T-Cell Receptors, Part Five: A Receptor For All Cells?

Back to Part 4: More Odd TCR Transcripts

This last installment in this week of blogging covers the more recent, and potentially more controversial stories I've found of wandering TCRs.

The claims made in the following papers are big, and could have far reaching connotations if they bear out. All of these papers report whole, functional TCRs, being expressed and used by white blood cells. Just the wrong ones.

It all starts in 2006, with a sentence. A tantalising, if near-painfully vague sentence:

"A series of control experiments prompted us to test the hypothesis that human neutrophils express components of the TCR machinery."

A series of control experiments ey, that old chestnut.Wait, what, neutrophils?

These cells are one of the major components of the innate immune system, and account for the majority of circulating white blood cells at any one given time, despite their rapid turnover.

Their job is to turn up early to places where the body is in trouble (such as inflamed or infected areas) and do some damage control, which largely consists of eating any pathogens they can find, and making it harder for any they can't find to survive or spread.

Previously they'd been thought to act completely via innate immune receptors. The presence of one of the hallmark receptors of adaptive immunity on them is a little surprising to say the least.

Using antibodies directed against the alpha and beta constant regions, they report that around five to eight percent of freshly isolated neutrophils express αβ-TCR, seemingly in a manner comparable to typical TCR expression.

They seem to express a varied repertoire, when looking for different Vα and Vβ transcription by RT-PCR, that are revealed to be rearranged as per normal. There's CD3 components, CD28, all seemingly upregulated by TCR agonists, as well as a number of proteins required for TCR signalling.

So what are these TCRs supposed to be doing in these neutrophils? Upon TCR stimulation (with anti-CD3 and anti-CD28 antibodies, which is thought to represent a fairly physiological level of stimulation), and watched what the neutrophils did next.

What they did next was live long and prosper; it seemed stimulating the TCRs inhibited neutrophil apoptosis and increased secretion of IL-8, the chemokine responsible for recruiting more neutrophils to the danger zone.

So, the theory goes that some neutrophils express TCR, which presumably helps them recognise either specific pathogens, or a broader swathe of pathogens, which can then find bugs quicker, helping to recruit more neutrophils to the threat.

The same group (Wolfgang's Kaminski's group from Heidelberg) has had a couple of follow up papers on this, during which time it gets re-dubbed the TCRL_n, as it's TCR-'like', and in neutrophils, which somewhat solves the quandry of the 'TC' in TCR.

One of these papers is really just a long observation that the repertoire of different TCRs expressed in neutrophils starts off broad, and shrinks with increasing age.

The next is just a case study of a patient with a nasty autoimmune condition (where their red blood cells get targetted and destroyed by their own antibodies), whose number of TCR positive neutrophils had jumped from 5% to 80%.

There's also one final neutrophil paper from a dental group, who noted that oral neutrophils also seem to have a higher expression of TCR than do their circulating cousins, which ties in with their previous work showing that oral neutrophils have a different phenotype.

It takes three years after the start of the neutrophil story before the next cell joins the party. Eosinophils, a kind of granulocyte (mostly responsible for killing parasitic worms) enter the fray, wielding not αβ, but γδ-TCRs.

It's another frustrating start; this time they were on the hunt for γδ TCRs in eosinophils because of the "surprising similarities" between them. Either this is fantastically lucky fishing, or there's a few experiments they're not telling us about.

Either way, they find the γδ-TCR on the eosinophils by flow, along with CD3. Interestingly, they don't find much γδ in lymphocytes (1.4%, which is a few percent shy of usual), nor do they find αβ expressed on neutrophils, putting them at odds with the previously discussed papers.

It seems that while possessing all the bits and bobs needed for TCR recognition, these eosinophils don't produce nearly as much TCR message as T-cells, nor as diverse a range of TCRs. However, activation with TCR agonists caused eosinophils to do exactly what they're supposed to do when activated normally, with degranulation and the release of cytotoxic proteins and reactive oxygen species (ROS).

There's even a couple of figures showing exploring possible functions for the γδ receptor. The presence of γδ-blocking antibodies inhibits the ability of eosinophils to produce ROS in response to mycobacterium, or to induce apoptosis in a colorectal cancer cell line.

The final additions to the TCR club is reported by the same Heidelberg group, giving the impression they probably went on a TCR-testing rampage.

Macrophages are key cells responsible for maintaining immunity in the tissues, by phagocytosing and killing pathogens and presenting their antigens to T-cells. They differentiate from monocytes, which circulate in the body looking for signs for infection or inflammation.

These are classical examples of innate immune cells that bridge the gap to adaptive immunity, through their antigen presentation. These recent papers suggest that they might go one further, with some subpopulations expressing either the αβ or γδ TCRs.

The story unfolds much like the others. There's a small percentage of monocytes and macrophages expressing a limited repertoire of 'TCRL_m' (5% for αβ, 3% for γδ), along with other TCR signalling components.

The obligatory search for function touches on some big topics.

For the TCRL_mαβ, they investigate the intereaction with mycobacteria, which it seems upregulate the expression of the TCR. They go further, and stain lung sections from tuberculous patients, showing that the contacting edge of cells around the granulomas are enriched with TCR expressing macrophages.

TCRL_mγδ on the other hand was investigated during (murine) bacterial meningitis, during which the presence of γδ-macrophages was enriched in the CSF. They also found γδ-MΦs in atherosclerotic plaques, indicating a pretty broad range of possible interactions.

There's a lot to muddy the water in these papers. There's a couple of not-completely-convincing figures, and the obvious matter of the contradiction between the papers. They do all however go a long way to refute the possibility they're just seeing T-cell contamination (either by immunohistochemistry or FISH).

These papers seem to ask more questions than they answer. How does this happen. Do they undergo any selection, and if not, how do they avoid auto-reactivity? If so many other cells can make use of the TCR, why do we even need T-cells in the first place?

If true, these are hugely interesting findings. Here we have terminally differentiated myeloid cells, seemingly expressing one of the classical lymphoid, adaptive immune receptors. Having presented some of these at journal clubs I've seen first hand a bit of resistance to accepting these possibilities outright.

Personally, my view is that biology is a massively confusing thing, and the closer we look at it the more wierd, unexpected stuff we're going to find. Once biology has evolved a system such as the TCR, there's no reason why other cells within the same organism shouldn't make use of it. As technology increases the throughput and sensitivity at  which we can operate, more and more of our accepted models are going to gather inconvenient aberrations like these.

There we have it; the case of the wandering TCR. Whether or not you believe these stories (or more importantly, whether or not you think they have any biological signnificance), I hope you found them as interesting as I did.

While this week of blogging ballooned into a much bigger project than intended, I've enjoyed doing it. Over the coming months I hope to do some other key aspects of TCR biology, just maybe not all at once next time.