We can differentiate different types of T-cell depending on
whether they have encountered antigen or not and their location.
And we can divide them into what are called central memory and
effector memory T-cells after they have encountered the antigen.
So here we can see in a lymphoid tissue, a naïve T-cell,
in other words one that has not yet encountered antigen.
But you will also see that there is a dendritic
cell entering that lymphoid tissue and
that dendritic cell may well be carrying
antigen that the naïve T-cell is specific for.
At this early stage in its development,
the T-cell has a number of different
cell surface molecules present that can
define it as being a naïve T-cell.
So for example, it has a particular
splice variant of the molecule CD45.
And this splice variant
is called CD45RA.
So these cells are CD45RA
An alternative splice variant
of CD45 is called CD45RO.
And these cells, these naïve T-cells
do not express that variant.
So they are CD45RO negative (CD45RO-).
They also express to other important
molecules, the adhesion molecule CD62L.
And we’ve already mentioned that they
express the chemokine receptor CCR7.
Following their activation, these
naïve T-cells develop into effector
cells, but some of them also develop
into these all important memory cells
that are required for the adaptive
immune response and the secondary
immune response that is so characteristic
of the adaptive immune response.
These T-cells will initially be within the lymphoid
tissues, and they will sit there and we refer
to them as T central memory cells; central because
they’re actually within the lymphoid tissues.
They now express the CD45RO
splice variant of CD45.
So they are CD45RA-, CD45RO+.
They continue to
express CD62L and CCR7.
However they don’t have to stay in the lymph
nodes, they can move to peripheral tissues.
And when they do this, they can reside in those
peripheral tissues as T effector memory cells.
So if an infection occurs again with
the same pathogen, there’s some memory
T-cells that are already in the locations
that that infection may occur in.
You don’t have to wait for them
to come from the lymph nodes.
There’ll be some there as T central
memory cells, but there’ll also be some
that are actually going to be already
there in the location that you need them.
So they’re ready to go and can be
immediately responsive to the antigen.
And these cells are characteristically CD45RA-,
CD45RO+, and they lack expression of CD62L and CCR7,
because they don’t need those molecules anymore
because they’re already in the peripheral tissues.
And then following activation by re-encounter with the same
pathogen leading to a secondary immune response, they’ll
develop into T effector cells that are CD45RA+ now, lose
expression of CD45RO and maintain the absence of CD62L and CCR7.