High expression of the antigen recognized by the monoclonal antibody GB24 on human breast carcinomas: a preventive mechanism of malignant tumor cells against complement attack?

Complement C5b-9 and Cancer: Mechanisms of Cell Damage, Cancer Counteractions, and Approaches for Intervention

The interactions of cancer cells with components of the complement system are highly complex, leading to an outcome that is either favorable or detrimental to cancer cells. Currently, we perceive only the "tip of the iceberg" of these interactions. In this review, we focus on the complement terminal C5b-9 complex, known also as the complement membrane attack complex (MAC) and discuss the complexity of its interaction with cancer cells, starting with a discussion of its proposed mode of action in mediating cell death, and continuing with a portrayal of the strategies of evasion exhibited by cancer cells, and closing with a proposal of treatment approaches targeted at evasion strategies. Upon intense complement activation and membrane insertion of sufficient C5b-9 complexes, the afflicted cells undergo regulated necrotic cell death with characteristic damage to intracellular organelles, including mitochondria, and perforation of the plasma membrane. Several pro-lytic factors have been proposed, including elevated intracellular calcium ion concentrations and activated JNK, Bid, RIPK1, RIPK3, and MLKL; however, further research is required to fully characterize the effective cell death signals activated by the C5b-9 complexes. Cancer cells over-express a multitude of protective measures which either block complement activation, thus reducing the number of membrane-inserted C5b-9 complexes, or facilitate the elimination of C5b-9 from the cell surface. Concomitantly, cancer cells activate several protective pathways that counteract the death signals. Blockage of complement activation is mediated by the complement membrane regulatory proteins CD46, CD55, and CD59 and by soluble complement regulators, by proteases that cleave complement proteins and by protein kinases, like CK2, which phosphorylate complement proteins. C5b-9 elimination and inhibition of cell death signals are mediated by caveolin and dynamin, by Hsp70 and Hsp90, by the mitochondrial stress protein mortalin, and by the protein kinases PKC and ERK. It is conceivable that various cancers and cancers at different stages of development will utilize distinct patterns of these and other MAC resistance strategies. In order to enhance the impact of antibody-based therapy on cancer, novel precise reagents that block the most effective protective strategies will have to be designed and applied as adjuvants to the therapeutic antibodies.

Modulation of host CD59 expression by varicella-zoster virus in human xenografts in vivo

Varicella-zoster virus (VZV) is the causative agent of both chickenpox (varicella) and shingles (zoster). VZV survives host defenses, even with an intact immune system, and disseminates in the host before causing disease. To date, several diverse immunomodulatory strategies used by VZV to undermine host immunity have been identified; however, few studies have addressed the complement evasion strategies used by this virus. Here, we show that expression of CD59, which is a key member of host regulators of complement activation (RCA), is significantly upregulated in response to VZV infection in human T cells and dorsal root ganglia (DRG) but not in human skin xenografts in SCID-hu mice in vivo. This is the first report demonstrating that VZV infection upregulates host CD59 expression in a tissue-specific manner in vivo, which may aid VZV in complement evasion and pathogenesis.

CD55 limits sensitivity to complement-dependent cytolysis triggered by heterologous expression of α-gal xenoantigen in colon tumor cells

Engineering cancer cells to express heterologous antigen α-gal and induce the destruction of tumor cells depending on the complement cascade may be a promising strategy of tumor therapy. However, the feasibility and effect of using α-gal to induce colorectal adenocarcinoma cell line cytolysis is not yet known. In this study, we evaluated α-gal expression's ability to sensitize human colorectal adenocarcinoma cell lines to complement attack in cell lines LoVo, SW620, and Ls-174T. Nearly all α-gal-expressing LoVo and SW620 cells were killed by normal human serum (NHS), but α-gal-expressing Ls-174T cells showed no significant lysis. We analyzed the expression levels of membrane-bound complement regulatory proteins (mCRPs) on the three cell lines, and their protective role in α-gal-mediated activation of the complement. LoVo showed no expression of any of the three proteins. CD59 was strongly expressed by SW620 and Ls-174T. CD46 and CD55 varied between the two cell lines. CD46 on SW620 was only half the intensity of CD46 on Ls-174T. Ls-174T showed a notable expression of CD55, while expression of CD55 on SW620 was not detected. The sensitivity of Ls-174T expressing α-gal to NHS greatly increased following the downregulation of CD46 and CD55 with short hairpin RNA (shRNA). However, there is no increase in cell killing when CD59 expression was diminished. Our findings suggest that the use of α-gal as antigen to induce tumor cell killing may be a potential therapeutic strategy in colon cancer and that CD55 plays a primary role in conferring resistance to lysis.

Detection of cell membrane-bound CD46 using flow cytometry

CD46 is an important regulator of the complement system by preventing unwanted deposition of the complement activation products and opsonins C3b/C4b onto self-tissue. Recently, intracellular signals mediated by CD46 activation on several distinct human cell types have demonstrated that CD46 also plays decisive roles in immuneregulation. The growing recognition of CD46 as key regulator in several vital biological processes, led to increased demand in sensitive methods for monitoring CD46 expression and changes thereof on cells and in tissues. Here we describe a method, which allows for studying CD46 expression on the surface of cells using specific antibodies in combination with fluorescence-activated cell sorting (FACS) analysis.

Complement-induced protection: an explanation for the limitations of cell-based tumour immunotherapies

Complement is involved in the inflammatory response and clearance of infected or altered cells. It is therefore unexpected that complement-deficient animals are less susceptible to carcinogen-induced tumours and more readily control growth of injected tumour cell lines than their wild-type counterparts, implying that complement promotes tumour development and progression. Conversely, natural killer (NK) and CD8(+) T cells are known to limit progression of the same tumours. Previous studies indicate that sublytic levels of the complement membrane attack complex protect cells against further attack by lytic doses of complement and other pore-formers such as perforin. We hypothesise that inefficient attack by complement in vivo allows tumour cells to avoid lysis by both NK cells and antigen-specific cytotoxic T cells, thereby promoting tumour outgrowth. Complement could thus be limiting the efficacy of NK and T cell-targeted cancer therapies, and the inclusion of complement inhibitors could optimise these immunotherapeutic regimes.

Complement in cancer and cancer immunotherapy

Recently, there has been an increase of interest in the use of biological or immune-based therapies for patients with malignancies. This has been informed by the deeper understanding of the crosstalk between the host immune system and malignant tumours, as well as the potential advantages of immunotherapy-high specificity and less toxicity compared to standard approaches. The particular emphasis of this article is on the role of the complement system in tumour growth and antibody-based cancer immunotherapy. The functional consequences from overexpression of complement regulators by tumours and the development of strategies for overcoming this are discussed in detail. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of cancer.

Upregulated expression of complement inhibitory proteins on bladder cancer cells and anti-MUC1 antibody immune selection

Membrane complement inhibitors (CD46, CD55 and CD59) are upregulated in some human cancers indicating that they play a role in immune evasion. We investigated complement inhibitor expression in bladder cancer and examined the hypothesis that selective pressure of an antibody response (anti-MUC1) results in the upregulated expression of complement inhibitors on tumor cells. Paired samples of tumor and normal tissue from 22 bladder cancer patients were analyzed for expression of MUC1, CD46, CD55 and CD59, and matched serum samples analyzed for anti-MUC1 IgM and IgG levels. Relationships between anti-MUC1 antibody levels and complement inhibitor expression were investigated. MUC1 mRNA was upregulated in 86% of tumor samples. CD46 was upregulated in 77%, CD55 in 55% and CD59 in 59% of tumors. Low titer anti-MUC1 IgM was detected in normal human sera, but was elevated in 41% of the bladder cancer patients. Anti-MUC1 IgG was virtually absent from normal sera, but present in 32% of the cancer patients. There was a direct relationship between anti-MUC1 antibody titer and expression level of complement inhibitors. Analysis of the correlation of each antibody with the expression of each complement inhibitor by Spearman's rank test revealed a strong correlation between both anti-MUC1 IgM and IgG levels and increased expression of CD46 and CD55, and combined anti-MUC1 IgM/IgG levels correlated with increased expression of all 3 complement inhibitors. In conclusion, the data demonstrate upregulated complement inhibitor expression and the presence of an anti-MUC1 antibody response in bladder cancer patients and support the hypothesis of antibody-mediated immune selection.

p53 Regulates Cellular Resistance to Complement Lysis through Enhanced Expression of CD59

It has been recently hypothesized that the CD59 gene has two putative p53-responsive elements that may be involved in defense of host cells from damage by the complement system in inflammation. Here we have examined the roles of these putative p53-binding sequences within the CD59 gene in regulation of CD59 expression. We have shown that both of these potential responsive elements bind p53 in vitro. Knocking down expression of p53 using small interfering RNA led to a 6-fold decrease in CD59 protein expression in HeLa cells. We have previously observed a decrease of CD59 in camptothecin-induced apoptotic IMR32 cells, whereas expression was increased in the surviving fraction compared with untreated cells. Here, we have shown that these changes are associated with altered expression levels and acetylation status of p53. We have also shown that acetylation status of p53 regulates CD59 expression on cells exposed to inflammatory cytokines to model inflammation. Our data suggest that p53 and in vivo positive/negative regulators of p53 could be used to modulate susceptibility of tumor cells to complement lysis in chemotherapy.

Immunoplacental therapy, a potential multi-epitope cancer vaccine

The field of tumor immunology has made great advancements in recent years. A retrospective analysis of previous vaccine strategies combined with present knowledge may provide additional insight in this treatment modality. This article provides a review of immunoplacental therapy (IPT), a cancer vaccine consisting of chorionic villi extractions from the human placenta after a live full-term delivery. This therapy was first introduced in the 1970s by Valentin I. Govallo, M.D., Ph.D., who noted the immunological similarities between pregnancy and cancer. The goal of cancer immunotherapy, according to Govallo, is to view the fetal allograft as an "impregnating tumor" and create an immunological state in the oncological patient analogous to a spontaneous abortion in a pregnant women. The placenta shares identical growth mechanisms, antigenic determinants, and immune-escape properties with cancer cells; this includes numerous tumor-associated antigens, angiogenic growth factors, complement regulatory proteins, and defective apoptotic mechanisms which aid in their survival. Placental vaccination may function as a multi-epitope vaccine; the body recognizes the placental antigens of this vaccine as foreign, and thus stimulates a cross reactive humoral and cell-mediated immune response targeting cancer tumor-associated antigens as well as proteins that aid in cancer angiogenesis, complement regulation, and apoptotic resistance. With recent advancements in molecular and cellular cancer immunology, the model introduced by Govallo may provide an important strategic approach to cancer immunotherapy.

Do poor-prognosis breast tumours express membrane cofactor proteins (CD46)?

CD46 or membrane cofactor protein (MCP) is a complement regulatory protein that has been identified on all nucleated cells and which protects them from attack by autologous complement. Breast carcinomas are reported to consistently express CD46.

AIM AND METHODS

Our previous immunohistochemical study showed that in breast carcinomas, loss of CD59 and CD55 correlated with poor survival. This study aimed to investigate the prognostic significance of CD46 on breast tumours using a rabbit polyclonal anti-CD46 antibody with a standard immunohistochemistry method. A total of 510 breast tissues from patients with primary operable breast cancer diagnosed between 1987 and 1992 had previously been included in tissue microarrays. They included patients 70 years of age or less (mean = 54 years) with a long-term follow-up (median = 82 months).

RESULTS

Immunohistochemical study revealed that 507/510 (99.4%) of breast tumours expressed CD46. Strong immunoreactivity was exhibited by 136/510 (27%) tumours, while moderate and weak staining was observed in 43% and 29% of tumours, respectively. Intensity of CD46 expression was significantly associated with tumour grade (p < 0.05), histological type of tumour (p < 0.001) and tumour recurrence (p < 0.05). There was no correlation with lymph node stage or the presence of vascular invasion, nor with patient age or menopausal status. Interestingly, as most tumours expressed CD46, it would appear that poor-prognosis tumours that lose CD55 and CD59 still express CD46.

CONCLUSION

Breast tumours express high levels of CD46 that correlates with tumour grade and recurrence. It is therefore likely that loss of CD55 and CD59 could be compensated by expression of CD46. However, loss of CD55 and CD59, even for tumours that still express CD46, is still associated with a poor prognosis. This may suggest that CD46 alone can protect from complement lysis but that loss of CD55 and CD59 are associated with other roles in immune regulation.

Transfer of functional prostasomal CD59 of metastatic prostatic cancer cell origin protects cells against complement attack

BACKGROUND

Prostasomes are secretory granules produced, stored, and released, by the glandular epithelial cells of the prostate. They express the glycosylphosphatidylinositol (GPI)-anchored complement regulatory protein CD59, which has been shown to be transferred to spermatozoa and erythrocytes.

METHODS

The CD59 content of prostasomes isolated from seminal fluid and malignant prostate cells (PC-3, DU145, and LNCaP) and the transfer of prostasomal CD59 to rabbit erythrocytes (RE) and to PIPLC-treated and unmanipulated cancer cells were investigated using FACS. All prostasomes were also incubated with RE and tested in a hemolytic assay.

RESULTS

Prostasomes from cancer cells had higher expression of CD59 than those of normal cells. Prostasomal CD59 of different origin could be transferred to RE, malignant cell lines stripped of CD59 by PIPLC, or unmanipulated LNCaP cells. Malignant cell prostasomes had an increased ability to inhibit complement-mediated lysis compared to those from non-malignant cells.

CONCLUSIONS

These results point to a novel mechanism by which prostasomes can protect prostatic malignant cells from complement attack.

Loss of CD55 is associated with aggressive breast tumors

PURPOSE

CD55 is a complement regulatory protein expressed by cells to protect them from bystander attack by complement. CD55 is overexpressed on some tumor cell lines, and in colorectal carcinomas, it has been shown to be an indicator of poor prognostic.

EXPERIMENTAL DESIGN

A large set of samples (480) from patients with primary operable breast cancer followed for 4-192 months were included in the present study. The prognostic significance of CD55 was then investigated in these tumors using an anti-CD55 monoclonal antibody (RM1) that we raised against a synthetic peptide and a standard immunohistochemistry method.

RESULTS

Ninety-five percent of the breast carcinomas expressed CD55 (RM1) with intensity ranging from weak (51%) to strong (6%). High expression of CD55 was significantly associated with low-grade (grades 1 or 2; P = 0.001), lymph node negativity (P = 0.031), and good prognosis tumors (Nottingham Prognostic Index < 3.4; P < 0.001). Survival analysis showed CD55 overexpression was associated with a more favorable outcome and loss of CD55 being associated with poor survival (P = 0.001). Intensity of CD55 expression was significantly correlated (P = 0.002) with intensity of CD59 expression (as shown in a previous study) in these series of patients.

CONCLUSIONS

In conclusion, we found that loss of both CD55 and CD59 in breast carcinomas is associated with a worse prognosis.

Analysis of the level of mRNA expression of the membrane regulators of complement, CD59, CD55 and CD46, in breast cancer

We have examined the relative mRNA expression of the complement (C) regulatory proteins CD59, CD55 and CD46 in RNA isolated from 50 primary breast cancer specimens using a semiquantitative RT-PCR approach. Having normalized the mRNA expression levels of the C regulators relative to actin, we subsequently correlated their expression with estrogen receptor (ER) and various clinical, pathologic and biochemical features of the disease. CD59 and CD46 were detected in all clinical biopsies, while CD55 mRNA was detected in the majority of samples. The comparative levels of expression between the 3 regulators analyzed, using Spearman rank correlation test, revealed a significant association (p = 0.01; r = 0.36) between CD46 and CD59. CD46 exhibited the most striking pattern of association, with increased levels of expression being associated with ER-positive samples and lower levels of expression associated with a loss of differentiation and epidermal growth factor receptor positivity. Application of Spearman rank correlation test revealed CD46 expression was significantly associated with expression of ER at the level of protein (p = 0.031; r = 0.31) and mRNA (p < 0.001; r = 0.52). CD46 expression also correlated with insulin-like growth factor receptor-positive samples using Spearman rank correlation test (p = 0.016; r = 0.34), but negatively associated with tumor samples either exhibiting histologic grade 3 when compared to grades 1 or 2 or displaying elevated levels of inflammatory cell infiltrate. Immunohistochemical analysis of a limited series (n = 8) of paraffin-embedded breast cancers indicated that the level of CD46 protein expression directly associates with that of the mRNA and, where prominent, is localized in the tumor epithelial cell population, including at the plasma membrane. These data provide new information on expression of these important regulators in breast cancer and suggest that CD46 should be evaluated as a novel prognostic indicator.

Loss of CD59 expression in breast tumours correlates with poor survival

CD59 (protectin), a phosphatidylinositol-anchored glycoprotein, is a member of the cell membrane-bound complement regulatory proteins that inhibits the formation of the terminal membrane attack complex (MAC) of complement. In this study, the expression of CD59 was evaluated in 520 breast carcinomas from patients with a mean follow-up of 87 months. This expression was correlated with clinicopathological features and patient survival. Marked variation in the intensity of CD59 expression, which correlated with histological grade and Nottingham prognostic index (NPI), was found, with higher expression of CD59 found more often in well and moderately differentiated tumours and those of good prognosis (NPI < or = 3.4). In contrast, high grade and poor prognosis (NPI > 5.4) carcinomas significantly demonstrated lack of CD59 expression (p < 0.001). Moreover, it was found that the percentage of CD59-positive cells correlated significantly with patient survival, ie patients with a high percentage of positive cells (>50%) had a better overall survival (p = 0.006). A correlation was also found between the percentage of CD59-positive cells and tumour type and also the development of distant metastases. No association was found between either the intensity or the percentage of cells expressing CD59 and vascular invasion, lymph node stage, tumour size, patient age or menopausal status. In multivariate analysis, CD59 percentage positivity was of independent prognostic significance with grade and lymph node stage. These findings indicate that loss of CD59 may offer a selective advantage for breast cancers, resulting in more aggressive tumours and conferring a poor prognosis for patients.

Afa/Dr diffusely adhering Escherichia coli infection in T84 cell monolayers induces increased neutrophil transepithelial migration, which in turn promotes cytokine-dependent upregulation of decay-accelerating factor (CD55), the receptor for Afa/Dr adhesins

Ulcerative colitis and Crohn's disease are inflammatory bowel diseases thought to involve strains of Escherichia coli. We report here that two wild-type Afa/Dr diffusely adhering E. coli (DAEC) strains, C1845 and IH11128, which harbor the fimbrial F1845 adhesin and the Dr hemagglutinin, respectively, and the E. coli laboratory strain HB101, transformed with the pSSS1 plasmid to produce Afa/Dr F1845 adhesin, all induced interleukin-8 (IL-8) production and transepithelial migration of polymorphonuclear leukocytes (PMNL) in polarized monolayers of the human intestinal cell line T84 grown on semipermeable filters. We observed that after PMNL migration, expression of decay-accelerating factor (DAF, or CD55), the brush border-associated receptor for Afa/Dr adhesins, was strongly enhanced, increasing the adhesion of Afa/Dr DAEC bacteria. When examining the mechanism by which DAF expression was enhanced, we observed that the PMNL transepithelial migration induced epithelial synthesis of tumor necrosis factor alpha and IL-1beta, which in turn promoted the upregulation of DAF.

Expression of rat complement control protein Crry on tumor cells inhibits rat natural killer cell-mediated cytotoxicity

Crry is a rodent membrane-bound inhibitor of complement activation and is a structural and functional analog of the human complement inhibitors decay-accelerating factor and membrane cofactor protein. We found previously that expression of rat Crry on a human tumor cell line enhances tumorigenicity in nude rats. In this study, we investigated the effect that rat Crry expressed on tumor cells has on rat cell-mediated cytotoxicity and antibody-dependent cell-mediated cytotoxicity (ADCC). The expression of rat Crry on the surface of different human tumor cell lines inhibited ADCC mediated by rat natural killer (NK) cells. C3 opsonization is known to enhance NK cell-mediated cytolysis, and a potential mechanism for Crry-mediated inhibition of NK cell lysis is through Crry modulation of C3 deposition on target cells. However, the transfection of tumor cell lines with Crry enhanced their resistance to NK cell-mediated lysis in the absence of exogenous complement. The resistance of Crry-expressing tumor cells to NK cell-mediated ADCC could be reversed by treatment with anti-Crry F(ab)(2). In addition, anti-Crry F(ab)(2) enhanced the susceptibility of 13762 rat mammary adenocarcinoma cells (that endogenously express Crry) to ADCC mediated by allogeneic rat NK cells in the absence of added complement. We found no evidence that rat NK cells were a source of complement for target cell deposition during the in vitro cytolysis assay. These data suggest a novel function for rat Crry in tumor immune surveillance that may be unrelated to complement inhibition.

Regulation of the new coexpressed CD55 (decay-accelerating factor) receptor on stomach carcinoma cells involved in antibody SC-1-induced apoptosis

The human monoclonal antibody SC-1 was isolated from a patient with a diffuse-type adenocarcinoma of the stomach using somatic cell hybridization. The immunoglobulin (Ig)M antibody reacts specifically with diffuse- (70%) and intestinal-type (25%) gastric adenocarcinoma and induces apoptosis in vitro and in vivo. When used in clinical trials with stomach carcinoma patients, significant apoptotic and regressive effects in primary tumors have been observed with the antibody SC-1. The SC-1 receptor is a new 82 kd membrane-bound isoform of glycosylphosphatidylinositol (GPI)-linked CD55 (decay-accelerating factor, DAF). CD55 is known to protect cells from lysis through autologous complement and is coexpressed with the ubiquitously distributed 70 kd isoform. The SC-1-specific CD55 isoform is up-regulated shortly after antibody binding, followed by an internalization of the antibody/receptor-complex, whereas the membranous expression of wild-type CD55 remains unchanged. The apoptotic process is marked by cleavage of cytokeratin 18, indicating the involvement of caspase-6 in the apoptotic process. In contrast to other apoptotic pathways, a cleavage of poly(ADP-ribose)polymerase (PARP) is not observed. The expression of the cell-cycle regulator c-myc becomes up-regulated, whereas expression of topoisomerase IIalpha is down-regulated. Induction of apoptosis leads to an increase in the internal Ca(2+) concentration, which is not necessary for the apoptotic process but for the transport of newly synthesized SC-1-specific CD55 isoform to the membrane.

Immune evasion of tumor cells using membrane-bound complement regulatory proteins

Membrane-bound complement regulatory proteins (mCRPs) play an important role in the protection of cells from complement-mediated injury. It is now apparent that malignant tumor cells also express these proteins to escape complement attack. Here, Arko Gorter and Seppo Meri discuss the implications of complement resistance for the immunotherapeutic treatment of solid tumors with monoclonal antibodies.

Expression of membrane cofactor protein (MCP, CD46) in human liver diseases

Membrane cofactor protein (MCP, CD46) is one of the complement regulatory proteins, and is widely distributed in human organs and protects cells from complement-mediated cytotoxicity. We analysed the distribution and the intensities of MCP in liver diseases and evaluated the role of MCP during hepatocarcinogenesis. Western blot analysis revealed that relative densities (density of the sample/density of the standard sample) of MCP in 27 HCC, 18 liver cirrhosis, nine chronic hepatitis and 12 normal liver were 0.63+/-0.23, 0.21+/-0.07, 0.25+/-0.10 and 0.11+/-0.03 (mean+/-s.d.) respectively. MCP expression in hepatocellular carcinoma (HCC) was significantly higher than that in both liver cirrhosis and chronic hepatitis (P < 0.01). The difference in the tumour sizes, the grades of differentiation and viral marker status did not affect the expression. Immunohistological analysis revealed that MCP was distributed mainly in the basolateral membrane of the hepatic cord in non-cancerous liver, along with endothelial cells and bile duct cells. In HCC, the protein was observed on the membrane in a non-polarized fashion. These data suggest that HCC cells acquire the increased MCP expression in a development of HCC and may escape from tumour-specific complement-mediated cytotoxicity.

Protection of human breast cancer cells from complement-mediated lysis by expression of heterologous CD59

CD59, decay accelerating factor (DAF) and membrane cofactor protein (MCP) are widely expressed cell surface glycoproteins that protect host cells from the effects of homologous complement attack. Complement inhibitory activity of these proteins is species-selective. We show that the human breast cancer cell line MCF7 is relatively resistant to lysis by human complement, but is effectively lysed by rat or mouse complement. CD59, DAF and MCP were all shown to be expressed by MCF7. The species-selective nature of CD59 activity was used to demonstrate directly the effectiveness of CD59 at protecting cancer cells from complement-mediated lysis. cDNAs encoding rat and mouse CD59 were separately transfected into MCF7 cells, and cell populations expressing high levels of the rodent CD59 were isolated by cell sorting. Data show that rat and mouse CD59 were highly effective at protecting transfected MCF7 cells from lysis by rat and mouse complement, respectively. Data further reveal that rat CD59 is not effective against mouse complement, whereas mouse CD59 is effective against both mouse and rat complement. These studies establish a model system for relevant in vivo studies aimed at determining the effect of complement regulation on tumourigenesis, and show that for effective immunotherapy using complement-activating anti-tumour antibodies, the neutralization of CD59 and/or other complement inhibitory molecules will probably be required.

A Bispecific Monoclonal Antibody Directed Against Both the Membrane-Bound Complement Regulator CD55 and the Renal Tumor-Associated Antigen G250 Enhances C3 Deposition and Tumor Cell Lysis by Complement

Tumor cells may inhibit the induction of a complement-mediated inflammatory response through overexpression of membrane-bound regulators of complement activation. Therefore, it is of interest to determine the most efficient approach to block these membrane-bound complement regulators on tumor cells. In the present study, we first generated a bispecific mAb directed against both CD55, using the functional blocking mAb MBC1, and the highly expressed HLA class I molecule as a model for a tumor-associated Ag, using the mAb W6/32. Tumor cells opsonized with bispecific mAb W6/32*MBC1, then exposed to complement and subsequently stained for C3 deposition, were assessed by flow cytometric analysis. We found that opsonization with W6/32*MBC1 resulted in a 92% enhancement of C3 deposition on renal tumor cells as compared with opsonization with W6/32 alone and a 17% enhancement of the C3 deposition as compared with incubation with a mixture of both parental mAb. Based on these results, we developed a bispecific mAb recognizing both CD55 and the relatively low expressed renal tumor-associated Ag G250. Increasing concentrations of the bispecific mAb G250*MBC1 resulted in a 25 to 400% increase in C3 deposition on renal tumor cells as compared with C3 deposition in the presence of the parental mAb G250 alone. G250*MBC1 enhanced C3 deposition by 21% in comparison with a mixture of both parentals. Furthermore, opsonization of tumor cells with G250*MBC1 rendered these cells more sensitive to complement-mediated lysis. In conclusion, the bispecific mAb G250*MBC1 induces deposition of C3 and tumor cell lysis more efficiently than G250 alone.

Tumor heterogeneity and immunotherapy of cancer

Tumor heterogeneity is the presence of intercellular differences, either from clonal origin or present within subpopulations of tumor cells. Recent advances in immuno-histology, flow cytometric analysis, molecular biological techniques, and tissue culture methods makes it possible to investigate tumor heterogeneity in detail. In this review data are presented to document that this hallmark of neoplastic disease results from DNA-instability and the interactions of tumor cells with their environment. The present inability to treat most patients effectively with immunotherapy may partly be due to the occurrence of tumor heterogeneity. Therefore, the heterogeneity of the tumor phenotype is discussed in conjunction with the various immunotherapeutic treatment modalities. In addition to local cytokine production by immune cells and tumor cells, and limited access of either antibodies or immune cells into the tumor, tumor heterogeneity is an important factor that determines the progress of immunotherapy of cancer. Therefore, accurate quantitative methods using antibodies and molecular probes to identify HLA-associated target peptides, tumor-associated antigens and accessory molecules, to predict which patients will have a high probability of responding to treatment, are needed.