|From A History of British Fish (William Yarrell, 1835)|
I’ve talked about lamprey immune systems several times (here, here, and here). I find them fascinating because it shows both how our own immune system developed, and also shows alternate routes that can lead to a pretty good, but very different, immune system.
Quick background: In order of evolutionary appearance you have sea urchins, lampreys, sharks, reptiles, mammals. (Note that this is not true, it’s no more than a sloppy shorthand for common ancestry, but it’s a handy shorthand for this purpose. See a phylogenetic tree here.) Mammals have a form of adaptive immune system that includes T lymphocytes and antibodies, and at first glance this whole complex system arose, almost fully-formed, in sharks.1
This has always amazed me, because an adaptive immune system doesn’t work in isolation; the pieces don’t work alone. You need all kinds of moving parts — all the complex molecular pieces that chop and snip DNA to form T cell receptors and antibodies, all the multiple parts of a thymus that screen T cells for functional and safe receptors, the MHC molecules that the receptors see and all the pieces that snip and shuffle around peptides for that system, the spleen and lymph nodes that let lymphocytes interact with other cells, — and it seemed that all these pieces abruptly appeared and put themselves together, like a fine watch, in one evolutionary blink.
When I first learned about this, some 15 or 20 years ago, I told myself that this was an illusion, that once more species were looked at we’d see the history of these moving parts in other common ancestors. Of course, this is exactly what’s happened since then. We see accidental, random parts in sea urchin genomes (I talk about that here) and we see other bits and pieces arising in lampreys and hagfish (in the links at the top).
So in reality, the adaptive immune system didn’t arise all that suddenly after all; the pieces gradually were added over a hundred million years or more, sometimes purely by chance, sometimes for other purposes altogether, and sometimes as components of a prototypic immune system that acted as a foundation for the whole shark thing.2
So that’s the first part of the background: In lampreys, which diverged from the mammalian lineage maybe 450 million years ago, we see many of the pieces of a mammalian adaptive immune system. There are cells that look a lot like lymphocytes, there is something that looks like a spleen. But, as I say, there are none of the familiar pieces that we think of as an adaptive immune system. Lampreys flatly do not have our adaptive immune system:
Nevertheless, the cardinal elements of adaptive immunity, namely Ig, TCR, RAG1 and 2, and MHC class I and II, were conspicuously absent.3
|Lamprey variable receptor with bound antigen4|
But step back a little, and look a little deeper, and we see some familiar parts. Lampreys do, in fact, have variable receptors, just like T cell receptors and antibodies, and those receptors are made by chopping and shuffling genome DNA, just like TcR and antibodies, and are expressed in their lymphocyte-like cells, and some are secreted (like antibodies and B cells) and some are cell-associated (like T cell receptors).
And here’s the other amazing thing: At the molecular level, the lamprey receptors are completely unlike T and B cell receptors. The lamprey lineage came up with a completely different system that allows them to do pretty much the same thing as the shark lineage. Their receptors are different kinds of molecules, and the system that shuffles the genomic DNA is different. 5 Yet, the functional end product is the same — a system that has immunological memory. An adaptive immune response, that’s quite alien to our own, but that works pretty damn well.
Although the Ig-based and VLR-based adaptive immune systems in jawed and jawless vertebrates use different genes and assembly mechanisms, both systems generate diverse repertoires of anticipatory receptors capable of recognizing almost any Ag through the combinatorial assembly of large arrays of partial gene segments. The development of clonally diverse lymphocytes allows for Ag-specific responses and memory, which are lacking in innate immunity.3
There is still a lot we don’t know about lamprey immunity (how does it present self-reative receptors, with no thymus?) but what we do know is just so amazing, I’m completely fascinated by it. It beautifully illustrates two of the basic features of evolution — building on previous structures, whether related or not; and alternate solutions to the same problem. Herrrin and Cooper have a short and dense, but very interesting, review, 3 that prompted this particular post.
- That is, in the common ancestor of sharks and mammals, to use a slightly less-sloppy terminology.[↩]
- And of course, the system has continued to evolve. The mammalian system is remarkably similar to the shark in broad strokes, but it’s also very different in many ways.[↩]
- Herrin, B., & Cooper, M. (2010). Alternative Adaptive Immunity in Jawless Vertebrates The Journal of Immunology, 185 (3), 1367-1374 DOI: 10.4049/jimmunol.0903128[↩][↩][↩]
- B. W. Han, B. R. Herrin, M. D. Cooper, I. A. Wilson (2008). Antigen Recognition by Variable Lymphocyte ReceptorsScience, 321 (5897), 1834-1837 DOI: 10.1126/science.1162484[↩]
- Though there are some common pieces that hint at a common ancestor of the two systems, maybe.[↩]