We know that the immune system can destroy tumors. We also know, unfortunately, that by the time we see a tumor, immunity probably won’t destroy the tumor. There are lots of reasons for that. One is that tumors are essentially part of the normal body, so it’s normal for the immune system to ignore them. It looks as if you need to have immunity that’s just right to get rid of a tumor.
Tumors arise from normal self cells, that the immune response has been programmed to ignore. Now, the process of becoming a tumor is not normal, and so tumors are not entirely normal self any more — meaning that there are likely to be some targets in most if not all tumors. But in all but the most reckless tumors the differences between abnormal and normal are relatively small, compared to, say, a virus-infected cell that contains many potential targets.
There’s actually a long list of known tumor antigens; the T-cell tumor peptide database lists many hundreds of them. But most are not truly specific for the tumor. The’re actually normal self antigens; they’re derived from proteins that are overexpressed in tumors, or that are differentiation antigens or “cancer-germline” antigens that are normally also found in self tissues. What’s more, these normal self antigens are the most interesting tumor antigens, as far as clinical utility is concerned. Mutations can make brand-new, non-self targets for the immune system, but they’re going to be sporadic targets, often unique to individual tumors — not something you can prepare for. The normal antigens, though, are likely to be predictable, common targets; it’s conceivable that tumor vaccines can be prepared in advance.
|Human melanoma cell
If these antigens were common (which they are, in some tumor types — like melanoma), and they were good targets for the immune system, then we wouldn’t see much cancer. We do see melanomas quite often, and part of the reason may be that the immune system generally responds quite weakly to these antigens. Why is that? And, more to the point, how can we make the immune system respond more strongly? A recent paper in the Journal of Experimental Medicine offers answers for both of these questions.
From work in the past couple of years, we now have decent estimates of how many T cells there are that can react with any particular target. (See here and here for my discussion of the earlier papers.) A reasonably strong immune response to a non-self epitope might originate from maybe 100 or so precursor T cells. There’s a rather wide range of frequency for these precursor cells, say from 20 to 1000; and to some extent, the fewer T cells there are the weaker (the less immunodominant) the immune response.
We expect T cells against normal self targets to be less common, because they should be eliminated as they mature in the thymus. Some may survive, though, and we would count on these survivors to attack the normal (albeit overexpressed, or abnormally present) target in the cancer cells. But just how rare are they?
Rizzuto et al say they’re really rare (this was in mice, by the way); at least ten times less abundant than T cells against non-self antigens. If you look at the range I gave for “normal” precursors, that could mean there are fewer than 5 or 10 precursors. If the average is “fewer than five”, then quite possibly some mice have only two, or one, or no precursors. You can’t have much of a response with no precursors.
So there’s a weak anti-tumor response because there aren’t many T cells in the body that can respond to the normal self targets in the tumor. That’s not really a surprise, but it does raise the question, What if there were more of the T cells? To ask that question, Rizzuto et al. tried transferring more of these precursor T cells into tumor-bearing mice — starting at around the normal level for a precursor to non-self antigen, and going up from there — and then vaccinating with the appropriate target.
The effects were pretty dramatic. With no supplemental T cells (that is, with the natural, very low, level of T cell precursors) the mice all died of the tumor quickly. At the middle of the range, almost all of the mice rejected the tumor. And at the highest levels of transfers? The mice all died again. Having enough T cells to respond was protective, but putting in too many made them useless.
These results identify vaccine-specific CD8+ precursor frequency as a remarkably significant predictor of treatment and side-effect outcome. Paradoxically, above a certain threshold there is an inverse relationship between pmel-1 clonal frequency and vaccine-induced tumor rejection.
|Mouse melanoma cell
(My emphasis) This paradoxical effect is probably because the numerous T cells started to compete with each other so that none of them were properly activated; they only saw effective-looking polyfunctional T cells at the lower transfer levels.
In other words, if you’re going to transfer T cells to try to eliminate a tumor, more is not necessarily better. Quality and quantity are both important factors, and quantity helps determine quality.
One question I have is how this relates to tumor immune evasion. Many tumor types acquire mutations, as they develop, that block presentation of antigen to T cells. Are these mutations perhaps only partially effective — giving the tumors sufficient protection against the tiny handful of natural precursors they “expect” to deal with, but not against a larger attack after, say, vaccination — or are they more complete, and protective even if the optimal number of T cells are transfered? I’d guess that it would depend on the tumor, but it looks as if it might be a relevant question and it would be nice to have more than a guess.
Our results show that combining lymphodepletion with physiologically relevant numbers of naive tumor-specific CD8+ cells and in vivo administration of an effective vaccine generates a high-quality, antitumor response in mice. This approach requires strikingly low numbers of naive tumor-specific cells, making it a new and truly potent treatment strategy.