Proteolytic elimination of decay-accelerating factor (DAF): lytic abnormality coincides with removal of DAF in papain-treated human erythrocytes

A Human Erythrocyte-based Haemolysis Assay for the Evaluation of Human Complement Activity

The complement system consists of at least 50 proteins that serve as one of the first lines of defence against foreign, or damaged, cells and invading microorganisms. Its dysregulation underlies the pathophysiology of many different diseases, which makes functional assays of complement activity crucial; they are, however, underutilised. Standard haemolysis assays for the analysis of complement function employ sensitised non-human erythrocytes (e.g. from the sheep, guinea-pig or rabbit), the use of which raises animal welfare concerns. To provide an alternative to the use of such animal-derived products for complement function assays, we developed a method that employs modified human erythrocytes to evaluate the activity of complement pathways. Human erythrocytes were subjected to various chemical and/or proteolytic treatments involving 2,4,6-trinitrobenzene sulphonate (TNBS) and pancreatin. Haemolysis assays demonstrated that sequential treatment with TNBS and pancreatin resulted in significantly greater complement-mediated haemolysis, as compared to TNBS or pancreatin treatment alone. Evidence that lysis of the modified erythrocytes was complement-mediated was provided by the chelation and subsequent restoration of calcium in the plasma. Thus, such modified human erythrocytes could be used as an alternative to animal-derived erythrocytes in haemolysis assays, in order to evaluate complement activity in human plasma during, for example, the screening of patients for complement deficiencies and other abnormalities in a clinical setting.

Micromolar A and B blood group active trisaccharides abolish human complement haemolytic activity assayed by erythrocytes

BACKGROUND

At present, there is really no satisfactory treatment of severe haemolytic transfusion reactions involving the ABO system other than the use of steroids that at best are palliative in their effects. In contrast, the use of micromolar concentrations of A or B blood group active trisaccharides that are inexpensive and readily available may prevent lysis by generating soluble immune complexes (ICs) that consume complement. The purpose of this study was to determine the total lytic activity of human serum and to estimate the extent to which trisaccharides can exhaust this capacity.

METHODS

We measured complement consumption by (ICs) formed between anti-blood group antibodies and A or B blood group active sugars on erythrocytes (solid phase) or soluble components carrying trisaccharides (fluid phase) in AB serum. A direct complement-mediated lysis (DCL) assay measured the solid-phase reaction and an indirect complement consumption assay (CCA) allowed determination of the fluid-phase reaction. In CCA, the residual lytic activity of AB serum was measured following preincubation with various ICs.

RESULTS

Based on over 4000 data points a new mathematical model of complement consumption was formulated. Its predictions deviated by less than 6% in DCL and 9% in CCA when compared with the experimentally accessible data. The new model describes the dynamics of complement consumption including the soluble phase of the reaction.

CONCLUSIONS

The mathematical model extrapolation predicts that: (1) in undiluted human serum up to 40% of the physiological erythrocyte concentration could be lysed (when antibodies did not limit lysis); and (2) a 40 microM (or less) concentration of blood group active trisaccharides per patient is sufficient to block the haemolytic complement activity.

Soluble CD46 (membrane cofactor protein, MCP) in human reproductive tract fluids

CD46 (membrane cofactor protein, MCP) is a cell surface complement regulatory protein which may have an additional role in human sperm-egg interaction. A soluble form (sCD46) has also been detected in a number of biological fluids, most notably seminal plasma. The present study has employed a monoclonal antibody-based ELISA to assay sCD46 in reproductive tract fluids in normal and pathological conditions. Large amounts of sCD46 were detected in seminal plasma of both fertile and infertile men (combined mean, 4859 ng/ml). Vasectomized men had lower levels (mean, 2421 ng/ml), indicating contributory sources both before and after the vas deferens ligation site. Pre-colostrum also contained relatively high quantities (mean, 445 ng/ml), whereas breast milk (mean, 117 ng/ml), peritoneal fluid (mean, 154 ng/ml) and follicular fluid (mean, 107 ng/ml), as well as uterine (mean, 208 ng/ml), umbilical (mean, 166 ng/ml) and peripheral (mean, 206 ng/ml) blood plasma, had sCD46 levels within a comparable range. Amniotic fluid had low sCD46 concentrations (mean, 22 ng/ml). In endometriosis, peritoneal fluid levels of sCD46 were significantly raised (mean, 199 mg/ml). These results indicate distinctive fluid compartmentalisation of sCD46 consistent with a biological function in human reproductive tract fluids.

Protease-modified erythrocytes: CD55 and CD59 deficient PNH-like cells

The increased susceptibility to homologous complement in paroxysmal nocturnal haemoglobinuria (PNH) is known to be associated with the deficiency of the membrane complement inhibitors CD59 and CD55. Proteases have been used in this study to modify normal human RBC to complement sensitive PNH-like cells. To investigate the protective role of CD59 and CD55, the relationship between the content of CD59 and CD55 and the complement susceptibility of the PNH-like cells has been determined. The differential resistance of the enzyme-treated RBC to complement-mediated injury was measured by acidified serum lysis. Pronase-treated erythrocytes lacked both CD59 and CD55 and were very susceptible to complement-mediated lysis. Papain treatment of RBC reduced the CD55 content but did not affect CD59 and induced slight susceptibility to complement-mediated lysis. Trypsin treatment of RBC destroyed 80% of CD59, had little effect on CD55 (unless incubation was extended) and slightly increased susceptibility to lysis. Thus, partial CD55 and CD59 activity was sufficient to protect cells from complement-mediated lysis. In the reactive lysis assay, anti-CD55 and anti-CD59 induced haemolysis, anti-CD59 having the more pronounced effect. Lysis was enhanced when RBC were treated by both antibodies simultaneously.

An indirect effect of an antibody on complement deposition and lysis of differently sensitized surrounding cells

Lysis of papain-treated group A and B erythrocytes by human complement was studied by an anti-A (BRIC. 131) and an anti-B (BRIC. 30) IgM monoclonal antibody in 51Cr release assays. The indirect effect of membrane-bound antibody, i.e. its influence on complement binding to sensitized surrounding cells, was examined in a cold target competition test in which sensitized, non-labelled cells are present along with sensitized labelled cells and complement. The mode by which anti-A antibodies indirectly suppressed lysis of sensitized B cells up to 20-fold was studied by following C1q and C3b binding. C1q binding to both types of erythrocytes was not altered in mixed populations of erythrocytes in the presence of both antibodies. Binding of C3b to a mixture of both cell types was, however, suppressed, when both antibodies were present. C3b deposition in mixed cell populations did not reach a significantly higher extent than deposited to one type of erythrocyte alone. This was consistent with the results from competitive lysis and suggests that the anti-A captured most C3b at high anti-A concentrations and deprived the similarly sensitized B erythrocytes of complement. We think that this phenomenon is not due to an uneven removal of complement regulatory proteins from A and B erythrocytes by papain. Instead, the phenomenon might be due to an inherent property of anti-A mAb to better produce nucleation sites for C3 convertases which, upon binding factor B, better compete for the limiting factor D. A mathematical analysis of cold target competition experiment (containing 2430 individual measurements) also shows that the distribution of complement between the competing A and B erythrocyte population is uneven, since it predicts that in any given antibody combination the majority of complement is bound to A erythrocytes. This is consistent with the measured average percentage of lysis.

Identification and characterization of membrane cofactor protein (CD46) in the human kidneys

Membrane cofactor protein (MCP, CD46) is an integral protein that serves as a cofactor for factor I in inactivating C3b/C4b deposited on the same cell membrane as C3bi/C4c+C4d. This C3b/C4b inactivation is closely associated with self-protection of host cells from autologous complement attack. We have studied the distribution and properties of MCP in the normal human kidney by immunohistochemical and immunoblotting methods using monoclonal antibodies against MCP. MCP was predominantly expressed on the juxtaglomerular apparatus. Glomerular capillary walls, mesangial areas, and tubulus were also MCP positive. Glomerulus MCP was composed of two major bands of 45-65 kDa, which were similar to those of lymphocyte MCP. The proportion of the high and low molecular weight components in glomerulus MCP, however, was considerably different from that of lymphocyte MCP among the individual samples tested. Glomerular epithelial cells and mesangial cells from an individual having equal amounts of high and low molecular weight components in the lymphocytes were cultured separately and the properties of their MCP investigated. MCP in the mesangial cells and glomerular epithelial cells showed profiles in which the upper band was predominant. The results may explain the unique distribution of the high and low molecular weight forms in the glomerulus. These forms of MCP together with factor I were all capable of inactivating C3b to C3bi. Message analysis suggested that glomerular epithelial cells and mesangial cells synthesized a single species of mRNA of 4.2 kb from which the polymorphic MCP species were generated. Flow cytometric analysis suggested that MCP was minimal in mesangial cells. These results, taken together with the previous reports on the distribution of other complement regulatory proteins, infer that the distribution profile of MCP is rather similar to that of DAF but differs from those of CD59 and CR1 in the normal human kidney; this may reflect the differences between their roles or functional properties in renal tissue.

Levels of complement regulatory molecules in lung cancer: disappearance of the D17 epitope of CD55 in small-cell carcinoma

The levels of complement-regulatory molecules (complement receptor type one [CR1], decay-accelerating factor [DAF], membrane cofactor protein [MCP], and an inhibitor of membrane attack complex [CD59]) in lung cancer cells were analyzed to investigate the relation between their expression and histological subtypes, and the possibility of homologous complement deposition on cancer cells. In 25 cell lines (10 adenocarcinoma, 3 large-cell carcinoma, 7 small-cell lung cancer [SCLC], and 5 squamous cell carcinoma), flow cytometric analysis revealed that MCP was expressed in all cell lines, whereas none of the cell lines was CR1-positive. CD59 was detected in all cells. The DAF epitope defined by IA10 was expressed in all cells except one large cell carcinoma cell line. However, another epitope for anti-DAF monoclonal antibody, D17, was not detected in 5 (71.4%) SCLC and in 4 (22.2%) non-small-cell lung cancer. This disparity was seen in most cell lines, irrespective of histological subtypes. The loss of D17 reactivity seemed to be pertinent to malignant phenotype, because most of the normal pulmonary cells possessed the D17 epitope. Furthermore, a cell line lacking DAF (IA10-/D17-) allowed alternative pathway-mediated homologous complement (C3) deposition after pretreatment with anti-MCP antibody. This raises a new possibility for immunotargeting of cancer. These cell lines should be useful in studying the biology of lung cancer.

Preferential inactivation of the C5 convertase of the alternative complement pathway by factor I and membrane cofactor protein (MCP)

Human C3b bound to the ghost of sheep erythrocytes (E*) via activation of the alternative complement pathway (E*AC3b) consists of four major constituents on SDS-PAGE of 350, 260, 210 and 180 kDa. 350 kDa C3b is a dimeric form of C3b in which the alpha' chain of one C3b binds covalently to that of the other C3b. This complex is presumed to serve as a core for the alternative pathway C5 convertase. The other C3b populations are monomers complexed with membrane proteins or sugars. Using E*AC3b (C3b labeled) as a substrate, we have investigated functional properties of membrane cofactor protein (MCP), which is an integral membrane protein with C3b-binding and factor I-dependent cofactor activities. In conjunction with factor I, MCP was found to degrade the protein-bound C3b preferentially including the 350 kDa dimer. There was a similar but lesser tendency of this selective cleavage of C3b-dimer by CR1 but not by factor H or C4bp. In contrast to CR1 and factor H, detergent solubilization of EAC3b was required for MCP to fully express its cofactor activity for this selective degradation of C3b. We next separated the C3b dimer from the monomers and assessed their ability to assemble the alternative C5 convertase. The C3b dimer but not the monomers expressed C5 convertase activity following the addition of factors B and D, C5 and Ni2+. Kinetic analysis of the degradation of the C3b dimer by MCP and factor I suggested that only one C3b was efficiently converted to C3bi and this occurred concomitant with a decrease in C5 convertase activity. These results suggest that MCP has the ability to more efficiently interact with protein-bound C3b and that this may relate as well to its preferential ability to irreversibly inactivate the C5 convertase.