Complement
Fifteen
or more serum components constitute the complement system, the sequential activation and assembly into
functional units of which leads to three main effects: release of peptides
active in inflammation (top right); deposition of C3b, a powerful
attachment promoter (or ‘opsonin’) for phagocytosis, on cell membranes
(bottom right); and membrane damage resulting in lysis (bottom left).
Together these make it an important part of the defences against
microorganisms. Deficiencies of some components can predispose to severe
infections, particularly bacterial (see Fig. 33).
The upper half of the figure
represents the serum, or ‘fluid’ phase, the lower half the cell surface, where
activation (indicated by dotted haloes) and assembly largely occur. Activation
of complement can be started either via adaptive or innate immune recognition.
The former pathway is called ‘classic’ (because first described), and is
initiated by the binding of specific antibody of the IgG or IgM class (see Fig.
14) to surface antigens (centre left); the innate, and probably earlier evolutionary
pathways include the ‘alternative’ pathway, in which complement
components are activated by direct interaction with polysaccharides on some
microbial cell surfaces, or by a variety of pattern recognition receptors (PRRs; see Fig. 5) including ‘mannosebinding
lectin (MBL) and C-reactive protein (CRP; centre left). Some of the steps
are dependent on the divalent ions Ca2+ (shaded circles) or Mg2+ (black
circles). A key feature of complement is that it functions via a biochemical cascade:
a single activation event (whether by antibody or via innate pathways) leads to
the production of many downstream events, such as deposition of C3b.
Activation is usually limited to
the immediate vicinity by the very short life of the active products,
and in some cases there are special inactivators (represented here by
scissors). Nevertheless, excessive complement activation can cause unpleasant
side-effects (see Fig. 36).
Note that, in the absence of
antibody, many of the molecules that activate the complement system are
carbohydrate or lipid in nature (e.g.
lipopolysaccharides, mannose), suggesting that the system evolved mainly to
recognize bacterial surfaces via their non-protein features. With the
appearance of antibody in the vertebrates (see Fig. 46), it became possible for
virtually any foreign molecule to activate the system.
For many years this was the only
way in which complement was known to be activated. The essential feature is the
requirement for a specific antigen–antibody interaction, leading via components
C1, C2 and C4 to the formation of a ‘convertase’ which splits C3.
Ig IgM and some subclasses of IgG (in the human,
IgG1–IgG3), when bound to antigen are recognized by Clq to initiate the classic
pathway.
C1 A Ca2+-dependent union of three
components: Clq (MW 400 000), a curious protein with six valencies for Ig
linked by collagen-like fibrils, which activates in turn Clr (MW 170 000) and
C1s (MW 80 000), a serine proteinase that goes on to attack C2 and C4.
C2 (MW 120 000), split by C1s into small (C2b) and
large (C2a) fragments.
C4 (MW 240 000), likewise split into C4a (small)
and C4b (large). C4b then binds to C2, and
also, via a very unusual type of reactive thioester bond, to any local
macromolecule, such as the antigen– antibody complex itself, or to the membrane
in the case of a cell-bound antigen. This tethers the C4bC2 complex forming a
‘C3 convertase’. Note that some complementologists prefer to reverse the names
of C2a and b, so that for both C2 and C4 the ‘a’ peptide is the smaller one.
C3 (MW 180 000), the central component of all
complement reac- tions, split by its convertase into a small (C3a) and a large
(C3b) fragment. Some of the C3b is deposited on the membrane, where it serves
as an attachment site for phagocytic polymorphs and macro- phages, which have
receptors for it; some remains associated with C2a and
C4b, forming a ‘C5 convertase’. Two ‘C3b inactivator’ enzymes rapidly
inactivate C3b, releasing the fragment C3c and leaving membrane bound C3d.
C5 (MW 180 000), split by its convertase into C5a,
a small peptide that, together with C3a (anaphylatoxins), acts on mast cells, polymorphs
and smooth muscle to promote the inflammatory response, and C5b, which
initiates the assembly of C6, 7, 8 and 9 into the membrane damaging or ‘lytic’
unit.
CR Complement receptor. Three types of molecule
that bind different products of C3 breakdown are found on cell surfaces: CR1 is
found on red cells, and is important for the removal of antibody–antigen
complexes from blood; CR1 and CR3 on phagocytic cells, where they act as
opsonins (see Fig. 9); and CR2 on B lymphocytes where it has a role in
enhancing antibody production but is also, unfortunately, the receptor via which the Epstein–Barr virus
(glandular fever) gains entry (see Fig. 27).
The principal features
distinguishing this from the classic pathway are the lack of dependence on
calcium ions and the lack of need for C1, C2 or C4, and therefore for specific
antigen–antibody interaction. Instead, several different molecules can initiate
C3 conversion, notably lipopolysaccharides (LPS) and other bacterial products,
but also including aggregates of some types of antibody such as IgA (see Fig.
20). Essentially, the alternative pathway consists of a continuously ‘ticking
over’ cycle, held in check by control molecules, the effects of which are
counteracted by the various initiators. which are counteracted by the various initiators.
B Factor B (MW 100 000), which complexes with C3b, whether produced via
the classic pathway or the alternative pathway itself. It has both structural
and functional similarities to C2, and both are coded for by genes within the
very important major histocompatibility complex (see Fig. 11). In birds, which
lack C2 and C4, C1 activates factor B.
D Factor D (MW 25 000), an enzyme that acts on
the C3b–B complex to produce the active convertase, referred to in the language
of com- plementologists as C3bBb.
Pr Properdin (MW 220 000), the first isolated
component of the alter- native pathway, once thought to be the actual initiator
but now known merely to stabilize the C3b–B complex so that it can act on
further C3. Thus, more C3b is produced which, with factors B and D, leads in
turn to further C3 conversion, a ‘positive feedback’ loop with great amplifying
potential (but restrained by the C3b inactivators factor H and factor I).
MBL and other pathways
MBL Mannose-binding lectin (also variously referred
to as mannose binding protein or mannan-binding protein), a C1q-like molecule
that recognizes microbial components such as yeast mannan and activates C1r and
C1s, and hence the rest of the classic pathway. MBL deficiency predisposes children
to an increased incidence of some bacterial infections.
CRP C-reactive protein, produced in large amounts
during ‘acute- phase’ responses (see Fig. 7), binds to bacterial
phosphorylcholine and activates C1q.
Lytic pathway
Lysis of cells is probably the
least vital of the complement reactions, but one of the easiest to study. It is
initiated by the splitting of C5 by one of its two convertases: C3b–C2a–C4b
(classic pathway) or C3b– Bb–Pr (alternative pathway). Thereafter the results
are the same, however caused.
C6 (MW 150 000), C7 (MW 140 000) and C8 (MW 150 000) unite
with C5b, one molecule of each, and with
10 or more molecules of C9 (MW 80 000). This ‘membrane attack complex’
is shaped some- what like a cylindrical tube and when inserted into the membrane
of bacteria, red cells, etc. causes leakage of the contents and death by lysis.
Needless to say, some bacteria have evolved various strategies for avoiding
this (see Fig. 29).
Complement inhibitors
In order to prevent over-activation
of the complement cascade, there are numerous inhibitory mechanisms regulating
complement. Some of these, like C1q inhibitor, block the activity of complement
proteinases. Others cleave active complement components into inactive fragments
(factor I). Yet others destabilize the molecular complexes that build up during
complement activation. Genetic manipulation has been used to make pigs carrying
a transgene coding for the human version of one such important regulatory
protein, DAF (decay accelerating factor); results suggest that tissues from
such pigs are less rapidly rejected when transplanted into primates, increasing
the chances of carrying out
successful xenotransplantation (see Fig. 39).
