So how are these peptides
Well they’re generated
by antigen processing.
And that antigen can
come from two locations.
The action can be within a cell, in that
case we refer to it as endogenous antigen.
And processing of antigens that are within the cell; endogenous
antigens produces peptides that are
eight to nine amino acids long.
And this is a necessary length for
presentation by MHC Class I because the
peptide binding groove in MHC Class I
specifically binds peptides of this length.
In contrast, antigen that is
present outside of cells, exogenous
antigen that is then taken up for example by a phagocytic cell.
These are processed into peptides that are rather
longer, approximately 15 amino acids, can be 20 or so.
But typically around about 15 amino acids,
sometimes a little bit shorter but
you know on average around about 15 amino acids
long for presentation by MHC Class II.
Let’s look now at the actual enzymes and structures
that are involved in this processing of protein antigen.
So here we have an example
of an endogenous antigen.
It could be a protein from a virus
that’s infecting a cell for example.
The first thing that happens is that
this protein becomes ubiquitinated.
It has a tag, an
ubiquitine tag added to it.
This causes it to be fed into a
structure that is called the proteasome.
Now all cells have proteasomes in them and they
are complexes of enzymes that form a tube-like
structure and proteins are fed in at one end
and come out the other end being chopped up.
And it’s a way in which cells get
rid of sort of worn out proteins.
But when cells become infected with viruses or other
pathogens, they actually modify the constitutive proteasome.
They add some units to it that
then mean it’s a immunoproteasome.
And the difference between the constitutive
protein and the immunoproteasome
is that the immunoproteasome is specialized to produce peptides
of the length that’s required to
bind into the MHC Class I groove; in
other words, peptides of around about
eight or nine amino acids long.
So we have have an infected cell here
because it’s got an immunoproteasome.
That tells us it must be infected because we
only see immunoproteasomes in infected cells.
The protein is then fed
into the immunoproteasome.
Meanwhile, in the endoplasmic reticulum,
all of our nucleated cells are all the time
producing many, many, many different proteins
and one of those proteins will be MHC Class I.
And there are other proteins that are produced in the
endoplasmic reticulum that associate with MHC Class I.
And three of the important ones are tapasin,
calreticulin and a protein called Erp57.
And they help keep the conformation of the
MHC Class I correct for peptide binding.
So the protein’s being fed
into the immunoproteasome.
And then at the other end, we get these peptides
that are eight or nine amino acids long.
Now that’s good, we need peptides that
are eight or nine amino acids long.
And we’ve got the MHC, so that’s good.
But there’s a kind of problem here isn’t there,
because the peptides are in the cytosol.
But the MHC Class I is in
the endoplasmic reticulum.
So we have to get those together.
And the way that that happens is that these
peptides, as they’re being fed out of the end of the
immunoproteasome, they are picked up by two molecules
that span the membrane of the endoplasmic reticulum.
And these two molecules are called TAP1 and TAP2 - the
Transporters associated with Antigen Processing 1 and 2.
And these TAP1 and TAP2 molecules pick up
the peptides on the cytoplasmic face of the
membrane of the endoplasmic reticulum and take
them across into the endoplasmic reticulum.
So you can see the peptide’s being taken up and
now getting into the endoplasmic reticulum.
And they can be put into the peptide
binding groove of MHC Class I.
This complex of peptide plus MHC then buds off from the
endoplasmic reticulum and vesicles are transported through
the cell, ultimately to the cell surface where the
peptide MHC can be recognized by the T-cell receptor.
And in this case, you will need a CD8+
T-cell to recognize the MHC Class I molecule.
So the T-cell receptor recognizes peptide MHC, the
CD8 recognizes the non-polymorphic region of Class I.
Let’s have a look at processing of exogenous
antigen, antigen coming from outside the cell.
So for example a phagocytic cell phagocytosing
an antigen or a cell endocytosing an antigen.
The protein antigen is taken up
by endocytosis or phagocytosis.
And meanwhile in professional antigen
presenting cells, as well as making
MHC Class I in the endoplasmic reticulum,
they’re making MHC Class II.
The MHC Class II associates with a molecule that’s
called invariant chain (Ii) - big I little i.
This acts as a kind of cork
in a bottle type of situation.
It acts as a stopper, because you’ll recall
from the previous slides that you’re feeding
peptide into the endoplasmic reticulum, peptides
that have come from the immunoproteasome.
You don’t want those peptides to bind into the MHC Class
II groove, you want them to bind into the MHC Class I groove.
So to prevent peptide binding in the endoplasmic
reticulum, the peptides that are destined to meet up
with MHC Class II, there’s this stopper in the peptide
binding groove for Class II called invariant chain.
Meanwhile following endocytosis or
phagocytosis, there are various enzymes
that are produced within the endosomes
and the phagosomes that will
degrade the protein to peptides that
are approximately 15 amino acids
long, in other words the right length
to be presented by MHC Class II.
The vesicles bud off from the
endoplasmic reticulum with the MHC Class II
and the invariant chain, and
eventually these are going to meet up.
And they fuse within the cytoplasms of
the vesicle containing the peptides,
fuses with the vesicle containing the
MHC Class II plus the invariant chain.
At this point in time, the invariant
chain gets partly degraded and most of it
is chewed away by enzymes, leaving just a
little bit of the invariant chain left.
And that little bit of the
invariant chain is called CLIP.
Subsequently two molecules called DM and DO remove CLIP
and insert the peptide into the peptide binding groove.
And then the exchange occurs
between CLIP and the peptides.
And the peptide MHC complex then moves to the cell
surface where it can be recognized by CD4+ T-cells.
The T-cell receptor recognizing the
combination of peptide plus MHC Class II, and
the CD4 recognizing the non-polymorphic
regions of the MHC Class II molecule.