In the Immune System of a Reptile, the Ruin lizard
François Vanhoutte
Each vertebrate organism has to preserve the
integrity of its body from the intrusion of
microorganisms or pathogens, each one responsible
for a particular disease or pathology : viruses,
bacteria, pathogenic fungi, and other large and
complex organisms collectively termed parasites. All
the means to avoid an infection belong to the
immunity system. This system is so efficient as even
if each vertebrate continually enters in contact with
pathogens, the effects of this contact are rarely
visible. Immunity is shared in two phases. Innate
immunity is the first one. It protects from the
entrance and invasion by microorganisms. The
second phase is adaptative immunity. Contrary to
innate immunity, this phase needs activation by
effectors and receptors on cells to start. It takes more
time for adaptative immunity to be efficient
compared to innate immunity that starts as soon as a
pathogen enters in contact. The cells of adaptative
immunity are called lymphocytes. Lymphocytes B
(LB) produce proteins that are specific to adaptive
immunity. They are antibodies. Only one type of
antibody is produced by each LB. It matches to the
antigen, a part of the pathogen that is recognized by
the antibody. This fixation leads to the destruction of
the pathogen during adaptative immunity.
Antibodies belong to the immunoglobulin (Ig)
family. Igs molecules are part of it and are presented
on the LB surface. They have the same potential to
bind the antigen as the antibody but they possess
another part which allow them to be in the cell
membrane. Igs (and antibodies) are built from four
chains, two light (L) and two heavy (H) chains. The
L chain has one variable (V) and one constant (C)
part. The H chain has one V and three or four C
parts. L and H chains gather to give a complete Ig
with a variable region made from the variable parts
and a constant region made from the C parts. The
variable region gives the specificity of binding to the
antigen, the constant region determines the type of
immunoglobulin.
There are different types, called isotypes, for
each class of Vertebrates. Each one with different
functions. In birds, there are three isotypes : IgM,
IgY and IgA. In reptiles that are considered as the
ancestor of birds, very little is known : IgM is
assumed to be present as in any vertebrate that
possesses Igs, but others isotypes are unknown. In
amphibians that have a common ancestor with birds
and reptiles, there are IgM, IgY and IgX. The aim of
this project was to determine the isotypes of H chain
in reptiles and to compare the sequences with those
of birds and amphibians to discuss the way Igs
evolved between these three vertebrate classes. As
amphibians and birds possess both IgM and IgY, I
was expecting to find these isotypes in reptiles too.
But the question was about the last isotype : IgA as
in birds or IgX as in amphibians?
To determine the isotypes of H chain in
reptiles, I worked on the Ruin lizard (Podarcis
sicula). I had a lizard library that represented all the
proteins that can be found in spleen (organ that
contains many lymphocytes). This library was
screened with a probe from a duck IgA H chain. The
probe hybridised with clones that were lizard IgM H
chains. A new probe was made from one of this IgM
H chain to have a specific lizard probe. A lizard IgY
H chain was identified from the library.
Comparisons of the lizard IgM and IgY H
chains with those of the duck and the chicken
(birds), the xenopus and the axolotl (amphibians),
could put forward different conclusions. The
structure of the lizard IgM H chain is very conserved
between birds, reptiles and amphibians, more than
the lizard IgY H chain with its homologues. It seems
to be more than one C part possible for the lizard
IgM H chain, consequences of the duplication of the
genome in tetrapods. After having built a
phylogenetic tree that represents the evolution
relationships between IgM, IgY, IgE H chains from
mammals, birds, fishes and other vertebrates, reptiles
are confirmed to be closer to birds than amphibians
and to be ancestors of birds. Moreover, as proposed
in many articles, IgY seems to be the ancestor of IgG
and IgE in mammals : same structure, analogical
functions. I didn’t succeed to isolate the lizard IgA,
which seems to be due to an absence of the IgA
protein in the spleen library. From an intestine
library, in which a lot IgA is secreted, future
experiments should be successful.
Degree project in biology, University of Uppsala, spring 2003
BioMedical Center, Department of Cell and Molecular Biology, Immunology Programme
Supervisor : Lars Pilström