Thursday, 21 February 2013

T-Cell Receptors, Part Four: More Odd TCR Transcripts

After the discovery of TARP, things were a quiet for a little while; it looked like TCRs seemed happy to remain in the T-cells. However, there were a couple of blips on the radar, around the same time, that never really developed into fully-fledged stories.

The first of these two papers was in 2002, working off the back of an interesting observation from a previous study; when analysing the expression of TCR expression in T-cells that adhere to stromal bone marrow cells, they found that the control group of stromal cells alone seemed to express TCR as well as the T-cells (NB, in mouse) .

Closer investigation revealed that mesenchymal cells expressed αβ TCR (both primary cells and cell lines) mRNA, as well as CD3, revealed by RT-PCR. However, neither of the RAG proteins are expressed, and what little TCR there is expressed (by T-cell standards) isn't re-arranged, which you'd expect without the necessary recombinases present.

Once again we see that the transcripts are made of (different) J regions correctly spliced onto constant regions, akin to what we saw with TARP. As the whole constant region was present, they were afforded the luxury of easily looking for protein on cells, as constant region antibodies exist; lo and behold MEFs (as a model mesenchyme system) stain positive for TRBC, while MEFs from TCRβ-/- mice don't (although no T-cell staining as a positive control? Seriously?).

Curiously, mesenchyme cell lines that expressed more of the TCR (at RNA level) seemed to have different growth properties to those that didn't, both proliferating more and causing a greater incidence of cancer when injected intradermally into nude mice.

The next case we see of wandering TCRs takes us to the relatively young field of neuroimmunology.

While we all know and appreciate the role MHC proteins and CD3 play in immunology, it turns out they're required for a number of neurological roles, such as correct synaptic formation and hypothalamus development.

This, and similar observations, prompted Harvard researchers to wonder if perhaps there might be some TCR lurking in the CNS, to act as the ligand for these seemingly important neuronal MHC proteins.

A series of in situ hybridisation experiments shows that there was indeed TCRβ expressed in brain segments, and that it is indeed localised to the neurons. Examination of the message revealed a curious thing about the transcripts (and I think you've guessed it); they're non-recombined, consisting of a (specific) J region spliced to the constant region.

They find no TCRα mRNA, and detect no constant region protein by immunostaining. In addition, TCRβ knock out mice fail to show a similar neurological phenotype to MHC or CD3 knock out mice, which seems to put paid to the idea that this neuronal TCRβ expression provides the ligand for MHC in the CNS.

The authors offer a few suggestions for why the CNS might be expressing this TCR; maybe the RNA serves a purpose, maybe the protein or activity is just very low and undetectable, or maybe the expression is just part of some vestigal transcriptal program, playing no role but doing no real harm. Not the most earth-breaking of papers, but another curious case of TCR message getting around and seeing the sights.

(As a brief aside, this reminds me of an interesting neuroimmunology paper I saw a little while ago, where a genome wide investigation into narcolepsy revealed the major associative polymorphisms mapped to the TCRα locus. This was only an association study, but it does speak to possible non-immune roles for the TCR.)

Both of these stories are a little reminiscent of the B-cell TCR expression we saw in part 3, expression of funny TCR transcripts (spliced but lacking Vs), in cells that we wouldn't expect them in. Interestingly, there's even a link to increased cell proliferation, which echoes the TARP story somewhat.

However, there's very little (at present) to convince me these transcripts are doing much, if anything. Personally, from my own experience sequencing TCR repertoires, I know in T-cells we get a bunch of odd transcripts popping up, the type of which have been known about for a long time. It's always hard to say whether these things biologically important or just interesting looking enough to give postgrads false hope.

Chances are good they just represent the noise in the machine, little ripples of expression as a stone is dropped nearby in the transcriptional pond, unintended byproducts of another pathway. Either way, these papers raise some interesting possibilties, and pretty convincingly show TCR sequence popping up where it's not supposed to, so they've got a welcome entry in this series.

The next (and final) entry into this week of TCR blogging brings us up to the present crop of non-T-cell TCRs, including the papers that actually set off my interest in the topic. Having presented some of the papers to my own journal club I won't be surprised by a mixed response; who wouldn't want to double check the presence of TCRs in no fewer than three other non-lymphoid white blood cell types?

On to part 5, where we finish exploring the landscape of where TCRs emerge

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