232
6 Immunoproteins
6.6 The Complement System
With its 20 or so proteins, the complement system
plays an effective part in the defence against microorganisms by causing partial destruction of the
cell membrane and, thereby, cell lysis. The activation of the complement system can occur in two
reaction cascades, the "classical" and the "alternative", in each step of which a zymogen is proteolytically converted to an active protease. The
enzymatically active C-terminal fragment is
designated by addition of the suffix "b" to the
complement component number, and the cleaved
N-terminal partial peptide by addition of the suffix "a". The specificity of complement action is
maintained through the triggering of the activation cascade, either via the classical pathway by
antigen-bound immunoglobulin or via the alternative pathway by the lipopolysaccharides of the
bacterial cell wall. The classical pathway results in
amplification of the immune reaction, and the
alternative pathway can be initiated before the
immune reaction. The complement system is
remarkable in that its lytic activity requires the
association of five different proteins [98, 120].
Three phases can be distinguished in the classical pathway and these occur at different sites on
the membrane. In the first phase, a recognition
complex is formed on antigen-bound antibodies
by the accumulation and proteolytic activation of
the components Clq, Clr and Cls. Only certain
immunoglobulin classes or subclasses are able to
bind complement: in man these are IgGl, IgG3
and IgM; in the mouse IgG2a and IgM; in the
guinea-pig IgG2; and in ruminants IgG 1
[98, 120]. The Cl complex contains one molecule
of the hexameric Clq and two molecules each of
Clr and Cls. The Cl complex is activated by
interaction with the Fc regions of several Ig molecules; in the process a bond is cleaved at about
one-third of the length in each of the zymogens
Clr and Cls. The smaller N-terminal cleavage
fragment remains bound to the enzymatically active C-terminal fragment by a disulphide bridge
[103]. The second phase is initiated when the activated Cls produces the fragments C4b and C2b
from C4 and C2; these become enzymatically active (C3 convertase) after binding to a neighbouring binding site on the membrane. The cleavage
fragment C3b, produced from C3, binds to the
C4b,2b complex and modifies its activity to that
of a C5 convertase. The fragment C5b, produced
from C5, accumulates at a third membrane site
and becomes associated with one molecule each
of C6, C7 and C8. Binding of several (up to 16)
C9 molecules results in the formation of a channel structure which perforates the membrane
[65].
In the alternative pathway, the C5 convertase is
formed via a feedback loop involving bound C3b,
factor B, factor D and free C3. C3b, bound to
lipopolysaccharide of the bacterial cell wall or
another activator, forms an inactive C3b,B complex with factor B. Cleavage of B by factor D
results in the complex C3b,Bb, which as an active
C3 convertase produces more C3b. Finally, a further C3b binds to the C3b,Bb complex; the
resulting C3b,Bb,C3b complex is the C5 convertase of the alternative pathway. This produces
C5b and thereby allows the formation of the
membrane-attacking complex [98, 120]. Because
of its C3 convertase activity, the C3b,Bb complex
represents a positive feedback system which is
regulated by factors H, I and P (properdin). Factor H binds C3b, displacing the Bb subunit and
making C3b accessible for proteolytic inactivation by factor I. ClInh and the C4b-binding protein function as inhibitors of the classical pathway; the former inhibits interaction of Cl with
the immune complexes. The decay-accelerating
factor (DAF), an inhibitor in both pathways, is a
membrane protein found on almost all cells of the
body, and together with factor I it proteolytically
inactivates C3b and C4b [70, 148].
The proteins of the complement system are
well known in man and several rodents (Table 6.2). Many have been sequenced via their
cDNA, and therefore conclusions can be drawn
about their relationships to each other and to
other proteins. The subunits of the hexameric
Clq molecule are made up in each case of three
proline-rich polypeptide chains; the subunits
together form a stem with a collagen-like structure and a head, which interacts with the Fc
region of immunoglobulins. Clr, Cls, C2, C6,
factor B and factor D are serine proteases. Clr
and Cls are very similar in their domain structures and agree in about 40 % of their amino acids.
The domains I and III are apparently the products of a gene duplication; domain II corresponds
to the sequence of the EGF; domains IV and V
resemble a repeated sequence of about 60 amino
acids which has also been detected in other complement components and other proteins, e.g. C2,
factor H and its homologues CR1, C4b-binding
protein and DAF, factor B, the Bz-glycoprotein I
and the IL-2 receptor [148]. The C-terminal catalytic domains of Clr and Cls are typical serine
proteases [103, 189]. The sequences of C2 (732
6 Immunoproteins
6.6 The Complement System
With its 20 or so proteins, the complement system
plays an effective part in the defence against microorganisms by causing partial destruction of the
cell membrane and, thereby, cell lysis. The activation of the complement system can occur in two
reaction cascades, the "classical" and the "alternative", in each step of which a zymogen is proteolytically converted to an active protease. The
enzymatically active C-terminal fragment is
designated by addition of the suffix "b" to the
complement component number, and the cleaved
N-terminal partial peptide by addition of the suffix "a". The specificity of complement action is
maintained through the triggering of the activation cascade, either via the classical pathway by
antigen-bound immunoglobulin or via the alternative pathway by the lipopolysaccharides of the
bacterial cell wall. The classical pathway results in
amplification of the immune reaction, and the
alternative pathway can be initiated before the
immune reaction. The complement system is
remarkable in that its lytic activity requires the
association of five different proteins [98, 120].
Three phases can be distinguished in the classical pathway and these occur at different sites on
the membrane. In the first phase, a recognition
complex is formed on antigen-bound antibodies
by the accumulation and proteolytic activation of
the components Clq, Clr and Cls. Only certain
immunoglobulin classes or subclasses are able to
bind complement: in man these are IgGl, IgG3
and IgM; in the mouse IgG2a and IgM; in the
guinea-pig IgG2; and in ruminants IgG 1
[98, 120]. The Cl complex contains one molecule
of the hexameric Clq and two molecules each of
Clr and Cls. The Cl complex is activated by
interaction with the Fc regions of several Ig molecules; in the process a bond is cleaved at about
one-third of the length in each of the zymogens
Clr and Cls. The smaller N-terminal cleavage
fragment remains bound to the enzymatically active C-terminal fragment by a disulphide bridge
[103]. The second phase is initiated when the activated Cls produces the fragments C4b and C2b
from C4 and C2; these become enzymatically active (C3 convertase) after binding to a neighbouring binding site on the membrane. The cleavage
fragment C3b, produced from C3, binds to the
C4b,2b complex and modifies its activity to that
of a C5 convertase. The fragment C5b, produced
from C5, accumulates at a third membrane site
and becomes associated with one molecule each
of C6, C7 and C8. Binding of several (up to 16)
C9 molecules results in the formation of a channel structure which perforates the membrane
[65].
In the alternative pathway, the C5 convertase is
formed via a feedback loop involving bound C3b,
factor B, factor D and free C3. C3b, bound to
lipopolysaccharide of the bacterial cell wall or
another activator, forms an inactive C3b,B complex with factor B. Cleavage of B by factor D
results in the complex C3b,Bb, which as an active
C3 convertase produces more C3b. Finally, a further C3b binds to the C3b,Bb complex; the
resulting C3b,Bb,C3b complex is the C5 convertase of the alternative pathway. This produces
C5b and thereby allows the formation of the
membrane-attacking complex [98, 120]. Because
of its C3 convertase activity, the C3b,Bb complex
represents a positive feedback system which is
regulated by factors H, I and P (properdin). Factor H binds C3b, displacing the Bb subunit and
making C3b accessible for proteolytic inactivation by factor I. ClInh and the C4b-binding protein function as inhibitors of the classical pathway; the former inhibits interaction of Cl with
the immune complexes. The decay-accelerating
factor (DAF), an inhibitor in both pathways, is a
membrane protein found on almost all cells of the
body, and together with factor I it proteolytically
inactivates C3b and C4b [70, 148].
The proteins of the complement system are
well known in man and several rodents (Table 6.2). Many have been sequenced via their
cDNA, and therefore conclusions can be drawn
about their relationships to each other and to
other proteins. The subunits of the hexameric
Clq molecule are made up in each case of three
proline-rich polypeptide chains; the subunits
together form a stem with a collagen-like structure and a head, which interacts with the Fc
region of immunoglobulins. Clr, Cls, C2, C6,
factor B and factor D are serine proteases. Clr
and Cls are very similar in their domain structures and agree in about 40 % of their amino acids.
The domains I and III are apparently the products of a gene duplication; domain II corresponds
to the sequence of the EGF; domains IV and V
resemble a repeated sequence of about 60 amino
acids which has also been detected in other complement components and other proteins, e.g. C2,
factor H and its homologues CR1, C4b-binding
protein and DAF, factor B, the Bz-glycoprotein I
and the IL-2 receptor [148]. The C-terminal catalytic domains of Clr and Cls are typical serine
proteases [103, 189]. The sequences of C2 (732