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IMMUNOGLOBULIN/FIBRONECTIN
SUPERFAMILY
One of the first problems that animals had
to solve was how to be multicellular. In a multicellular
organism, cells must interact with each other.
Once primitive animal cells began to express the immunoglobulin
domain on their cell surfaces, other cells expressing the same (or slightly
modified) domains could interact with them.
As vertebrate animals began to live longer (and put off reproduction
until later in life), the issue of distinguishing between one’s own cells
and foreign cells became ever more important and important families of
immunoglobulins within the immunoglobulin superfamily
evolved such as MHC proteins, antibodies,
and T-Cell receptors. A very early duplication of ancestral immunoglobulin/fibronectin
molecules produced fibronectin and immunoglobulin families. The functional domains of each of these molecules
became incorporated into a variety of multi-domain proteins which can
have single or multiple fibronectin domains
and single or multiple immunoglobulin domains.
There are many molecules (such as the CAMs,
contactin, nephrin, myomesin, MERTK, PUNC, TIE2, ROBO1, CRLF1, contactin3) which
contain both fibronectin and immunoglobulin
domains (depicted in green below). |
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A number of proteins in the fibronectin branch of this superfamily
have been adapted to complex immune reactions.
Macrophage mannose receptors allow macrophages to recognize carbohydrates
and lymphocyte antigen 75, which is expressed in a number of tissues,
is used in antigen presentation by dendritic
cells (Online Mendelian Inheritance in Man). The immunoglobulin branch of the immunoglobulin/fibronectin
superfamily includes genes known in both vertebrates
and invertebrates, including the simplest invertebrates.
An immunoglobulin-like domain is present in the extracellular
part of the receptor tyrosine kinase in some
sponges (Schacke, 1994). Two Ig-like
molecules are known from fruit flies, another from squid, and from sponges
(the C2 set of Ig domains) (Schacke,
1994a). Immunoglobulins
perform a variety of functions in the human body including roles in the
nervous system (such NCAM, the neural cell adhesion molecule, homologs
of which exist in insects and the mollusk Aplysia) and the placenta (such
as the family of pregnancy specific glycoproteins,
PSG, whose mutations can cause complications in pregnancy). Some immunoglobulin genes are expressed by leukocytes
and other cells as well. PECAM,
for example, is expressed in plateletes, monocytes,
neutrophils, and some T cells and can interact
with collagen. It is also expressed
in endothelial cell junctions which may contain about a
million molecules of PECAM (OMIM).
Basigin is the protein which composes the OK blood group.
It is widely expressed and mutations in mice cause infertility
and abnormalities of the CNS (in memory and sensation).
It is possible that primordial members of the immunoglobulin family
were similar to basigin (Miyauchi, 1990). Included in the wide variety
of human cells which express immunoglobulins
are white blood cells which function in innate immune responses. For example, Paired Immunoglobulin-Like Receptor
a is expressed on monocytes, granulocytes, and dendritic
cells while FCGR1A, FCGR1B, and FCGR1C are expressed on macrophages and
monocytes. CD47 is
involved in the increased calcium concentration in the cytoplasm of cells
once they have bound to the extracellular matrix. Macrophages require this protein to fuse in
order to become osteoclasts. It is critical as a marker of self for red blood
cells. Red blood cells lack MHC
proteins and would be destroyed by macrophages at the spleen and natural
killer cells if they did not express CD47 (OMIM). Although acquired immunity is considered
to be a characteristic of vertebrates, many higher invertebrates possess
humoral proteins called agglutinins which function in the
immune response by can clump foreign cells together. (Hoar,
1983). Although tunicates
lack acquired immunity, they do possess genes which participate in the
acquired immune responses of vertebrates such as complement proteins,
lectins, 2 interleukin receptors, opsonins,
agglutinins, cytokines, and hemolysins. (Burighel, from Harrison, 1997, p. 269;
Dehal, 2002). ACQUIRED IMMUNITY The acquired immunity of higher vertebrates
such as ourselves involves incredibly complex
interactions between immunoglobulin bearing cells, such as T cells, B
cells, and natural killer cells. It
also involves the ability to distinguish between self and nonself,
which is mediated through other members of the immunoglobulin superfamily, the MHC proteins (major histocompatibility
complex). The components of these
complex immune systems have simpler homologs
in invertebrates and primitive vertebrates. Sponges can distinguish between self and
non-self in that a sponge can reject a graft made from another sponge. Wandering phagocytes combat
microbes in sponges and some flatworms, indicating that a primitive immune
resistance predated the evolution of circulatory systems. Invertebrates do not have a lymphatic system
or any aspects of acquired immunity, but certain parts of their immune
systems do display characteristics observed in vertebrates. Although there is no complement system, there
is a prophenoloxidase system which involves
a cascade, including enzymes which kill microbes and clot blood. Although invertebrates do not possess antibodies,
there are molecules called lectins (found in
bacteria, plants, and all animals) which can cause foreign cells to clump
by binding to sugar groups. In tunicates, both macrophages
and morula cells participate in the rejection
of foreign cells. The numbers of
morula cells can increase four times within
the first 2 hours of the reaction. Morula cells are similar to vertebrate lymphocytes in this
feature and in some morphological characterisitics
(Rinkevich, 1998). Lectins are a group
of cell surface proteins which, like the immunoglobulin domain, have been
utilized by many animals for immune reactions (Hofer, 2001). Sponges possess lectins
homologous to those found in vertebrates (Gamulin,
1994; Schacke, 1994a). In invertebrates, lectins bind to foreign particles and facilitate phagocytosis (as do antibodies in vertebrates). The human protein collectin
(related to the lectins) coats foreign particles
before phagocytosis, just as antibodies do (OMIM). Mannose-binding lectin
(MBL) is produced by liver hepatocytes and functions
in the blood (hepatocytes are pictured below). |
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MBL can bind Staphylococcus aureus,
many Streptococcus, E. coli, Klebsiella, Haemophilus influenzae type b, yeast, other fungi, and viruses such
as HIV,
IAV, and RSV. Although MBL binds
to microbial monosaccharides which also exist in higher animals,
these monosaccharides do not occur in the repetitive
manner in vertebrates in which they are found in many microbes. Cancer cells and virally infected cells can
change their cell surface sugars and MBL can bind to these cells and inhibit
tumor growth. Deficiency of MBL
in humans lessens complement binding to microbes, Thus, MBL behaves in a way similar to
antibodies in many respects (Lu, 2002; Gadieva,
2001). Transplantation reactions occur in urochordates. Urochordates express a gene homologous to natural killer cell
receptors on some of their blood cells.
In fact, this receptor (CD94) is considered a marker for natural
killer cells in humans (Khalturin, 2003). Nonspecific cytotoxic
cells (NCC cells) in teleosts and amphibians
are equivalent to natural killer cells in mammals and seem to possess
evolutionarily conserved receptors (Harris, 1991). Natural killer cell receptors are encoded in
a leukocyte receptor cluster region (LRC) of mammalian chromosomes. Teleost fish have
homologs of mammalian natural killer cell receptors (C-type
lectin receptors) that are located in a homologous
gene cluster (Yoder, 2001; Sato, 2003). OPSONIZATION The antibodies of higher vertebrates function in opsonization. When
antibodies bind to a cell or molecule, it signals white blood cells to
perform phagocytosis. |
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Antibodies are not the
only molecules which can serve as opsonins;
lectins are the primary opsonins
in tunicates. Lectins
also perform opsonization in mollusks and arthropods
(Pearce, 2001). In addition to
attracting white blood cells through opsonization,
lectin-bound microbes attract other factors in the innate
immune system, the complement proteins.
(Lu, 2002). COMPLEMENT In higher vertebrates, antibodies which have
bound to a microbe’s cell membrane can initiate the response of a cascade
of blood proteins, referred to as complement proteins, which can generate
a hole on the microbial membrane and kill it.
This complement cascade is an important component of adaptive immunity.
All gnathostomes possess adaptive immunity
while jawless fish lack it. Although the complement system is an important
component of adaptive immunity, it also functions in innate immunity.
Jawless fish possess complement proteins which function in the
lectin pathway.
Cartilaginous fish were the first vertebrates to develop a classical
pathway of complement proteins and this pathway had developed components
similar to the mammalian pathway by the evolution of bony fish. Complement pathways involve enzymes called
proteases which cleave inactive proteins to produce their active forms. Hemolymph coagulation
is part of innate immunity in many invertebrates. Horeshoe crab proteases
involved in the coagulation cascade possess homologous sequences to those
found in complement cascades. Some
ectotherms have duplications of complement genes
and a more elaborate innate set of immune mechanisms while mammals have
elaborated their adaptive immune mechanisms (Zarkadis,
2001). The complement system is thought to have
evolved from a simple mechanism similar to that found in lectin
and alternative pathways. |
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Three proteins might
have functioned in this system: a C3-like protein (magenta, in the above
drawing), a serine protease similar to factor B (green), and a complement
receptor on immune cells (red). Complement
proteins (factor B and C3) are known in echinoderms and complement-like
proteins are known in more primitive invertebrates.
The complement system seems to have evolved from a simple pathway
involved in opsonization before the evolution
of antibodies (Zarkadis, 2001). C3 is the main component in all 3 complement
pathways. In all three pathways,
it is cleaved (by C3bBb in the alternative pathway and C4bC2a in the classical
and lectin pathways) into two fragments, and exposes its thioester region which binds the target molecule (Nakao, 2003). The
thioester bond of C3 can bind to microbes serving in opsonization and as the site for the late complement proteins
(C5 through C9 to generate a membrane pore) (Fujita, 2004). Although primitive invertebrates possess the
domains used by the complement system, a complement system did not evolve
until the deuterostomes. Primitive deuterostomes
evolved the components of the complement cascade and the genome duplications
early in the history of the vertebrates allowed the integration of several
related pathways (Fujita, 2004). |
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The
complement pathway can also be activated by the serum mannose binding
lectin (MBL, a C-type lectin) to
the carbohydrates of microbes. The
serine protease MASP (MBL-associated serine protease) activates complement C3 which can either result in opsonization
or the assembly of the complement factors which put a hole in the microbial
membrane (Vasta, 1999). Sponge molecules possess the SCR/CCP domains
which are found in complement proteins (Zarkadis,
2001). In primitive deuterostomes,
it is not obvious that the lectin and alternative
pathways have different functions (Zarkadis,
2001). Elements of the lectin
pathway, MBL and MASP, are known since protochordates. Elements of the alternative pathway, C3 and
factor B, are known in echinoderms and factor D in bony fish. (Zarkadis, 2001; Lundqvist, 1999;Pearce, 2001). The sea urchin protein SpBf
is a complement protein which possesses SCR domains, a von Willebrand
factor domain, and a serine protease domain.
Sea urchins possess complement C3 proteins which seem to function
in opsonization and whose levels increase in response to infection.
Sea urchin proteins possess a thioester
region, which in vertebrate complement proteins C3, C4, and C5, is the
region which are exposed and bind target molecules
in complement activation. Some insect proteins have molecules similar
to complements with thioester regions (Smith,
2002). Urochordates possess
proteins pertaining to both the alternative pathway (C3, Bf) and lectin pathway (MBL, MASP). Although tunicates lack acquired immunity, they
do possess complement genes, lectins, and 2
interleukin receptor genes which probably function in innate immunity
(Dehal, 2002). Tunicates
seem to have homologs of complement receptors
and complement proteins are known to mediate phagocytosis
in bony fish (Zarkadis, 2001). The lectin-based
opsoinzation pathway seems to be the original
complement pathway. Tunicates seem
to have a minimal complement system involving a lectin
which binds to serine proteases (forming GBL-MASP complex), C3, and a
C3 receptor on blood cells (Fujita, 2004).
Complement proteins α2M/C3/C4/C5
form a gene family. Tunicates have
2 molecules similar to α2M and two which are similar to C3. They also possess 3 linked Bf genes, nine ficolin-like molecules, and 2 C1q like molecules (Fujita,
2004). Hagfish possess a protein CLP (complement-like
protein) which functions in immune defenses is structurally similar to
a mammalian complement protein (Hanley, 1992). Lampreys possess homologs
of C3, MASP, factor B (which has 3 SCR domains like those of gnathostomes and unlike sea urchins which have 5), and probably
possess complement receptors. Both
hagfish and lamprey C3 function in opsonization
(Zarkadis, 2001).
Hagfish possess a protein CLP (complement-like protein) which functions
in immune defenses is structurally similar to a mammalian complement protein
(Hanley, 1992). In mammals, the
C1q complement binds immunoglobulin in the classical pathway. Lampreys possess C1q, apparently as a part of
the innate reaction (Matsushita, 2004).
Classical and lytic pathways not known in
lampreys (Fujita, 2004). Sharks possess homologs
of C3, C4, and MASP, (Zarkadis, 2001). Of the classical pathway which involves C1,
C2, and C4, C4 is known in sharks and bony fish. Of the terminal lytic
proteins C5-C9, C5 and C8 are known in sharks (Zarkadis,
2001). Factors C6, C7, C8, and C9 form a gene family (Fujita, 2004). Complement factors C1r and C1s bind to form
a heterotetramers with two subnits
of each enzyme (Presanis, 2003). In teleosts, many
of the complement proteins genes are duplicated resulting in a greater
functional diversity than possessed in mammals (Nakao,
2003; Zarkadis, 2001). Most of the components of the classical complement
pathway have paralogs which function in the
lectin or alternate complement pathways, suggesting
that large-scale gene duplications led to three pathways derived from
a single ancestral pathway (Fujita, 2004). The gnathostome
complement pathways were established before the branching of cartilaginous
fish from the other lineages. The
C1q which binds antibodies in the classical pathway is a lectin
homolgous to those which activate the lectin pathway (Fujita, 2004). It seems that all of the genes of the natural
killer cell complex have evolved from duplications of a primordial gene
with a C-type lectin domain (such as that of
the hepatic asialoglycoprotein receptor). On human chromosome12p, there is a 2 Mb region
which contains at least 18 genes for lectin-like
receptors, In
this gene family, there is a subfamily of lectin-like
genes which are expressed in monocytes, dendritic cells, and/or endothelia. Thus the receptors required for NK function
have functions outside NK cells (Hofer, 2001).
MHC Acquired immunity is only found in vertebrates
and depends on B and T cell receptors. While the immunoglobulin receptors on B cells
can recognize proteins, receptors on T cells recognize peptides which
are attached to MHC molecules. In
general, MHCI proteins (composed of MHCIa and b2 microglobulin chains) bind
peptides in the cytoplasm. These
peptides are produced when proteins are broken down in the cytoplasm and
taken to the endoplasmic reticulum where they bind the MHC proteins. The MHC I proteins “present” these peptides
on the cell membrane, predominantly to cytotoxic
T cells. MHCII proteins (which
are heterodimers composed of a and b chains) bind peptides in lysosomes
and endosomes.
These peptides are typically obtained from the breakdown of foreign
proteins, originating outside the cell.
MHCII proteins present their peptides to T Helper cells. The MHC proteins are encoded by HLA genes (human
leukocyte antigens) and have changed little in structure during their
evolution. b2 microglobulin can function by itself as a chemotactic
factor and may have evolved before the MHC proteins. ANTIBODIES AND
T CELL RECEPTORS (TCRs) One of the characteristics of jawed vertebrates
is the possession of adaptive immunity which allows them to predict a
great diversity of potential antigens to defend against, attack antigens
with specific humoral proteins, and retain a
memory of the antigens they have encountered.
In addition to the MHC and complement factors already considered,
this system depends on two novel types of immunoglobulin known only in
gnathostomes: antibodies and T cell receptors.
These receptors are common in the
human lymphocytes pictured below. |
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While these lymphocytes are the second most
abundant type of leukocyte in the blood, the majority of them are found
outside the blood in sites such as lymph nodes, the spleen, tonsils, the
appendix, and Peyers patches of the small intestine.
The following image is of the spleen. |
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Lymphocytes
can generate a diversity of 1016 different receptors and each
lymphocyte carries 105
copies of one receptor (Laird, 2000). Where did these complex proteins come
from? Antibodies and T cell receptors
are assembled from smaller chains, known as constant (C), variable (V),
diversity (D), and joining regions (J).
Some of these components evolved long before antibodies and T cell
receptors. Variable
immunoglobulin domains are known in cell surface molecules of plants and
fungi in addition to a diversity of animals, including the most primitive
animals, the sponges (Muller, 2001; Muller, 2001a).
The V and C regions of immunoglobulins
resulted from the duplication of an ancestral region of about 110 amino
acids (Schluter, 1997). The
ancestral Ig/TCR receptor probably had a V domain
and a C domain. The C1 domain of antibodies and TCRs
is known in MHC molecules I and II, tapasin,
and SIRPS (Du Pasquier, 2004). An
antibody is depicted in the following illustration. |
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The J chain of vertebrate antibodies is thought
to function in the dimerization of antibody
subunits and their transport. The
gene for the J chain is expressed in a number of protostome
invertebrates as well as in diverse groups of vertebrates. In invertebrates, it is expressed on macrophage-like
blood cells and epithelial surfaces. Since these animals lack antibodies, it appears
that the J chain functions in other capacities as well (Takahashi, 1996).
The J chain is expressed one week
before the mu chain in human fetal development,
consistent with the suggestion that it serves multiple functions (Takahashi,
1996). Vertebrates possess two types of immunoglobulin
which are thought to be similar to the ancestral immunoglobulin which
gave rise to antibodies and TCR genes, the JAM/CTX and the nectin/poliovirus receptor (PVR) families. These proteins serve as cell adhesion molecules
and virus receptors (such as polio, Coxsackie virus, and reoviruses). Apparently,
one ancestral pair of these genes was linked and subsequently duplicated
to produce the four paralagous linkage groups
known in the human genome. JAM,
CTX, and nectin molecules can form dimers, as can the subunits of antibodies and T cell receptors
(Du Pasquier, 2004).
Drosophila possesses a protein with V and C chains showing homology
to nectin (Du Pasquier, 2004). Tunicates, among the most primitive chordates,
possess JAM, CTX and PVR genes which possess both a V and a C domain (Du
Pasquier, 2004). Jawless fish have no organized lymphatic
tissue and lack both a spleen and thymus.
Lampreys and hagfish seem to lack antibodies, T Cell
receptors, MHC proteins, and RAG. (Zarkadis,
2001). They
do, however, possess serum heterodimeric proteins
which resemble both antibodies and T cell receptors (Varner, 1991). These receptors are formed from two different
heavy chains attached to two different light chains, unlike the antibodies
and TCRs of vertebrates. While
hagfish can reject foreign skin grafts, they do so with very modest increases
in the amount of antibody-like moelcules in
their blood (0.3% of serum protein compared to 50% in sharks) (Varner,
1991). T cell receptors are depicted
below. |
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Hagfish also possess
proteins recognized by anti-IgM antisera
(which bind IgM antibodies of higher vertebrates
(Rumfelt, ). Lamprey blood contains cells which are morphologically
indistinguishable from mammalian lymphocytes although no MHC, T cell receptor,
or antibodies are known from lampreys. These cells express genes associated with mammalian
lymphocytes and occur in tissues (such as the intestines) where lymphocytes
are common in higher vertebrates (Mayer, 2002). Lymphocyte-like cells in lampreys are small
with little cytoplasm and produce lymphocyte transcription factors (Spi and Ikaros), CD45, BCAP, and
CAST (which in mammals are primarily expressed in lymphocytes), CD98 and
CD9 (which mammals use in lymphocyte proliferation and migration), proteasome subunits (PSMB4, PSMB7, 26S subunit pUb-R3, PSMA2,
PSMA6, and PSMF1), and ABC9 (similar to the ABC proteins which mammals
use to transport peptides to the MHC)., the complement protein C1q, and
a number of other genes expressed in mammalian lymphocytes (hepsin,
sygin 2, RAMP4, and talin)
(Mayer, 2002). Lampreys not only have a Spi
gene (a gene which is specific to the differentiation of lymphocytes in
higher vertebrates), it is expressed on the surface of the lymphocyte-like
cells (Shintani, 1999). In addition to antibodies such as IgM and IgW, sharks synthesize the
antibody-like molecules IgNARC (new antigen
receptor in cartilaginous fish) and NAR (new shark antigen receptor) (Schluter, 1997). The
NAR immunoglobulin (new antigen receptor) known in nurse sharks functions
as an independent molecule without forming a dimer
with other immunoglobulins. Its variable region seems to be equally related
to both antibody and TCR variable regions and may be derived from the
ancestor of both (Roux, 1998). Sequence
comparisons suggest that NAR
separated from the antibody group of molecules before the
classes of antibodies were produced. ( In sharks, B cells make 4 classes of antibody,
including IgM, which humans also produce. Shark antibodies can be secreted and can function
on the cell membrane. While higher
vertebrates create antibody diversity by possessing large numbers of antibody
parts in gene clusters which will be randomly shuffled 50 V, 30 D, 6 J,
8 C), sharks possess more than 100 clusters which do not contain this
diversity (1 V, 2 D, 1 J, 1 C) and are not reshuffled (Hohman,
1993). To express it another way,
sharks possess (VH-D-D-JH-CH)n
while bony fish and tetrapods possess (VH)n-(D)n-(JH)n-(CH)n. (Magor,
1999). |
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The antibody light chains
in sharks and teleost fish form a number of
clusters which share a V-J-C organization (Pitstrom, 2002).
One type of immunoglobulin in sharks, IgW
displays characteristics which might be expected from a primordial immunoglobulin
(Berstein, 1996)
In addition to IgM, lungfish possess
immunoglobulins similar to the IgW
previously known only in sharks. Thus,
IgW and IgM must have
duplicated early in the evolution of gnathostomes
(Ota, 2003). Both B and T cells are known in bony fishes. |
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While the shuffling of
the components of immunoglobulins are
essential to the aquired immunity of higher
vertebrates, immunoglobulins which aren’t shuffled
are also capable of immune function in vertebrate groups ranging from
sharks to mammals. Mammals possess
families of receptors such as paired Ig-like
receptors (PIR), Ig-like transcripts (ILT; also known as leukocyte inhibitory
receptors LIR), and killer inhibitory receptors (KIR) which can interact
with MHC I and II proteins. These
Ig-like molecules do not rearrange themselves
(like Ig and TCR receptors) yet they function
in immune responses (Laird, 2000). In summary, the complex immune reactions of higher vertebrates depend on a variety of genes in the immunoglobulin and lectin superfamilies. Both of these gene families were present in the most primitive animals, as were phagocytic cells and some ability to distinguish between self and nonself. Many of the molecular components required for the acquired immunity of higher vertebrates are present in invertebrates (especially primitive chordates) which lack acquired immunity. Many of the components of the complex humoral and cell mediated reactions of higher vertebrates are present in jawless and cartilaginous fish whose immune responses are simpler than those of higher vertebrates. The most complex aspects of the human immune system supplement, rather than replace, other components which are homologous to the immune mechanisms of more primitive animals (such as the fibronectin-containing macrophage mannose receptors, lectin-containing receptors, and immunoglobulin-like receptors whose components are not rearranged). |
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