The activation mechanism of the alternative pathway of the human complement system by the immune precipitate formed with F(ab')2 of rabbit IgG antibody: the generation of C3- and C5-cleaving enzymes on the immune precipitate

Complement C3b contributes to Escherichia coli-induced platelet aggregation in human whole blood

Introduction

Platelets have essential functions as first responders in the immune response to pathogens. Activation and aggregation of platelets in bacterial infections can lead to life-threatening conditions such as arterial thromboembolism or sepsis-associated coagulopathy.

Methods

In this study, we investigated the role of complement in Escherichia coli (E. coli)-induced platelet aggregation in human whole blood, using Multiplate^® aggregometry, flow cytometry, and confocal microscopy.

Results and Discussion

We found that compstatin, which inhibits the cleavage of complement component C3 to its components C3a and C3b, reduced the E. coli-induced platelet aggregation by 42%-76% (p = 0.0417). This C3-dependent aggregation was not C3a-mediated as neither inhibition of C3a using a blocking antibody or a C3a receptor antagonist, nor the addition of purified C3a had any effects. In contrast, a C3b-blocking antibody significantly reduced the E. coli-induced platelet aggregation by 67% (p = 0.0133). We could not detect opsonized C3b on platelets, indicating that the effect of C3 was not dependent on C3b-fragment deposition on platelets. Indeed, inhibition of glycoprotein IIb/IIIa (GPIIb/IIIa) and complement receptor 1 (CR1) showed that these receptors were involved in platelet aggregation. Furthermore, aggregation was more pronounced in hirudin whole blood than in hirudin platelet-rich plasma, indicating that E. coli-induced platelet aggregation involved other blood cells. In conclusion, the E. coli-induced platelet aggregation in human whole blood is partly C3b-dependent, and GPIIb/IIIa and CR1 are also involved in this process.

The Role of Systems Biologic Approach in Cell Signaling and Drug Development Responses-A Mini Review

The immune system is an integral aspect of the human defense system and is primarily responsible for and involved in the communication between the immune cells. It also plays an important role in the protection of the organism from foreign invaders. Recent studies in the literature have described its role in the process of hematopoiesis, lymphocyte recruitment, T cell subset differentiation and inflammation. However, the specific molecular mechanisms underlying these observations remain elusive, impeding the elaborate manipulation of cytokine sequential delivery in tissue repair. Previously, the discovery of new drugs and systems biology went hand in hand; although Systems biology as a term has only originated in the last century. Various new chemicals were tested on the human body, and studied through observation. Animal models replaced humans for initial trials, but the interactions, response, dose and effect between animals and humans could not be directly correlated. Therefore, there is a need to form disease models outside of human subjects to check the effectiveness and response of the newer natural or synthetic chemicals. These emulate human disease conditions wherein the behavior of the chemicals would be similar in the disease model and humans.

Opinion paper: Stimulation of complement amplification or activation of the alternative pathway of complement?

In this opinion paper, we suggest that the scheme of the complement system should be redrawn in order to better illustrate its potencies. This can be achieved by putting the amplification loop of the alternative complement pathway at the center of the complement system. This arrangement emphasizes that C3b molecules, generated by any pathway, can stimulate complement amplification. Furthermore, it allows one to differentiate between this type of stimulation of amplification and that driven by those immune complexes that capture dimeric C3b molecules, which are more potent C3 convertase precursors than C3b. Schemes similar to the one drawn may help to better illustrate the interplay of the pathways and convey a clearer comprehension of the mechanics of the complement system.

Complement amplification revisited

Complement amplification in blood takes place not only on activating surfaces, but in plasma as well, where it is maintained primarily by C3b2-IgG complexes. Regular products of C3 activation in serum, these complexes are inherently very efficient precursors of the alternative pathway C3 convertase. Moreover, they can bind properdin bivalently, thus creating preferred sites for convertase formation. C3b2-IgG complexes have a half-life that is substantially longer than that of free C3b, since both C3b molecules are partially protected from inactivation by factor H and I. These complexes are preferentially generated on certain naturally occurring and induced antibodies that exhibit a paratope-independent affinity for C3/C3b. Such antibodies are known to stimulate alternative complement pathway activation. We have assembled the evidence for the generation and the functional potency of the C3b2-IgG complexes, which have been studied during the last two decades. We illustrate their roles in immune complex solubilization, phagocytosis, immune response, and their ability to initiate devastating effects in ischemia/reperfusion and in aggravating inflammation.

The covalent interaction of C3 with IgG immune complexes

Antigens (Ags) are converted into immune complexes (antigen-antibody complexes, IC) as soon as they encounter their specific antibodies (Abs). In fluids containing complement, the process of IC formation and fixation of complement components occur simultaneously. Hence, the formation of Ag-Ab-complement complexes is the normal way of eliminating Ags from a host. C3b-C3b-IgG covalent complexes are immediately formed on interaction of serum C3 with IgG-IC. These C3b-C3b dimers constitute the core for the assembly of C3/C5-convertase on the IC, which are subsequently converted into iC3b-iC3b-IgG by the complement regulators. These complexes are detected on SDS-PAGE by two bands of molecular composition, C3alpha65-C3alpha43 (band A) and C3alpha65-heavy chain of the Ab (band B), which correspond to C3b-C3b and C3b-IgG covalent interaction respectively, and that identify opsonized IC (C3b-IC). C3b can attach to Fab and Fc regions of the Ab molecule with similar efficiency. The presence of multiple C3b binding regions on IgG is considered an advantageous characteristic that facilitates the elimination of Ags in the form of C3b(n)-IC. Ab molecules on the IC recognize the Ag, and also serve as a very good acceptor for C3b binding. In this way, Ags, even if they have no acceptor sites for C3b, can be efficiently processed and removed. When C3 is activated in serum by IC or other activators, secondary C3b-IgG covalent complexes are generated, with bystander monomeric circulating IgG, and thus constitute, physiological products of complement activation. These complexes gain importance when IgG concentration is extremely high as in cases of infusion of intravenous IgG (IVIG) in several pathologies. The covalent attachment of activated complement C3 (C3b, iC3b, C3 d,g) to Ags or IC links innate and adaptative immunity by targeting Ags to different cells of the immune system (follicular dendritic cells, phagocytes, B cells). Hence C3b marks Ags definitively, from the earliest contact with the innate immune system until their complete elimination from the host.

C3 binds with similar efficiency to Fab and Fc regions of IgG immune aggregates

The covalent binding reaction of the third component of complement (C3) with rabbit IgG immune aggregates has been studied by enzymic digestion of C3b-IgG adducts. In these adducts C3b was radioactively labeled in the free thiol group generated during activation of the internal thioester of C3. Trypsin digestion of 14C-labeled C3b-IgG adducts degrades C3b to a small antibody-bound 14C-labeled C3 fragment (14C-C3frg), whereas the antibody remains unaltered. Papain digestion of trypsin-treated 14C-C3frg-IgG complexes generated Fc and Fab fragments bearing equivalent amounts of covalently bound 14C-C3frg (43% and 40%, of the total C3 present in the aggregates, respectively). Hydroxylamine treatment of the 14C-C3frg-Fab and 14C-C3frg-Fc complexes released a 14C-C3frg of similar size (about 3-4 kDa) in which the N-terminal residue was the radiolabeled Cys1010. A fragment with the same radioactive N terminus and characteristics was obtained by sequential trypsin and papain digestion of purified C3 labeled with iodo-[14C] acetamide. Affinity-purified 14C-C3frg-Fc complexes digested with pepsin generated a mixture of radioactive peptides, most probably complexes formed by 14C-C3frg and C gamma 2 or the hinge digestion products, and 14C-C3frg-pFc' complexes. The latter was also immunoprecipitated with anti-Fc-Sepharose from the pepsin digestion supernatants of 14C-labeled-C3b-IgG complexes. Taken together these data indicate that, during complement activation through the alternative pathway by IgG immune aggregates, C3 is not bound to a single site on the antibody molecule. Both Fab and Fc regions of IgG are equally efficient targets for C3 anchorage. In addition, the data confirm the pFc' as a region of C3 attachment within the Fc portion, and strongly suggest that C3b is bound either to the C gamma 2 domain or the hinge or both.

Formation of covalently linked C3-C3 dimers on IgG immune aggregates

Upon activation of the complement system by IgG immune aggregates several components become tightly bound to the aggregates. The covalent interaction of C3 with immune complexes is essential for the solubilization and inhibition of immune precipitation of the complexes. It has recently been reported that on erythrocytes that have a fixed complement, activated C3 can become involved in the formation of C3b-C3b covalent dimers, which acts as high-affinity binding sites for C5 (Kinoshita, T., Takata, Y., Kozono, H., Takeda, J., Hong, K. and Inoue, K., J. Immunol. 1988 141: 3895). To characterize the molecular composition of immune aggregates that have fixed complement by the alternative pathway, we have investigated whether such C3b-C3b dimers are formed in IgG immune complexes. For this purpose immune aggregates bearing covalently bound C3 were analyzed by two-dimensional gel electrophoresis and the resolved bands transferred to polyvinylidene difluoride membranes and sequenced. When immune aggregates were incubated with serum for 15 min at 37 degrees C, the major high-molecular mass bands detected by gel electrophoresis corresponded to heavy chain-C3 alpha 65 and C3 alpha 65-C3 alpha 43 (derived from iC3b-iC3b-IgG) covalent complexes. If K76COONa, an inhibitor of factor I, was added to the serum, before incubation with the immune complexes, then the major C3 alpha fragment detected on the complexes corresponded to the C3 alpha' chain (105 kDa) and not C3 alpha 65. Hence C3b-C3b covalent dimers are readily formed on the immune aggregates incubated with normal human serum, and are degraded to iC3b-iC3b by factor I. The second C3b molecule was shown to be bound to the C3 alpha 43 region (C-terminal portion of the C3 alpha' chain) of the first C3b molecule, which was itself covalently bound to the heavy chain of IgG. Covalent complexes of heavy chain-(C3 alpha 65)2 molecular composition were also detected, but their precise bonding pattern has not been established.

C3 binds covalently to the C gamma 3 domain of IgG immune aggregates during complement activation by the alternative pathway

Ovalbumin-antiovalbumin IgG immune aggregates were incubated with normal human serum in the presence of iodo[1-14C]acetamide, in conditions in which only the alternative pathway of complement was activated. The [14C]C3b-IgG covalent complexes formed were digested with pepsin, and analysed by SDS/polyacrylamide-gel electrophoresis and fluorography. Covalent complexes of [14C]C3-Fd and [14C]C3-pFc' were visualized, demonstrating that, during complement activation by the alternative pathway, C3 is covalently incorporated into the C gamma 3 domain of IgG, as well as into the Fd region. The C gamma 2 domain becomes protected from pepsin action by the bound C3b. All the covalent linkages between C3 and the IgG were sensitive to hydroxylamine. When [14C]C3-pFc' covalent complexes were treated with 1 M-NH2OH and loaded onto a Bio-Gel P-4 column, a radioactive peak of 3 kDa was obtained. The material released from [14C]C3-pFc' and [14C]C3-F(ab')2 complexes after treatment with 1 M-NH2OH was mixed and analysed in the Bio-Gel P-4 column. A similar radioactive peak of 3 kDa was obtained. When this peak, either from [14C]C3-pFc' alone or from the mixture of [14C]C3-F(ab')2 and [14C]C3-pFc', was fractionated by h.p.l.c., virtually the same radioactive peptide profile was obtained, indicating that very similar C3 peptides remained covalently bound to both regions (Fab and C gamma 3) of the antibody molecule. It is suggested that C3 bound to the C gamma 3 domain of IgG may interfere with the Fc-Fc interactions of immune aggregates and thus may be involved in several biological properties displayed by these complement-activating aggregates.

Immunoglobulin fragments, F(ab')2, that are cytotoxic to enzyme-treated cells

Bivalent immunoglobulin fragments of IgG, F(ab')2, prepared from normal murine sera were found to be cytotoxic to neuraminidase-treated cells. The fragments were cytotoxic to both allogenic and syngeneic targets (with respect to the source of the sera), suggesting that the antigen bound by the F(ab')2 is not related to the major histocompatibility locus of mice (H-2).

Antigen-antibody reactions in the eye involving complement binding IgG antibodies and their non-complement binding F(ab')2 fragments

An immunogenic inflammation was induced in the eyes of inbred Wistar-Furth rats by intravenous injection of anti-human serum albumin IgG antibodies and intravitreal injection of human serum albumin. The ocular inflammation was compared to the findings observed following the intravenous injection of F(ab')2 fragments of anti-human serum albumin IgG antibody and intravitreal injection of human serum albumin. The dose of antigen and IgG immunoglobulin antibody molecules used in the experiments caused a consistent inflammatory response that reached a peak at 24 hours and lasted for approximately seven days. The experimental animals, in which an intraocular reaction of the antigen with F(ab')2 antibody fragments occurred, did not show any immunogenic inflammatory response, indicating an absence of complement activation by either the classical or the alternative pathways.

Activation of the alternative complement pathway by the immune precipitate formed with F(ab')2 fragment of human IgG antibody

The complement fixing ability of the F(ab')2 fragment of human IgG was studied using an immune precipitate (Ippt) formed between tetanus toxoid and the F(ab')2 of high-titer IgG antibody against tetanus toxin. A major subclass of the specific IgG antibody against tetanus toxin, which was separated by affinity column chromatography, was identified as IgG1. On incubation of normal human serum (NHS) with the Ippt formed at equivalence, a dose-dependent consumption of CH50, C3 and C5 activities was observed without significant loss of the early acting complement components. A similar consumption of CH50, C3 and C5 activities was found in NHS reacted with Ippt formed at any antigen/antibody ratio. The Ippt formed at antibody excess was more efficient in complement consumption than the Ippt formed at antigen excess. An apparent consumption of C3 and C5 activities was also noted in C4-deficient guinea pig serum treated with Ippt. When Ippt was incubated with Mg2+--EGTA-treated NHS, both C3 and C5 convertases of the alternative pathway were generated on the Ippt. From these results, it was concluded the the F(ab')2 of human IgG antibody, especially IgG1 antibody, when it formed an Ippt with antigen, could activate the alternative complement pathway.

In-vivo phagocytosis: enhancement of bacterial clearance by native and enzyme-treated immunoglobulins

The enhancement of bacterial blood clearance in mice by native and enzymatically derived fractions of rabbit anti-E. coli hyperimmune serum was tested. Native immune serum, the corresponding IgG, F(ab')2 and Facb fractions strongly augmented the phagocytosis rate of bacteria, whereas Fab/Fc, Fab, Fc fragments, corresponding preparations from normal serum, and E. coli-absorbed preparations showed no marked enhancing capacity. In one experiment opsonization by IgM was demonstrable. Mice that were injected with fatal doses of bacteria could be protected by subsequent treatment with IgG or F(ab')2 preparations of rabbit anti-E. coli serum. Conclusion is drawn that the Fc region of the IgG molecule is not predominantly responsible for opsonized clearance while the intact divalent antigen cross-linking F(ab')2 fragment - presumably by virtue of complement activation via the alternate pathway - could mediate enhanced bacterial clearance.

The binding of complement component C3 to antibody-antigen aggregates after activation of the alternative pathway in human serum

Preformed immune aggregates, containing antigen and either IgG (immunoglobulin G) or F(ab')2 rabbit antibody, were incubated with normal human serum under conditions allowing activation of only the alternative pathway of complement. Both the IgG and F(ab')2 immune aggregates bound C3b, the activated form of the complement component C3, in a similar manner, 2-3% of the C3 available in the serum being bound to the aggregates as C3b, and the rest remaining in the fluid phase as inactive C3b or uncleaved C3. It was found that the C3b was probably covalently bound to the IgG in the aggregates, since C3b-IgG complexes could be demonstrated on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, after repeated washing with buffers containing high salt or boiling under denaturing conditions. Incubation of the C3b-antibody-antigen aggregates in buffers known to destroy ester linkages had little effect on the C3b-IgG complexes, which suggested that C3b and IgG might be linked by an amide bond. Two main types of C3b-IgG complexes were found that had apparent mol.wts. of 360000 and 580000, corresponding to either one to two C3b molecules respectively bound to one molecule of antibody. On reduction of the C3b-IgG complexes it was found that the beta-chain, but not the alpha'-chain, of C3b was released along with all the light chain of IgG but only about half or less of the heavy chain of IgG. These results indicate that, during activation of the alternative pathway of complement by immune aggregates containing IgG antibody, the alpha'-chain of C3b may become covalently bound at one or two sites in the Fd portion of the heavy chain of IgG.

The separation of functionally distinct forms of the third component of human complement (C3)

Complement component C3 prepared by the method of Tack & Prahl [(1976) Biochemistry 15, 4513-4521] was found to contain the following trace contaminants: C3b, haemolytically inactive C3 with intact alpha- and beta-chains (C3u) and degraded C3 (apparent mol.wt. 140000) with an intact beta-chain but with a fragmented alpha-chain. The proportion of C3u in the C3 is increased on standing and by freezing and thawing. These contaminants could be separated from each other and from native C3 by chromatography on sulphated Sepharose. They have been characterized by their susceptibility to C3b inactivator in the presence of beta 1H, their ability to be cleaved by C3 convertase and their ability to form alternative-pathway C3 convertase in solution. Incubation of C3b or C3u with beta 1H and C3b inactivator resulted in cleavage of the C3 species; the alpha'-chain of C3b was cleaved to fragments of apparent mol.wts. 67000 and 43000, the alpha-chain of C3u was cleaved to fragments of apparent mol.wt. 75000 and 43000. Native C3 and degraded C3 were unaffected by incubation with beta 1H and C3b inactivator. C3u, unlike C3, was not cleaved to C3b by the classical- or alternative-pathway C3 convertase in solution. When C3b or C3 was incubated with factors B and D, forming C3 convertase, the initial rate of factor-B cleavage was several order of magnitude lower in the presence of C3 than in the presence of C3b. The slow rate observed for C3 could be decreased by preincubation with beta 1H and C3b inactivator or by rechromatography of the C3. The degraded C3 did not support factor-B cleavage by factor D.

The activation mechanism of human complement system by immune precipitate formed with rabbit IgG antibody

Immune precipitate (Ippt) formed between egg albumin and rabbit IgG antibody activated both pathways of the human complement system. On incubation with diluted serum, Ippt combined with several factors in the serum to form a complex which acquired C3- and C5-cleaving activities. In serum chelated with ethyleneglycol tetraacetic acid (EGTA), C3- and C5-cleaving activities of properdin system enzymes were formed on Ippt. Kinetic studies on the formation and the decay of C3- and C5-cleaving enzymes on Ippt revealed that C3- and C5-cleaving activites were almost dependent on the properdin system enzymes. The experiments in which C3-cleaving activity formed on Ippt was inhibited by anti-properdin or anti-B but not by anti-C4 supported the above results. The participation of the classical pathway was considered to accelerate the assembly of the properdin system enzymes.