Growth-supporting activity of fragment Ba of the human alternative complement pathway for activated murine B lymphocytes

Complement-activated fragment Ba functions as an antibacterial protein and mediates immune responses in lower vertebrates

The complement system plays an important role in antibacterial infection and immune regulation. Ba, an important complement component, is produced and released by the cleavage of complement factor B during complement activation. However, the immune functions of Ba are unclear. In this study, we reported that recombinant Ba exerted direct bactericidal and immune regulatory effects. Recombinant Paralichthys olivaceus Ba (rPoBa) bound bacteria via interaction with the bacterial wall component lipopolysaccharide, resulting in bacterial membrane permeabilization and bacterial death. Furthermore, rPoBa exhibited bactericidal activity against Gram-negative bacteria in a manner that depended on concentration, time, temperature, pH, and metal ions. Structure prediction analysis showed that PoBa contained three distinct complement control protein (CCP) domains. CCP1 was mainly responsible for binding to lipopolysaccharide, and both CCP1 and CCP3 might be required for bacterial membranous permeabilization. The bactericidal effects of Ba were observed only in lower vertebrates, with no such effects observed in mammals. In addition, rPoBa could protect P. olivaceus against Vibrio harveyi infection both in vitro and in vivo by significantly improving the immune activity of peripheral blood leukocytes and reducing tissue bacterial loads. Consistently, when PoCFB expression in P. olivaceus was knocked down, the PoBa production and complement activity were decreased, and bacterial replication was significantly enhanced. In conclusion, this study revealed that the complement-activated recombinant Ba fragment improved the immune defense against bacterial infection and provided a potential strategy to control disease in lower vertebrates.

Two different complement Factor B (Bf) alleles of the orangutan major histocompatibility complex (MHC) are also conserved in chimpanzee and humans showing importance in primate immunity

BACKGROUND

Major Human Histocompatibility complex (MHC or HLA in humans) has been associated to autoimmune diseases. However, only statistical phenomenological and no pathogenetic description has been reached after decades. This shows that MHC single locus association studies are probably useless for HLA/diseases association. Extended HLA (class I and class II genes) haplotypes should also be studied conjointly with class III or complement alleles (complotypes). Complotypes in humans are defined as alleles belonging to C2, C4 and Bf (Factor B) genes/proteins (class III). Also, the placing of MHC class I and class II genes close together with complement genes from at least birds to humans shows existence of a strong selection to gather conjointly these loci that fight microbes, help self-maintenance and avoid autoimmunity. In this paper we aim to study Bf alleles in primates in order to rise again interest to study the role of Bf alleles together with other MHC genes in their physiopathology and evolution.

METHODS

Orangutan (Pongo pygmaeus, Popy) cell lines RNA from 6 different individuals were retrotranscribed, PCR amplified, cloned and DNA sequenced in order to study Bf alleles.

RESULTS

A Bf allele identical to that found in chimpanzee (Patr-Bf*A01) and human (rs641153) was found in two of the six studied orangutans: Popy-Bf*A01 and Popy-Bf*A02. This polymorphism is placed in Factor B codon 32 that defines BF*S and Bf*F proteins in man and produce Leu instead of Arg (Bf*S) or Gln (Bf*F). In addition, each new orangutan allele present synonymous differences with each other at codon 25: Popy-Bf*A01 shows ACG while Popy-Bf*A02 bears ACA, both codifying for Thr.

CONCLUSIONS

The selection for about 15 million years (time gap of evolutionary appearance between orangutan and hominids) shows the importance of this particular allele conservation in immune and self defense in primates. The complotypes (Bf,C2 and C4 loci) alleles together with other MHC class I and Cass II loci alleles are often transmitted in block to the germinal line: this indicates that all specific alleles from the MHC different loci may work together to accomplish MHC functions. All MHC loci alleles should be studied together to unveil their physiopathology and also maintenance of specific alleles (like the one described in this paper) for so long time in evolution should be further studied in Bf and the other neighbouring complement loci (C2 and C4).

The alternative complement pathway activation product Ba as a marker for transplant-associated thrombotic microangiopathy

BACKGROUND

Transplant-associated thrombotic microangiopathy (TA-TMA) occurs after hematopoietic stem cell transplantation (HSCT) and is characterized by microvascular thrombosis and end-organ injury particularly of the kidneys. TA-TMA is challenging to diagnose and treat, which can lead to long-term complications and death in patients with severe disease. Studies have shown that genetic abnormalities of the alternative complement pathway (AP) are associated with TA-TMA. We hypothesized that patients with TA-TMA may generate elevated levels of the AP activation product, Ba, compared with HSCT patients without TA-TMA.

PROCEDURE

We longitudinally measured plasma levels of complement activation products C3a, Ba, and C5a in 14 HSCT patients: 7 with TA-TMA and 7 without TA-TMA. We assessed renal function by calculating estimated glomerular filtration rate (eGFR) and correlated the extent of AP activation with renal dysfunction in both patient populations.

RESULTS

The median days from HSCT to study enrollment were 154 (39-237) in the TA-TMA group and 84 (39-253) in the HSCT group without TA-TMA. Median Ba levels (ng/mL) at enrollment were 1096.9 (826.5-1562.0) in the TA-TMA group and 725.7 (494.7-818.9) in the HSCT group without TA-TMA (P = 0.007). Over the study duration, Ba levels inversely correlated with eGFR. There were no differences in C3a, C5a, or sC5b9 levels between the two populations at any measured interval.

CONCLUSIONS

We conclude in this preliminary study that Ba protein may serve as a marker for TA-TMA, and furthermore, that components generated in the early phase of AP activation may be involved in the pathogenesis of renal endothelial injury in TA-TMA.

Molecular characterization, expression and antimicrobial activity of complement factor D in Megalobrama amblycephala

Complement factor D (Df) is a serine protease, which can activate the alternative pathway by cleaving complement factor B, and involves in the innate defense against pathogens infection in teleost. In this study, we cloned, characterized the Df gene from blunt snout bream (Megalobrama amblycephala) (Mamdf), and examined its expression pattern and antimicrobial activity. The open reading frame (ORF) of Mamdf was 753 bp, encoding 250 amino acids with a molecular mass of 27.2 kDa. Mamdf consisted of a single serine protease trypsin superfamily domain, 3 substrate binding sites and 3 active sites, but no potential N-glycosylation site. Pairwise alignment showed that Mamdf shared the highest identity (94%) with grass carp (Ctenopharyngodon idellus). Phylogenetic analysis indicated that Mamdf and other vertebrate Df had a common ancestral origin. Mamdf structured with 4 introns and 5 exons. The Mamdf mRNA expressed relatively high at the intestine appearance stage during early development and constitutively expressed in various tissues with the highest expression in the kidney in healthy adults. After challenged with Aeromonas hydrophila, significant changes of Mamdf at both mRNA and protein levels in the kidney, spleen, liver and head-kidney were observed. The recombinant Mamdf protein showed antimicrobial activity against both gram-positive bacteria and gram-negative bacteria. The above results suggested the immune function of Mamdf, and would benefit further detailed Df function research in the immune process in teleost.

A teleost complement factor Ba possesses antimicrobial activity and inhibits bacterial infection in fish

Complement factor B (Bf) is a component of the complement system. Following activation of the alternative pathway of the complement system, factor B is cleaved into Ba and Bb fragments. In fish, the Bf of rainbow trout is known to act as a C3 convertase, but the function of the Ba fragment is essentially unknown. In this study, we examined the expression patterns of tongue sole Cynoglossus semilaevis Bf (named CsBf) and the biological activity of the Ba fragment of CsBf (named CsBa). CsBf possesses the conserved domains of Bf and shares 39.9%-56.4% sequence identities with other fish Bf. CsBf expression was high in liver, muscle, and heart, and low in intestine, blood, and kidney. Bacterial infection significantly induced CsBf expression in kidney, spleen, and liver in a time-dependent manner. Recombinant CsBa (rCsBa) exhibited apparent binding capacities to bacteria and tongue sole peripheral blood leukocytes, and binding of rCsBa to bacteria inhibited bacterial growth. When overexpressed in tongue sole, CsBa significantly reduced bacterial dissemination in fish tissues. Together these results indicate for the first time that a fish Ba possesses antibacterial effect as well as immune cell-binding capacity, and thus probably plays a role in host immune defense against bacterial infection.

BF*F allotype of the alternative pathway of complement: A marker of protection against the development of antiphospholipid antibodies in patients with systemic lupus erythematosus

Background

B factor (BF) from the alternative complement pathway seems to participate in the pathophysiology of systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS).

Objective

To study the allotypic variability of BF in SLE and their associations with clinical and autoantibodies profile.

Methods

BF allotypes were determined by high-voltage agarose gel electrophoresis, under constant cooling, followed by immunofixation with anti-human BF antibody, in 188 SLE patients and 103 controls. Clinical and serological data were obtained from medical examination and records.

Results

No significant differences of BF variants between patients and controls were found, neither in relation to epidemiologic or clinical manifestations. Associations of phenotype BF SS07 and allotype BF*S07 were found with anticardiolipin IgM (aCl-IgM) antibodies ( p = 0.014 and p = 0.009 respectively), but not with aCl-IgG, lupus anticoagulant (LA), anti β2GPI or clinical APS. A significant decrease in BF*F allotype ( p = 0.043) and BF SF phenotype ( p = 0.018) was detected in patients with anti-phospholipid antibodies as a whole (aCl-IgG, aCl-IgM, LA and anti β2GPI).

Conclusions

There is a link between phenotype BF SS07 and allotype BF*S07 with aCl-IgM in SLE patients; BF*F allotype could be considered a marker of protection against the development of antiphospholipid antibodies in these patients.

The complement inhibitor, CRIT, undergoes clathrin-dependent endocytosis

Complement C2 receptor inhibitor trispanning (CRIT) is a receptor for the second component of complement and is found in various tissues and hemopoietic cells. On binding to CRIT, C2 cannot be activated to potentially form a variant-C3 convertase as it is rendered non-cleavable by C1s. CRIT thus limits the amount of C3 convertase formed on the cell surface. In this study we have shown, using flow cytometry and immunofluorescence microscopy, that human CRIT undergoes endocytosis from the plasma membrane. The endocytosis, possibly ligand mediated, occurs via clathrin-coated pits as it can be inhibited by prior incubation of cells in hypertonic medium or with chlorpromazine, at 37 degrees C. However, inhibition of endocytosis was not possible after treatment with nystatin, or filipin, inhibitors of caveolae/raft-dependent endocytosis. In the presence of C2 alone, CRIT associates with the adapter protein, beta-arrestin-2, and whether in association with C2 or not, then appears in the perinuclear region, but does not appear to be translocated into the nucleus. Apart from the C3aR and C5aR that internalize the anaphylatoxic peptides, this is the first report of the internalization via the clathrin pathway of a receptor for a complement serum protein.

Complement: central to innate immunity and bridging to adaptive responses

The complement system, a pillar of innate immunity, has belatedly become recognised as a key modulator of adaptive immunity, acting to direct, modulate and modify the responses of lymphocytes to stimuli. These effects are mediated by interactions between complement components or activation-derived fragments and specific binding proteins--complement receptors and regulators--on the target cells. This review will describe the current state of knowledge in this swiftly moving field. It is hoped that the recognition of these properties will help to establish complement in the role it richly deserves as the lynchpin of immunity.

Parasite interaction with host complement: beyond attack regulation

Many orthologous proteins of known mammalian receptors have been discovered in parasites. Besides disguising the parasite as self in terms of the host immune system, evidence is accumulating that these receptors link to signalling pathways in parasites that appear to be involved in their growth or development. Recently, several proteins of the host complement system, which forms part of the innate defence against invading microorganisms, have been shown to possess alternative functions. These complement proteins interact with signalling pathways involved in early development and differentiation, as well as organ and tissue regeneration. By altering cellular interactions and responses, complement is being shown to have novel roles besides the originally described inflammatory role. The possibility exists that, as for other host factors interacting with parasites and affecting their growth or development, host complement proteins could also have such an influence.

Complement factor B gene regulation: synergistic effects of TNF-alpha and IFN-gamma in macrophages

Complement factor B (Bf) plays an important role in activating the alternative complement pathway. The inflammatory cytokines, in particular TNF-alpha and IFN-gamma, are critical in the regulation of Bf gene expression in macrophages. In this study, we investigated the mechanisms of Bf gene regulation by TNF-alpha and IFN-gamma in murine macrophages. Northern analysis revealed that Bf mRNA expression was synergistically up-regulated by TNF-alpha and IFN-gamma in MH-S cells. Truncations of the 5' Bf promoter identified a region between -556 and -282 bp that mediated TNF-alpha responsiveness as well as the synergistic effect of TNF-alpha and IFN-gamma on Bf expression. Site-directed mutagenesis of a NF-kappaB-binding element in this region (-433 to -423 bp) abrogated TNF-alpha responsiveness and decreased the synergistic effect of TNF-alpha and IFN-gamma on Bf expression. EMSAs revealed nuclear protein binding to this NF-kappaB cis-binding element on TNF-alpha stimulation. Supershift analysis revealed that both p50 and p65 proteins contribute to induction of Bf by TNF-alpha. An I-kappaB dominant negative mutant blocked Bf induction by TNF-alpha and reduced the synergistic induction by TNF-alpha and IFN-gamma. In addition, the proteasome inhibitor MG132, which blocks NF-kappaB induction, blocked TNF-alpha-induced Bf promoter activity and the synergistic induction of Bf promoter activity by TNF-alpha and IFN-gamma. LPS was found to induce Bf promoter activity through the same NF-kappaB cis-binding site. These findings suggest that a NF-kappaB cis-binding site between -433 and -423 bp is required for TNF-alpha responsiveness and for TNF-alpha- and IFN-gamma-stimulated synergistic responsiveness of the Bf gene.

Expression profiling in knockout mice: lymphotoxin versus tumor necrosis factor in the maintenance of splenic microarchitecture

Expression profiling provides a powerful approach to define the underlying molecular mechanisms in disease. Several techniques referred collectively to as gene profiling may be also helpful in the analysis of the phenotype of mice with targeted mutations, especially if applied to distinct histological compartments, to specific cell types or to evaluate the effect of specific challenges, such as infection. Here we review several of the existing techniques applicable to genetic knockout studies, and share our experience from the study of mice with tumor necrosis factor (TNF) and lymphotoxin (LT) deficiencies, with specific emphasis on the distinction between TNF- and LT-mediated signalling pathways in vivo. Gene expression profiling analysis of TNF/LT-deficient mice supports the notion that TNF and LT, originally discovered as distinct biological activities, manifest both distinct and redundant functions in vivo.

Modulation of renal disease in MRL/lpr mice genetically deficient in the alternative complement pathway factor B

In systemic lupus erythematosus, the renal deposition of complement-containing immune complexes initiates an inflammatory cascade resulting in glomerulonephritis. Activation of the classical complement pathway with deposition of C3 is pathogenic in lupus nephritis. Although the alternative complement pathway is activated in lupus nephritis, its role in disease pathogenesis is unknown. To determine the role of the alternative pathway in lupus nephritis, complement factor B-deficient mice were backcrossed to MRL/lpr mice. MRL/lpr mice develop a spontaneous lupus-like disease characterized by immune complex glomerulonephritis. We derived complement factor B wild-type (B+/+), homozygous knockout (B-/-), and heterozygous (B+/-) MRL/lpr mice. Compared with B+/- or B+/+ mice, MRL/lpr B-/- mice developed significantly less proteinuria, less glomerular IgG deposition, and decreased renal scores as well as lower IgG3 cryoglobulin production and vasculitis. Serum C3 levels were normal in the B-/- mice compared with significantly decreased levels in the other two groups. These results suggest that: 1) factor B plays an important role in the pathogenesis of glomerulonephritis and vasculitis in MRL/lpr mice; and 2) activation of the alternative pathway, either by the amplification loop or by IgA immune complexes, has a prominent effect on serum C3 levels in this lupus model.

A longitudinal study of C3, C3d and factor Ba in burn patients in Hong Kong Chinese

A longitudinal study of serum C3, C3d and fragment Ba was carried out in 53 burn patients of Chinese origin whose total burn surface area ranged from I to 45%. Complement C3 was found to be activated on or before day 7 post-burn. The sharp increase in C3d suggested an acute inflammatory response. In addition factor Ba was increased in these patients, suggesting that the alternative pathway was also activated following thermal injuries. The fluctuations of C3, C3d and Factor Ba observed about 1 year after injury suggested chronic inflammation associated with long term of outcome of healing in the burn sites.

Mice deficient for the complement factor B develop and reproduce normally

Factor B is an essential component of the complement cascade which forms the C3 and C5 convertase of the alternative pathway. Factor B cleavage products also function as cofactors in antibody-independent monocyte-mediated cytotoxicity, macrophage spreading, plasminogen activation and proliferation of B lymphocytes. Several healthy kindreds heterozygous for the factor B null or non-functional allele have been reported but the absence of homozygous factor B deficiency in humans or in animals has been speculated to be caused by the lethality of the phenotype. Here we report the generation of factor B-deficient mice by gene targeting in vivo. These mice were born at the expected Mendelian ratio and they both develop and breed normally in a conventional animal facility. These mice represent a model of complete alternative pathway deficiency. This model enables the dissection of the complement cascade in vivo and the elucidation of the relative contribution of this complement pathway in the various physiological and pathological phenomena ascribed to the complement system.

A targeted disruption of the murine complement factor B gene resulting in loss of expression of three genes in close proximity, factor B, C2, and D17H6S45

Factor B is a serine protease, essential for the function of the alternative pathway of complement activation. To study further the importance of the alternative pathway of complement activation in vivo and to help elucidate any additional functions of factor B or its activation fragments we developed, by homologous recombination in embryonic stem cells, mice with a disrupted factor B gene. Factor B-deficient mice produced no detectable factor B mRNA or protein and had no detectable factor B enzymatic activity or alternative pathway function in their serum. Further studies revealed that the two adjacent genes, complement component C2 and D17H6S45, had been down regulated as a result of the disruption. The down-regulation of C2 gene expression was sufficient to cause a complete loss of classical pathway function as determined by the failure of sera from the deficient mice to opsonize antibody-sensitized sheep erythrocytes and by impairment of immune complex processing in vivo. The resulting mouse is deficient in both factor B and C2, and hence the alternative and classical pathways of complement activation, and adds to the repertoire of models for studying the in vivo role of complement in the immune system.

Abrogation of the alternative complement pathway by targeted deletion of murine factor B

To investigate the role of complement protein factor B (Bf) and alternative pathway activity in vivo, and to test the hypothesized potential genetic lethal effect of Bf deficiency, the murine Bf gene was interrupted by exchange of exon 3 through exon 7 (including the factor D cleaving site) with the neor gene. Mice heterozygous for the targeted Bf allele were interbred, yielding Bf-deficient offspring after the F1 generation at a frequency suggesting that Bf deficiency alone has no major effect on fertility or fetal development. However, in the context of one or more genes derived from the 129 mouse strain, offspring homozygous for Bf deficiency were generated at less than expected numbers (P = 0.012). Bf-deficient mice showed no gross phenotypic difference from wild-type littermates. Sera from Bf-deficient mice lacked detectable alternative complement pathway activity; purified mouse Bf overcame the deficit. Classical pathway-dependent total hemolytic activity was lower in Bf-deficient than wild-type mice, possibly reflecting loss of the alternative pathway amplification loop. Lymphoid organ structure and IgG1 antibody response to a T-dependent antigen appeared normal in Bf-deficient mice. Sensitivity to lethal endotoxic shock was not significantly altered in Bf-deficient mice. Thus, deficiency of Bf and alternative complement activation pathway led to a less dramatic phenotype than expected. Nevertheless, these mice provide an excellent model for the assessment of the role of Bf and the alternative pathway in host defense and other functions in vivo.

Constitutive expression of murine complement factor B gene is regulated by the interaction of its upstream promoter with hepatocyte nuclear factor 4

Factor B (Bf) is a constituent of the alternative pathway of complement activation encoded within the major histocompatibility complex. Transcription of the murine gene from two initiation sites generates two Bf mRNA species differing in size and tissue distribution. Striking genetic, tissue-specific differences in Bf mRNA levels at extrahepatic sites (kidney and intestine) among mouse strains correlate with a DNA sequence polymorphism in the 5'-flanking region of the gene and differential nuclear protein binding at the Bf upstream transcriptional initiation site (UIS). To ascertain the functional consequences of this polymorphism in the Bf promoter, we analyzed the effects of strain-specific sequences in the Bf 5' region on the expression of a chloramphenicol acetyltransferase (CAT) reporter gene transfected in human and mouse hepatoma cells. The CAT activity and mRNA level produced when transcription was driven by the sequence of strains with high extrahepatic expression were reduced to background levels when the sequence specific to the low expressor strains was used. Eighty percent of this difference was accounted for by a point substitution that affects DNA-protein interaction at the UIS, the sequence of higher affinity conferring higher expression. Hepatocyte nuclear factor 4 (HNF-4), derived from HepG2, mouse liver and kidney or cell-free translation of HNF-4 RNA, is the nuclear protein that preferentially binds to the high expressor UIS. Bf-CAT is not expressed in cells that lack HNF-4 (CV-1). However, co-transfection of HNF-4 into CV-1 cells drives Bf-CAT expression and reproduces the differences derived from the substitution that affect HNF-4 binding in vitro. These data show that interaction of HNF-4 with polymorphic variants of the upstream Bf promoter is the major determinant of strain-specific extrahepatic factor B expression.

cis and trans elements differ among mouse strains with high and low extrahepatic complement factor B gene expression

Factor B (Bf), an enzyme of the alternative pathway of complement activation, is one of four major histocompatibility complex (MHC) class III genes. To ascertain the genetic mechanism for tissue-specific constitutive and regulated expression of Bf, we sequenced the regulatory regions 5' of the gene from mice of different H-2 MHC haplotypes and assessed trans-acting factors, specific DNA binding nucleoproteins, in liver and kidney. Striking tissue-specific differences in constitutive expression of Bf were demonstrated in mice of H-2f or H-2z haplotypes when compared with H-2d or H-2u (kidney and intestinal Bf in H-2d or H-2u much greater than H-2f or H-2z). These differences correlated with a point nucleotide substitution 3 bp downstream of the upstream Bf initiation site that affects interaction with a DNA binding protein. This and additional cis differences localize the sequence substitutions responsible for previously identified restriction fragment length polymorphisms among inbred mouse strains and also reveal two previously unrecognized polymorphisms generated by SmaI and HinfI digestion. Evidence for differences in trans was found in a comparison of DNA binding nucleoproteins from kidney, but not liver, of B10.PL when compared with B10.M. These data, together with the high degree of sequence homology between human and mouse Bf 5' flanking regions, should prompt a search for polymorphic restriction sites and cis binding elements in the Bf promoter that could serve as markers of human MHC-associated renal pathology and variants in local MHC class III gene expression.

C5 does not play a major role in the immune response of mice to SRBC in vivo

The role of complement component C5 in the immune response of mice to sheep red blood cells (SRBC) was investigated. Congenic C5-sufficient and C5-deficient B10. D2 mice and genetically C5-deficient DBA/2 mice, as such or supplemented with C5-sufficient serum, were used as experimental animals. C5-substitution of the C5-deficient mice resulted in measurable C5 levels for days. The functional half-life of C5 in C5-deficient DBA/2 mice was about 21 h. No significant differences between the IgM-responses of C5-bearing and naive C5-deficient animals were observed. This suggests that C5 does not play a major role in the primary humoral immune response of mice in vivo, although C5 seems to do so in in vitro experiments, even with the same antigen. Antigen-induced C5-production by C5-deficient mice as one of the explanations of the in vitro/in vivo discrepancy could not be confirmed experimentally.

Adipsin and complement factor D activity: an immune-related defect in obesity

Adipsin is a serine protease that is secreted by adipocytes into the bloodstream; it is deficient in several animal models of obesity, representing a striking example of defective gene expression in this disorder. Recombinant mouse adipsin was purified and its biochemical and enzymatic properties were studied in order to elucidate the function of this protein. Activated adipsin has little or no proteolytic activity toward most substrates but has the same activity as human complement factor D, cleaving complement factor B when it is complexed with activated complement component C3. Like authentic factor D, adipsin can activate the alternative pathway of complement, resulting in red blood cell lysis. Decreased (58 to 80 percent) complement factor D activity, relative to lean controls, was observed as a common feature of several experimental models of obesity, including the ob/ob, db/db, and monosodium glutamate (MSG)-injected mouse and the fa/fa rat. These results suggest that adipsin and the alternative pathway of complement may play an unexpected but important role in the regulation of systemic energy balance in vivo.

Expression of the binding molecules for factor B and its fragment, Bb, of human complement on human monocytes after in vitro cultivation

We have examined the expression of the binding molecules for Bb of human complement on the surface of resting and cultured human monocytes. Flow cytometry using biotinylated anti-B antibodies and phycoerythrin-avidin showed that although resting monocytes did not bind Bb significantly, monocytes cultivated for 24 h in the presence of fetal calf serum were capable of binding with Bb and its precursor B, but not with Ba fragment. The Bb-binding molecules were pronase-sensitive, suggesting that membrane proteins are associated with the Bb-binding molecules. In addition to human monocytes, liquid paraffin-induced guinea pig inflammatory macrophages were also found to express Bb-binding molecules on their surface. This implies that Bb receptor-like molecules become expressed during activation and differentiation of monocytes, just as observed with the C3d-receptor of monocytes.