There is a group of cells that share
characteristics of T-cells, which are cells of the
adaptive response, but also natural killer
cells which are cells of the innate response.
And I think this highlights a very important
aspect of the immune system, is that we try and
force things into little boxes, we try and put a
label on something saying, ‘This is a T-cell.
This is an NK cell’, and so on.
But actually it’s all a kind of continuum
really, and there are things that
don’t fit neatly into the little divisions
that we would like to put them in.
And that’s true I think
of all of biology.
These NKT-cells are a distinct subset of
alpha beta (αβ) T-cells, and they branch
from conventional T-cells at the double
positive stage of thymocyte development.
A thymocyte by the way is simply a young
T-cell that’s developing within the thymus.
The NKT-cells express markers, in
other words cell surface molecules
that we can use for identification of different cell types.
These markers are associated with
both T-cells, for example they have
a TCR - T-cell receptor which is
very characteristic of a T-cell.
But they also have molecules on their
surface that are characteristic
of natural killer cells, for example
C-lectin-type receptor NK1.1.
Interestingly, the T-cell receptor
of NKT-cells is mostly composed
of what is referred to as a semi-invariant T-cell receptor.
In other words, the T-cell receptor doesn’t vary
nearly as much as it does on a conventional T-cell.
They tend not to recognize peptide plus MHC,
like the conventional T-cells, instead they
recognize glycolipids presented by the
nonpolymorphic MHC class I-like molecule, CD1d.
So a little bit like the gamma delta
(γδ) T-cells that we just referred to.
And when activated, they secrete cytokines including
gamma interferon (IFNγ) and interleukin-4 (IL-4).
And when we discuss T-cells in a little bit
more detail we’ll see that we can divide T-cells
into different types, Th1, Th2 and so forth,
on the basis of the cytokines they produce.
But these NKT-cells don’t fit neatly into these
different divisions of Th1 and Th2, because
Th1 cells would typically secrete gamma interferon,
Th2 cells would typically secrete IL-4.
But look, these NKT-cells here, they’re
secreting both those types of cytokine.
And although the exact details of how natural killer T-cells
contribute to immunity is still very much being investigated by
researchers, their primary function may well be to act as
regulatory cells to regulate other parts of the immune response.