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Know Your Enemy

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By Hsei Di Law

New research has revealed a key mechanism by which our immune system turns against us.

Letter for letter, every single 75 trillion of your cells has identical DNA. However, one group of cells not only has DNA that reads differently to every other cell of the body, but also differently from one another. These are a type of white blood cell known as B cells.

B cells have a central role within the immune system. They are the ones responsible for producing antibodies – the molecules that fight off infections. One estimate says that a single B cell can secrete more than 2000 molecules of antibody per second.

What is also unique to B cells is that the letters of their DNA have been deliberately scrambled. It is a cell’s natural impulse to protect its DNA sequence tooth and nail, making this feature of B cells a remarkable and curious phenomenon.

B cells are constantly being formed in the bone marrow. In each developing B cell, a pair of enzymes homes in on the region of DNA that encodes antibody molecules. Here they act together to randomly cut out and rejoin bits and pieces of DNA.

This scrambling of the DNA creates millions of random combinations of sequences. What results is a troop of B cells, each of which possesses a different DNA sequence for antibodies. Thus each and every one expresses a different and unique kind of antibody.

Owing to this process of randomising DNA, each of us has millions of different antibodies making the rounds at any one time. This tremendous diversity is the reason our immune system works.

Antibodies operate by binding to foreign molecules with a matching shape and attacking them. Possessing an assortment of antibody shapes means we can fight off millions of different infection agents.

However, the basis of antibody manufacture has a double-edged sword. Because antibody shapes are randomly determined, we not only produce antibodies that bind to foreign molecules, but also those that can bind to parts of the own body. These antibodies are capable of attacking the self as vehemently as antibodies with the “right” shapes would attack microbes. B cells that produce this kind of “self-reactive” antibodies are the culprits of autoimmune diseases such as systemic lupus erythematosus and Graves’ disease.

As you might have guessed, our body has built-in ways of weeding out self-reactive B cells (although these pathways have gone wrong in autoimmune patients). The first of these involves a screening process of newly formed B cells in the bone marrow. Those identified as self-reactive are immediately removed before they have any chance of causing harm. Elimination of these cells occurs by a mechanism called apoptosis, an extraordinary spectacle in which a cell self-destructs. The rest of the B cell population, those that are “foreign-reactive”, filter through this screening process and leave the bone marrow as our true fighters.

Antibody molecules serve another function – as a receiving “aerial” on the cell’s surface. These antibody aerials pick up signals from the environment that are then passed via a number of proteins to the nucleus. Whether a B cell survives or dies depends on the signals it receives.

Led by PhD student Yogesh Jeelall and supervisors Professor Chris Goodnow and Dr Keisuke Horikawa at the John Curtin School of Medical Research, The Australian National University, we are trying to determine what happens to these signals along the way and whether they can be influenced. Understanding these signals is important as survival signals that are too strong or death signals that are too weak are thought to spur unwanted cell division, which can lead to cancer.

Our team has hypothesised that among the chain of proteins that conveys the signal to the nucleus, there is one that functions as the master control. This protein would act as a switch between cell death or survival. In order to test this theory, we have been trying to flip this protein switch and then watch whether cells destined to die would revert to growth.

In order to find this switch, we turned to B cell lymphomas, a type of cancer where B cells divide above and beyond control. Combing through genetic data, we found that patients of this disease frequently carry mutations in a protein known as CARD11. Importantly, CARD11 is one of the proteins among the conveyor belt of signalling proteins in B cells. The association of CARD11 with lymphoma hints at a potential for it to ignore death signals or even convert them into growth.

For our experiments to work we needed some cells that were predestined to die. The source of these was a laboratory strain of mice that possess self-reactive B cells. It is a feature of these self-reactive cells that they have been pre-programmed to die as part of their built-in mechanism against autoimmunity.

We were interested to know if CARD11 could reverse the fate of these condemned cells. Using genetic manipulation techniques, we inserted mutated CARD11 genes into the self-reactive cells of these mice. We then sat back and watched the lifespan of these cells.

Far from being killed off, we observed not only the survival of these self-reactive cells but also a 40-fold increase in their growth. In effect, the experiments mimicked what happens in patients with autoimmune disease, where self-reactive cells overturn death signals into growth, eventually becoming abundant enough to start attacking the self.

Other proteins in the same signalling chain were also tested, but these did not affect the signal, let alone convert it to growth. CARD11 seems unique in its ability to lie at the root of opposite effects – no such protein “switch” has been reported elsewhere so far.

The B Cell

Discovering the influence of CARD11 means we have potentially uncovered a central player in the development and incidence of autoimmunity. The next step for us is to use DNA sequencing technology to screen autoimmune patients for CARD11 mutations. This will allow a link, if any, to be made between CARD11 and human autoimmune disease.

Our research suggests that CARD11 is a key regulator of cell death and growth. It is particularly important to pin down critical molecules like CARD11 that can single-handedly cause an abnormality in the lifespan of cells because they are excellent drug targets and can reveal the underlying mechanisms of death and growth.

We hope that unravelling CARD11 will unlock new therapies and diagnostic tools for both autoimmune diseases and immune cell cancers.

Hsei Di Law is a research technician at the John Curtin School of Medical Research, The Australian National University.