Secret Army: More Manoeuvres Revealed

 

I don’t know about you but I find it difficult to grasp the idea that there are more bugs in my body than there are ‘me’ cells. That is, microorganisms (mostly bacteria) outnumber the aggregate of liver, skin and what-have-you cells. They’re attracted, of course, to the warm, damp surfaces of the cavities in our bodies that are covered by a sticky, mucous membrane, e.g., the mouth, nose and especially the intestines (the gastrointestinal tract).

The story so far

Over the last few years it’s become clear that these co-residents — collectively called the microbiota — are not just free-loaders. They’re critical to our well-being in helping to fight infection by other microrganisms (as we noted in Our Inner Self), they influence our immune system and in the gut they extract the last scraps of nutrients from our diet. So maybe it makes them easier to live with if we keep in mind that we need them every bit as much as they depend on us.

We now know that there are about 2000 different species of bacteria in the human gut (yes, that really is 2,000 different types of bug) and, with all that diversity, it’s not surprising that the total number of genes they carry far exceeds our own complement (by several million to about 20,000). In it’s a small world we noted that obesity causes a switch in the proportions of two major sub-families of bacteria, resulting in a decrease in the number of bug genes. The flip side is that a more diverse bug population (microbiome) is associated with a healthy status. What’s more, shifts of this sort in the microbiota balance can influence cancer development. Even more remarkably, we saw in Hitchhiker Or Driver? That the microbiome may also play a role in the spread of tumours to secondary sites (metastasis).

Time for a deep breath

If all this is going on in the intestines you might well ask “What about the lungs?” — because, and if you didn’t know you might guess, their job of extracting oxygen from the air we inhale means that they are covered with the largest surface area of mucosal tissue in the body. They are literally an open invitation to passing microorganisms — as we all know from the ease with which we pick up infections.

In view of what we know about gut bugs a rather obvious question is “Could the bug community play a role in lung cancer?” It’s a particularly pressing question because not only is lung cancer the major global cause of cancer death but 70% lung cancer patients have bacterial infections and these markedly influence tumour development and patient survival. Tyler Jacks, Chengcheng Jin and colleagues at the Massachusetts Institute of Technology approached this using a mouse model for lung cancer (in which two mutated genes, Kras and P53 drive tumour formation).

In short they found that germ-free mice (or mice treated with antibiotics) were significantly protected from lung cancer in this model system.

How bacteria can drive lung cancer in mice. Left: scheme of a lung with low levels of bacteria and normal levels of immune system cells. Right: increased levels of bacteria accelerate tumour growth by stimulating the release of chemicals from blood cells that in turn activate cells of the immune system to release other effector molecules that promote tumour growth. The mice were genetically altered to promote lung tumour growth (by mutation of the Kras and P53 genes). In more detail the steps are that the bacteria cause macrophages to release interleukins (IL-1 & IL-23) that stick to a sub-set of T cells (γδ T cells): these in turn release factors that drive tumour cell proliferation, including IL-22. From Jin et al. 2019.

As lung tumours grow in this mouse model the total bacterial load increases. This abnormal regulation of the local bug community stimulates white blood cells (T cells present in the lung) to make and release small proteins (cytokines, in particular interleukin 17) that signal to neutrophils and tumour cells to promote growth.

This new finding reveals that cross-talk between the local microbiota and the immune system can drive lung tumour development. The extent of lung tumour growth correlated with the levels of bacteria in the airway but not with those in the intestinal tract — so this is an effect specific to the lung bugs.

Indeed, rather than the players prominent in the intestines (Bs & Fs) that we met in Hitchhiker Or Driver?, the most common members of the lung microbiome are Staphylococcus, Streptococcus and Lactobacillus.

In a final twist Jin & Co. took bacteria from late-stage tumours and inoculated them into the lungs of mice with early tumours that then grew faster.

These experiments have revealed a hitherto unknown role for bacteria in cancer and, of course, the molecular signals identified join the ever-expanding list of potential targets for drug intervention.

References

Jin, C. et al. (2019). Commensal Microbiota Promote Lung Cancer Development via γδ T Cells. Cell 176, 998-1013.e16.

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Fantastic Stuff

 

It certainly is for Judy Perkins, a lady from Florida, who is the subject of a research paper published last week in the journal Nature Medicine by Nikolaos Zacharakis, Steven Rosenberg and their colleagues at the National Cancer Institute in Bethesda, Maryland. Having reached a point where she was enduring pain and facing death from metastatic breast cancer, the paper notes that she has undergone “complete durable regression … now ongoing for over 22 months.”  Wow! Hard to even begin to imagine how she must feel — or, for that matter, the team that engineered this outcome.

How was it done?

Well, it’s a very good example of what I do tend to go on about in these pages — namely that science is almost never about ‘ground-breaking breakthroughs’ or ‘Eureka’ moments. It creeps along in tiny steps, sideways, backwards and sometimes even forwards.

You may recall that in Self Help – Part 2, talking about ‘personalized medicine’, we described how in one version of cancer immunotherapy a sample of a patient’s white blood cells (T lymphocytes) is grown in the lab. This is a way of either getting more immune cells that can target the patient’s tumour or of being able to modify the cells by genetic engineering. One approach is to engineer cells to make receptors on their surface that target them to the tumour cell surface. Put these cells back into the patient and, with luck, you get better tumour cell killing.

An extra step (Gosh! Wonderful GOSH) enabled novel genes to be engineered into the white cells.

The Shape of Things to Come? took a further small step when DNA sequencing was used to identify mutations that gave rise to new proteins in tumour cells (called tumour-associated antigens or ‘neoantigens’ — molecular flags on the cell surface that can provoke an immune response – i.e., the host makes antibody proteins that react with (stick to) the antigens). Charlie Swanton and his colleagues from University College London and Cancer Research UK used this method for two samples of lung cancer, growing them in the lab to expand the population and testing how good these tumour-infiltrating cells were at recognizing the abnormal proteins (neo-antigens) on cancer cells.

Now Zacharakis & Friends followed this lead: they sequenced DNA from the tumour tissue to pinpoint the main mutations and screened the immune cells they’d grown in the lab to find which sub-populations were best at attacking the tumour cells. Expand those cells, infuse into the patient and keep your fingers crossed.

Adoptive cell transfer. This is the scheme from Self Help – Part 2 with the extra step (A) of sequencing the breast tumour. Four mutant proteins were found and tumour-infiltrating lymphocytes reactive against these mutant versions were identified, expanded in culture and infused into the patient.

 

What’s next?

The last step with the fingers was important because there’s almost always an element of luck in these things. For example, a patient may not make enough T lymphocytes to obtain an effective inoculum. But, regardless of the limitations, it’s what scientists call ‘proof-of-principle’. If it works once it’ll work again. It’s just a matter of slogging away at the fine details.

For Judy Perkins, of course, it’s about getting on with a life she’d prepared to leave — and perhaps, once in while, glancing in awe at a Nature Medicine paper that does not mention her by name but secures her own little niche in the history of cancer therapy.

References

McGranahan et al. (2016). Clonal neoantigens elicit T cell immunoreactivity and sensitivity to immune checkpoint blockade. Science 10.1126/science.aaf490 (2016).

Zacharakis, N. et al. (2018). Immune recognition of somatic mutations leading to complete durable regression in metastatic breast cancer. Nature Medicine 04 June 2018.

Holiday Reading (3) – Stopping the Juggernaut

The mutations that drive cancers fall into two major groups: those that reduce or eliminate the activity of affected proteins and those that have the opposite effect and render the protein abnormally active. It’s intuitively easy to see how the latter work: if a protein (or more than one) in a pathway that tells cells to proliferate becomes more efficient the process is accelerated. Less obvious is how losing an activity might have a similar effect but this comes about because the process by which one cell becomes two (called the cell cycle) is controlled by both positive and negative factors (accelerators and brakes if you will). This concept of a balancing act – signals pulling in opposite directions – is a common theme in biology. In the complex and ever changing environment of a cell the pressure to reproduce is balanced by cues that ask crucial questions. Are there sufficient nutrients available to support growth? Is the DNA undamaged, i.e. in a fit state to be replicated? If the answer to any of these questions is ‘no’ the cell cycle machinery applies the brakes, so that operations are suspended until circumstances change. The loss of negative regulators releases a critical restraint so that cells have a green light to divide even when they should not – a recipe for cancer.

Blanc sides.004

The cell cycle.

Cells are stimulated by growth factors to leave a quiescent state (G0) and enter the cell cycle – two growth phases (G1 & G2), S phase where DNA is duplicated and mitosis (M) in which the cells divide to give to identical daughter cells. Checkpoints can arrest progression if, for example, DNA is damaged. 

We’re all familiar with this kind of message tug-of-war at the level of the whole animal. We’ve eaten a cream cake and the schoolboy within is saying ‘go on, have another’ whilst the voice of wisdom is whispering ‘if you go on for long enough you’ll end up spherical.’

Because loss of key negative regulators occurs in almost all cancers it is a high priority to find ways of replacing inactivated or lost genes. Thus far, however, successful ‘gene therapy’ approaches have not been forthcoming with perhaps the exception of the emerging field of immunotherapy. The aim here is to boost the activity of the immune system of an individual – in other words to give an innate anti-cancer defense a helping hand. The immune system can affect solid cancers through what’s become known as the tumour microenvironment – the variety of cells and messengers that flock to the site of the abnormal growth. We’ve referred to these as ‘groupies’ and they include white blood cells. They’re drawn to the scene of the crime by chemical signals released by the tumour – the initial aim being to liquidate the intruder (i.e. tumour cells). However, if this fails, a two-way communication sees would-be killers converted to avid supporters that are essential for cancer development and spread.

Blanc sides.002

The tumour microenvironment. Tumour cells release chemical messengers that attract other types of cell, in particular those that mediate the immune response. If the cancer cells are not eliminated a two-way signaling system is established that helps tumour development.

There is much optimism that this will evolve into a really effective therapy but it is too early for unreserved confidence.

The obstacle of reversing mutations that eliminate the function of a gene has led to the current position in which practically all anti-cancer agents in use are inhibitors, that is, they block the activity of a protein (or proteins) resulting in the arrest of cell proliferation – which may ultimately lead to cell death. Almost all these drugs are not specific for tumour cells: they hit some component of the cell replication machinery and will block division in any cell they reach – which is why so many give rise to the side-effects notoriously associated with cancer chemotherapy. For example, the taxanes – widely used in this context – stick to protein cables to prevent them from pulling duplicated DNA strands apart so that cells, in effect, become frozen in final stages of division. Other classes of agent target different aspects of the cell cycle.

It is somewhat surprising that non-tumour specific agents work as well as they do but their obvious shortcomings have provided a major incentive for the development of ‘specific’ drugs – meaning ones that hit only tumour cells and leave normal tissue alone. Several of these have come into use over the past 15 years and more are in various stages of clinical trials. They’re specific because they knock out the activity of mutant proteins that are made only in tumour cells. Notable examples are Zelboraf® manufactured by Roche (hits the mutated form of a cell messenger – called BRAF – that drives a high proportion of malignant melanomas) and Gleevec® (Novartis AG: blocks a hybrid protein – BCR-ABL – that is usually formed in a type of leukemia).

These ‘targeted therapies’ are designed to not so much to poke the blancmange as to zap it by knocking out the activity of critical mutant proteins that are the product of cancer evolution. And they have produced spectacular remissions. However, in common with all other anti-cancer drugs, they suffer from the shortcoming that, almost inevitably, tumours develop resistance to their effects and the disease re-surfaces. The most remarkable and distressing aspect of drug resistance is that it commonly occurs on a timescale of months.

And being outwitted

Tumour cells use two tactics to neutralize anything thrown at them before it can neutralize them. One is to treat the agent as garbage and activate proteins in the cell membrane that pump it out. That’s pretty smart but what’s really staggering is the flexibility cells show in adapting their signal pathways to counter the effect of a drug blocking a specific target. Just about any feat of molecular gymnastics that you can imagine has been shown to occur, ranging from switching to other pathways in the signalling network to short-circuit the block, to evolving secondary mutations in the target mutant protein so that the drug can no longer stick to it. Launching specific drugs at cells may give them a mighty poke in a particularly tender spot, and indeed many cells may die as a result, but almost inevitably some survive. The blancmange shakes itself, comes up with a counter and gets down to business again. This quite extraordinary resilience of tumour cells derives from the infinite adaptability of the genome and we might do well to reflect that in trying to come up with anti-cancer drugs we are taking on an adversary that has overcome the challenges involved in generating the umpteen million species to have emerged during the earth’s lifetime.

Not the least disheartening aspect of this scenario is that when tumours recur after an initial drug treatment they are often more efficient at propagating themselves, i.e. more aggressive, than their precursors.

Self Help – Part 1

It’s not easy to find good things to say about cancer and humour is equally elusive, as those of us who lecture on the subject know very well. But most people are aware of one cheering fact: cancers aren’t transmissible between humans – that is, they’re not like ’flu, venereal diseases and lots of other nasty things we pass around. Thus, if you transplant tumours from one animal to another of the same species (usually mice) generally they don’t grow – in much the same way that transplanted organs (livers, etc.) are rejected by the recipient’s immune system. Transplant rejection occurs because the body mounts an immune response to the foreign (i.e. ‘non-self’) organ: transplantation works when that is reduced by matching donor to recipient as closely as possible and combining that with immunosuppressant drugs.

But here’s an obvious thought: if tumours transferred between animals don’t grow, their immune systems must be doing a pretty good job of recognizing them as ‘non-self’ and killing them off. If that’s true, how about trying to boost the immune response in cancer patients as a therapeutic strategy? It’s such a good idea it’s become the trendiest thing in cancer science, the field being known as immunotherapy.

Immunotherapy

The aim is to give a patient’s immune response a helping hand so it can kill their tumours. The stars of the show are a subset of white blood cells called T lymphocytes: that’s because some of them have the power to kill – they’re ‘cytotoxic T cells’. So the simple plan is to boost either the number or the efficiency of these tumour-killing T cells. The story is complicated by there being lots of sub-types of T cells – most notably T Helper cells (that do what their name suggests: activate cytotoxic T cells) and Suppressor T cells that shut down immune responses.

To get the hang of immunotherapy we need only focus on ways of boosting T Helpers but in passing we can hardly avoid asking “why so complicated?” Well, the immune system has evolved on a tight-rope, trying on the one hand to kill invading organisms whilst, on the other, leaving the cells and tissues of the host untouched. It works amazingly well but it can fall off both ways when either it’s overcome by the genomic gymnastics of cancer or when it exceeds its remit and causes auto-immune diseases – things like type 1 diabetes in which the immune system destroys the cells in the pancreas that make insulin.

Shifting the balance

We’ve seen that T cells (of all varieties) are among the ‘groupies’ attracted to the scene of growing solid tumours (in Cooperative Cancer Groupies and Trouble With The Neighbours) and so the name of the game is how to tweak the balance in that environment towards more efficient tumour cell killing.

Broadly speaking, there are two forms of cancer immunotherapy. In one T cells are removed from the patient, grown to large numbers and then put back into the circulation – called ‘adoptive cell therapy’, we’ll come to it in Part 2. The more widespread approach, sometimes called ‘checkpoint blockade’, uses agents that block inhibitory pathways switched on by tumours – in effect releasing molecular brakes that prevent T cell hyperactivity and autoimmunity. So ‘checkpoint blockade’ is a systemic method – drugs are administered that diffuse throughout the body to find their targets, whereas next time we’ll be talking about ‘personalized medicine’ – using the patient’s own cells to fight his cancer.

There’s one further method – viral immunotherapy – which I wasn’t going to mention but has been in the news lately to the extent that I feel obliged to make a trio with “Blowing Up Cancer” to follow Parts 1 & 2.

There’s nothing new about this general idea. Over 100 years ago the New York surgeon William Coley noticed that occasionally tumours disappeared when patients accidentally picked up post-operative bacterial infections and, from bugs grown in the lab, he made extracts that, injected into solid tumours, caused about one in ten of them to regress, with some patients remaining well for many years thereafter.

A new era

Even so, it took until 1996 before it was shown that blocking an inhibitory signal could unleash the tumour killing power of T cells in mice and it was not until 2011 that the first such agent was approved by the U.S. Food and Drug Administration for treating melanoma. In part the delay was due to the ‘agent’ being an antibody and the time taken to develop ‘humanized’ versions thereof. Antibodies (aka immunoglobulins) are large, Y-shaped molecules made by B lymphocytes that bind with high specificity to target molecules – antigens – humanized forms being engineered so that they are made almost entirely of the human protein sequence and therefore do not provoke an immune response.

92 FigCheckpoint Blockade Activates Anti-Tumour Immunity

Interactions between Receptors A and a suppress T cell activity. Antibodies to these receptors block this signal and restore immune activity against tumour cells.

Unblocking the block

We picture the tumour microenvironment as a congregation of various cell types with chemical messengers whizzing to and fro between them. In addition, some protein (messenger) receptors on cell surfaces talk to each other. The receptors themselves become messengers thus drawing the cells together – essential to bring killer cells into contact with their target. You can think of all these protein-protein interactions as keys inserting into locks or as molecular handshakes – a coming together that passes on information. Antibodies come into their own because they bind to their targets just as avidly as the normal signaling molecules – so they’re great message disruptors.

The sketch shows in principle how this works for two interacting receptors, A and a. The arrival of a specific antibody (anti-A or anti-a) puts a stop to the conversation – and if the upshot of the chat was to decrease the immune response, bingo, we have it! Targeting a regulatory pathway with an antibody enhances anti-tumour responses.

Putting names to targets, CTLA-4 and PD-1 are two key cell-surface receptors that, when engaged, trigger inhibitory pathways and dampen T-cell activity. Antibodies to these (ipilimumab v. CTLA-4; pembrolizumab and nivolumab v. PD-1) have undergone a number of clinical trials and the two in combination have given significant responses, notably for melanoma. So complex is immune response control that it presents many targets for manipulation and a dozen or so agents (mostly antibodies) are now in various clinical trials.

Déjà vu

So the era of immunotherapy has well and truly arrived but, as ever with cancer, it is not quite time to break open the champagne and put our feet up. Whilst combinations of antibodies have given sustained responses, with some patients remaining disease-free for many years, at the moment immunotherapy has only been shown to work in subsets of cancers and even then only a small fraction (about 25%) of patients respond. My correspondent Dr. Markus Hartmann has pointed out that the relatively limited improvements in survival rates following immunotherapy might be significantly enhanced if we took into account the specific genetic background of patients and determined which genes of interest are expressed or switched off. This information should reveal why some patients benefit from immunotherapy whilst others with clinically similar disease do not.

The challenge, therefore, is to characterise individual tumours and their supporting bretheren in terms of the cell types and messengers involved so that the optimal targets can be selected – and, of course, to make the necessary agents. It’s a tough ask, as the sporting fraternity might put it, but that’s what science is about so onwards and upwards with William Coley’s words of 105 years ago writ large on the lab notice board: “That only a few instead of the majority showed such brilliant results did not cause me to abandon the method, but only stimulated me to more earnest search for further improvements in the method.”

I’m grateful to Dr. Markus Hartmann  (Twitter: @markus2910) for constructive comments about this post.

References

Coley, W. B. (1910). The Treatment of Inoperable Sarcoma by Bacterial Toxins (the Mixed Toxins of the Streptococcus erysipelas and the Bacillus prodigiosus). Proceedings of the Royal Society of Medicine  3, 1-48.

Twyman-Saint Victor, C. et al. (2015). Radiation and dual checkpoint blockade activate non-redundant immune mechanisms in cancer. Nature 520, 373–377.

Wolchok, J.D. et al. (2013). Nivolumab plus Ipilimumab in Advanced Melanoma. N. Eng. J. Med., 369, 122-133.

Beware of Greeks …

Finding the words

One of the pitfalls of writing is repetition. Be it book, blog or broadsheet, most authors must dread someone gleefully chirruping ‘You used that exact phrase in 1999.’ I wonder if The Immortal Bard suffered likewise – having to resort to ferreting through piles of dusty manuscripts, finally in desperation shouting ‘Anne, Anne – got this great new line If you prick us do we not bleed? – Heard it before?’

‘Yes, of course dear. You used it in that thing about the Italian moneylender.’

‘Damn. Thought it sounded familiar. Where would I be without your memory – make me immortal with a kiss.’

‘Give over you daft beggar – even you know that’s one of that Marlowe bloke’s lines!’

‘Doh!’

I’m something of a sitting duck here, partly through not being Shakespeare but also because of the habit of often talking about biology. Take one simple example, Will’s tiny pinprick of blood – in which there will be about fifty million cells. That’s fifty million separate little sacs swirling around in a bead you wouldn’t notice if it wasn’t bright red. Isn’t that stunning for starters? Indeed, but it’s when we turn to molecular cancer that nature’s capacity to amaze is unfettered, the remarkable becomes the norm and even the English language can seem inadequate.

Finding the exit

A study of a mouse model of one form of leukemia is the most recent contribution to have us sifting Shakespeare’s superlatives to do justice to the discovery. Blood cells start life within the bone marrow but, until they’ve matured, they’re corralled by a marrow–circulation barrier, also made of cells. Adult cells normally make proteins on their surface that attach to the barrier, and these help them to squeeze past into the freedom of the circulation. In leukemia abnormal levels of white blood cells are present in the circulation, which means that those cells have also found a way through the bone barrier. A Prague group have shown that one type of leukemic cell has come up with an astonishingly novel escape mechanism in which they release fragments of their own DNA. That’s pretty staggering because not only is DNA generally locked in the nucleus but it comes in large chunks called chromosomes. So two very unusual things have happened to get to this stage: (1) some of DNA has been shattered and (2) these pieces have crossed not only the membrane that encloses the nucleus but also the outer boundary of the cell itself.

DNA fragments from tumour cells enter barrier cells and kill them, releasing tumour cells into the circulation

DNA fragments from tumour cells enter barrier cells and kill them, releasing tumour cells into the circulation

But then something even more extraordinary happens: having tunneled their way out of the tumour cell, the escaped bits of DNA do a kind of reverse reprise by entering the cells that form the barrier between bone marrow and circulating blood. It’s as though the barrier cells see the passing packages of DNA as presents and gobble them up. Alas! They should have read their Virgil – or at least Dryden’s summary of the tale of the wooden horse of Troy: ‘Trust not their presents, nor admit the horse’ – for the barrier cells pay the ultimate price for their gluttony. The DNA fragments are sensed as something abnormal – as indeed they are – and this provokes a stress response – and a pretty extreme one at that – because the cells are so overwhelmed by the influx that they commit suicide.

The capacity for individual cells to switch on a death program is an important part of life – it’s essential in normal development and it’s also the best cancer defence we have. In other words, if things get out of control, kill the cell – because that eliminates the danger and cells can be replaced. But here we have an almost stupefying paradox: in the tumour cells this defence is neutralized – but they’ve come up with a way of turning it on in the normal cells they have to get past in order to spread around the organism.

It’s another astounding example of the plasticity of our genetic material and the incredible adaptability of cancer cells. Even Mr. S. might feel adjectivally challenged!

Reference

Dvořáková, M. et al., (2012). DNA released by leukemic cells contributes to the disruption of the bone marrow microenvironment. Oncogene 10 December 2012; doi: 10.1038/onc.2012.553