Immune complex binding to erythrocyte-CR1 (CD 35), CR1 expression and levels of erythrocyte-fixed C3 fragments in SLE outpatients

Consequences of dysregulated complement regulators on red blood cells

The complement system represents the first line of defense that is involved in the clearance of pathogens, dying cells and immune complexes via opsonization, induction of an inflammatory response and the formation of a lytic pore. Red blood cells (RBCs) are very important for the delivery of oxygen to tissues and are continuously in contact with complement proteins in the blood plasma. To prevent complement activation on RBCs, various complement regulatory proteins can be found in plasma and on the cell membrane. RBCs are special cells without a nucleus and having a slightly different make-up of complement regulators than nucleated cells, as membrane cofactor protein (MCP) is not expressed and complement receptor 1 (CR1) is highly expressed. Decreased expression and/or function of complement regulatory proteins may result in unwanted complement activation and accelerated removal of RBCs. This review describes complement regulation on RBCs and the consequences when this regulation is out of balance.

The atherogenic bacterium Porphyromonas gingivalis evades circulating phagocytes by adhering to erythrocytes

A relationship between periodontitis and coronary heart disease has been investigated intensively. A pathogenic role for the oral bacterium Porphyromonas gingivalis has been suggested for both diseases. We examined whether complement activation by P. gingivalis strain ATCC 33277 allows the bacterium to adhere to human red blood cells (RBCs) and thereby evade attack by circulating phagocytes. On incubation with normal human serum, the P. gingivalis strain efficiently fixed complement component 3 (C3). Incubation of bacteria with washed whole blood cells suspended in autologous serum resulted in a dose- and time-dependent adherence to RBCs. The adherence required functionally intact complement receptor 1 (CR1; also called CD35) on the RBCs and significantly inhibited the uptake of P. gingivalis by neutrophils and B cells within 1 min of incubation (by 64% and 51%, respectively) and that by monocytes after between 15 min and 30 min of incubation (by 66% and 53%, respectively). The attachment of C3b/iC3b to bacterium-bearing RBCs decreased progressively after 15 min, indicating that conversion of C3 fragments into C3dg occurred, decreasing the affinity for CR1 on RBCs. We propose that P. gingivalis exploits RBCs as a transport vehicle, rendering it inaccessible to attack by phagocytes, and by doing so plays a role in the development of systemic diseases.

Measurement of erythrocyte C4d and complement receptor 1 in systemic lupus erythematosus

OBJECTIVE

C4-derived activation fragments are the only complement ligands present on the surfaces of normal erythrocytes. The significance of this observation is unknown, and the role of erythrocyte-bound C4 (E-C4) in human disease has not been explored. More than any other human disease, the pathogenesis of systemic lupus erythematosus (SLE) has been characterized by defects in clearance of complement-bearing immune complexes via erythrocytes expressing complement receptor 1 (CR1). This study was undertaken to determine whether these functional defects might be reflected by abnormal patterns of E-C4 and E-CR1 expression on erythrocytes of patients with SLE.

METHODS

We conducted a cross-sectional study of 100 patients with SLE, 133 patients with other diseases, and 84 healthy controls. Erythrocytes were characterized by indirect immunofluorescence and by flow cytometry for determination of levels of C4d and CR1.

RESULTS

Patients with SLE had higher levels of E-C4d and lower levels of E-CR1 than did patients with other diseases (P < or = 0.001) or healthy controls (P < or = 0.001). The test was 81% sensitive and 91% specific for SLE versus healthy controls and 72% sensitive and 79% specific for SLE versus other diseases, and it had an overall negative predictive value of 92%.

CONCLUSION

This is the first report of abnormal levels of E-C4d in human disease. We found that abnormally high levels of E-C4d and low levels of E-CR1 are characteristic of SLE, and combined measurement of the 2 molecules has high diagnostic sensitivity and specificity for lupus. Determination of E-C4d/E-CR1 levels may be a useful addition to current tests and criteria for SLE diagnosis.

Complement receptor expression and activation of the complement cascade on B lymphocytes from patients with systemic lupus erythematosus (SLE)

It has previously been reported that the expression of the complement receptors, CR1 on erythrocytes and blood leucocytes and CR2 on B cells, is reduced in patients with SLE, and that the reduced expression of CR1 on erythrocytes is related to disease activity. We have earlier demonstrated that normal B cells are capable of activating the alternative pathway (AP) of complement in a CR2-dependent fashion. In this study we have investigated whether disturbances in this activity may be related to the altered phenotype of SLE B cells. Flow cytometry was used to measure expression of complement receptors and regulatory proteins on B cells from SLE patients, as well as the deposition of C3 fragments occurring in vivo or after in vitro AP activation. We have confirmed, for a proportion of the patients studied, reduced expression of CR1 and CR2 on B cells, and shown a consistency between low CR2 expression and reduced in vitro AP activation in the presence of homologous, normal serum. In addition, the B cells, like erythrocytes, bear raised levels of in vivo-deposited C3dg, but not C3b fragments, compared with normal B cells. The erythrocytes from SLE patients were unable to inhibit in vitro AP activation by B cells in homologous serum. Finally, we demonstrated an inverse relationship between SLE disease activity index (SLEDAI) and the expression of complement receptor 2 (CR2) on SLE B cells. Thus, determination of CR2 on B cells may emerge as an additional laboratory tool in the assessment of SLE activity.

Complement

The complement system mediates a wide range of important biological functions. The use of modern techniques in protein chemistry and molecular biology has greatly facilitated our understanding of the interactions between the fluid phase and cell-bound components of the system. Structural and genetic analysis has shown that while many of these components are polymorphic, there are major similarities between many of the proteins serving enzymatic and regulatory roles in both the alternative and classical pathways. The regulation of complement activation and Class III genes, on chromosomes 1 and 6 respectively, encode nine of the major proteins in the system. The genetic basis of C4 and C3 polymorphisms is now well established, and further study may reveal functional differences between polymorphic variants of other components. The study of individuals with either genetic or acquired deficiencies of complement proteins and receptors has provided insight into the function of these components, leukocyte adherence deficiency (LAD) providing the best example. An appreciation of the genetics, structure and functions of the regulatory proteins decay-accelerating factor (DAF) and homologous restriction factor has enhanced our understanding of the pathogenesis of paroxysmal nocturnal haemoglobinuria. The full importance of CD59 glycoprotein, the newest member of the complement family, remains to be determined.