Yersinia pestis Δail Mutants Are Not Susceptible to Human Complement Bactericidal Activity in the Flea

Ail confers serum resistance in humans and is a critical virulence factor of Y. pestis, the causative agent of plague. Here, the contribution of Ail for Y. pestis survival in the flea vector was examined. Rat or human but not mouse sera were bactericidal against a Y. pestis Δail mutant at 28°C in vitro. Complement components deposited rapidly on the Y. pestis surface as measured by immunofluorescent microscopy. Ail reduced the amount of active C3b on the Y. pestis surface. Human sera retained bactericidal activity against a Y. pestis Δail mutant in the presence of mouse sera. However, in the flea vector, the serum protective properties of Ail were not required. Flea colonization studies using murine sera and Y. pestis KIM6+ wild type, a Δail mutant, and the Δail/ail+ control showed no differences in bacterial prevalence or numbers during the early stage of flea colonization. Similarly, flea studies with human blood showed Ail was not required for serum resistance. Finally, a variant of Ail (AilF100V E108_S109insS) from a human serum-sensitive Y. pestis subsp. microtus bv. Caucasica 1146 conferred resistance to human complement when expressed in the Y. pestis KIM6+ Δail mutant. This indicated that Ail activity was somehow blocked, most likely by lipooligosaccharide, in this serum sensitive strain. IMPORTANCE This work contributes to our understanding of how highly virulent Y. pestis evolved from its innocuous enteric predecessor. Among identified virulence factors is the attachment invasion locus protein, Ail, that is required to protect Y. pestis from serum complement in all mammals tested except mice. Murine sera is not bactericidal. In this study, we asked, is bactericidal sera from humans active in Y. pestis colonized fleas? We found it was not. The importance of this observation is that it identifies a protective niche for the growth of serum sensitive and nonsensitive Y. pestis strains.

[The effects of complement C3f segment on expression and secretion of collagen I, III and transforming growth factor-beta1 in human embryonic lung fibroblast]

OBJECTIVE

To observe the effects of complement fragment C3f on expression and secretion of collagen I, III and transforming growth factor( TGF)-beta1 in human embryonic lung fibroblast (MRC-5) cells.

METHODS

MRC-5 cells were cultured with C3f (the synthetic 17 peptides fragments of complement C3). The extracellular and intracellular expression levels of type I, III collagens and TGF-beta1 in MRC-5 cultures were detected by ELISA and immunohistochemistry, respectively.

RESULTS

The expression levels of type I, III collagen and TGF-beta1 in the supernatant of MRC-5 cultures decreased significantly with the concentrations of C3f as compared with controls (P < 0.05). Also the expression level of TGF-beta1 in MRC-5 cytoplasm reduced significantly as compared with controls (P < 0.05).

CONCLUSION

The results of present in vitro study showed that the complement fragment C3f could reduce the formation of TGF-beta1 and type I, III collagens in MRC-5 cells, and inhibit the lung tissue fibrosis.

Cutting edge: Complement (C3d)-linked antigens break B cell anergy

C3dg adducts of Ag can coligate complement receptor type 2 (CR2; CD21) and the B cell Ag receptor. This interaction significantly amplifies BCR-mediated signals in Ag-naive wild-type mice, lowering the threshold for B cell activation and the generation of humoral immune responses as much as 1000-fold. In this study we demonstrate that CR2-mediated complementation of BCR signals can also overcome B cell anergy. Unlike Ag alone, BCR/CR2 costimulation (Ars-CCG/C3dg complexes) of anergic Ars/A1 B cells led to Ca(2+) mobilization in vitro and the production of autoantibodies in vivo. Interestingly, the in vivo immune response of anergic cells occurs without the formation of germinal centers. These results suggest that the Ag unresponsiveness of anergic B cells can be overcome by cross-reactive (self-mimicking) Ags that have been complement-opsonized. This mechanism may place individuals exposed to complement-fixing bacteria at risk for autoimmunity.

[The immune enhancement of the C3 to the Escherichia coli vaccine]

C3b was separated and purified from the SPF chicken serum. It was linked with E. coli antigen by the glutaral. 11 days aged SPF chicken were immunized by the complex antigen and the chickens of control group were immunised by the FCA- E. coli antigen . They were boosted at the age of 18 day. The immune response was monitored by an enzyme-linked immunosorbent assay(ELISA) for anti-E. coli anitibody. The ELISA results indicated that during the early several weeks, IgG titers elicited by FCA (FCA-E. coli) were higher than those elicited by C3 (C3b-E. coli), but decreased rapidly after a peak around the end of 4th week from being immunized. Chickens immunized with C3b always gave increased response, and the IgG titers were equal to that of FCA at the end of 5th week from being immunized and then higher and higher than that of FCA. Thus the adjuvant effect of C3b is different from that of FCA, it could induce production of memory cell and make the antigens stimulate immune cells consistently and stably.

Localised PtdIns(3,4,5)P3 or PtdIns(3,4)P2 at the phagocytic cup is required for both phagosome closure and Ca2+ signalling in HL60 neutrophils

Several events accompany integrin-mediated phagocytosis by myeloid cells. These include local pseudopod and phagocytic cup formation followed by Ca2+ signalling. However, there is also a role for localised phosphatidylinositol (3,4,5) trisphosphate [PtdIns(3,4,5)P3] production. Here we report that in neutrophilic HL-60 cells expressing PH-Akt-GFP, binding of iC3b-coated zymosan particles (2 μm in diameter) via β2 integrin induces an incomplete phagocytic cup to form before either PtdIns(3,4,5)P3 or phosphatidylinositol (3,4) bisphosphate [PtdIns(3,4)P2] production or Ca2+ signalling. These phosphoinositides then accumulated locally at the site of the phagocytic cup and Ca2+ signalling and phagosome closure follows immediately. Although photobleaching showed that PH-Akt-GFP was freely diffusible in the cytosol and able to dissociate from the phagocytic cup, it was restricted to the plasma membrane of the formed but open phagosome and failed to diffuse into the surrounding plasma membrane or neighbouring phagocytic cups even if connected. Inhibition of phosphoinositide (PI) 3-kinase or depletion of membrane cholesterol inhibited both Ca2+ signalling and phagosome closure, but had no effect on particle binding or phagocytic cup formation. We therefore conclude that PtdIns(3,4,5)P3 or PtdIns(3,4)P2 generation was not required for the events that initiate the formation of the phagocytic cup, but that anchoring of PtdIns(3,4,5)P3 at the phagocytic cup is an essential step for phagosome closure and Ca2+ signalling.

The human complement regulator factor H binds pneumococcal surface protein PspC via short consensus repeats 13 to 15

The innate ability of Streptococcus pneumoniae to resist complement activation and complement-mediated phagocytosis may be a direct consequence of the ability of the bacteria to bind components of the complement regulatory system. One such component, factor H (fH), is a crucial fluid-phase negative regulator of the alternative pathway of complement and is utilized by a number of pathogenic organisms to resist complement attack. The pneumococcal surface protein C (PspC [also known as CbpA] and SpsA) has been shown to bind fH, although the exact binding site within one or more of the 20 short consensus repeats (SCRs) of the molecule is not known. The purpose of the current study was to map specific SCRs on fH responsible for this binding. Initial experiments utilizing type 2 pneumococcal strain D39 and its isogenic PspC-negative derivative (D39/pspC mutant) showed that fH binding was PspC dependent. A purified recombinant protein derivative of PspC that lacked the proline-rich region (PspCDeltaPro) had a reduced binding efficiency for fH, thereby directly showing the importance of this region for the fH interaction. We have specifically shown by inhibition experiments that SCRs responsible for heparin and C3b binding of fH are not involved in binding PspC and the interaction between fH and PspC is largely hydrophobic, since no inhibition was observed in the presence of high concentrations of NaCl. Construction of SCR proteins encompassing the whole fH molecule showed that SCRs 8 to 15 (SCR 8-15) mediated binding to PspC. Further localization experiments revealed that SCR 13 and SCR 15 were required for full binding, although partial binding was retained when either SCR was removed.

A new apoptotic pathway for the complement factor B-derived fragment Bb

Apoptosis is involved in both the cellular and humoral immune system destroying tumors. An apoptosis-inducing factor from HL-60 myeloid leukemia cells was obtained, purified, and sequenced. The protein found has been identified as a human complement factor B-derived fragment Bb, although it is known that factor B is able to induce apoptosis in several leukemia cell lines. Monoclonal antibodies against fragment Ba and Bb inhibited the apoptotic activity of factor B. When the purified fragment Bb was used for apoptosis induction, only the anti-Bb antibody inhibited Bb-induced apoptosis, and not the anti-Ba antibody. The apoptosis-inducing activity was found to be enhanced under conditions facilitating the formation of Bb. Blocking TNF/TNFR or FasL/Fas interactions did not interfere with the factor B-induced apoptosis. CD11c (iC3bR) acts as the main subunit of a heterodimer binding to fragment Bb in the apoptosis pathway, and the factor B-derived fragment Bb was found to possess the previously unknown function of inducing apoptosis in leukemic cells through a suicide mechanism of myeloid lineage cells during the differentiation stage.

Replenishment of RANTES mRNA expression in activated eosinophils from atopic asthmatics

Eosinophils have been shown to express the gene encoding regulated upon activation, normal T-cell expressed and secreted (RANTES), a potent eosinophilotactic chemokine. RANTES protein expression in eosinophils has previously been shown to be up-regulated by a number of agonists, including complement-dependent factors (C3b/iC3b) and interferon-gamma (IFN-gamma). We hypothesized that gene expression of RANTES is regulated in these cells by eosinophil-specific agonists. We analysed RANTES mRNA expression by reverse-transcription polymerase chain reaction (RT-PCR) in human peripheral blood eosinophils obtained from mild atopic asthmatics following stimulation over time. In resting eosinophils, a low level of RANTES mRNA was found to be constitutively expressed in all the atopic donors tested in this study (n = 6). Following stimulation with C3b/iC3b (serum-coated surfaces), eosinophils released measurable levels of RANTES, while sustained transcript expression was detected for up to 24 hr of stimulation. In contrast, IFN-gamma (5 ng/ml) transiently and significantly (P<0.05, n = 3) depleted relative amounts of RANTES PCR product (compared with beta2-microglobulin) after 1-4 hr of stimulation. RANTES transcript was again detectable after 24 hr of IFN-gamma incubation, suggesting that the pool of RANTES mRNA had been replenished. Other eosinophil-active cytokines, interleukin-3 (IL-3), IL-4, IL-5 and granulocyte-macrophage colony-stimulating factor, did not appear to modulate RANTES mRNA expression after 1 hr of incubation. The effect of IFN-gamma on RANTES mRNA was reversed by cycloheximide, suggesting that IFN-gamma may act by increasing the rate of translation of RANTES mRNA. These findings indicate that IFN-gamma may induce a rapid and transient effect on the translation and replenishment of RANTES mRNA in eosinophils. This novel observation supports the notion that eosinophils have the potential to replenish their stored and released bioactive proteins.

Different stimulating effects of complement C3b and complete Freund's adjuvant on antibody response

Upon activation, complement C3 undergoes a conformational change and acquires the capacity to covalently bind to other proteins such as antigen and to interact with specific receptors; therefore, C3 is involved in cell mediated immune response. The adjuvant effect produced by linking C3-fragments to antigen has recently been described. We injected C3b-Ag complexes consisting of one molecule of C3b ester linked to one molecule of HEL to immunised mice, and we compared the C3b adjuvant activity with that of complete Freund's adjuvant. IgG titers elicited by HEL emulsified in CFA (HEL + CFA) were higher than those elicited by HEL-C3b, but decreased rapidly after a peak response around day 45 whereas HEL-C3b resulted in a continuous increase of anti-HEL response. Mice immunised with HEL + CFA then boosted with HEL-C3b gave significantly higher response than those boosted with HEL + CFA, indicating more efficient memory cell restimulation by C3b. HEL + CFA leads to better priming than HEL-C3b when mice are boosted with HEL-C3b. Thus, adjuvant effect of C3b is different from that of CFA, leading to more stable IgG production and better memory stimulation.

The influence of IL-3, IL-5, and GM-CSF on normal human eosinophil and neutrophil C3b-induced degranulation

The priming effect of interleukin-3 (IL-3), interleukin-5 (IL-5), and granulocyte/macrophage-colony stimulating factor (GM-CSF) on eosinophil and neutrophil degranulation was studied. Granulocytes were obtained from normal donors, and degranulation was induced by incubation with serum-opsonized Sephadex particles. The released amounts of eosinophil cationic protein (ECP), eosinophil protein X (EPX), myeloperoxidase (MPO), and lactoferrin (LF) were measured by radioimmunoassay (RIA). The effect of IL-5 was dose- and time-dependent, with a maximal enhancement of ECP and EPX release of 71% (P < 0.03) and 66% (P < 0.03), respectively. Neutrophil degranulation, however, was unaffected. IL-3 was marginally effective, whereas GM-CSF seemed to act as a secretagogue for both eosinophil and neutrophil degranulation. We conclude that IL-5 selectively primes eosinophil degranulation, whereas IL-3 and GM-CSF seem to act as secretagogues for eosinophils and neutrophils. The results indicate that IL-5 may be involved in the priming of eosinophils as observed in patients with asthma and hypereosinophilic syndrome (HES).

Kinetics of the biosynthesis of complement subcomponent C1q by murine macrophages: LPS, immune complexes, and zymosan alone and in combination with interferon-gamma

We investigated the effects of bacterial lipopolysaccharide (LPS), immune complexes (IC), and C3b opsonized zymosan (AZ) alone and in combination with interferon-gamma (IFN-gamma) priming on macrophage synthesis and secretion of C1q. Our results indicated that LPS, IC, and AZ alone stimulated C1q mRNA and secretion in the absence of IFN-gamma. The increase in mRNA accumulation was detectable after 3 h, peaked at 6 h and was maintained at constitutive levels for 24 h. There was a corresponding early burst of increased secretion of functional C1q after 3 to 6 h which declined rapidly after 9 to 24 h culture of LPS-stimulated macrophages. Priming of macrophages with IFN-gamma and simultaneous triggering with LPS, IC, or AZ produced additive rather than synergistic increases in C1q mRNA accumulation. These same agents inhibited constitutive secretion of C1q in the absence of IFN-gamma priming as determined by autoradiographic analysis of metabolically radiolabeled secretory C1q. Triggering of IFN-gamma primed macrophages with LPS, IC, or AZ also markedly suppressed the increased rate of C1q secretion induced by IFN-gamma in a dose-related fashion. A corresponding dose-dependent increased accumulation of endogenous C1q in cell lysates was detected by Western blot analysis of macrophages which had been stimulated by LPS, IC, or AZ alone or in combination with IFN-gamma. Our findings indicate that LPS as well as FcR and C3bR triggering agents stimulate early and sustained C1q synthesis accompanied by an early and short-lived burst of C1q secretion which rapidly diminished and results in an increased intracellular accumulation of C1q due to ongoing synthesis. IFN-gamma appeared to further amplify the same kinetics of increased C1q mRNA accumulation and decreased extracellular accumulation mediated by LPS, IC, and ZM. Our results suggest that LPS, IC, and AZ alone or in combination with IFN-gamma stimulate early C1q production to modulate macrophage effector functions followed by an inhibition of C1q secretion when the activation process has been culminated.

[The characteristics of Staphylococcus aureus peptidoglycan in the spectrum of the reactivity of human lymphocytes to bacterial peptidoglycans]

The sensitivity of lymphocytes of healthy persons to S. aureus peptidoglycan as compared with that to the polyclonal stimulator zymosan C3b and peptidoglycans of other bacteria (Streptococcus faecalis, Escherichia coli, Bacterium bifidum) was analyzed with a test system permitting the determination of specific reactivity to peptidoglycans. The analysis showed that at the peak of luminol-dependent chemiluminescence (25-30 minutes) individual reactivity to S. aureus peptidoglycan varied within wide limits (the coefficient of lymphocyte stimulation was 1.4-9.6, 3.5 +/- 0.6), exceeding sensitivity to other bacteria, as well as the values obtained in the negative control. The conclusion of the wide spread of sensitization to S. aureus peptidoglycan and the possibility of using this preparation for the study of cell-mediated immunity reactions was made.

Selective inactivation of human neutrophil elastase by synthetic tannin

Tannins of natural or synthetic origin are well-known adjuvants in topical anti-inflammatory therapy of skin diseases. In this study, the influence of synthetic tannin on neutrophil accumulation, enzyme release, and on the proinflammatory activity of neutrophil-derived enzymes was investigated. The results show that synthetic tannin (Tamol) specifically inhibits the neutrophil serine protease human leukocyte elastase (HLE) in an irreversible manner with a half-maximal inhibitory concentration (IC50) of 0.3 microgram/ml. Exogenous protein partially abolished the tannin-dependent HLE inhibition (IC50 of Tamol at 1% protein-concentration:1.0 microgram/ml). Synthetic tannin did not influence the activities of other neutrophil enzymes like Cathepsin G, beta-glucuronidase, and myeloperoxidase. The specificity of Tamol for HLE was further substantiated by the lack of inhibition of other serine proteases. Additionally, Tamol had no effect on f-met-leu-phe-induced neutrophil chemotaxis and did not alter enzyme degranulation of neutrophils in response to f-met-leu-phe and opsonized zymosan. We conclude from our results that the anti-inflammatory properties of synthetic tannin may at least in part be due to inactivation of the proinflammatory protease HLE.

The use of fluorescence quenching in flow cytofluorometry to measure the attachment and ingestion phases in phagocytosis in peripheral blood without prior cell separation

Flow cytofluorimetry identifies and quantifies cell markers of different leukocyte subpopulations by combining cytofluorimetry with the differences in the light scattering properties of the leukocytes in mixed populations. In the phagocytic assay, reported in this paper, the experimental conditions were selected in such a way that it was possible to analyse the phagocytic function of granulocytes in peripheral blood without time-consuming cell separation. The percentage of phagocytosing granulocytes was not dependent on the concentration of granulocytes at the selected incubation time and particle (yeast-C3b) concentration. Furthermore, it was possible to adapt a previously described fluorescence quenching technique (FQ method) to differentiate between attachment and ingestion. Crystal violet, originally used in the FQ method, could not be used in this assay due to its lysomotropic effect. Trypan blue at a concentration of 0.25 mg/ml or higher at pH 4.5 showed a plateau effect in fluorescence quenching indicating an effect on attached but not ingested particles. This assay offers a simple technique to screen the functional properties of phagocytic cells in peripheral blood.

Prostaglandin E2, prostaglandin E1 and thromboxane B2 release from human monocytes treated with C3b or bacterial lipopolysaccharide

C3b or lipopolysaccharide treatment of human peripheral blood monocytes in culture stimulates an early release of thromboxane B2 and a delayed release of prostaglandin E into culture supernatants. Immunoreactive thromboxane B2 release is maximal from 2-8 h, whereas prostaglandin E release is maximal from 16-24 h after stimulation of monocytes in culture. We further examined this process by comparing the time course of labelled prostaglandin E2, prostaglandin E1 and thromboxane B2 release from human monocytes which were pulse or continuously labelled with [3H]arachidonic acid and [14C]eicosatrienoic acid. The release of labelled eicosanoids was compared with the release of immunoreactive prostaglandin E and thromboxane B2. The time course of prostaglandin E2 release was virtually identical to the release of prostaglandin E1 in all culture supernatants regardless of labelling conditions. However, release of immunoreactive prostaglandin E paralleled the release of labelled prostaglandin E1 and E2 only for continuously labelled cultures. The release of labelled prostaglandin E1 and E2 from pulse labelled cultures paralleled the release of thromboxane B2 and not immunoreactive prostaglandin. In contrast, labelled and immunoreactive thromboxane B2, quantitated in the same culture supernatants, demonstrated similar release patterns regardless of labelling conditions. These findings indicate that the differential pattern of prostaglandin E and thromboxane B2 release from human monocytes is not related to a time-dependent shift in the release of prostaglandin E1 relative to prostaglandin E2. Because thromboxane B2 and prostaglandin E2 are produced through cyclooxygenase mediated conversion of arachidonic acid, these results further suggest that prostaglandin E2 and thromboxane B2 are independently metabolized in human monocyte populations.

Inhibition of lymphocyte proliferation by fluid-phase C3b as influenced by macrophages

The effect of fluid-phase C3b on mitogen-induced lymphocyte proliferation in the presence and absence of macrophages was studied. In general, C3b inhibited the proliferation of lymphocytes when monocytes (macrophages) were present. The degree of inhibition by C3b was different for B and T lymphocytes and varied for different subpopulations of lymphocyte classes. In the absence of monocytes (macrophages), there was insignificant inhibition by C3b of lymphocyte proliferation, and thus the observed inhibition appeared to be due to the effect of C3b on the monocytes/macrophages present in the mixed lymphocyte preparations.

Substances that can trigger activation of the alternative pathway of complement have anti-melanoma activity in mice

Intraperitoneal (i.p.) injection of substances that can ignite the alternative pathway of complement, namely isolated human C3b or C3(H2O), guinea-pig C3(H2O) or cobra venom factor, or conventionally prepared zymosan, will reproducibly and very significantly increase the mean survival time of C57BL mice previously inoculated i.p. with melanoma cells. The effect is greater at higher doses and earlier post-inoculation (p.i.) administration, but the substances are active at low doses (30-100 micrograms/mouse) if given early enough. It is likely that C3b or C3(H2O) was the previously unidentified anti-tumour factor activated in serum by S. aureus treatment or serum fractionation and described elsewhere. Activation of the alternative pathway of complement appears to have potential interest for cancer therapy.

Electrophoretic isolation of a membrane-bound NADPH oxidase from guinea-pig polymorphonuclear leukocytes

Electrophoretic isolation of a membrane-bound NADPH oxidase of guinea-pig polymorphonuclear leukocytes was attempted with the O2- -generating membranes of cells unstimulated or stimulated with C3b-zymosan or sodium dodecyl sulfate, and also with the phagosomes isolated from the phorbol myristate acetate-coated latex particle-phagocytosing cells. When these vesicles were subjected to discontinuous polyacrylamide gel electrophoresis in the presence of Triton X-100 and then assayed for NADPH-Nitroblue tetrazolium reducing activity, the activity was detected by the appearance of a single, blue band of the reduced dye on the gel, independent of the source of vesicles. In addition, the enzyme was able to generate O2- and its activity was significantly augmented with the homologous liver microsomal cytochrome b5. Its activity was heat-labile and inactivated by N-ethylmaleimide and p-chloromercuribenzene sulfonate. The enzyme, with an apparent molecular weight of 150 000, in the phagosomes was easily susceptible to limited proteolysis by trypsin and formed an active fragment with a molecular weight of 70 000, accompanying the loss of O2- -generating activity of the vesicles.

Inhibition of neutrophil function by fluid phase C3b of complement

A high-molecular-weight fragment of C3 was isolated from normal human serum by column chromatography, was generated by incubation of serum at 37 degrees C with inulin, and was produced from highly purified C3 by limited digestion with trypsin. This product was shown to inhibit the antibacterial function of neutrophils by using Escherichia coli O75 as the main test organism. The inhibitor reacted with anti-C3b and anti-C3c, but not with anti-C3B (anti-native C3) or anti-C3a. The manner of preparation of the inhibitor, the sodium dodecyl sulfate-polyacrylamide gel electrophoresis pattern, and the amino acid composition of the inhibitor indicated that it was fluid phase C3b. The inhibitor of neutrophil function (fluid phase C3b) was shown to bind to C3b receptors or acceptors on sheep erythrocytes in a model system.

Human monocyte spreading induced by factor Bb of the alternative pathway of complement activation. A possible role for C5 in monocyte spreading

The central serine esterase of the alternative pathway of complement (APC) activation, activated factor B (Bb), has been shown recently to induce murine macrophages and human monocytes to become spread on a glass substrata. It has also been established that to induce the spreading reaction, the catalytic site of the Bb enzyme must be structurally intact since treatment of Bb with heat (56 degrees C for 30 min) or diisopropylfluorophosphate (10(-3) M) destroyed both enzymatic and spreading activities. In the C3b,Bb complex, Bb exhibits restricted substrate specificity for C3 and C5. With this in mind, the role of C3 and C5 in the monocyte spreading reaction was explored in the present study. Expression of C3 and C5 on the surface of human peripheral blood monocytes was investigated by the direct fluorescent antibody technique employing fluorescein isothiocyanate-conjugated anti-C3 or C5 F(ab')2 antibody fragments. It was found that C3 and C5 were present on 6 +/- 7% of freshly prepared monocytes and that expression of C5, but not C3, increased to 70 +/- 6% when monocytes were incubated for 3 d in serum-free medium. Biosynthesis of C5 was indicated when it was found that under serum-free conditions, monocytes incorporated [3H]leucine into immunoprecipitable C5 with an apparent mol wt of 180,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The role of C3 and C5 in the monocyte spreading reaction induced by factor Bb was explored by testing for the ability of anti-C3 and anti-C5 Fab' antibody fragments to block monocyte spreading. It was found that anti-C5 Fab' inhibited by up to 100% the 3-h human monocyte spreading reaction induced by Bb; in contrast, anti-C3 Fab' or anti-C4 Fab' inhibited by less than 10%. That the inhibitory effect of anti-C5 Fab' was exerted directly on the monocyte was established when it was found that the 3-h monocyte spreading reaction was significantly inhibited by pretreating monocytes with anti-C5 Fab' for 20 min and then washing before the addition of Bb. The specificity of the inhibitory effect of anti-C5 Fab' was established by quantitatively absorbing the antibody fragments with polyacrylamide gel-purified C5 antigen: greater than 4 microgram of C5 absorbed by 100% the inhibitory activity of 10-20 microgram of anti-C5 Fab'. That factor Bb exerted its effect on monocytes by interacting directly with cell surface C5 was indicated when it was found that purified C5 inhibited the monocyte spreading reaction induced by Bb; greater than 25 microgram of C5 inhibited by 100% the spreading reaction induced by 3 microgram factor Bb.

Membrane sialic acid on target particles modulates their phagocytosis by a trypsin-sensitive mechanism on human monocytes

Monolayers of human peripheral blood monocytes in the absence of exogenous proteins ingest a variety of natural particulate activators of the human alternative complement pathway. Sheep erythrocytes, which do not ordinarily activate the human alternative complement pathway or initiate a direct monocyte phagocytic response, can be modified to exhibit both functions by the deletion or alteration of membrane sialic acid residues. Enzymatic removal of the sialic acid residues with sialidase or their conversion to heptulosonic acid derivatives by limited oxidation with NaIO4 and reduction with BH4- have equivalent dose-response effects on the capacity of the altered sheep erythrocytes to initiate the phagocytic response by human monocytes or to activate the alternative pathway in human serum. The deposition of C3b on native sheep erythrocytes had little effect on their ingestion by human monocytes, whereas the fixation of C3b on desialated sheep erythrocytes had a synergistic effect on the percentage of monocytes ingesting such a particle. The monocyte receptor essential for ingestion of desialated sheep erythrocytes or desialated sheep erythrocytes bearing C3b was inactivated by concentrations of trypsin that also prevented the monocytes from ingesting natural activators of the human alternative complement pathway, but did not alter the receptors for C3b or the Fc portion of IgG. The capacity of the nonimmune host to respond to desialated particles by initiating the monocyte ingestive process and by activating the alternative complement pathway to provide the synergy afforded by C3b deposition on that particle represents a primitive biochemical basis for differentiation of nonself from self.