Mystery Rays from Outer Space

Clonal evolution during in situ to invasive breast carcinoma progression1

What’s a tumor?

In some ways, that’s a bad question (never mind the answer) because it implies that a tumor is a single thing. But we know that’s not true. A tumor, by the time we can detect it, is a collection of many cells, at least billions of them, and those cells are not all the same. I’m not even talking about the normal cell types that are incorporated into a tumor (things like blood vessels and support cells). Even cells that are unambiguously cancerous are very different within a tumor. And of course, that’s important for the things we’re most interested in, prognosis and treatment, because it’s not the average tumor cell that we’re most concerned about, it’s the subset of tumor cells that are most resistant to treatment, or that are most aggressive.

The development of this variation is really fundamental to how we understand tumor formation and tumor growth. Cancerous cells don’t just appear, fully ready to metastasize and grow. What happens is that a normal cell mutates slightly and gains a little advantage. Most of its progeny stay like that, but one of them mutates again and changes a little more, and then one of that cell’s progeny mutates again, and so on. It probably takes at least a half-dozen mutations, over many cell generations, before a normal cell has progressed through to a detectably cancerous cell.  (I’ve talked about this before, here.)

Also, since truly normal cells simply don’t mutate that many times — there are too many checks and repair systems to allow a half-dozen mutations to accumulate in a single human’s² lifetime — one of the mutations is probably in the check/repair system, turning the cancerous pathway into a mutator pathway as well.

So we expect tumors to be made up of many different cell types, and this is indeed what we see:

With rare exceptions, human malignancies are thought to originate from a single cell, yet by the time of diagnosis, most tumors display startling heterogeneity in cell morphology, proliferation rates, angiogenic and metastatic potential, and expression of cell surface molecules. ¹

So how diverse are tumors?

That’s been a hard question to answer, because you’d need tools to look at individual cells, and you’d also need some way of expressing that diversity. A recent paper¹ looked at diversity in breast cancer using some individual-cell tools, which I’m not going to discuss, and took an interesting approach to describing the variability:

… we applied diversity measures from the ecology and evolution sciences to our copy number data. These diversity measures estimate the number and distribution of species in a certain geographical area or environmental niche. In our context, a species is a cancer cell population … Hence, a region of a tumor containing cancer cells with 3 different copy number ratios is interpreted to contain 3 distinct “species.” ¹

They suggest that this way of describing tumors could be a useful aid to prognosis and to predicting response to therapy, offering a quantitative description of tumor variability (which might correlate with the tumor’s potential for spread and escaping treatment).

I hadn’t thought of tumors as ecosystems before, but I wonder if the analogy could be taken further by considering, say,cytotoxic T lymphocytes as predators …

1. Park, S., Gönen, M., Kim, H., Michor, F., & Polyak, K. (2010). Cellular and genetic diversity in the progression of in situ human breast carcinomas to an invasive phenotype Journal of Clinical Investigation DOI: 10.1172/JCI40724[↩][↩][↩][↩]
2. let alone a mouse’s lifetime[↩]

We’re just dipping our toes into the oceans of information from large-scale genome sequencing. We’re at the point now where sequencing a human genome is, not routine, but not extraordinary. The most recent examples of this are two groups who sequenced the genome of a cancer (one group did a lung cancer, the other did a melanoma), and compared to the person’s normal cells. ¹   This lets you see where the cancer cells are mutated.

How many mutations are there in a cancer? We already know that cancer is a multi-step process, involving probably at least 7 or 8 distinct stages. We also know that cancer cells have far more mutations than are needed for these minimals steps. How many is “more”?

• Over 20,000 mutations – 23,000 mutations in the lung cancer, 33,000 in the skin cancer.

Where did these mutations come from? What drives mutagenesis in a cancer cell?

• Cigarettes and UV light. They can point out the typical kinds of mutagenesis for each and show that the lung cancer mutations are tobacco-induced, the skin cancer mutations are UV-induced.

How often do cigarettes cause mutations?

• “… an average of one mutation for every 15 cigarettes smoked.”

(I question this figure, or rather, question whether the implied causation is that direct. But it’s not impossible, given their data.)  From an immunological viewpoint, the 20,000 mutations is interesting because it suggests that cancers should have lots of targets for the immune system. This was already pretty clear, but this helps nail it down.

(By the way, the poster at the top, like the research in question, comes from the Wellcome Trust Institute.)

1. Pleasance, E., Stephens, P., O’Meara, S., McBride, D., Meynert, A., Jones, D., Lin, M., Beare, D., Lau, K., Greenman, C., Varela, I., Nik-Zainal, S., Davies, H., Ordoñez, G., Mudie, L., Latimer, C., Edkins, S., Stebbings, L., Chen, L., Jia, M., Leroy, C., Marshall, J., Menzies, A., Butler, A., Teague, J., Mangion, J., Sun, Y., McLaughlin, S., Peckham, H., Tsung, E., Costa, G., Lee, C., Minna, J., Gazdar, A., Birney, E., Rhodes, M., McKernan, K., Stratton, M., Futreal, P., & Campbell, P. (2009). A small-cell lung cancer genome with complex signatures of tobacco exposure Nature DOI: 10.1038/nature08629

   Pleasance, E., Cheetham, R., Stephens, P., McBride, D., Humphray, S., Greenman, C., Varela, I., Lin, M., Ordóñez, G., Bignell, G., Ye, K., Alipaz, J., Bauer, M., Beare, D., Butler, A., Carter, R., Chen, L., Cox, A., Edkins, S., Kokko-Gonzales, P., Gormley, N., Grocock, R., Haudenschild, C., Hims, M., James, T., Jia, M., Kingsbury, Z., Leroy, C., Marshall, J., Menzies, A., Mudie, L., Ning, Z., Royce, T., Schulz-Trieglaff, O., Spiridou, A., Stebbings, L., Szajkowski, L., Teague, J., Williamson, D., Chin, L., Ross, M., Campbell, P., Bentley, D., Futreal, P., & Stratton, M. (2009). A comprehensive catalogue of somatic mutations from a human cancer genome Nature DOI: 10.1038/nature08658 [↩]

TRegs infiltrate into a tumor

One of the reasons the immune system doesn’t destroy tumors is the presence of regulatory T cells (TRegs) that actively shut down the anti-tumor response.  For once, there’s a little bit of encouraging news on that front.

TRegs are normal parts of the immune system.  They actively prevent other T cells (and so on) from attacking their target. ¹  What’s more, TRegs are antigen-specific.  That is, they recognize a specific target, just as do other T cells, but instead of responding by, say, destroying the cells (like  cytotoxic T lymphocyte) or by releasing interferon (like a T helper cell) a TReg’s response to antigen is to prevent other T cells from doing anything in response to that antigen.  In other words, TRegs cause an antigen-specific inhibition of the conventional immune response. ²

Back to tumors.  We know that immune responses don’t routinely eliminate tumors by the time they’re detectable.  There is some evidence that lots of very small, proto-tumors, are in fact destroyed by the immune system very early on, before they’re clinically detectable, but those tumors that survive that attack seem to be pretty resistant to immune control.  At least part of that resistance is because TRegs get co-opted into the tumor’s control (see here, and references therein, for more on that).

So if TRegs are antigen-specific, and TRegs control immune responses to the tumor, what are the tumor antigens that are driving the TRegs?

I would have assumed that TRegs are looking at many, many tumor antigens, including both normal self antigens³ as well as classical tumor antigens.⁴  But a recent paper⁵ suggests, to my surprise, that this assumption is wrong.  Instead, “Tregs in tumor patients were highly specific for a distinct set of only a few tumor antigens“. ⁵ What’s more, eliminating TRegs cranked up the functional immune response, but only to those antigens TRegs recognized — as you’d expect, if the suppression is indeed antigen specific.

This is interesting for several reasons.  If TRegs can be specific for tumor antigens, then at least in theory ((In practice, we don’t quite have the tools yet, I think) it should be possible to turn off these TRegs while leaving the bulk of TRegs intact (and therefore not precipitating violent autoimmunity).  It also suggests that if the TRegs are only suppressing a subset of effector T cells, there’s something else preventing most effector T cells from, well, effecting.  Maybe those are antigen non-specific TRegs, or maybe there’s something else we need to know about.

I’d like to see this sort of study replicated, and a little more fine-tuning on identifying the TReg’s targets (the readout was intentionally fairly coarse here, in order to identify as many as possible).  Still, it’s an unexpected, and potentially very useful, observation.

1. It’s still not quite clear how they do this[↩]
2. There are also antigen-nonspecific TRegs, but we will ignore them for now.  They’re not as effective as the antigen-specific sort, anyway.[↩]
3. Because TRegs, unlike most immune cells, can be stimulated by normal self antigens[↩]
4. That is, antigens that are mutated, or dysregulated, and that therefore act as standard targets for immune cells[↩]
5. Bonertz, A., Weitz, J., Pietsch, D., Rahbari, N., Schlude, C., Ge, Y., Juenger, S., Vlodavsky, I., Khazaie, K., Jaeger, D., Reissfelder, C., Antolovic, D., Aigner, M., Koch, M., & Beckhove, P. (2009). Antigen-specific Tregs control T cell responses against a limited repertoire of tumor antigens in patients with colorectal carcinoma Journal of Clinical Investigation DOI: 10.1172/JCI39608[↩][↩]

TRegs in normal skin

Tumors are supposed to be destroyed by our immune system. So how come we still see tumors?

A big part of the answer is probably that our immune system is very good at destroying proto-tumors, but is not so good at handling those that manage to sneak through and grow to the point of detectability. That splits the first question into two questions: Why do some proto-tumors manage to sneak through, not being eliminated by the immune system? And why is it that detectable tumors are not effectively handled?

The first part, I think, may often be related to cell-intrinsic immune escape mutations. That is, pre-cancerous cells are constantly being attacked by the immune system; in turn (if they survive long enough) they constantly mutate, doing things like damaging the antigen-presentation pathway that makes them recognizable by the immune system. Eventually, they find some configuration that reduces the rate at which they’re killed. Once cancer cell replication is even fractionally greater than destruction,¹ a tumor can begin to grow.

So that’s probably the earliest stage of tumor growth. But once tumors reach a certain size, a second factor kicks in. Chronic immune responses are dangerous; after all, the whole point of the immune system is to kill things. The chronic immune response against the growing tumor is now shut down. This has been understood for quite a while — the immune system often becomes “tolerant” of a tumor. More recently, it’s become clear that it’s not merely “tolerance” (which implies that the immune system is simply benignly ignoring the tumor); the presence of a tumor actively forces the immune system to shut itself down, slamming on the brakes rather than just peacefully coasting by.

Brakes are a fundamental part of an active immune response. If you look at diagrams of normal immune responses, they show inverted “U” shaped curves (in here and here, for example), where the response is triggered, rapidly ramps up, hopefully does its thing, and then just as rapidly shuts down to near-background levels once again. There used to be a sort of general feeling that this was a rather passive thing — pathogen stimulates response, response destroys pathogen, no more stimulus, response goes away — but now we understand that the shut-down phase is just as active and dynamic as the upward curve. Just as with the upward phase, there are all kinds of different mechanisms to control the response; one of the most important is the “Regulatory T cell” (TReg).  And it’s pretty clear that TRegs are involved in controlling the immune response to tumors (I talked about that here, and links therein).

TRegs have been known for a while (I gave a brief history, including the I-J fiasco, here). The usual description of a TReg includes a number of markers;² one of the most basic is CD4. CD4 T cells used to be lumped together as “T Helper” cells, but now we have multiple sub-specialties in the CD4 category, and TRegs are one of those specialities.

More recently, TRegs — or at least cells that function the same way as TRegs — have been described in the CD8 population of T cells.³ CD8 T cells are traditionally called “Cytotoxic T lymphocytes” (CTL) (although it’s been increasingly clear that cytotoxicity is just one of many functions a CD8 T cell can offer), but it seems that these variants of CD8s can actively shut down an ongoing immune response, in a specific and targeted way. There seems to be a trend to calling these cells “suppressor cells” rather than “TRegs”. “Suppressor T cells” is an older term that was out of favor for a while, but it’s probably useful to bring it back and distinguish between the natural TRegs and some of the other cells that can do something similar but that have different sources and origins.

At least some of the CD8 suppressor T cells can arise from apparently-conventional CD8 T cells. That is, you can pull CD8 T cells out of a normal mouse’s spleen, and depending on what those cells see and are exposed to, they could progress to being conventional CTL — killing tumor cells, producing interferon and other cytokines, generally being a destructive force — or they could become suppressor CD8 T cells, and actively prevent that destruction from happening.

It turns out that one of the forces that can drive a CD8 T cell into being a suppressor T cell is a tumor. A recent paper from Arthur Hurwitz’s lab⁴ shows this quite clearly. They had shown previously that transferring specific CD8 T cells into a tumor-bearing mouse resulted in what they called “tolerance”.⁵ But now they demonstrate that it’s more than that; the transferred CD8s are converted into suppressor T cells that actively shut down immune responses.

Tumor-infiltrating TcR-I cells suppressed the in vitro proliferation of both melanoma Ag-specific CD8+ (37B7) T cells and OVA-specific CD4+ (OT-II) T cells. … Even at a ratio of one TcR-I cell to four responder T cells, we observed 30% suppression of proliferation. ⁴

This isn’t the only way that tumors escape immune recognition, but (at least for some tumors) it may be an important one. It’s clearly an important consideration for things like tumor vaccines and immune therapy, because it suggests that immunizing with tumor antigens (and thereby generating lots of tumor-specific CD8 T cells) may actually increase the suppressive effect of the tumor.

The conversion of CD8+ effector T cells into suppressor cells may be one mechanism by which tumors restrict the immune response from effectively controlling tumor growth. As subsequent effectors infiltrate the tumor, either following peripheral sensitization or as a result of adoptive transfer therapy, the induced regulatory cells may suppress these new effectors and reduce their ability to confer tumor immunity. This cyclic suppressive process may contribute to the profound loss of antitumor responses following adoptive immunotherapy. ⁴

(My emphasis.) On the other hand, if this is a common mechanism, then overriding it — which should be possible, using cytokines, specific T cell subsets, and/or targeted receptor ligands — may switch the suppressive population abruptly back into an effector group, turning the brainwashed traitors into resistance fighters.

1. Destruction would include far more than immune destruction, of course — it would include cells that become differentiated and no longer replicated, cells that outgrow their oxygen supply, cells that undergo apoptosis, and so on[↩]
2. FoxP3, CD25, and so on[↩]
3. I’m not sure who made the first identification; this looks as if it’s one of those fields where there were incremental advances, hinting more and more strongly at the presence of these cells, but with no single clearcut starting point. Papers in the early 2000s start to point at regulatory CD8s, and by 2004 a handful of relatively high-profile papers fairly solidly identified them. A 2004 review paper is
   Zimring, J., & Kapp, J. (2004). Identification and Characterization of CD8+ Suppressor T Cells Immunologic Research, 29 (1-3), 303-312 DOI: 10.1385/IR:29:1-3:303[↩]
4. Shafer-Weaver, K., Anderson, M., Stagliano, K., Malyguine, A., Greenberg, N., & Hurwitz, A. (2009). Cutting Edge: Tumor-Specific CD8+ T Cells Infiltrating Prostatic Tumors Are Induced to Become Suppressor Cells The Journal of Immunology, 183 (8), 4848-4852 DOI: 10.4049/jimmunol.0900848[↩][↩][↩]
5. Anderson MJ, Shafer-Weaver K, Greenberg NM, & Hurwitz AA (2007). Tolerization of tumor-specific T cells despite efficient initial priming in a primary murine model of prostate cancer. Journal of immunology (Baltimore, Md. : 1950), 178 (3), 1268-76 PMID: 17237372[↩]

A new technique called optical frequency domain imaging (OFDI) provides amazing images of tumors (especially their blood vessels) in situ.

“(a) OFDI images of representative control and treated tumors 5 d after initiation of antiangiogenic VEGFR-2. The lymphatic vascular networks are also presented (blue) for both tumors. (b) Quantification of tumor volume and vascular geometry and morphology in response to VEGFR-2 blockade.”

– Vakoc, B., Lanning, R., Tyrrell, J., Padera, T., Bartlett, L., Stylianopoulos, T., Munn, L., Tearney, G., Fukumura, D., Jain, R., & Bouma, B. (2009). Three-dimensional microscopy of the tumor microenvironment in vivo using optical frequency domain imaging Nature Medicine DOI: 10.1038/nm.1971

Pregnant woman (Ivory Coast, West Africa)

Recently I’ve mentioned a few cases of transmissible tumors — that is, cases where tumors actually spread from their original host, to other individuals. The two most dramatic transmissible tumors are Canine Transmissible Venereal Tumor (CTVT) and Tasmanian Devil Facial Tumor (TDFT), where the original tumor can spread widely throughout the entire species. (See this post, this one, and this one, for more detail.) There’s also at least one case of a tumor that accomplished a single transmission, from the original patient to the surgeon who operated on him. ¹

Tumors aren’t supposed to be able to spread in this way, because they’re essentially foreign transplants — they should be rapidly rejected, as if they were, say, a skin graft between two random people. In this post I talked about how CVTV and TDFT might have arisen. (There are also a number of cases of tumors that spread to immuno-suppressed individuals, such as after organ transplants, but those cases are easier to understand from an immunologic viewpoint.)

When checking up on references for the last post, I ran across a set of transmissible tumors I hadn’t known about: Vertically transmitted tumors, in which tumors spread from a pregnant mother to the fetus in utero.²

This also, I’m glad to say, seems to be very rare, as you’d expect. Even though mother and child are partially tissue-matched, and even though pregnancy is a very special situation, immunologically, the parent and her child are not genetically identical, and should reject grafts from each other pretty efficiently. (Transplants from parent to child still require immune suppressive treatment.) The review I ran across lists a total of 14 cases of vertical spread of tumors, from 1866³ to 2002.⁴ Although they do note that:

Given the lag time between birth and diagnosis in several of the infants, cases of maternal–fetal transmission may not be as rare as the literature would suggest, and the number of cases could be higher as the detection of metastatic tumor in the fetus may go undetected in cases of abortion or maternal–fetal demise. ²

Malignancy during pregnancy isn’t all that uncommon (0.1% of pregnancies, it says here), so the handful of cases with actual spread of the tumor to the fetus are “numerically inconsequential”. What was different about these 14 cases? We don’t really know, in general. Almost all of the described cases are earlier than 1965,⁵ predating the molecular era of medicine. Perhaps some, or many, of the infants were immune compromised, as the authors note:

Fetuses with a congenital immunodeficiency are likely to be at an even higher risk for the engraftment of such tumor cells.⁶ Other factors that may affect the likelihood of tumor cells entering the fetal circulation include maternal homozygosity for one of the fetal HLA haplotypes,⁷ metastatic potential of the maternal tumor, and a high maternal blood and/or placental tumor load. ²

The outcome of this transmission was very poor; only 3 of the 14 children survived the disease.

I don’t really have any lesson to draw from these cases. Without an extensive molecular workup that isn’t available for almost all of these cases, I don’t know that we can learn much about tumor transmission. Still, these stories are worth keeping in mind when thinking about mechanisms of tumor transmission.

1. Gartner HV, Seidl Ch, Luckenbach C, et al. Genetic analysis of a sarcoma accidentally transplanted from patient to a surgeon. N Engl J Med 1996;335:1494–1496.[↩]
2. Tolar J, & Neglia JP (2003). Transplacental and other routes of cancer transmission between individuals. Journal of pediatric hematology/oncology : official journal of the American Society of Pediatric Hematology/Oncology, 25 (6), 430-4 PMID: 12794519[↩][↩][↩]
3. Friedreich N. Beitrage zur pathologie des Krebses. Virchows Arch 1866; 36:465–477.[↩]
4. Tolar J, Coad JE, Neglia JP. Transplacental transfer of small cell carcinoma of the lung. N Engl J Med 2002; 346:1501–1502.[↩]
5. Not saying the molecular medicine abruptly switched on in 1965, it’s just a convenient cutoff[↩]
6. Pollack MS, Kirkpatrick D, Kapoor N, et al. Identification by HLA typing of intrauterine-derived maternal T cells in four patients with severe combined immunodeficiency. N Engl J Med 1982; 307:662–666.[↩]
7. Osada S, Horibe K, Oiwa K, et al. A case of infantile acute monocytic leukemia caused by vertical transmission of the mother’s leukemic cells. Cancer 1990; 65:1146–1149.[↩]

[Cancer] mortality has been systematically decreasing among younger individuals for many decades. … the cancer mortality rates for 30 to 59 year olds born between 1945 and 1954 was 29% lower than for people of the same age born three decades earlier.  … substantial changes in cancer mortality risk across the life span have been developing over the past half century in the United States. … this analysis suggests that efforts in prevention, early detection, and/or treatment have significantly affected our society’s experience of cancer risk.¹

But:

The mortality decline we describe in this paper cannot therefore be attributed to an overall decline in cancer incidence. Rather, the net improvement in cancer mortality in birth cohorts born since 1925 seems to reflect a succession of public health and medical care efforts. ¹

They point to reduction in smoking, effective treatment of childhood leukemias and lymphomas and testicular cancers of young adulthood, and “increasingly successful screening programs for breast, prostate, and colon cancer” as important factors, and add “We are optimistic that ongoing efforts in very early cancer prevention (such as use of HB and human papillomavirus vaccines), as well as ongoing clinical trials of targeted therapies, will preserve the downward trend of cancer mortality“.

1. Kort, E., Paneth, N., & Vande Woude, G. (2009). The Decline in U.S. Cancer Mortality in People Born since 1925 Cancer Research, 69 (16), 6500-6505 DOI: 10.1158/0008-5472.CAN-09-0357[↩][↩][↩]

Canine Venereal Tumor phylogeny

Bayman commented, after reading this post:

So isn’t the real question why can’t all tumors be transmissible? If you believe the tumor immunologists, all tumors should be capable of avoiding T cell attack…no??

I don’t have answers, but I can speculate a little. ¹

Very quick background: In general, tumors are unique. They arise independently each time, and when their host dies, the tumor dies too. That’s in contrast to pathogens, whic may or may not kill their hosts, but which survive and are transmitted to a new host; pathogen infections are not unique, they have a long evolutionary history reaching back through many individual hosts. Tumors can’t do this, for the same reason that skin grafts are rejected by unrelated animals — tumors are essentially unrelated grafts, and should be very rapidly rejected by the new host.

But in very rare circumstances — there are two known instances, and a couple of other possible ones — tumors have arisen that can be transmitted from one host to another.  The two cases are canine transmissible venereal tumor, and Tasmanian Devil facial tumor.  There have been suggestions that these tumors are unique in some immunological way, but I am not convinced by those arguments: See this post and this one for more background.  That’s not to say that these tumors have no ways of evading the immune system; what I am saying, is that virtually all tumors have some way of evading the immune system, and the functions that have been convincingly described for the transmissible tumors don’t seem all that exceptional for tumors in general.

So, if these tumors can be transmitted, and they aren’t all that extraordinary immunologically, what does make them extraordinary?  As I say, I don’t know, but based on tumor immunology as I understand it, I can make some guesses.

The most important factor, I suspect, has nothing to do with immunology.  These tumors are unusual in that they have a built-in way of contacting new hosts. TDFT is spread through bites, CTVT is spread sexually.  There’s no similar way that, say, a liver tumor, or a brain tumor, could be spread.  So that immediately rules out the vast majority of tumors; even if they could survive after transmission, there’s no chance of a transmission chain. ²  But still, most tumors would be rejected even if they did manage to be transmitted.

The Three E’s of tumor immunity

What seems to happen with most tumors³ is that proto-tumors appear quite early, but are controlled by the immune system – perhaps for years — and never become detectable.  Many such proto-tumors are completely eliminated by the immune system, and we have no way of telling that they even existed.  Many more are controlled at the half-dozen cell stage, much too small to detect; they aren’t eliminated by the immune system, but they can’t escape and grow either.  A very small percentage of these equilibrium tumors, though, eventually find a way of at least partially escaping from immune control, and begin to grow. (Perhaps the immune system kills 99% of the new cells, but a 1.01% growth rate compounds itself fast enough to be eventually detectable.)  This is the “Three E’s” theory of tumor growth (discussed more here and here) — “Elimination, Equilibrium, Escape”.

Regulatory T cells and cancer

The Three E’s apply to the very small proto-tumors. But there is probably another factor that kicks in once the tumor becomes larger.  Tumors are themselves immunosuppressive — they shut down immunity throughout the entire body, to some extent, but they shut down immunity to themselves very powerfully.  The immune system has powerful safeguards that prevent it from attacking its own body; broadly speaking, tumors are their own body, and in many cases tumors probably also have been selected to massively amplify the normal protective signals. ⁴ (See this post, and this one, for more on that.)

So here⁵ is my speculation.  We suspect that the ability to be transmitted is present in several tumors, but they never get the opportunity to transmit.  Of those rarities that do get transmitted, most are rapidly rejected, as foreign grafts.  But a tiny minority of this minority may be able to survive because they have powerful immune suppression abilities on top of their common immune evasion abilities.

Why did the Tasmanian Devil cross the road?

Were CTVT and TDFT just lucky — just happened to have the right immune suppressive abilities?  I don’t think so.  I think they were in the right place at the right time.  They were tumors that had a mechanism for transmission, and that had some ability to immune suppress, but they would normally have been rejected as foreign grafts.  Except that both of these tumors, I think, arose at a time and place where their population was highly inbred.  CTVT arose, we speculate, as dogs were becoming domesticated; probably a small, inbred, closely-related population.  Tasmanian Devils in general may not be closely related, but I suspect there are sub-populations⁶ that were closely related and that would not have rapidly rejected skin grafts.  The early version of the respective tumors would not have been rapidly rejected by these closely-related new hosts, giving them a chance to establish their own immune suppressive regime.

Now we have the chance for natural selection of the tumors.  Variants with more powerful immune suppression could spread to a wider range of hosts; variants with standard immune suppression died out with their victims.  In dogs, this natural selection could occur over time; as dogs became gradually more variable, there would be continuous new selection for new tumors that could keep up with the dogs. ⁷ With the Devils, the selection would be over space: The tumors would be selected for their ability to spread within new sub-populations of the Devils, perhaps through gradually more distantly-related subgroups. Eventually, we see the tumors as being capable of transmission and growth throughout the entire population, but the original tumor might not have had this ability.

Rapid MHC diversity in Channel Island Foxes

This model suggests that humans are probably not at great risk of having a transmissible tumor spread in us; and the same is true for most species.  You need the combination of an inbred sub-population with a mechanism of tumor spread and the right kind of tumor. And inbred populations are usually a transient thing; MHC becomes diverse very rapidly, and then the window for tumor establishment is closed.

But this is just a guess, so don’t be too comforted.

1. And by the way, I disagree with Bayman’s suggestion here (“Tumor Immunology Is A Waste of Time”) that he “find[s] it impossible to believe that effective therapy will ever achieved by artificially stimulating the immune system to attack weak and largely self antigens.” But this post is already too long, so I’ll save my answer for another time.[↩]
2. There’s at least one case of a surgeon who apparently contracted a patient’s tumor after cutting himself during surgery — given the option, perhaps many more tumors could be transmissible, but don’t get the chance.[↩]
3. Not necessarily those induced artificially, with high doses of carcinogens or with powerful oncogenes, but with those that arise naturally, in older individuals[↩]
4. I suspect that tumors have many ways of achieving this localized immune suppression.  I also suspect that different tumors have different dependence on this localized immune suppression.  Those tumors that were highly successful as proto-tumors might already be very good at avoiding immunity — for example, they may secrete tons of TGF?, or otherwise have very powerful TReg-inducing abilities — and only need to shut down a little.  Those that barely squeaked by as proto-tumors, may have very potent immune suppression.  I don’t think the mechanisms for this tumor-based immune suppression are very well understood, though over the next couple years they probably will be. [↩]
5. Finally![↩]
6. Subpopulations that are now, probably, extinct, because of the tumors[↩]
7. I’m told that CTVT is eliminated faster or slower in different dogs.  It would be very interesting to correlate this with MHC types, to see if there’s still some effect of rejection even after 50,000 years of selection on these tumors.[↩]

We know that viruses cause a significant minority of human cancers, but we don’t know quite how many, or which, cancers are viral. It’s not as easy as you might think to tell.

The link between viruses and cancer was one of the major breakthroughs in cancer biology, but you could also make a case that that link set cancer research back several years. (Cancer viruses were shown, in chickens, in 1911¹, but it wasn’t until the 1960s that interest in the concept took off, with Epstein’s demonstration of Epstein-Barr virus (EBV)² in some human tumors. ) Studying viral oncogenesis has led to huge advances in our understanding of the fundamental biology of cancers. The problem was that there was a general assumption, in the 1960s and 1970s, that viruses were directly responsible for the vast majority of human tumors. If so, all we needed to do was to identify the viruses, develop vaccines against them, and voila! No more cancer!

Of course, it wasn’t that easy. For all the fundamental advances from this concept, it’s been relatively unproductive as far as the bedside is concerned. Most human tumors are not caused by viruses,³ and even those that are, have been really resistant to treatment via direct anti-viral approaches.⁴ The research units that were established in the 1970s to look at the virus/tumor connection have mostly either disbanded, or taken a different direction now.

Mouse polyomavirus

In spite of that, there’s still new and exciting stuff coming out. Most recently,⁵ Yuan Chang and Patrick Moore identified a new cancer-causing virus of humans. (This was their second breakthrough virus; in 1994⁶ they also found the second-most recent cancer-causing virus of humans, Kaposi’s Sarcoma Herpesvirus.)) This brings to six the number of clearly-linked human cancer viruses: papillomaviruses, HTLV-1, hepatitis B virus, EBV, Kaposi’s Sarcoma Herpesvirus, and the new one, Merkel cell polyomavirus.

I keep meaning to talk about the discovery of Merkel cell polyomavirus, but I’ll set that aside for now. Very briefly, last fall Chang and Moore showed that the presence of the virus is linked to a fairly rare tumor, Merkel cell carcinoma, and they and others have now confirmed the epidemiological association and demonstrated a mechanism for causing tumors. We’re at the stage now of trying to understand the normal biology of the virus. As with most (and all human) cancer viruses, the virus is much more widespread than the tumor. How widespread it is? How does it spread? Does it cause other disease — in particular, is it involved in other tumors?

That last is a particularly interesting question, because although Merkel cell tumors are rare, there are a number of common tumors that could, conceivably, also be caused by the virus; and if the virus causes even a subset of those, then it could be a common cause of cancer rather than an unusual one. So far, though, I think the evidence suggests that the virus is fairly limited in the damage it causes; quite a few groups have looked for the virus in other types of cancer, and although it may be occasionally found⁷, that most likely reflects general background infection with the virus rather than a causative role.

One exception is a recent paper⁸ which suggested that the virus might be associated with a subset of squamous cell carcinomas. SCC are a pretty common form of skin tumor, so if MCPyV is a cause of even a subset of SCC it might be a significant cause of cancer.

The problem is that the other studies on MCPyV have shown that it’s out there even in normal, non-diseased humans⁹ — as you’d expect; the virus must be able to spread and circulate within the human population somehow, and the version of the virus found in Merkel cell tumors is damaged and likely can’t spread, so there must be virus replicating in normal tissues. In this paper, the authors find the virus in a subset of SCC cancers, but I would like to know how many they’d find in normal human skin using the same techniques –  the 15% of positive tumor samples may or may not be significantly different. On the other hand, the SCC-associated virus did show a similar molecular signature to that found in Merkel cell cancer,¹⁰ suggesting a causative role. Right now, I’m not completely convinced, but am definitely intrigued.

One really interesting point to add is that — if MCPyV really does cause a significant number of tumors — then it’s been missed all these years, despite high interest in searching for human cancer viruses, until new techniques were applied to the right samples in the right way. Are there other human cancer viruses out there, waiting for the right technique? Is it still possible that most human cancers are caused by viruses? I think that’s pretty unlikely, but the door is still open a crack.

1. A sarcoma of the fowl transmissible by an agent separable from the tumor cells. Rous, P. 1911. J. Exp. Med. 13:397–411.[↩]
2. Virus particles in cultured lymphoblasts from Burkitt’s lymphoma. MA Epstein, BG Achong and YM Barr. Lancet 1 (1964), pp. 702–703.[↩]
3. The usual educated guess is 20%[↩]
4. One major exception, of course, being papilloma virus vaccines, which are a direct outcome of this line of research. But it’s taken a long time to come to fruition. You could also point to vaccination against hepatitis B virus, and a number of veterinary vaccines, as clinical advances arising from the virus/cancer research. But I think it’s fair to say that the energy put into this has been fundamentally, but not clinically, well spent.[↩]
5. Feng, H., Shuda, M., Chang, Y., & Moore, P. (2008). Clonal Integration of a Polyomavirus in Human Merkel Cell Carcinoma Science, 319 (5866), 1096-1100 DOI: 10.1126/science.1152586[↩]
6. Identification of herpesvirus-like DNA sequences in AIDS-associated Kaposi’s sarcoma.
   Chang Y, Cesarman E, Pessin MS, Lee F, Culpepper J, Knowles DM, Moore PS.
   Science. 1994 Dec 16;266(5192):1865-9.[↩]
7. For example, Human Merkel cell polyomavirus infection I. MCV T antigen expression in Merkel cell carcinoma, lymphoid tissues and lymphoid tumors.
   Shuda M, Arora R, Kwun HJ, Feng H, Sarid R, Fernández-Figueras MT, Tolstov Y, Gjoerup O, Mansukhani MM, Swerdlow SH, Chaudhary PM, Kirkwood JM, Nalesnik MA, Kant JA, Weiss LM, Moore PS, Chang Y.
   Int J Cancer. 2009 Sep 15;125(6):1243-9[↩]
8. Dworkin, A., Tseng, S., Allain, D., Iwenofu, O., Peters, S., & Toland, A. (2009). Merkel Cell Polyomavirus in Cutaneous Squamous Cell Carcinoma of Immunocompetent Individuals Journal of Investigative Dermatology DOI: 10.1038/jid.2009.183[↩]
9. Human Merkel cell polyomavirus infection II. MCV is a common human infection that can be detected by conformational capsid epitope immunoassays.
   Tolstov YL, Pastrana DV, Feng H, Becker JC, Jenkins FJ, Moschos S, Chang Y, Buck CB, Moore PS.
   Int J Cancer. 2009 Sep 15;125(6):1250-6[↩]
10. Shuda, M., Feng, H., Kwun, H., Rosen, S., Gjoerup, O., Moore, P., & Chang, Y. (2008). T antigen mutations are a human tumor-specific signature for Merkel cell polyomavirus Proceedings of the National Academy of Sciences, 105 (42), 16272-16277 DOI: 10.1073/pnas.0806526105 [↩]

A year or so ago, the first time I mentioned transmissible tumors, I dismissed Tasmanian Devil Facial Tumor disease as not particularly surprising. As I said more recently, I’ve changed my mind about that, almost entirely because of a chat with Elizabeth Murchison at the Origins of Cancer symposium last month, who told me a couple of things I hadn’t known about the situation.

TDFT is a transmissible tumor, a cancer that arose in one Tasmanian Devil some time ago (at least a decade ago, and maybe much longer) and that is now spreading throughout the Tasmanian Devil population — killing a vast number of them. It’s a tumor that’s spread when these highly territorial animals¹ bite each other on the face; the tumor then grows on the bitten animal’s face and eventually makes it unable to eat properly. Nearly 90% of Tasmanian Devils, in affected areas, have died. This is a catastrophe.

(The good news, such as it is, may be that the Devils as a population are adapting to the tumor to some extent — not by resisting the tumor, though, but through rapid selection for much earlier breeding. ² By reproducing earlier, the population may be maintained even though the Devils are dying as adults. Of course, the population that “survives” won’t be the same as those the existed earlier — they’ll be younger, they’ll have all kinds of different characteristics because they’re no longer being selected for adult survival — but at least it might open a window to save the species.)

Cancers are not, in general, transmissible.  Each new cancer is a new event, that arises from normal cells of the affected individual.  A transmissible cancer would be like an organ transplant — it would be rapidly rejected, because individuals in a population have different MHC molecules, and the immune system rapidly and aggressively rejects cells with the wrong MHC. So there are two possible reasons why a tumor could, in fact, become transmissible. Either the individuals in a population do not have different MHC molecules on their cells, or the tumor can transmit between individual in spite of different MHC molecules.

The explanation for the spread of TDFT seemed to be the former: It was claimed that Tasmanian Devils have very little MHC diversity, so that they can’t reject the tumors³. Even though MHC is normally highly diverse in a population, there are certainly populations — especially small, island, threatened populations, like Tasmanian Devils — that have very limited MHC diversity, so this seemed reasonably plausible. ⁴ This would be a fairly well-understood mechanism of tumor spread, which is why I said it wasn’t that interesting.

The other possibility is that the tumor could spread between individuals in spite of them having different MHC molecules. This is not supposed to happen, according to our understanding of MHC and organ transplants; but in fact, it’s the way the only other extant transmissible tumor (Canine Transmissible Venereal Tumor) spreads.  We have no idea why that happens, which makes it interesting. (A number of ways have been proposed by which CTVT avoids rejection. I won’t go into them in any detail here: none of them, as far as I can see, are unique to CTVT, but rather are very common molecular changes in tumors of all kinds, and yet other tumors are not transmissible; so I don’t consider any of these suggestions to explain the unique feature of CTVT.)

Anyway, as I say, the explanation for TDFT was simple enough: The tumor spread because Tasmanian Devils aren’t genetically diverse. But there are three major arguments, I’ve learned, that say that explanation is wrong.

First: Tasmanian Devils are not, in fact, spectacularly genetically homogenous. ⁵ They’re not as diverse as you’d like to see, but they’re not completely homogenous. In particular, there are two clear, genetically distinct, populations of Devils in Tasmania, one in the East, and another in the Northwest.

Second: Murchison told me that Tasmanian Devils — even those in the same sub-population — vigorously reject each others’ skin grafts. This is what’s supposed to happen with skin grafts, of course. It implies that the Devils do not, in fact, have the same MHC; and in my opinion it’s a much stronger experiment than those in the original homogenous-MHC paper. ³ If Devils reject skin grafts from each other, then they ought to reject tumors from each other — in other words, even if the tumor can take in one individual, then it should be rejected in another, so the tumor should not spread throughout the population. The skin graft finding hasn’t, as far as I know, been published, but if it holds up, it’s a strong argument against homogenous MHC.

Third: Murchison also told me that the tumor is spreading into Devils in the Northwest. ⁶ As I said, the Northwestern and Eastern Tasmanian Devils are clearly distinct populations, with different genetic characteristics. ⁷  If the tumor can spread between them, then the tumor isn’t relying on genetic homogeneity for its transmission.

So the evidence for MHC homogeneity is not good, the evidence that the tumor requires MHC homogeneity is not good, and the only precedent we know of, CTVT, does not require MHC homogeneity for its transmission.

It seems that the two transmissible tumors we know of may be much more similar than I had thought.

1. Incidentally, when I was looking for images of Tasmanian Devils, I found it interesting that the great majority of images I found using Google Image Search showed an open-mouthed, angry Devil, about to take a chomp out of something.  I was going to comment on that, and then I went to Flickr and did the same search, finding instead thousands of pictures of peaceful, timid, close-mouthed Devils.  I think the popular images, put by Google at the top of the list, as so popular because they reinforce the Devil stereotype, and the Flickr photos are a more accurate reflection of their true nature.[↩]
2. Jones, M., Cockburn, A., Hamede, R., Hawkins, C., Hesterman, H., Lachish, S., Mann, D., McCallum, H., & Pemberton, D. (2008). Life-history change in disease-ravaged Tasmanian devil populations Proceedings of the National Academy of Sciences, 105 (29), 10023-10027 DOI: 10.1073/pnas.0711236105[↩]
3. Siddle, H. V., Kreiss, A., Eldridge, M. D., Noonan, E., Clarke, C. J., Pyecroft, S., Woods, G. M., and Belov, K. (2007). Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial. Proc Natl Acad Sci U S A 104:16221-16226[↩][↩]
4. Olivia Judson had a very good summary of the argument in her blog; she also made the important point that MHC diversity, or lack of it, is clearly not the only factor involved in the tumor spread.[↩]
5. JONES, M., PAETKAU, D., GEFFEN, E., & MORITZ, C. (2004). Genetic diversity and population structure of Tasmanian devils, the largest marsupial carnivore Molecular Ecology, 13 (8), 2197-2209 DOI: 10.1111/j.1365-294X.2004.02239.x[↩]
6. Again, as far as I know this isn’t published yet.[↩]
7. The authors of the 2004 paper didn’t specifically look at MHC diversity, though; so it remains possible, if unlikely, that the MHC is relatively homogenous while the rest of the genome is diverse.  This is completely the opposite of the usual situation, so I think it’s not likely.[↩]