CHECKPOINT FOR B CELL SURVIVAL IN HUMAN GUT

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B cells are cells of the immune system. B cells fight infections by identifying infectious particles via very specific receptors on the B cell surface, called the B cell receptors. B cell receptors recognise infectious agents by their individual shape and they bind to them in a lock-and-key way. Millions of B cells circulate through the blood and tissues of the body and each B cell has a B cell receptor that is unique to that B cell. As a consequence, no matter what infectious agent finds its way into the body, there will be a B cell with a receptor of the complementary shape to bind it. If a B cell binds to an infectious particle via its B cell receptor, the B cell may become activated and secrete its B cell receptor that will bind to and fight the infectious agent. When the B cell receptor is secreted it is referred to as antibody.The process that generates the huge and diverse set of B cells with unique receptors has a major associated hazard. B cells can be produced that bind to the body's own cells and tissues and can attack them. Other cells of the immune system called T cells have the capacity to regulate B cells and they themselves can discriminate well between self and non-self. However, many B cells can make T cell independent responses and these are particularly dangerous if not properly regulated for specificity.The infectious agents that activate B cells in a T-cell independent way include those that cause some types of pneumonia. B cells have an additional trick to recognise these agents because they have a repeating pattern of shapes on the surface. B cells may be able to recognise the shape through the B cell receptor, but also the regularity with which the shape is presented. Experiments in our lab suggest that B cell selection that would prevent self-reactivity and promote responsiveness to particles that activate B cells independent of T cells happens in the gut. The gut contains a lot of 'friendly' bacteria that constantly stimulate the immune system it contains. Our experiments suggest that this environment supports stages in B cell development that are largely ignored in models of human B cell immunology or assessment of human disease. We call this a 'checkpoint' because it is a stage of B cell development where only cells that have a required set of properties are allowed to pass. The gut is involved in the development of B cells in different ways in many species, including chickens, mice, sheep and rabbits. Therefore an influence of the gut on human B cell development is important and highly likely to occur, but as yet totally mysterious.The aims of experiments described in this application are to understand how B cells mature in the gut and how this is regulated.