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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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.

 

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.