TRegs infiltrate a tumor
TRegs infiltrate into a tumor

There’s increasing evidence supporting the notion that tumors are often not rejected by the immune system because regulatory T cells actively block the immune response to the tumor cells. 1

That means that within the tumor, two branches of the immune response are fighting it out. If the TRegs win, the tumor will not be rejected (and may eventually kill the host); if the rejection branch2 wins, the tumor may be rejected and the host may survive a little longer.

Both TRegs and rejection-branch T cells are driven by specific antigen. That is, as opposed to the general patterns that drive innate immune responses, the T cells are activated by peptides associated with major histocompatibility complexes (mainly class II MHC, for the TRegs).

So that raises an interesting question: What specific peptides activate the TRegs in the tumors, and are they different from the ones that activate rejection-type CD4s?

The question is even more interesting than it may seem at first glance, because3 there are different TReg subsets with different peptide preferences. One set of TRegs likes to see ordinary self-peptides: Peptides that are naturally present, and that should not be rejected because, well, they’re part of you. “Normal” rejection-type T cells don’t see those peptides, because those that do are killed during their development (or are converted into TRegs during development, probably). The other group of TRegs sees foreign peptides, that would be expected to be rejected. You need these TRegs as well, because there are times when a chronic immune response, even to a foreign invader, is more harmful than the invader itself; so under those circumstances, some rejection-type T cells get converted into TRegs, and those can shut down the response to the invader, hopefully to reach a happy accommodation.

Are the TRegs in tumors the first kind, that are activated by the normal self-antigens that are present in the tumor cells (which are, remember, originally you to start with)? Or are they the second type, responding to the foreign antigen present in the tumor (mutated proteins, say, or over-expressed growth factors) but converted into a TReg type from a rejection-type when the tumor foreign antigens proved to be a chronic stuimulus?

Reservoir Dogs StandoffA recent paper4 suggests it’s the latter:

This allows us to ask whether tumor-associated Treg cells arise from the repertoire of TCRs used by natural Treg cells or from the repertoire used by effector cells. We show that Treg population in tumors is dominated by T cells expressing the same TCRs as effector T cells. These data suggest that Treg in tumors are generated by expansion of a minor subset of Treg cells that shares TCRs with effector T cells or by conversion of effector CD4+ T cells and thus could represent adaptive Treg cells. 4

If this is generally true (and the authors do offer a helpful series of caveats) it has a very important implication. There’s a huge amount of interest in tumor vaccines — identify an antigen specific for the tumor, and induce a potent immune response to it, in the hope that T cells will then reject the tumor. But you see the problem: If the TRegs are stimulated by the same antigen, then your vaccine is going to increase both sides — the rejection branch and the TReg branch — and you’re no further ahead than when you started! This may be one of the reasons that tumor vaccines have been only intermittently effective. But it does make even more attractive another approach toward cancer immunization, where TRegs are specifically blocked, hopefully allowing the already-present rejection-type5 T cells to kick in and, maybe, eliminate the tumor:

This further suggests that improved cancer immunotherapy may depend on the ability to block tumor-antigen induced expansion of a minor Treg subset or generation of adaptive Treg cells, rather than solely on increasing the immunogenicity of vaccines. 4

  1. I’m not quite comfortable with the phrasing here, but I can’t come up with a non-lawyerly, succinct way to phrase it. TRegs are part of the immune system, and so when they’re active the immune system isn’t blocked, it’s highly functional. What’s being blocked is what we traditionally think of as an immune response — the aggressive response that causes inflammation and that kills targets — while the TReg form is the branch of the immune response that prevents all those things. When TRegs are dominant, the immune response isn’t easily visible, but it’s still an active immune response.[]
  2. Again, not happy with the term; if anyone has a more felicitious phrase, let me know[]
  3. My qualifier here is “For now”, because this is a rapidly-changing field that has kind of outstripped my ability to follow it right now; I’m not quite sure whether this is the consensus view any more[]
  4. Kuczma, M., Kopij, M., Pawlikowska, I., Wang, C., Rempala, G., & Kraj, P. (2010). Intratumoral Convergence of the TCR Repertoires of Effector and Foxp3+ CD4+ T cells PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013623[][][]
  5. Having typed that a dozen times here, I like it less than ever[]