Natural Killer T-Cells (NKT-Cells) – Lymphocyte Development

00:01 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.

00:16 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.

00:27 This is an NK cell’, and so on.

00:29 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.

00:37 And that’s true I think of all of biology.

00:41 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.

00:55 A thymocyte by the way is simply a young T-cell that’s developing within the thymus.

01:01 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.

01:29 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.

02:41 But look, these NKT-cells here, they’re secreting both those types of cytokine.

02:48 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, although some NTK-Cells are cytotoxic.