 72-129 AU for SOL. These methods have been standardized for clinical use in relation to impaired complement function and immune complex disease, and adapted for measuring complement mediated binding of immune complexes to erythrocytes. They are sensitive, easy to perform and do not require expensive facilities. By measuring the interaction of complement with immune complexes, these methods may highlight aspects of the classical and the alternative pathway that are different from those detected using haemolysis as an endpoint.