Deposition Of Complement Components Research Articles

Protective function of complement against alcohol-induced rat liver damage

The complement system can promote tissue damage or play a homeostatic role in the clearance and disposal of damaged tissue. We assessed the role of the terminal complement pathway in alcohol-induced liver damage in complement C6 (C6−/−) genetically deficient rats. C6−/− and corresponding C6+/+ rats were continuously exposed to ethanol by feeding ethanol-supplemented liquid diet for six weeks. Liver samples were analyzed for histopathology and complement component deposition by immunofluorescence microscopy. Prostaglandin E receptors and cytokine mRNA levels were analyzed by RT-PCR and plasma cytokines by ELISA. Deposition of complement components C1, C3, C8 and C9 was observed in C6+/+ rats, but not in C6−/− animals. The histopathological changes, the liver weight increase and the elevation of the plasma pro-/anti-inflammatory TNF-α/IL-10 ratio were, on the other hand, more marked in C6−/− rats. Furthermore, ethanol enhanced the hepatic mRNA expression of the prostaglandin E receptors EP2R and EP4R exclusively in the C6−/− rats. Our results indicate that a deficient terminal complement pathway predisposes to tissue injury and promotes a pro-inflammatory cytokine response. This suggests that an intact complement system has a protective function in the development of alcoholic liver damage.

Biological activities of naturally occurring antibodies reactive with Candida albicans mannan.

Sera from normal adult humans may contain high levels of antibody reactive with Candida albicans mannan. This study examined selected biological activities of such antibodies, focusing on sera that were collected from 34 donors and analyzed individually. The results showed that antimannan titers were normally distributed. Reactivity as determined by enzyme-linked immunosorbent assay with serotype A mannan generally paralleled reactivity with serotype B. Analysis of the kinetics for activation of the complement system and deposition of complement component 3 (C3) onto serotype A and serotype B cells showed a decrease in the lag time that occurred before the onset of rapid accumulation of C3 that correlated with increasing antimannan titers. In contrast, there was a decrease in the overall rate of accumulation of C3 on serotype A cells that was strongly correlated with increasing antibody titers; serotype B cells showed no such decrease. An evaluation of the contribution of mannan antibody to opsonophagocytic killing showed that mannan antibody in individual sera and antimannan immunoglobulin G (IgG) affinity purified from human plasma contributed to killing by neutrophils in a dose-dependent fashion in the absence of a functional complement system. However, affinity-purified antibody in very high concentrations was inhibitory to both complement-dependent and complement-independent opsonophagocytosis, and this finding suggests a prozone-like effect. In contrast, if the complement system was functional, antimannan IgG was not needed for opsonophagocytic killing. These results suggest that naturally occurring mannan antibodies and the complement system are functionally redundant for opsonophagocytic killing by neutrophils.

Critical role of the complement system in group B streptococcus-induced tumor necrosis factor alpha release.

Group B Streptococcus (GBS) is a major cause of newborn sepsis and meningitis and induces systemic release of tumor necrosis factor alpha (TNF-alpha), believed to play a role in morbidity and mortality. While previous studies have shown that GBS can induce TNF-alpha release from monocytes and macrophages, little is known about the potential modulating effect of plasma or serum on GBS-induced TNF-alpha release, and there are conflicting reports as to the host receptors involved. In a human whole-blood assay system, GBS type III COH-1 potently induced substantial monocyte TNF-alpha release in adult peripheral blood and, due to a higher concentration of monocytes, 10-fold-greater TNF-alpha release in newborn cord blood. Remarkably, GBS-induced TNF-alpha release from human monocytes was enhanced approximately 1000-fold by heat-labile serum components. Experiments employing C2-, C3-, or C7-depleted serum demonstrated that C3 activation via the alternative pathway is crucial for potent GBS-induced TNF-alpha release. Accordingly, whole blood from C3-deficient mice demonstrated significantly reduced GBS-induced TNF-alpha release. Preincubation with human serum enhanced the TNF-alpha-inducing activity of GBS in a C3- and factor B-dependent manner, implying deposition of complement components via the alternative pathway. GBS-induced TNF-alpha release was inhibited by monoclonal antibodies directed against each of the components of CR3 and CR4: the common integrin beta subunit CD18 and the alpha subunits CD11b (of CR3) and CD11c (of CR4). Blood derived from CR3 (CD11b/CD18)-deficient mice demonstrated a markedly diminished TNF-alpha response to GBS. We conclude that the ability of plasma and serum to greatly amplify GBS-induced TNF-alpha release reflects the activity of the alternative complement pathway that deposits fragments on GBS and thereby enhances CR3- and CR4-mediated monocyte activation.

Distinct time pattern of complement activation and cytotoxic T cell response in Guillain-Barré syndrome.

Humoural and cellular immune mechanisms are involved in the pathogenesis of Guillain-Barré syndrome (GBS). While activation of complement has been implicated in the initiation of myelin damage, we provide data here on the role of cellular cytotoxicity in GBS. Archival autopsy tissues including spinal roots, dorsal root ganglia and peripheral nerve were examined from 11 subjects who died 1 day to 8 weeks after onset of symptoms from GBS exhibiting a primary demyelinating pathology. In order to study the extent of humoural and cellular immune processes with regard to disease duration, a broad panel of antibodies to immunological and cellular markers was used to visualize the stage of demyelination, the deposition of complement components and expression of CD59, and to characterize cell infiltrates. Deposits of C9neo antigen on degenerating myelin sheaths were predominantly detected in acute cases. Expression of CD59 was upregulated on demyelinating fibres, but did not correlate with the presence of C9neo antigen or duration of disease. Quantitative analysis of endoneurial T cells showed a correlation between the density of CD3+ T cells per square unit and the degree of demyelination, but not with the duration of disease. The ratio of CD8+ to CD3+ T cells, however, was significantly increased in cases of GBS with a subacute course. Granzyme B positive lymphocytes and upregulation of MHC class I molecules on Schwann cells and myelin sheaths were detected in cases with more than 4 weeks disease duration. These findings implicate an important role of cytotoxic T cell responses for myelin damage in subacute stages of GBS.

Complement and renal disease

The complement system is involved in several aspects of renal disease, including primary renal diseases, dialysis and renal transplant rejection. Initially, a role for complement in renal disease was inferred solely from the deposition of complement components in affected kidneys and changes in complement levels in the circulation during active disease. Recent studies have shown how complement modulates the onset, development and the resolution of renal disease. Current research provides clues on the role of individual complement components and activation pathways as well as possible modes of complement regulation in the management of renal disease.

Activation of Complement Components and Reduced Regulator Expression in Alcohol-Induced Liver Injury in the Rat

The purpose of this study was to evaluate the possible contribution of complement-mediated inflammation to the development of alcoholic liver disease. Male Wistar rats were fed ethanol by liquid diet in a model that results in continuous ethanol intoxication and induces early signs of alcoholic liver injury. After a six-week study period liver samples were analyzed for the deposition of complement components (C1, C3, and C8) and expression of cell membrane-bound regulators (Crry and CD59). Activation of the homologous complement system in vitro was tested by treating frozen liver sections with normal rat serum (NRS). Immunohistochemical analysis showed deposits of C8 in the liver sections of ethanol-treated rats. When frozen liver sections from these rats were treated with NRS, periportal deposition of both C3 and C8, but only slight C1 deposition, was observed. Immunohistochemical and Western blot analysis both revealed a reduced expression of the complement regulators Crry and CD59. These results suggest an induction of complement-activating capacity in the liver after chronic ethanol treatment. Lack of C1 deposition in the lesions suggests that complement activation occurs primarily via the alternative pathway. The reduced expression of the critical complement regulatory proteins Crry and CD59 may sensitize the liver to complement-mediated damage.

The complement system at the feto‐maternal interface: friend or foe?

The complement system is an essential component of the innate immune system and functions as a double‐edged weapon providing on the one hand an effective means of defence and yet causing cell and tissue damage. The liver represents the major source of complement components present in the blood that circulates in decidua and in intervillous space. We have examined macrophages, endothelial and stromal cells and trophoblast as the most likely candidates for local production of complement components. All these cells produce primarily C4 and C3 and various other components under the control of locally available cytokines. We have recently examined the distribution and the role of C1q and of the related proteins, MBL and emilin, a component of the elastic fibers. While emilin is evenly distributed in the decidual stroma, both C1q and MBL appear to be present preferentially on the wall of blood vessels. The results of in vitro experiments have shown that extravillous trophoblast use C1q and emilin to adhere and to migrate in decidua. Deposits of complement components are seen particularly on the wall of spiral arteries. C1q can be detected on the endothelium and also in the subendothelium, where we detect deposition also of the terminal complex. Placental cells are protected from the undesired effect of complement activation products by acquiring complement regulators from the fluid phase or by expressing regulators on the cell surface including DAF, MCP and CD59.

Expression of complement regulating factors in gastric cancer cells

Aims: To investigate the deposition of complement components, C3d and C5b–9, and the expression of complement regulating factors (S protein, membrane cofactor protein (MCP; CD46), protectin (CD59), decay accelerating factor...

Deletion of wboA enhances activation of the lectin pathway of complement in Brucella abortus and Brucella melitensis.

Brucella spp. are gram-negative intracellular pathogens that survive and multiply within phagocytic cells of their hosts. Smooth organisms present O polysaccharides (OPS) on their surface. These OPS help the bacteria avoid the bactericidal action of serum. The wboA gene, coding for the enzyme glycosyltransferase, is essential for the synthesis of O chain in Brucella. In this study, the sensitivity to serum of smooth, virulent Brucella melitensis 16M and B. abortus 2308, rough wboA mutants VTRM1, RA1, and WRR51 derived from these two Brucella species, and the B. abortus vaccine strain RB51 was assayed using normal nonimmune human serum (NHS). The deposition of complement components and mannose-binding lectin (MBL) on the bacterial surface was detected by flow cytometry. Rough B. abortus mutants were more sensitive to the bactericidal action of NHS than were rough B. melitensis mutants. Complement components were deposited on smooth strains at a slower rate compared to rough strains. Deposition of iC3b and C5b-9 and bacterial killing occurred when bacteria were treated with C1q-depleted, but not with C2-depleted serum or NHS in the presence of Mg-EGTA. These results indicate that (i) OPS-deficient strains derived from B. melitensis 16M are more resistant to the bactericidal action of NHS than OPS-deficient strains derived from B. abortus 2308, (ii) both the classical and the MBL-mediated pathways are involved in complement deposition and complement-mediated killing of Brucella, and (iii) the alternative pathway is not activated by smooth or rough brucellae.

Serum amyloid P component bound to gram-negative bacteria prevents lipopolysaccharide-mediated classical pathway complement activation.

Although serum amyloid P component (SAP) is known to bind many ligands, its biological function is not yet clear. Recently, it was demonstrated that SAP binds to lipopolysaccharide (LPS). In the present study, SAP was shown to bind to gram-negative bacteria expressing short types of LPS or lipo-oligosaccharide (LOS), such as Salmonella enterica serovar Copenhagen Re and Escherichia coli J5, and also to clinical isolates of Haemophilus influenzae. It was hypothesized that SAP binds to the bacteria via the lipid A part of LPS or LOS, since the htrB mutant of the nontypeable H. influenzae strain NTHi 2019-B29-3, which expresses a nonacetylated lipid A, did not bind SAP. This was in contrast to the parental strain NTHi 2019. The binding of SAP resulted in a clear inhibition of the deposition of complement component C3 on the bacteria. SAP inhibited only the activation of the classical complement pathway; the alternative route remained unaffected. In the classical route, SAP prevented the deposition of the first complement component, Clq, probably by interfering with the binding of Clq to LPS. Since antibody-mediated Clq activation was not inhibited by SAP, SAP seems to inhibit only the LPS-induced classical complement pathway activation. The SAP-induced inhibition of C3 deposition strongly diminished the complement-mediated lysis as well as the phagocytosis of the bacteria. The binding of SAP to gram-negative bacteria, therefore, might influence the pathophysiology of an infection with such bacteria.

A model of xenograft hyperacute rejection attenuates endothelial nitric oxide production: a mechanism for graft vasospasm?

Background The deposition of complement components within grafts, complement consumption, and prolongation of graft function by complement inactivation imply a pivotal role for complement in xenograft hyperacute rejection. The current investigations examined the endothelial production of vasoactive substances in pulmonary arteries during simulated hyperacute rejection. Methods and results Canine pulmonary arteries were suspended in organ chambers and exposed to either autologous canine serum for 90 minutes or heterologous porcine serum for 30, 60, or 90 minutes. Following serum exposure, the vessels were allowed a one-hour equilibration in buffered crystalloid solution. Dose-response curves were obtained with acetylcholine, sodium nitroprusside, and calcium ionophore A23187 following contraction with phenylephrine (10 −6M) in the presence of indomethacin (10 −5M). Receptor-dependent, endothelial-dependent relaxations to acetylcholine (10 −9–10 −4M) were impaired with 30-, 60-, or 90-minute porcine serum exposure when compared to vessels exposed to autologous canine serum ( n = 10, 7, 9, respectively; p < .05; 2-way ANOVA). Receptor-independent, endothelial-dependent relaxations to calcium inophore (10 −9–10 −6M) were significantly impaired at 60- and 90-minute porcine exposures only ( n = 7, 8; p < .05). Endothelial-independent relaxations to sodium nitroprusside (10 −9–10 −4M) were not impaired with either canine or porcine serum exposure. Oxyhemoglobin (10 −6M) abolished acetylcholine-mediated relaxations, indicating that nitric oxide was the predominant mediator. Conclusions Simulated hyperacute xenograft rejection impairs endothelium-dependent relaxation of canine pulmonary arteries. Both basal and stimulated production of nitric oxide is impaired by heterologous serum exposure and, subsequently, complement activation. Reduced production of nitric oxide may explain, in part, the vasospasm and thrombosis of xenografts during hyperacute rejection.

Characterization of a murine monoclonal antibody to Cryptococcus neoformans polysaccharide that is a candidate for human therapeutic studies.

The murine monoclonal antibody (MAb) 18B7 [immunoglobulin G1(kappa)] is in preclinical development for treatment of Cryptococcus neoformans infections. In anticipation of its use in humans, we defined the serological and biological properties of MAb 18B7 in detail. Structural comparison to the related protective MAb 2H1 revealed conservation of the antigen binding site despite several amino acid differences. MAb 18B7 was shown by immunofluorescence and agglutination studies to bind to all four serotypes of C. neoformans, opsonize C. neoformans serotypes A and D, enhance human and mouse effector cell antifungal activity, and activate the complement pathway leading to deposition of complement component 3 (C3) on the cryptococcal capsule. Administration of MAb 18B7 to mice led to rapid clearance of serum cryptococcal antigen and deposition in the liver and spleen. Immunohistochemical studies revealed that MAb 18B7 bound to capsular glucuronoxylomannan in infected mouse tissues. No reactivity of MAb 18B7 with normal human, rat, or mouse tissues was detected. The results show that both the variable and constant regions of MAb 18B7 are biologically functional and support the use of this MAb in human therapeutic trials.

Antifungal activity of a human antiglucuronoxylomannan antibody.

The human immunoglobulin M (IgM) monoclonal antibody (MAb) 2E9 binds the glucuronoxylomannan (GXM) of Cryptococcus neoformans serotypes A, B, and D. This study was undertaken to determine the opsonic efficacy of 2E9 and its ability to promote the antifungal activity of human polymorphonuclear neutrophils (PMNs) against C. neoformans. We incubated purified PMNs with fluorescein isothiocyanate-labeled C. neoformans cells that were treated with the GXM IgM 2E9, IgM antibodies that do not bind GXM, and rabbit and human factor-B-deficient serum as complement sources. PMN-associated C. neoformans cells fluoresced and were detected with a fluorescence-activated cell sorter. The amount of phagocytosis was defined as the percent fluorescing PMNs, which was 37% for yeast cells opsonized with 2E9 plus rabbit serum and 57% for yeast cells opsonized with 2E9 plus factor-B-deficient serum. Phagocytosis was significantly greater for yeast cells that were treated with 2E9 plus a complement source than for yeast cells treated with the complement sources alone or treated with the control IgMs alone or with the complement sources. Fluorescence quenching and light and electron microscopy of the phagocytosis mixtures revealed that 2E9-opsonized yeast cells were internalized by PMNs. Maximal inhibition of C. neoformans growth occurred when PMNs were cocultured with yeast cells that were opsonized with 2E9 plus a complement source. Our data demonstrate that the human GXM IgM 2E9 can mediate PMN phagocytosis and C. neoformans growth inhibition in vitro. These findings strongly suggest that antibody-mediated deposition of complement components on the cryptococcal capsule can augment PMN complement receptor-mediated antifungal activity. Antibody activation of complement-mediated effector cell antifungal mechanisms may play a role in host defense against cryptococcosis and represents a goal for the use of MAbs to treat or prevent human C. neoformans infections.

Protection of Gingival Epithelium against Complement-mediated Damage by Strong Expression of the Membrane Attack Complex Inhibitor Protectin (CD59)

Adult periodontitis (AP) is a chronic inflammatory disease of the tooth-supporting apparatus. Activation products of the inflammation-inducing complement system have been detected in the gingival crevicular fluid at the site of gingival inflammation. In the present study, we examined whether evidence for ongoing complement activation in gingival tissues of patients with AP can be obtained. In light of the potential tissue-damaging effects of the complement system, we also examined how the gingival tissue is protected against the cytolytic activity of complement. Surgical and autopsy samples of AP (n = 18) and healthy (n = 11) gingiva were analyzed for the expression or deposition of the complement regulators protectin (CD59) and vitronectin (S-protein) and complement components C3d and C9 by indirect immunofluorescence microscopy with specific antibodies. In healthy gingiva, protection was strongly expressed on the membranes of epithelial cells and on the vascular endothelia of the underlying connective tissue. In AP, protectin was also strongly expressed by endothelial cells, but in the epithelia the expression was granular and weaker than in the healthy gingiva. Coarse granular deposits of complement components were seen in the subepithelial tissues of 61% (C3d), 39% (C9), and 33% (vitronectin) of AP patients, compared with 9% (one case in 11) in healthy controls. In addition, deposits of C3d, C9, and vitronectin were observed on the basement membranes of both pocket and oral epithelium of healthy and AP gingiva but not at sites of protectin expression. The results suggest an increased turnover of the complement system in the gingival tissues of AP patients. The gingival epithelium and connective tissue endothelia are well-protected against damage by the membrane attack complex of complement (MAC). Protection of the underlying connective tissue is insufficient, however, and may allow for deposition of MAC and autologous tissue damage in AP.

Deposition of complement components on Streptococcus agalactiae in bovine milk in the absence of inflammation.

Bovine milk is generally considered to be almost devoid of complement, on the basis of undetectable hemolytic activity, unless inflammation recruits plasma components. This study examines the deposition of complement components on a mastitis-causing isolate of Streptococcus agalactiae by enzyme-linked immunosorbent assay (ELISA). Neat milk from mid-lactating, uninflamed mammary glands (normal milk) effected marked C3 deposition on bacteria. Kinetic studies showed a protracted lag period (30 to 45 min) preceding C3 deposition, which required about 2 h to reach a maximum. Experiments with diluted serum suggested that this slow C3 deposition resulted mainly from the low concentration of certain components of complement in milk. Bacteria incubated in neat milk readily bound bovine conglutinin, indicating the presence of iC3b. Elution of covalently bound C3 fragments with hydroxylamine confirmed the deposition of C3b and iC3b on bacteria. Deposition of C4 on bacteria was not detected in neat milk, suggesting that C3 deposition did not result from the activation of the classical pathway. This was not the result of a lack of antibodies. However, C4 deposition could be obtained by adding purified bovine C1q to normal milk, and C3 deposition was accelerated, suggesting the participation of the classical pathway. The deposition of C1q on antibody-sensitized bacteria was impeded by milk compared with that of C1q diluted in phosphate-buffered saline. Concentrations of C1q in normal milk were very low, ranging from 150 to 250 ng/ml. Overall, these findings indicate that C1q was a limiting factor of the classical pathway in normal milk. The capacity of milk to deposit C3 fragments on mastitis-causing S. agalactiae prompts further studies to investigate its role in opsonophagocytosis.

Complement component C7 is a plasminogen-binding protein.

Ab deposition, whether by reaction with the specific Ag or by preformed immune complexes, is followed by activation and deposition of complement components. Tissue destruction is observed in the Ab- and complement-induced lesions. The proteolytic enzyme plasmin is thought to participate in the Ab- and complement-mediated organ pathology. Plasmin is generated from plasma-derived plasminogen by cell-derived plasminogen activators (PAs). Two types of PAs are known, urokinase-type PA (uPA) and tissue-type PA (tPA). We investigated whether the PA system and the complement system can interact to promote local plasmin generation. Among the terminal complement components C5b6, C7, C8, and C9, the nonenzymatic component C7 is a plasminogen-binding protein. Radioligand binding studies revealed that the isolated component, as well as C7 after its incorporation into the terminal complement complex C5b-9, can bind plasminogen. Binding was inhibited by the lysine analogues 6-aminohexanoic acid and tranexamic acid, implicating the lysine binding sites of plasminogen into the binding interaction. tPA-mediated plasminogen activation was enhanced in the presence of C7. Based on these findings, an interaction is proposed between the complement system and the plasminogen activator system; a mechanism that may focus plasmin activity to structures that have been tagged by Ab and complement deposition.

Subinvolution of the uteroplacental arteries: an immunohistochemical study.

Subinvolution of uteroplacental arteries is a well-recognized cause of hemorrhage in the postpartum period. Although the physiological changes in these arteries during pregnancy are well documented, the sequence of events in normal involution is largely unknown. A recent immunohistochemical study has raised the possibility of an abnormal interaction between maternal uterine cells and fetal trophoblast in subinvolution. An indirect immunoperoxidase technique was used to compare deposition of complement components and immunoglobulin in subinvoluted and normally involuted uteroplacental arteries in 25 cases of postpartum hemorrhage. Deposits of C1q, C3d, C4, and C9 were detected within the walls of normally involuted vessels, whereas deposition of C1q, C3d, and C4 was absent in subinvoluted vessels; C9 was detected only focally. Deposition of immunoglobulins G, A, and M mirrored those described for complement components. The results suggest that immunological factors are necessary for the process of normal involution of uteroplacental arteries and are deficient in subinvoluted vessels.

Immunohistology of temporal arteritis: phenotyping of infiltrating cells and deposits of complement components

Deposition of complement factors, immunoglobulins and infiltrating cells was evaluated by immunohistochemical staining in 30 temporal artery biopsy specimens from patients suffering from temporal arteritis and/or polymyalgia rheumatica and in controls. In the temporal arteritis group infiltrating cells, classic complement, alternative complement and lytic complex activation were detected. In specimens from patients suffering from only polymyalgia rheumatica there was unexpected evidence of classic complement and lytic complex activation. We conclude that immuno-histochemistry provides support for the concept of temporal arteritis and polymyalgia being based on the same pathological process.

Characteristics of Nephritogenic IgA Immune Complexes

Studies were undertaken to elucidate the characteristics of IgA immune complexes responsible for glomerular deposition and complement activation. Monomeric IgA antidinitrophenyl (DNP) and the antigen DNP-Ficoll tended to form small soluble complexes that circulated in the blood without localizing in glomeruli of experimental animals. In contrast, polymeric IgA formed intermediate and large-sized complexes that caused glomerular immune deposits. Although covalently cross-linked large-sized IgA oligomers deposited in the glomeruli, they failed to induce concomitant deposition of complement components. Only the presence of a complement-activating antigen endowed the glomerular IgA immune deposits with the ability to activate complement. Thus, the size of the IgA complexes plays a primary role in initiating glomerular localization, and the nature of the antigen causes a secondary phase of inflammation.

Phase I trial of a mouse monoclonal antibody against GD3 ganglioside in patients with melanoma: induction of inflammatory responses at tumor sites.

Twenty-one patients were entered into a phase I trial to evaluate toxicity, antitumor effects, and biological responses at tumor sites during treatment of R24, an immunoglobulin G3 (IgG3) mouse monoclonal antibody (mAb) against GD3 ganglioside. Toxicity was related to dose of R24. Urticaria and pruritus were the most prominent side effects, with nausea, vomiting, and diarrhea occurring at the highest dose levels. Partial responses were observed in four patients lasting from 6 to 46 weeks, and mixed responses were seen in two patients. Responses occurred as early as 4 weeks and as late as 10 weeks after beginning treatment. Twenty of the 21 patients developed human IgG antibodies against R24. Antimouse Ig antibodies were first detected at a median of 14 days after starting treatment, but three of the four patients who had a partial response developed the antimouse Ig responses later than 20 days. Peak serum levels of R24 were related to dose and ranged from a mean of 0.9 micrograms/mL at the lowest dose level (1 mg/m2/d) to 44 micrograms/mL at the highest dose (50 mg/m2/d). The amount of R24 reaching tumor sites corresponded to the dose administered, and R24 could be detected in tumors as late as 30 days after finishing treatment. Inflammation at tumor sites was observed during treatment. Biopsies of tumors taken before, during, and after treatment revealed that R24 induced deposition of complement components, increased numbers of mast cells with mast cell degranulation, and infiltration of T lymphocytes. These results suggest that treatment with R24 can produce a localized inflammatory response at tumor sites that is capable of producing tumor regression.