Everyone has one of these, right? A signature scent. It’s that little something that alerts the senses that you have entered the room. Mine signature scent entails the woodiness of bergamot which, as the day continues, gives way to an enlivening flash of violet and nutmeg. It’s perfect! It’s me. My days begin with smoky, woody flavours of coffee, and eventually give way to endorphins and a furtive energy which gets the cytometer running, the cell cultures going and the ELISAs flashing the lemon-lime joy of a successful experiment.
I like to think that T-cells have their signature scents, too. Those little somethings about the cells that alerts the body that these particular cells have entered a particular compartment, and are ready to do work. I speak, via metaphor, of transcription factors.
One hears, of course, of T-Bet, GATA-3, RORγ and their indispensable impacts on the cytokine profiles of CD4+ T-cell subsets, these scents of a T-cell that dictate the outcome of an infection, an insult or, well, just the regulation of physiology in general. The one transcription factor that I am *obsessed* with of late is Promyelocytic Leukaemia Zinc Finger-1 (PLZF-1) which is a transcription factor of the Bric-a-brac Tramtrack Broad-complex Pox Virus Zinc Finger (BTB-POZ) family of transcription factors. The BTB-POZ’s, if you will, are known to be transcriptional repressors and, they find themselves at these exciting points-of-inflexion in the developmental biology of adaptive cells. As an example, the BTB-POZ protein Bcl6 regulates affinity maturation of B-cells. Th-POK (this protein, I kid you not, is sometimes called “Pokemon”) controls the T-cell decision of going CD4 or CD8.
PLZF, encoded by the Zbtb16 gene, is the “signature scent” of iNKT cells. It’s moments like these that keep me in awe of biology: a family of transcriptional factors whose affairs are deeply entwined in those of adaptive cells have a member who controls the development of a unique subset of T-cells that has both innateness and adaptivity. Features that are conferred upon these cells by this transcription factor.
The development of iNKT cells itself is a bit of a trip. We start, as we always do in such tales, in the thymus. “Stage zero” iNKT cells are positively selected via a recognition event facilitated by presentation of CD1d-borne alpha-linked glycosylceramides. What is “stage zero” for iNKT cells, for conventional T-cells, is the stage when they are CD4+.Stage “zero” iNKT precursors downregulate their CD69 and CD24. These cells, which are CD69lo CD24lo CD4+, do not join other CD4-expressing naïve T-cells in the periphery, but stay in the thymus. In the thymus, the iNKT cells proceed to stage two of their development which entails the upregulation of CD44—a cell-surface marker one associates with T-cells who’ve “seen” antigen, and also start transcribing genes for IL-4, IL-10 and IFNγ production. At stage two, these cells are ready to leave the thymus, and so they upregulate their S1P receptor, and hearken to the siren song of sphingosine-1-phosphate, and exit the thymus. Stage three of their development occurs in the periphery wherein they begin to upregulate cell-surface markers associated with NK cells (think NK1.1, NKG2D, NKG2A and such).
A wonderful, pioneering paper by Derek Sant’Angelo et al first identified PLZF as the signature transcription factor which is absolutely imperative for iNKT cell development. Compared to other αβ T-cell populations, PLZF seems restricted largely to iNKT cells, the proverbial signature scent, and a blockade of PLZF results in a strange, troubling reversion in phenotype: cells lacking PLZF Benjamin Button’d themselves back to a state in which some of them expressed CD4, had a pittance in the name of CD44 surface expression and literally no NK1.1., much like iNKT cells in an earlier stage of development. What was worse is that these PLZF-deprived iNKT cells secreted a whimper of IFNγ and a pathetic sigh of IL-4. So you see, without the signature scent, PLZF, the iNKT cells are, literally, not themselves!
There has been recent evidence to fortify the “signature scent” idea of iNKTs and PLZF. While the immensely clever Dr. Seddon Thomas has shown that the expression of LFA-1 on iNKT cells is under control of PLZF-1. My inventive and astute predecessor here in the Gumperz lab showed that iNKT cells can, under conditions of weak TCR stimulation, can produce a cytokine while maintaining mobility—a feature that is likely under the control of PLZF as well.
It is my hope that my work, too, will tell us more about the innate innateness of these cells: their ability to make cytokine under conditions that are thought to be anergizing for “conventional” T-cells, their ability to use integrin signaling to enliven the activities of antigen presenting cells, and, ultimately, how this adaptive innateness and innate adaptivity relate to the signature scent of iNKT cells.