So those are the two classically defined
processes of antigen presentation.
The presentation of endogenous antigen
to CD8+ T-cells using MHC Class I, and
the presentation of exogenous antigen
to CD4+ T-cells using MHC Class II.
But in fact, those pathways are not absolutely
set, they’re kind of interconnected in you like.
An exogenous antigen can be
presented using MHC Class I.
An endogenous antigen presented using MHC Class
II, as we can see on this particular slide.
And this isn’t a rare event.
This is happening all the time because often you
will want exogenous antigens to be presented to CD8+
T-cells and you’ll want endogenous antigens to be
presented to CD4+ helper and regulatory T-cells.
So the endogenous and exogenous pathways are
not completely separated from each other.
Endogenous antigens can enter the Class II pathway and
be presented to CD4+ helper and regulatory T-cells.
And exogenous antigens can enter the Class I
pathway and be presented to CD8+ cytotoxic T-cells.
Let’s just look at one example
of cross presentation.
Here we have a virus infected cell.
And antigens are being processed and presented
using MHC Class I on this virus infected cell.
However, you can have capture of these
antigens, either infected cells or viral
antigens that are picked up by the antigen
presenting cells in the infected individual.
So for example a dendritic cell can pick up antigen,
and these viral antigens can then enter the cytosol.
So they’ve been engulfed by endocytosis or phagocytosis, and
normally they would be given to the MHC Class II pathway.
But now they’re being put into the MHC Class I pathway because
the antigen escapes from the vesicles and enters the cytosol.
And then they can go through the normal Class I pathway
using that immunoproteasome that we heard about.
And you get this cross presentation so
that MHC Class I is presenting, using
a dendritic cell, MHC Class I is
presenting to virus specific CD8+ T-cells.
And you get co-stimulation and
activation of these cytotoxic T-cells.