T cell receptor (top) interacting with MHC

It would be nice if I could claim that advances in biology are driven by pure intellectual processes, by hermits on mountaintops achieving new theories through mediation and  deep, pure thoughts. Of course, that’s not the case.  I think its fair to say that many, if not most, of the advances in immunology and virology are driven by new technology. Every so often, some lab comes up with some new way of looking at cells (say, multicolor intracellular staining and flow cytometry) or measuring something about the cells (MHC tetramers, maybe), and we go back to look again at the problems we’ve been struggling, using this new approach, and sometimes the new approach cracks open the problem (usually revealing new and even more interesting problems inside, but that’s why we do this, right?)

(I’m not trying to say that all the advances in the field are technique-driven. Charlie Janeway’s “Dirty Little Secrets” essay didn’t rely on new techniques, and neither did the concept of cross-priming, or lots of others. I’m just saying that new techniques do have a huge influence.)

A particularly cool new technique was just described by Hidde Ploegh, in association with Rudi Jaenisch. 1 Basically, it’s a new way of making TcR-transgenic mice.  TcR transgenics have been around for a long time2 and have led to a quite a few advances in immunology  — they’re now just another tool that’s used in lots of basic research.

thymocytes in the hthymus
Thymocytes developing in the thymus

But making a TcR transgenic mouse is a fair bit of work.  You need to find the T cell you’re interested in, clone out both TcR chains, clone them into the right transgenic vector and transfer them into a stem cell, then make a mouse from that and usually backcross it to a RAG knockout for a dozen generations before you can actually use it. And then you can ask whatever question you had, a couple of years ago when you started all this.

(If that didn’t mean much to you: The TcR is the T cell receptor. It’s what makes T cells specific. Each T cell as it comes out of the thymus has its own, distinct receptor.  It’s distinct at the protein level, and the reason its distinct is that the genome of the T cell is also unique.  The genome of T cells gets sliced and diced and glued back together in a unique way.  If you want to get a duplicate of that receptor you grab that DNA, for both halves of the receptor [that is, the alpha and the beta chains] and plunk it back into other t cells, and hen those t cells will all recognize the same thing.  This is wildly oversimplified, of course, but it’s close enough.)

Ploegh’s group figured a way around most of this, just by cloning a mouse straight from the T cell.

(By the way, saying “just cloning a mouse” — I know we’re living in the future right now. 3  It reminds me of when I was in Worcester, in the early 2000s when Advanced Cell Technology was cloning cattle, seeing a group of protesters at the corner of my street holding signs protesting cloning.  “Ban cloning! No to flying cars! Martians go home!”)

T cell receptor
T cell receptor

Anyway, here in the future, we, or at least Jaenisch, have reached a point where they can quite routinely clone mice from somatic cells; that is, from skin cells or from, say, T cells.  So that’s what they did. They took a T cell that recognized the specific antigen they were interested in,4 and used that to clone out a mouse.  Since the T cell had already undergone its genome rearrangement (and since that can only happen once), all the cells in the new, cloned, mouse ended up with the properly rearranged DNA.  That means the TcR in these mice is fixed, so all the T cells in these mice will recognize the antigen you want, instead of several trillion different antigens.

Essentially these are TcR transgenics, only faster and better.  Better, because, for example, there’s no endogenous TcR to eliminate, so no back-crossing to RAG mice — though they did have to do some back-crossing — and the TcR gene is in the right place under all the right regulation and so on.  They also point out that the standard procedures for making TcR has to start with activated T cells that have been through repeated rounds of stimulation, whereas this approach lets you start with naive cells.  I’m not quite sure this is a huge factor, but I won’t argue the point.

It’s one of those things that seems fairly obvious once it’s done, but (at least to me) was not at all obvious until they actually did it.  I have a feeling that it’s probably not quite as easy as they made it sound, but is still doable by most labs if they really want to do it; so I think it’s something we’re going to see quite a bit of in the next few years. It should help move the mouse field into testing more relevant and accurate systems.

The time-consuming generation of transgenic mouse models has led to the widespread use of a limited number of surrogate antigens, such as ovalbumin (recognized by the OT-I and OT-II transgenic mice) to study the immunobiology of infectious disease. Pathogens engineered to produce fragments of ovalbumin, and the immune reaction against it, are unlikely to capture all essential aspects of the physiological response. 1

  1. Kirak O, Frickel EM, Grotenbreg GM, Suh H, Jaenisch R, & Ploegh HL (2010). Transnuclear mice with predefined T cell receptor specificities against Toxoplasma gondii obtained via SCNT. Science (New York, N.Y.), 328 (5975), 243-8 PMID: 20378817[][]
  2. I think the first one was von Boehmer’s, in 1988:
    Kisielow P, Blüthmann H, Staerz UD, Steinmetz M, & von Boehmer H (1988). Tolerance in T-cell-receptor transgenic mice involves deletion of nonmature CD4+8+ thymocytes. Nature, 333 (6175), 742-6 PMID: 3260350[]
  3. Also, living in the future-wise, I wrote this a first draft of this on my iPad while sitting at my son’s soccer practice. (The iPad turns out to be fine for typing, but not so much for WordPress input — links and images are a problem.)  []
  4. This is the new part — previous clones have been made from lymphocytes, but the target was unknown.[]