The regulation of complement on cell surfaces

Does dysregulated complement activation contribute to haemolytic uraemic syndrome secondary to Streptococcus pneumoniae?

We describe two patients with haemolytic uraemic syndrome (HUS) associated with invasive Streptococcus pneumoniae infection. Both patients had transiently reduced serum concentrations of complement C3. One had reduced expression of CD46 and never recovered renal function. No constitutive defect in regulation of the alternative pathway of complement activation was demonstrated in the second patient but there was an apparent improvement in her condition after administration of eculizumab. The most widely accepted mechanism for pneumococcal HUS is endothelial cell damage by pre-formed antibodies against the Thomsen-Friedenreich antigen. This explanation does not bear rigorous scrutiny. We postulate that transiently dysregulated complement activation may play a role in the pathogenesis of pneumococcal disease. We further postulate that the mechanism could be enhanced binding of factor H to the neuraminidase-altered surface of endothelial cells or reduced binding of factor H to the endothelial cell surface mediated by competitive binding of factor H by pneumococcal surface protein C (pspC).

B-cell activity in children with malaria

Background

Recent studies implicate deficiency of red blood cell (RBC) complement regulatory proteins (CR1 and CD55) in the pathogenesis of malarial anaemia. This study explored the involvement of B cell CD21, which has an analogous role to RBC CR1.

Methods

In a case control study conducted in Kisumu District hospital, western Kenya, children with severe malaria anaemia (SMA) and those with uncomplicated malaria (UM) were assessed by flow cytometry for B cells (CD20+) numbers, expression levels of CD21 and deposition of C3dg and by ELISA for soluble CD21 (sCD21). Paired t tests were used to determine statistical significance at a = 0.05.

Results

Children with SMA had significantly higher lymphocyte count (9,627.7 ± 8786.1 SD vs. 5,507 ± 2436 SD, P = 0.04 in the UM group) and the computed geometric mean of mature B-cell numbers based on the absolute lymphocyte count was significantly higher for SMA group: 1,823 (1,126 to 2,982, 95% CI) and 826.6 (564 to 1,220, 95% CI)] for UM group (P = 0.003). SMA group also had a higher percentage of CD20+ B cells (26.8 ± 9.7SD vs 20.9 ± 9.01 SD in the UM) (P = 0.03), indicating considerable polyclonal B-cell activation. The CD21 median flourescence intensity was lower in the SMA (246.4 ± 87.4 SD vs 369 ± 137.7 SD) (P <0.0001), probably due to complement mediated shaving of CD21 by fixed tissue macrophages. The CD20+ B cells of SMAs had higher levels of the complement split product C3dg (18.35 ± 10 SD vs 11.5 ± 6.8 S.D), (P = 0.0002), confirming possible role of complement in CD21 removal. Unexpectedly, the SMAs had lower levels of sCD21 (226.5 ± 131.5 SD vs 341.4 ± 137.3 SD in the UM) (P < 0.0001), indicating that the shaved CD21 is not released to peripheral circulation.

Conclusions

These results implicate B-cell in pathophysiology of severe malaria that involves increased B-cell proliferation, increased complement deposition and subsequent loss of membrane-bound CD21. The loss of CD21 is not by the classical enzmatic cleavage.

Overexpression of complement receptor type I (CR1, CD35) on erythrocytes in patients with hemoplasma infection

In the present study, 12 patients with fever of undetermined origin, anemia and icterus were diagnosed with hemoplasma infection by light microscopy, transmission electron microscopy examination and PCR assay after being excluded from other usual febrile diseases. Complement receptor type I (CR1, CD35) expression on the surface of erythrocytes was assessed by flow cytometry using mouse anti-human CD35 antibody. Compared with healthy volunteers, the level of CD35 was significantly elevated in patients with severe hemoplasma infection at diagnosis, and decreased sharply after treatment. However, in latent infection cases without clinical manifestations, CD35 expression showed an ascending trend but had no statistical difference compared to the healthy controls. The present study demonstrated that hemoplasma infection can induce high levels of expression of CR1 on the membrane of red blood cells, which may be a reaction to the immunity challenge.

Overexpression of EpCAM in uterine serous papillary carcinoma: implications for EpCAM-specific immunotherapy with human monoclonal antibody adecatumumab (MT201)

We evaluated the expression of epithelial cell adhesion molecule (EpCAM) and the potential of MT201 (adecatumumab), a human monoclonal antibody against EpCAM, in uterine serous papillary carcinoma (USPC). EpCAM expression was evaluated by real-time PCR and immunohistochemistry in a total of 56 USPC fresh-frozen biopsies and paraffin-embedded tissues. EpCAM surface expression was also evaluated by flow cytometry and immunohistochemistry in six USPC cell lines. Sensitivity to MT201 antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity was tested against a panel of primary USPC cell lines expressing different levels of EpCAM in standard 5-h (51)Cr release assays. EpCAM transcript was significantly overexpressed in fresh-frozen USPC when compared with normal endometrial cells (NEC). Median (minimum-maximum) copy number was 943.8 (31.5-1568.3) in tumor samples versus 12.9 (1.0-37.0) in NEC (P < 0.001). By immunohistochemistry, EpCAM expression was found in 96% (26 out of 27) of USPC samples with significantly higher expression compared with NECs (P < 0.001). High surface expression of EpCAM was found in 83% (five out of six) of the USPC cell lines tested by flow cytometry. EpCAM-positive cell lines were found highly sensitive to MT201-mediated antibody-dependent cellular cytotoxicity in vitro, whereas primary USPC cell lines were resistant to natural killer cell-dependent cytotoxicity. Human plasma IgG did not significantly inhibit MT201-mediated cytotoxicity against USPC. EpCAM is highly expressed in uterine serous carcinoma at mRNA and protein levels, and primary USPC are highly sensitivity to MT201-mediated cytotoxicity. MT201 might represent a novel therapeutic strategy in patients harboring advanced/recurrent or metastatic USPC refractory to standard treatment modalities.

The pathophysiology of malarial anaemia: where have all the red cells gone?

Malarial anaemia is an enormous public health problem in endemic areas and occurs predominantly in children in the first 3 years of life. Anaemia is due to both a great increase in clearance of uninfected cells and a failure of an adequate bone marrow response. Odhiambo, Stoute and colleagues show how the age distribution of malarial anaemia and the haemolysis of red blood cells may be linked by an age-dependent increase in the capacity of red blood cells to inactivate complement components absorbed or deposited directly on to the surface of the red blood cell. In this commentary, we discuss what has been established about the role of complement deposition on the surface of red blood cells in the pathology of malarial anaemia, how genetic polymorphisms of the complement control proteins influence the outcome of malaria infection and how the findings of Odhiambo, Stoute and colleagues and others shed light on the puzzling age distribution of different syndromes of severe malaria.

Overexpression of epidermal growth factor type-1 receptor (EGF-R1) in cervical cancer: Implications for Cetuximab-mediated therapy in recurrent/metastatic disease

OBJECTIVES

To evaluate and compare epidermal growth factor type-1 receptor (EGF-R1) expression in short term and established cervical cancer cell lines generated from primary and metastatic/recurrent sites of disease. To evaluate the sensitivity of cervical cancer cell lines to treatment with a chimeric MAb against EGFR-1 (Cetuximab).

METHODS

EGFR-1 expression was evaluated by flow cytometry on 22 cervical cancer cell lines including 14 primary cervical cancer cell lines obtained from cervical biopsies (11 patients) and recurrent sites of disease (three patients) as well as eight established cell lines. Tumor cell lines were tested for sensitivity to Cetuximab-mediated complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) in 51Cr release assays. Finally, Cetuximab-mediated inhibition of cell proliferation was also tested.

RESULTS

Fourteen out of fourteen (100%) primary tumors and seven out of eight (87.5%) established cervical cancer cell lines expressed EGFR-1 by flow cytometry. Cell lines from recurrent/metastatic sites of disease expressed higher levels of EGFR-1 when compared to those obtained from primary sites (p>0.05). Minimal CDC was detected in the majority of cervical cancer cell lines exposed to complement+/-Cetuximab in the absence of peripheral blood lymphocytes (PBL). In contrast, cervical tumor cell lines were found highly sensitive to Cetuximab-mediated ADCC when challenged with PBL from either healthy donors or cervical cancer patients. Importantly, ADCC was further increased in the presence of complement. Finally, tumor proliferation was significantly inhibited by Cetuximab in all cervical tumors tested.

CONCLUSIONS

EGFR-1 is highly expressed in primary and recurrent cervical tumors. Cetuximab might be a novel and attractive therapeutic strategy in patients harboring chemotherapy-resistant, recurrent, or metastatic cervical cancer.

Critical role of the C-terminal domains of factor H in regulating complement activation at cell surfaces

The plasma protein factor H primarily controls the activation of the alternative pathway of complement. The C-terminal of factor H is known to be involved in protection of host cells from complement attack. In the present study, we show that domains 19-20 alone are capable of discriminating between host-like and complement-activating cells. Furthermore, although factor H possesses three binding sites for C3b, binding to cell-bound C3b can be almost completely inhibited by the single site located in domains 19-20. All of the regulatory activities of factor H are expressed by the N-terminal four domains, but these activities toward cell-bound C3b are inhibited by isolated recombinant domains 19-20 (rH 19-20). Direct competition with the N-terminal site is unlikely to explain this because regulation of fluid phase C3b is unaffected by domains 19-20. Finally, we show that addition of isolated rH 19-20 to normal human serum leads to aggressive complement-mediated lysis of normally nonactivating sheep erythrocytes and moderate lysis of human erythrocytes, which possess membrane-bound regulators of complement. Taken together, the results highlight the importance of the cell surface protective functions exhibited by factor H compared with other complement regulatory proteins. The results may also explain why atypical hemolytic uremic syndrome patients with mutations affecting domains 19-20 can maintain complement homeostasis in plasma while their complement system attacks erythrocytes, platelets, endothelial cells, and kidney tissue.

The role of complement in immunological demyelination of the mammalian spinal cord

STUDY DESIGN

Specificity of serum complement component to elicit immunological demyelination.

OBJECTIVES

To assess the role of complement components and pathways in experimental immunological demyelination of the adult rat spinal cord.

SETTING

ICORD, University of British Columbia, Vancouver, Canada.

SUBJECTS

We used 32 adult male Sprague-Dawley rats, of approximately 220 g weight.

METHODS

Rats received intraspinal infusions of demyelinating reagents, delivered by osmotic minipump, for a 7-day infusion at 0.5 microl/h. Reagents consisted of a polyclonal antibody to galactocerebroside and human serum complement. Complement sera deficient for a single component were used to assess the role of the alternative pathway, the classical pathway, and the membrane attack complex. Demyelination was assessed, at 7 days, ultrastructurally.

RESULTS

Removal of C3 protein, common to classical and alternative complement pathways, or C4 protein, a classical pathway protein, resulted in no demyelination. However, complement deficient in Factor B, an alternative pathway protein, produced effective demyelination. Upon removal of C5 or C6, membrane attack complex proteins, demyelination was also observed.

CONCLUSION

This suggests that the classical pathway is sufficient for the protocol to demyelinate the adult rat spinal cord, and that the membrane attack complex is also not required.

The antitumor immune response in HER-2 positive, metastatic breast cancer patients

The aim of this study was to determine the basis for anti-tumor immune reactivity observed in patients with human epidermal growth factor receptor-2 (HER-2) (3+) breast carcinoma using an in vitro model in which the role of the HER-2-specific monoclonal antibody Herceptin was also investigated. Patients with metastatic breast cancer who had their primary tumor resected were included in this study. Peripheral blood mononuclear cell (PBMC)-dependent cytotoxicity in the presence or absence of Herceptin were assessed using the survival of target breast adenocarcinoma MDA-MB-361 cells as a parameter in a (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) test. We observed a significant increase in PBMC-dependent cytotoxicity when autologous serum was introduced in the assay. Furthermore, the addition of Herceptin significantly increases their cytotoxicity. These data suggest that autologous serum constitutively contains factors that might affect PBMC-dependent cytotoxic activity against HER-2 positive cancer cells.

Complement-opsonized HIV: the free rider on its way to infection

The complement system (C) is one of the main humoral components of innate immunity. Three major tasks of C against invading pathogens are: (i) lysis of pathogens by the formation of the membrane attack complex (MAC); (ii) opsonization of pathogens with complement fragments to favor phagocytosis; and (iii) attraction of inflammatory cells by chemotaxis. Like other particles, HIV activates C and becomes opsonized. To escape complement-mediated lysis, HIV has adopted various properties, which include the acquisition of HIV-associated molecules (HAMs) belonging to the family of complement regulators, such as CD46, CD55, CD59, and the interaction with humoral regulatory factors like factor H (fH). Opsonized virus may bind to complement receptor positive cells to infect them more efficiently or to remain bound on the surface of such cells. In the latter case HIV can be transmitted to cells susceptible for infection. This review discusses several aspects of C-HIV interactions and provides a model for the dynamics of this process.

Clinical features and pathogenesis of severe malaria

A major change in recent years has been the recognition that severe malaria, predominantly caused by Plasmodium falciparum, is a complex multi-system disorder presenting with a range of clinical features. It is becoming apparent that syndromes such as cerebral malaria, which were previously considered relatively clear cut, are not homogenous conditions with a single pathological correlate or pathogenic process. This creates challenges both for elucidating key mechanisms of disease and for identifying suitable targets for adjunctive therapy. The development of severe malaria probably results from a combination of parasite-specific factors, such as adhesion and sequestration in the vasculature and the release of bioactive molecules, together with host inflammatory responses. These include cytokine and chemokine production and cellular infiltrates. This review summarizes progress in several areas presented at a recent meeting.

Cutting edge: localization of the host recognition functions of complement factor H at the carboxyl-terminal: implications for hemolytic uremic syndrome

Incidents of hemolytic uremic syndrome (HUS) include a subset of patients that exhibit mutations in C factor H. These mutations cluster in the C-terminal domains of factor H where previous reports have identified polyanion and C3b-binding sites. In this study, we show that recombinant human factor H with deletions at the C-terminal end of the protein loses the ability to control the spontaneous activation of the alternative C pathway on host-like surfaces. For the pathology of HUS, the findings imply that mutations that disrupt the normal functions of the C-terminal domains prevent host polyanion recognition. The resulting uncontrolled activation of complement on susceptible host tissues appears to be the initiating event behind the acute renal failure of familial HUS patients.

Molecular mechanisms of target recognition in an innate immune system: interactions among factor H, C3b, and target in the alternative pathway of human complement

In the alternative pathway of complement (APC) factor H is the primary control factor involved in discrimination between potential pathogens. The APC deposits C3b on possible Ags, and the interaction with factor H determines whether the initial C3b activates the APC. Factor H is composed of a linear array of 20 homologous short consensus repeats (SCR) domains with many functional sites. Three of these sites are involved in binding C3b and regulating complement activation; others bind to sialic acid and/or heparin and are responsible for host recognition. Using site-directed mutations we have examined the contributions of each of these sites to target discrimination and to functional activities of factor H. Decay acceleration by SCR1-4 of C3/C5 convertases bound to nonactivators was strongly dependent on SCR domains 11-15 and 16-20. Loss of these regions caused a 97% loss of activity, with SCR16-20 being the most critical (>90% loss). On APC activators the pattern of site usage was different and unique on each. On yeast, deletion of the 10 C-terminal domains (SCR11-20) had no effect on specific activity. On rabbit erythrocytes, this deletion caused loss of 75% of the specific activity. An examination of binding affinity to C3b on the four cell types demonstrated that factor H exhibits a unique pattern of SCR involvement on each cell. The results reveal a complex molecular mechanism of discrimination between microbes and host in this ancient innate defense system and help explain the different rates and intensities of APC activation on different biological particles.

Red cell surface changes and erythrophagocytosis in children with severe Plasmodium falciparum anemia

Severe anemia is one of the most lethal complications in children infected with Plasmodium falciparum. The pathogenesis of this anemia is not completely understood. Experimental data from malaria-infected humans and animal models suggest that uninfected red cells have a shortened life span. This study looked for changes in the red cell surfaces of children with severe malarial anemia that could explain this accelerated destruction. A prospective case-control study was conducted of children with severe P falciparum anemia (hemoglobin of 5 g/dL or lower) admitted to a large general hospital in western Kenya. Children with severe anemia were compared with children who had symptoms of uncomplicated malaria and with asymptomatic children. Cytofluorometry was used to quantify in vitro erythrophagocytosis and to measure red cell surface immunoglobulin G (IgG) and the complement regulatory proteins CR1, CD55, and CD59. Red cells from patients with severe anemia were more susceptible to phagocytosis and also showed increased surface IgG and deficiencies in CR1 and CD55 compared with controls. Red cell surface CD59 was elevated in cases of severe anemia compared with asymptomatic controls but not as compared with symptomatic controls. The surface of red cells of children with severe P falciparum anemia is modified by the deposition of IgG and alterations in the levels of complement regulatory proteins. These changes could contribute to the accelerated destruction of red cells in these patients by mechanisms such as phagocytosis or complement-mediated lysis.

The complement system in liposome clearance: Can complement deposition be inhibited?

The activation of complement results in the opsonization of particles for removal by the reticuloendothelial system. Experimental evidence suggests that complement-mediated clearance of liposomal systems may significantly contribute to liposome biodistribution. Because of the multiplicity of complement activation mechanisms and the large number of proteins in the pathway, there are multiple opportunities to reduce or eliminate the opsonic effects of complement activation. This review addresses the state of our understanding of the interaction of liposomes with complement proteins and suggests some approaches to minimize complement activation.

Controlling the complement system in inflammation

Inappropriate or excessive activation of the complement system can lead to harmful, potentially life-threatening consequences due to severe inflammatory tissue destruction. These consequences are clinically manifested in various disorders, including septic shock, multiple organ failure and hyperacute graft rejection. Genetic complement deficiencies or complement depletion have been proven to be beneficial in reducing tissue injury in a number of animal models of severe complement-dependent inflammation. It is therefore believed that therapeutic inhibition of complement is likely to arrest the process of certain diseases. Attempts to efficiently inhibit complement include the application of endogenous soluble complement inhibitors (C1-inhibitor, recombinant soluble complement receptor 1- rsCR1), the administration of antibodies, either blocking key proteins of the cascade reaction (e.g. C3, C5), neutralizing the action of the complement-derived anaphylatoxin C5a, or interfering with complement receptor 3 (CR3, CD18/11b)-mediated adhesion of inflammatory cells to the vascular endothelium. In addition, incorporation of membrane-bound complement regulators (DAF-CD55, MCP-CD46, CD59) has become possible by transfection of the correspondent cDNA into xenogeneic cells. Thereby, protection against complement-mediated inflammatory tissue damage could be achieved in various animal models of sepsis, myocardial as well as intestinal ischemia/reperfusion injury, adult respiratory distress syndrome, nephritis and graft rejection. Supported by results from first clinical trials, complement inhibition appears to be a suitable therapeutic approach to control inflammation. Current strategies to specifically inhibit complement in inflammation have been discussed at a recent meeting on the 'Immune Consequences of Trauma, Shock and Sepsis', held from March 4-8, 1997, in Munich, Germany. The Congress (chairman: E. Faist, Munich, Germany), which was held in close cooperation with various national and international shock and trauma societies, was attended by about 2000 delegates from 40 countries. The major objective of the meeting was to provide an overview on the most state-of-the-art methods to prevent multiple organ dysfunction syndrome (MODS)/multiple organ failure (MOF) following the systemic inflammatory response (SIRS) to severe trauma. One of the largest symposia held within the Congress was devoted to current aspects of controlling complement in inflammation (for abstracts see: Shock 1997, 7 Suppl., 71-75). After providing the audience with information on the scientific background by addressing the clinical relevance of complement activation (G.O. Till, Ann Arbor, MI, USA) and discussing recent developments in modern complement diagnosis (J. Köhl, Hannover, Germany), B.P. Morgan (Cardiff, UK) introduced the symposium's special issue by giving an overview on complement regulatory molecules. Selected topics included overviews on the application of C1 inhibitor (C.E. Hack, Amsterdam, NL), sCR1 (U.S. Ryan, Needham, MA, USA), antibodies to C5 (Y. Wang, New Haven CT, USA) and to the anaphylatoxin C5a (M. Oppermann, Göttingen, Germany), and a report on complement inhibition in cardiopulmonary bypass (T.E. Mollnes, Bodø, Norway). The growing interest of clinicians in complement-directed anti-inflammatory therapy, and the fact that only some of the various aspects of therapeutic complement inhibition could be addressed on the meeting, has motivated the author to expand a Congress report into a short comprehensive review on recent strategies to control complement in inflammation.

Complement expression on astrocytes and astrocytoma cell lines: failure of complement regulation at the C3 level correlates with very low CD55 expression

Primary fetal human astrocytes and an astrocytoma cell line, U373-MG, expressed membrane cofactor protein (CD46), CD59, and low levels of decay-accelerating factor (CD55). Astrocyte CD55 was capable of regulating C3 deposition on the cell surface; albeit at a lower level than primary human fibroblasts. Negligible complement-mediated lysis of primary astrocytes and the U373-MG cell line was observed, even when large amount of astrocyte-specific, complement-activating antibodies were bound to the cells. Blocking the function of CD59 on astrocytes resulted in a > 90% cell lysis, while equivalent lysis of fibroblasts could only be achieved with additional blocking of CD55.

Complement activation by cold agglutinins

Purified monoclonal human IgM cold agglutinins (CA) of different specificities (anti-I, anti-i, anti-Pr) were investigated for their complement-activating capacity in a homologous system. Incubation of human RBC with excess of IgM CA in the cold, and subsequently with human serum at 37 degrees C, resulted in striking differences in hemolysis. Hemolysis did not correlate to the amount of antibodies bound to RBC at 4 or 20 degrees C. Despite the hemolytic inefficiency of anti-i and anti-Pr CA tested, C1 fixation and subsequent activation of the classical pathway of complement could be assessed in all cases. Absolute numbers of C3 molecules bound to RBC, exceeding the critical level to initiate the terminal sequence of the complement cascade, could not fully explain the differences in the hemolytic activity of the CA. Since C8 binding protein (C8bp) carries I determinants it is hypothesized that anti-I-induced complement-mediated hemolysis might also be favored by the binding of the autoantibody to and probably steric hindrance of this major regulatory protein of the terminal complement sequence. The prominent role of homologous restriction of complement-mediated lysis as a protective mechanism can also be deduced from the fact that rabbit as well as rat serum as a source of heterologous complement lysed cold agglutinin-sensitized red blood cells more efficiently than human serum.