Invisible Army Rouses Home Guard

Writing this blog – perhaps any blog – is an odd pastime because you never really know who, if anyone, reads it or what they get out of it. Regardless of that, one person that it certainly helps is me. That is, trying to make sense of the latest cancer news is one of the best possible exercises for making you think clearly – well, as clearly as I can manage!

But over the years one other rather comforting thing has emerged: more and more often I sit down to write a story about a recent bit of science only to remember that it picks up a thread from a piece I wrote months or sometimes years ago. And that’s really cheering because it’s a kind of marker for progression – another small step forward.

Thus it was with this week’s headline news that a ‘cancer vaccine’ might be on the way. In fact this development takes up more than one strand because it’s about immunotherapy – the latest craze – that we’ve broadly explained in Self Help Part-1Gosh! Wonderful GOSH and Blowing-up Cancer and it uses artificial nanoparticles that we met in Taking a Swiss Army Knife to Cancer.

Arming the troops

What Lena Kranz and her friends from various centres in Germany described is yet another twist on the idea of giving our inbuilt defence – i.e. the immune system – a helping hand to tackle tumours. They made small sacs of lipid called nanoparticles (they’re so small you could get 300 in the width of a human hair), loaded them with bits of RNA and injected them into mice. This invisible army of fatty blobs was swept around the circulatory system whereupon two very surprising things happened. The first was that, with a little bit of fiddling (trying different proportions of lipid and RNA), the nanoparticles were taken up by two types of immune cells, with very little appearing in any other cells. This rather fortuitous result is really important because it means that the therapeutic agent (nanoparticles) don’t need to be directly targetted to a tumour cell – thus avoiding one of the perpetual problems of therapy.

The second event that was not at all a ‘gimme’ was that the immune cells (dendritic cells and macrophages) were stimulated to make interferon and they also used the RNA from the nanoparticles as if it was their own to make the encoded proteins – a set of tumour antigens (tumour antigens are proteins made by tumour cells that can be useful in identifying the cells. A large number of have now been found: one group of tumour antigens includes HER2 that we met as a drug target in Where’s That Tumour?)

The interferon was released into the tumour environment in two waves, bringing about the ‘priming’ of T lymphocytes so that, interacting via tumour antigens, they can kill target cells. By contrast with taking cells from the host and carrying out genetic engineering in the lab (Gosh! Wonderful GOSH), this approach is a sort of internal re-wiring achieved by giving a sub-set of immune system cells a bit of genetic code (in the form of RNA).

TAgs RNA Nano picNanoparticle cancer vaccine. Tiny particles (made of lipids) carry RNA into cells of the immune system (dendritic cells and macrophages) in mice. A sub-set of these cells releases a chemical signal (interferon) that promotes the activation of T lymphocytes. The imported RNA is translated into proteins (tumour antigens) – that are presented to T cells. A second wave of interferon (released from macrophages) completes T cell priming so that they are able to attack tumour cells by recognizing antigens on their surface (Kranz et al. 2016; De Vries and Figdor, 2016).

So far Kranz et al. have only tried this method in three patients with melanoma. All three made interferon and developed strong T-cell responses. As with all other immunotherapies, therefore, it is early days but the fact that widely differing strategies give a strong boost to the immune system is hugely encouraging.

Other ‘cancer vaccines’

As a footnote we might add that there are several ‘cancer vaccines’ approved by the US Food and Drug Administration (FDA). These include vaccines against hepatitis B virus and human papillomavirus, along with sipuleucel-T (for the treatment of prostate cancer), and the first oncolytic virus therapy, talimogene laherparepvec (T-VEC, or Imlygic®) for the treatment of some patients with metastatic melanoma.

How was it for you?

As we began by pointing out how good writing these pieces to clarify science is for me, the question for those dear readers who’ve made it to the end is: ‘How did I do?’

References

Kranz, L.M. et al. (2016). Systemic RNA delivery to dendritic cells exploits antiviral defence for cancer immunotherapy. Nature (2016) doi:10.1038/nature18300.

De Vries, J. and Figdor, C. (2016). Immunotherapy: Cancer vaccine triggers antiviral-type defences.Nature (2016) doi:10.1038/nature18443.

 

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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.

Fast Food Fix Focuses on Fibre

If you’re like me you’re probably more bored than absorbed by the seemingly continuous stream of ‘studies’ telling us what we should and shouldn’t eat. No one’s going to argue it’s unimportant but gee, I wish they’d make their minds up. Of course the study of diet and its effects is tricky – as we noted in Betrayed by Nature – not least because you generally need enormous numbers of people to tease out significant effects.

Fortunately authoritative sources like The American Heart Association offer generally sane and simple advice: “eat a balanced diet and do enough exercise to match the number of calories you take in.”

A balanced diet includes fibre, sometimes called roughage, the stuff we eat but can’t digest that assists in taking up water and generally keeping our insides working. There’s much evidence that eating plenty of fibre helps to prevent bowel cancer – usually accumulated from vast numbers (e.g., the European Prospective Investigation into Cancer and Nutrition study involved over half a million people from ten European countries). But even for fibre, when you might just be thinking the answer’s clear-cut, there are other studies showing no protective effect.

So hooray for Stephen O’Keefe and friends from the University of Pittsburgh and Imperial College London for coming up with a dead simple experiment – and some pretty astonishing results (though to prevent panic we should reveal at the outset that they confirm that a high fibre diet can substantially reduce the risk of colon cancer).

Doing the obvious

The experiment compared what happened to two groups of 20, one African Americans, the other from rural South Africa, when they swapped diets for two weeks. So, in principle ‘dead simple’ but to describe it thus does a great injustice to the huge amount of effort involved – for a start they had to find two lots of 20 volunteers willing to have a colonoscopy examination before and after the diet swap. The Western diet was, of course, high protein, high fat, low fibre, whereas the typical African diet was high in fibre and low in fat and protein. Just to be clear, the American diet included beef sausage and pancakes for breakfast, burger and chips for lunch, etc. The traditional African diet comprises corn based products, vegetables, fruit and pulses, e.g., corn fritters, spinach and red pepper for breakfast.

B'fast jpegCompare and contrast.

A rural South African diet (corn fritters for breakfast) and the American diet (Getty images)

Shock – and horror

Almost incredibly, within the two weeks of these experiments there were significant, reciprocal changes in both markers for cancer development and in the bug army – the microbiota – inhabiting the digestive tracts of the volunteers. That is, the dreaded colonoscopy revealed polyps (tumour precursors) in nine Americans (that were removed) but none in the Africans. Cells sampled from bowel linings had significantly higher proliferation rates (a biomarker of cancer risk) in African Americans than in Africans. After the diet switch the proliferation rates flipped, decreasing in African Americans whilst the Africans now had rates even higher than in the starting African American group. These changes were paralled by an influx of inflammation-associated cells (lymphocytes and macrophages) in the now high-fat diet Africans whilst these decreased in the Americans on their new, high-fibre diet.

Equally amazing, these reciprocal shifts were also associated with corresponding changes in specific microbes and their metabolites. You may recall meeting our microbiota (in The Best Laid Plans of Mice and Men and It’s a Small World) – the 1000 or so assorted species of bacteria that have made you their home, mostly in your digestive tract, of which there are two major sub-families, Bacteroidetes and Firmicutes (Bs & Fs). We saw that artificial sweeteners in the form of saccharin shifts our bug balance: Fs down, Bs up. Here feeding Americans high-fibre diet was associated with a shift from Bs To Fs. As we noted before, the composition of the bug army is important because of the chemicals (metabolites) they produce – in this case the diet switch resulted in more short chain fatty acids (e.g., butyrate) in the American group and a reciprocal drop therein for the Africans.

The bottom line

It really is quite remarkable that these indicators of cancer risk manifest themselves so rapidly following a change to a typical Western diet. Of course ‘markers’ are one thing, cancer is another. As one of the authors, Jeremy Nicholson of Imperial College London, said: “We can’t definitively tell from these measurements that the change in their diet would have led to more cancer in the African group or less in the American group, but there is good evidence from other studies that the changes we observed are signs of cancer risk.”

Put less scientifically, “a nod’s as good as a wink to a blind horse.”

Reference

O’Keefe, S.J.D. et al. (2015). Fat, fibre and cancer risk in African Americans and rural Africans. Nature Communications 6, Article number: 6342 doi:10.1038/ncomms7342

Trouble With The Neighbours

It may seem odd to the point of negligence that a problem mankind has been grappling with since at least the time of the ancient Egyptians should, within the last ten years or so, be shown to have a whole new dimension, scarcely conceived hitherto. This hidden world, often now called the tumour microenvironment, is created as solid tumours develop and attract a variety of normal cells from the host to form a cellular cloud that envelops them and supports their growth (as we noted in Cooperative Cancer Groupies). We shouldn’t beat ourselves up for being slow to grasp its existence yet alone its importance – just take it as a reminder of the multi-faceted complexity that is cancer.

It’s true that over one hundred years ago the London physician Stephen Paget came up with his “seed and soil” idea – the notion that when cells escape from a primary tumour and spread to secondary sites (metastasis) they need to find a suitable spot that will nourish their growth, otherwise they perish – a fate that befalls most of them, fortunately for us.

But in the twenty-first century …

Perceptive though that idea was, it didn’t relate to the goings on in the vicinity of primary tumours – where the current picture is indeed of a cosmopolitan crowd of cellular groupies being recruited as the tumor starts to grow such that they infiltrate and closely interact with the cancer cells. The groupies are attracted by chemical messengers released by tumour cells – but it becomes a two-way communication, with messenger proteins shuttling to and fro between the different cell types.

Tumor uenvirThe tumour neighbourhood.

Two-way communication between host cells and tumor cells.

 White blood cells (e.g., lymphocytes and macrophages) are one group that succumbs to the magnetism of tumours. They’re part of the immune response that initially tries to eliminate the abnormal growth but, in an extraordinary transformation, when tumour cells manage to evade this defense the recruited cells change sides so to speak, switching their action to release signals that actively support tumor growth. The idea of boosting the initial anti-tumour response, thereby using the host defence system to increase the efficiency of tumour elimination, is the basis of immunotherapy, a popular research field at present to which we will return in a later piece.

Who’s who among the groupies

The finding that cells flooding into the ambience of a tumour can affect growth of the cancer has focussed attention on identifying all the constituents of the cellular cloud and unraveling their actions. Two recent studies by Claudio Isella from the University of Turin and Alexandre Calon from Barcelona, with their colleagues, have looked at a type of bowel cancer that has a particularly poor prognosis and used an ingenious ploy to lift the veil on who’s doing what to whom in the tumour milieu.

The tumours were initially classified on the basis of a genetic signature – that is, a snapshot of which genes are active in a tumour sample – ‘switched on’ or ‘expressed’ in the jargon – meaning that the information encoded in a stretch of DNA sequence is being used to make a functional gene product, usually a protein. They then used the crafty tactic of implanting human tumour cells into mice (the mice are ‘immunocompromised’ so that they don’t reject the human cells), separated the major types of cell in the tumours that grew and then looked at the genes expressed in those sub-sets. Remarkably, it emerged that, of the cell groupies that infiltrate into primary tumours, fibroblasts are particularly potent at driving tumour growth and metastasis. Fibroblasts are a cell type that makes the molecular scaffold that gives structure and shape to the various tissues and organs in animals – so it’s a surprise, to say the least, to find that cells with a rather mundane day job can play an important role in cancer progression. In this model system the sequence differences between corresponding human and mouse genes confirm that the predominant driver is mouse cells infiltrating the human tumours. Perhaps it shouldn’t be quite such a shock to find fibroblasts dabbling in cancer as we have met cancer-associated fibroblasts (CAFs) before as cells that, by releasing leptin, can promote the growth and invasion of breast cancer cells (in Isn’t Science Wonderful? Obesity Talks to Cancer).

How useful might this be?

As ever, this is just one more small step. However, the other key finding from this work is that a critical signal for the CAFs is a protein called transforming growth factor beta (TGFβ) and a small molecule that blocks its signal inhibits metastasis of human tumour cells in the mouse model. So yet again the cancer biologist’s best friend gives a glimmering of hope for human therapy.

References

Isella, C. et al. (2015). Stromal contribution to the colorectal cancer transcriptome. Nature Genet. http://dx.doi.org/10.1038/ng.3224

Calon, A. et al. (2015). Stromal gene expression defines poor-prognosis subtypes in colorectal cancer. Nature Genet. http://dx.doi.org/10.1038/ng.3225