One of the most encouraging developments in medical research has been the effort to help the immune system fight back, beating cancer at its own evolutionary game. That was a dominant theme last month at the annual meeting of the American Association for Cancer Research in Philadelphia as scientists discussed recent successes in immunotherapy while considering how far the field still has to go. Why have these treatments been working so well with some cancers but not others? And why, even in the best cases, do not all patients respond? The realisation that Darwinian forces, for good and bad, are at work inside us can be traced to the early 1950s, when Frank Macfarlane Burnet, an Australian virologist, was pondering how we manage to fight off a potentially infinite variety of invading microbes, tailoring an antibody against each one. One possibility was that when an interloper is identified, by its molecular bumps and grooves, the immune system systematically engineers an appropriate weapon. Nature doesn't work in such a methodical manner, and Burnet suggested a messier, more intuitive explanation: the clonal selection theory of immunity.

By shuffling DNA, the body is continually generating a random assortment of immune cells, each with a different shape. When one of them happens to encounter a microbe with a matching surface, the yin to its yang, it is activated. It begins multiplying to produce an army, called a clone, of identical progeny. These make the antibodies that fight the infection. Some of the newly created immune cells linger in the body, preserving a memory of the invader. The next time it appears, it will be quickly recognised and destroyed. In cancer, random variation and selection produce a different kind of progeny: clones of mutant cells. As normal cells divide, mutations inevitably occur and spread. The weaker mutants die, leaving the stronger ones to survive. The most aggressive evolve into cancerous tumours, continually devising ways to expand their territory and extend their lives.

There are several hypotheses to explain why we can't​ seek out and destroy these internal miscreants as effectively as those coming from outside. Because cancer arises from our own tissues, the immune system may not recognise a distinction between "self" and "other." Moreover, cancer, unlike infection, usually flourishes later in life. In the cold eyes of evolution, there is little selective advantage to keeping people alive past their reproductive prime. It's left to human wile to find ways of prodding the body into treating cancer as an enemy without destroying healthy tissue. In recent years, scientists have treated some patients by extracting their immune cells and re-engineering them to fight tumours. Sometimes, the cancer cells themselves are used to develop a custom vaccine, immunising a body against a destructive subset of itself. This is the intelligent design approach: the scientist-creator hovers overhead, crafting a remedy to each patient's internal trouble.

Much of the excitement at the Philadelphia meeting involved tricking the immune system in a more fundamental way. In the fight against bacteria and viruses, it is important for the attack to be swift and brief. Otherwise, the result might be chronic inflammation, a disaster of another kind. Cells have evolved molecular safeguards, called checkpoints, to keep the immune system from overstepping itself. If a cancer does manage to evoke a weak defensive response, the body – as if dealing with a common infection – quickly cools it off. The cancer, in the course of its own evolution, also may find ways of surviving by enacting the checkpoints on its own. Either way, the counterattack is blunted. With new drugs called checkpoint inhibitors, doctors are trying to remove the blockades.

So far, the most impressive results have been with advanced melanoma; some patients expected to die within months live on for years. That might be because melanoma cells have an unusually large number of mutations, giving the unleashed immune system plenty of targets. Lung cancer patients with the most genetic damage also appear to have the best chances of responding. In any case, the therapies seem to do best with cancers where an immunological reaction, however feeble, has somehow gotten underway. The difficulty lies in increasing its intensity without inflicting too much collateral damage. For the deadliest malignancies, the trade-offs are probably worth it. The challenge now is to confront a wider range of cancers – and to identify in advance the patients most likely to benefit. With every other treatment science has invented – chemotherapy, radiation, targeted drugs – cancer has shown an appalling ability to evolve workarounds​. The hope is that this time, it will be different. Like cancer itself, immunotherapy draws on the power of Darwinism – if only we can help it gain the upper hand.

New York Times