Download Biological functions of the complement system

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C O M P L E M E N T SYSTEM
Proteins of the Complement System
Society/Host/Peptide and Protein Group Joint Colloquium Organized and Edited by J. E. Fothergill
(University of Aberdeen) and K. B. M. Reid (University of Oxford)
Biological functions of the complement system
PETER J. LACHMANN
Molecirlur Immirrioputhology Unit, Medical Keseurch C'oirticil
('etitrc, Iiills R o d , C'urnhridge C'R2 2QII. U.K.
The biological firnctiori.~o f complement
T h e functions o f the complement system can be separated
into those of the MAC and those of the earlier steps of the
complement system. T h e functions of the MAC are unique
among the effector systems o f blood plasma. T h e conI t I trodirctiorI
sequence of the assembly of the MAC o n a membrane is to
T h e phenomenology of the complement system was quite make the membrane leak. This is associated with channel
extensively worked out by the early IY4Os when the four formation although the extent to which the membrane leak is '
classical complement components. C'l, C'4. C'3 and C'2, had due to the formation of a 'leaky patch' or of a true proteinbeen described as had the 'anlage' of the observations that lined transmembrane channel has never been fully resolved
there were multiple pathways of activation. In the lY6Os, the in spite o f many years of increasingly sophisticated investigacomplement components were purified as proteins and as tion.
antigens. T h e biochemistry of their interactions began to be
In erythrocytes the consequence of 'membrane attack' is
worked o u t and it became recognized that a number of t o lyse the cell. Lysis is largely, though not wholly, dependent
components were proteases.
upon the incorporation o f C9 and its subsequent unfolding
T h e first great surprise that this wave of work produced and insertion followed frequently by its circular polymerizawas the high concentration at which complement compo- tion to form the characteristic torus of poly-C9 which gives
nents occur in the serum. T h e one *hit' theory o f complement rise to the characteristic electron microscopic appearance of
lysis, put forward by Manfred Mayer and his group [ I ] , had complement lysis. A n entirely analogous lytic process is
until that time been interpreted in molecular terms as if the brought about by the MAC o n viruses that have lipoprotein
functional 'hits' represented individual protein molecules. membranes, although the lysis of viruses is accompanied by a
This had given rise t o the idea that the concentration of marked 'disassembly' o f the membrane. This presumably
complement in the blood was trivial and had discouraged reflects the lack o f cytoskeleton in the virus to maintain the
attempts at purification. It now became clear that this morphological integrity after lysis. Insertion of the MAC into
interpretation was wrong and that complement forms a bacteria will kill a range o f bacteria. T h e situation here is
significant part o f the plasma protein pool probably making substantially more complicated than on red cells since
up in total about S mg per 100 ml. T h e second surprise was bacteria have a cell wall as well as a plasma membrane
that there werc far more complement components than had (which is the essential layer damaged by complement) and
been previously known about. Fractionation showed that C I complement requires both to have access to the plasma
was made u p of three sub-components and that the compo- membrane and t o be able t o get inserted into it.
nent that had been called 'C3' was made u p of six separate
O n nucleated cells the actions o f the MAC arc also more
components: C3 itself and the five components of what is complex than on red blood cells. It has become apparent that
now known as the membrane attack complex (MAC): CS,
whereas an adequately rapid and intense complement attack
C6. C7, CX and CY.
does indeed produce lysis, less rapid formation of the
Then it was discovered that the alternative pathway of channels can be repaired by the cell and the effects of subcomplement activation had several components unique to lethal complement damage may activate the cell. It has been
itself notably the C2-like component. factor €3; and the C I - shown for polymorphonuclear leukocytes by Morgan er (11.
like component, factor D; as well as properdin. the original [ 61 that these temporary channels are, to some extent, selecmolecule discovered by Pillemer and his colleagues [ 2, 3 I.
tive for calcium and activate the cells by allowing calcium
It was also established that the complement system is quite entry into the cell. Similarly, it is possible that the activation
distinct from other plasma triggered enzyme cascades. Thus of calcium dependent enzymes may produce slow damage of
deficiencies of complement components d o not give rise to the cell. I t has further bccn demonstrated that MAC activamajor disturbances o f the kinin-generating fibrinolytic o r tion of nucleated cells gives rise t o liberation o f fatty acids
coagulation systems and tice t'ersu. T h e major exception t o from phospholipids and stimulates arachidonic acid metathis generalization is C 1 inhibitor, an essential control bolism.
molecule for complement but also an inhibitor of kallikrein,
One characteristic and unusual feature of complement
plasmin and certain clotting enzymes. T h e seminal discovery lysis is that it is subject to 'homologous restriction': complcin I YO2 by Landerman et (11.141and in 1963 by Donaldson ment from one species being inefficient in lysing cells o f the
& Evans [ S ]that deficiency o f this inhibitor gave rise t o the same species. Although this phenomenon has long been
disease known as hereditary angioedema. gave rise to a new known. it is only in recent years that it has been discovered
field o f investigation - which will be discussed later in thc that it is d u e to the presence o f specific membrane proteins
colloquium.
that interact with the terminal components. I will be discussAhhrcviations used: MAC. membrane attack complex; EBV, ing this phenomenon further at the end o f the colloquium
Epstein-Ihrr Virus.
(p. I I S Y ) .
Vol. I x
BIOCHEMICAL SOCIETY TRANSACTIONS
1144
It seems likely that the physiological action of the MAC is
largely that exerted on invading micro-organisms; and that
the effects o n mammalian cells and in particular in homologous mammalian cells are largely pathological.
Deficiency of the components of the MAC in man gives
rise to an increased susceptibility to infection with just one
family of micro-organisms, the Neisseria. Recurrent attacks
of mcningococcal meningitis allows deficiencies of the
terminal components to be ascertained. In the United States
and in Europe these deficiencies are all strikingly rare in
contrast with some third world countries where they are
much commoner. Thus, C6 deficiency is relatively common
among the coloured population in Cape Town [7] and C7
deficiency among North African and Scphardic Jews in
Israel 181.It would appear that there must be some selective
advantage maintaining the deficiency genes in such populations and this is borne out in the South African studv bv the,
observations that the homozygotes in individual sibships
occur at a frequency much closer to 0 . 5 than the expected
0.25. Although the nature of this selective advantage has not
been demonstrated convincingly it seems likely to be due to
the protection that deficiency of the MAC affords against
endotoxin shock produced by Gram-negative organisms.
Such protection was demonstrated many years ago in C6deficient rabbits 191. Although the contribution that the
complement system makes to endotoxin shock varies from
species t o species, it seems likely that in human populations
where deaths from shock following infantile gastroenteritis
are common, deficiency in the terminal components may
contribute t o survival. The pathological role of the MAC in
endotoxin shock appears to be related t o its capacity to
damage platelets and possibly t o generate cytokine production in cells that have been activated by non-lethal complement attack.
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Pro1eo1ysls 01 Ag
antibody formation
NO
Anfig*n
Release 01 auIMnliwnr
Cylokirvlr
(DNA- inhlbllor
Aclin)
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Fig. 1 . ScliettiriJi)rrhe huridliti~c ~ / ' i m r t i i r r i ci~orrrp1iixes
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(hmplemetir us at1 itiflumttiurory mediuror
The role o f complement in enhancing inflammation is a
function o f the early steps of the complement system
culminating in the cleavage of CS. The molecular mechanisms appear t o be two-fold: the actions o f the so-called anaphylatoxins CSa, C3a and, t o a much lesser extent, C4a; and
the reaction o f the covalently-bound complement fragments
C3b, C4b and their breakdown products with the complement receptors CRI, CR2, CR3 and CR4. The anaphylatoxins arc discussed by Dr Hugli (p. 1 154). CSa is the most
strikingly active and is a chemotactic factor as well as directly
activating a variety of cells and causing the degranulation of
mast cells and having a direct effect on their microvasculature. It remains a puzzle that in spite of the dramatic biological activities of C5a iti v i m , it is difficult to ascribe
clinical effects to CS deficiency that are different from those
seen in C6 o r C7 deficiency.
The interactions o f membrane bound C3b and C4b with
complement receptors have been studied extensively in
recent years. Two groups of molecules have such receptor
activity. CR I and CR2 are 'necklaces' of the 'short consensus
repeat' and it is interesting that their ligand-binding properties are somewhat different. CRI is essentially a C3b and
C4b receptor with weaker binding of iC3b. CR2 binds
primarily C3d,g and, more weakly, iC3b. CR2 is confined
essentially to cells o f the B lymphocyte series and its role lies
in the regulation of the immune response. CR1 is a widespread receptor and is of interest in being the only complement receptor on primate erythrocytes. CR 1 has cofactor
activity for factor I, the enzyme that splits C3b first to iC3b
and subsequently to C3d,g. For this important second split,
CRI is thc only physiological cofactor. Although C R l is a
good adherence receptor its role in phagocytosis is limited.
The other family of complement rcccptors (of which CR3
is the best studied). bclongs to the 8 2 intcgrins. a group of
molecules containing, besides CR3 and CR4, the lymphocyte
marker LFA- 1. These molecules share a common B-chain,
and a group of closely linked a-chains. The importance of
these receptors is demonstrated by the conscquenccs of their
genetic absence - the leukocyte adhesion deficiency
syndrome. Children with this deficiency suffer from delayed
separation o f the umbilical cord, hypertrophied gums. a
tendency t o have indolent staphylococcal skin infections
which heal with 'paper thin' scars and they often succumb to
staphylococcal septicacmias. The prognosis is more sinister
than was originally thought s o that affected children arc now
regarded as suitable subjects for bone-marrow transplantation. The major reason for the lack o f inflammatory response
is that polymorph or monocyte emigration into inflammatory
sites fails to occur. This may be related t o the lack o f binding,
not to the complement ligand, but rather t o ligands o n
endothelial cells. The ligand for LFA-I (I-CAMI) is now
well known and there may be yet other ligands for CR3.
There is now great intcrest in the usc of antibodies t o these
molecules, both to CR3 and to LFA-I and t o the ligand
I-CAM I , in preventing inflammatory injury in non-infective
situations, for example transplantation and reperfusion
injury. CR3 as a complement receptor is highly specific f o r
iC3b. Part of this binding is associated with a carbohydratebinding site o n CR3 which also binds @-glucan.Thc binding
site is complex and P-glucan does not inhibit iC3b rosctting,
although iC3b does inhibit the binding o f the glucan o r o f
zymosan. Both bindings can also be inhibited by adequate
concentrations o f N-acetyl D-glucosamine. It has bccn
claimed that there is an 'RGD'-binding site in CR3 but this
has not been widely accepted.
The combination o f zymosan with CR3 ;IS a powerful
stimulant of the respiratory burst and CR3 is believed t o bc
an important inflammatory receptor.
C'omplemetir arid rhe proper hatidlirig of immirtir cwnplcJxcJs
A major paradox in complement function derives from the
parallel observations that, o n the one hand, complement is
necessary for the production o f immune complex inflammation at all extra-renal sites while, o n the other hand, genetic
deficiencies o f the early components o f the classical pathway
have, as their major clinical association. autoimmune
immune complex disease. This association has concentrated
attelltion on the role that an intact complemeiit system has in
the handling o f immune complexes. Two major functions
have been identified. One is that complemcnt has an
important influence on immune complex size and causes
C O M P L E M E N T SYSTEM
immune complexes to be formed that arc much smaller and
more soluble than those formed in its absence. Very small
complcxes are not able to generate immune complex disease
and the production of more small complexes in itself
probably reduces their pathogenicity. Complexes that are
potentially phlogistic are those of intermediate size and it
appears to be of considerable importance that these are
transported to the liver where they are removed by the
Kuppfer cells which d o not function as antigen-presenting
cells since there is n o lymphatic tissue in the liver. It seems
that transport to the liver occurs optimally for immune
complexes bound to erythrocyte C R 1 presumably because
such erythrocyte-bound complexes d o not come into contact
with the endothelium o f blood vessels and therefore are less
likely to contact vascular endothelium where they can give
rise t o inflammation and feedback antibody formation. This
scheme is shown in Fig. 1 and has been discussed in greater
detail elsewhere by Lachmann [ 101.
I t does appear to be one of the major activities of the
complement system in vivo t o prevent the generation of
immunc complex disease and it is noteworthy that to bc
dcficient in C1 and C4 is a more or less sufficient predispossession to get these diseases. While the great majority
o f patients with autoimmune immune complex disease arc
not homozygous for complcmcnt deficiency it has been
shown that over half of them carry a null allele at the C 4 A
locus. C 4 A is the isotype of C4 which binds to immune
complexes.
7'1ic role of cwmplernerit in tlie iridirctiori of tlic iriirnirrie
respor i s e
This is a topic that has excited much interest since it was
dcmonstratcd by Pepys [ 1 I ] that cobra vcnom treatment of
mice reduced their antibody formation t o sheep erythrocytes
and since it was recognized that C R 2 is identical to the
Epstein Barr Virus ( E B V ) rcccptor. EBV is the most potent
B cell stimulator in man.
Nevertheless it has proved difficult to define the exact role
o f complcmcnt in thc induction of immune responscs. It has
been demonstrated that in guinca-pigs deficient in C4. C2
and particularly in C3. there is difficulty in making antibodies t o low concentrations o f antigen in the absence of
efficient adjuvants but that the difficulty can gcncrally be
overcome by increasing antigen dose o r using powerful
adjuvants. In complement deficient humans there is generally
1145
no clinically significant antibody deficiency and overall
immunoglobin levels are normal. It has however more
recently been shown that lgG4 levels are low in patients with
total deficiencies of C1, C 4 , C2 and C3. Since IgG4 is the
last IgG isotype t o be made in an immune response, this may
reflect a difficulty in generating efficient B cell memory.
Of the various explanations advanced for how complement influences antibody formation, two are now considered
to be important. T h e first is the observation made originally
by Klaus & Humphrey [ 121, that it requires both antibody
and complement to allow the localization o f antigen on the
follicular dendritic cells in the germinal centres of lymphoid
follicles and that this antigen localization seems to be necessary for the development of efficient B cell memory. T h c
other mechanism is the activation of B cells via CR2. This
can be demonstrated with aggregated C3d,g as the ligand (as
well as with E B V ) and some antibodies to CR2 also have a
stimulating effect in vitro. However, patients deficient in
Factor I, who cannot generate C3d.g, have no defect in antibody formation. Also those patients with nephritic factor
who have high circulating levels of (monomeric) C 3 d g
present continuously also make normal antibodies.
1 . Mayer. M. M. ( I96 1 ) in Immrinoc~hernicciIA p p r o d i e s t o l'rohlcws in Microbiology (Heidelherger, M. & Plescia. O., eds.),
pp. 268-279. Kutgers University Press, New 13runswick
2. Pillemer, L.. Blum, L., Lepow, 1. H., Ross, 0.A,, Todd, E. W. &
Wardlaw, A. C. ( 1954) Science 120.279-285
3. Pillemer, L.. Schoenherg, M. D., Wum, L. 6i Wurz. L. (lY54)
Scicvice 122. 545-549
4. Landerman. N. S.. Wehster. M. E.. Becker. E. L. & Katcliffe,
H. E. (1962)J . Allergj 33,330-341
5. 1)onaldson. V. H. & Evans, R. R. ( 1963)Am. J. Mcd. 35. 37-44
6 . Morgan, B. P.. Luzio. J. P. & Campbell, A. K. ( I Y X h ) ('c,ll
('ctlcirtm I . 399-41 I
7. Orren, A,, Potter, P. C., Cooper, K. C. & du l o i t , E. ( 1987)
Immitnologj~62.249-253
8. Schlesinger. M.. Nave. Z.. Levy, Y.. Slater. P. E. 6i Fishelson. Z.
(1990) <'/in. Lvp. Immtrnol. 81. 423-427
9. Brown. D. L. & Lachmann. P. J. ( I Y73) I t i t . Arch. ~ l l ~ ~ ~ ~ ~ ~ l p p l ,
Immrtnol. 45. 193-205
10. Lachmann, P. J. ( 1990) ('liem. I m m r i r i o l . 49, 245-263
I 1. Pepys. M. B. ( 1972) Ntrtiirc, (Loticloti) New Niol. 231. 157- I59
12. Klaus, G. G. B. & Humphrey. J. H. (1977) I m m i i n o l o ~33,
~
3 1-30
Received 1 August I990
Interaction of Clq, and other proteins containing collagen-like domains, with the C l q receptor
RAJNEZESH MALHOTRA, R O B E R T B. SIM and
KENNETH B. M. REID*
M. K.C'. Itiir~iirriochemistr?,Unit, Ileppcrrtmerit of Hiochcrnistn~.
Osfbrd Uni\ier.sity. South Parks Road. Oxford 0x1 3QU,
U .K.
Human C l q (460 kDa) is composed of 18 polypeptide
chains ( 6 A . 6 B and 6C) cach o f which is approx. 225 amino
acid residues long and is composed of a short N-terminal
non-collagen-like region ( 3 t o 9 residues) followed by a
collogcn-like region of approx. 8 1 residues and a C-tcrminal
Ahhrcviations uzed: MHP. monnan hinding protein: SP-A.
xurfnctant protein A; SP-I). surfactant protein I>.
*l'o whom correspondence should he addreszed.
VOl. 1 8
non-collagen-like region o f approx. 136 residues I I ] . T h e 18
chains form a molecule which is seen in the electron
microscope t o have six globular heads linked through six
collagen-like stalks to a fibril-like region 12, 31. This hexameric structure results from the formation of six triple-helical
regions. from thc 8 1-residue-long collagen-like scquenccs
present in the 6A-. 6B- and 6C-chains which are seen as the
central-fibril like region and stalks, while cach o f the six globular heads is formed by association o f the ('-terminal regions
o f one A-, one B- and one C-chain 14. 51. T h c complcte
amino acid sequences o f A-, B- and C-chains o f C I q have
been established by: protein sequencing (the complete A- and
B-chains and collagen-like region of the C-chain 16-81;
sequencing o f c D N A (the complete B-chain and part o f the
A-chain 19, 101, G. C. Scllnr, D. J. Blake XL K. B. M. Reid.
unpublished work); sequencing o f genomic D N A (the com-