What is the function of CTLA-4 endocytosis?

The immune response is a powerful weapons system usually targeted against foreign invaders such as bacteria and viruses. However, control of this weaponary is crucial to avoid collateral damage to our own bodies. As with all systems, faults sometimes occur and it is thought that some diseases such as rheumatoid arthritis or insulin-dependent diabetes, arise when the immune system is not controlled properly. These are known as autoimmune diseases and occur when immune cells, called 'T cells', are triggered inappropriately. We are studying a major 'off switch' on these cells known as CTLA-4, which is extremely important in preventing collateral damage by T cells. However, CTLA-4 is similar to a powerful 'on switch' called CD28 so pressing the right button is critical. Surprisingly CTLA-4 is found inside cells, not on the cell surface like CD28, this makes it difficult to understand how it can work. One idea is that CTLA-4 gets delivered to the cell surface when it's needed, however our experiments show that it doesn't stay at the cell surface but continues to go back inside. We are therefore wondering whether it needs to be inside the cell to work properly. The experiments in this proposal will study the mechanisms used by CTLA-4 to get inside the cell and the bits of CTLA-4 that are needed for this process. Understanding these features will, in the future, allow us to directly test how important internalisation of CTLA-4 is to its function.

CTLA-4 is an essential regulator in the immune system and mice lacking CTLA-4 develop profound autoimmune disease dying by 3-4 weeks of age. Whilst this has been established for a decade or more, the actual mechanisms of action of CTLA-4 are still under debate. Indeed it seems likely that CTLA-4 can work in several ways, for example, by helping CD25+ regulatory T cells to suppress other (potentially CTLA-4 negative) T cells or by 'cell intrinsic' mechanisms which directly inhibit the activity of cells that express CTLA-4. In the latter scenario, it is possible that CTLA-4 may recruit phosphatases to the TCR complex or possibly exclude signalling molecules from lipid rafts, however its there is no single widely accepted mechanism and numerous conflicting reports. In this proposal we test the hypothesis that a highly conserved feature of CTLA-4, namely its endocytosis from the plasma membrane, may be required for its inhibitory function, rather than being simply a mechanism for limiting its surface expression. Such an hypothesis is in line with growing evidence that endocytosis can be utilised to inhibit critical growth stimuli such as those through the EGF receptor. We therefore propose to conduct a detailed study of the mechanisms of CTLA-4 endocytosis, determine the regions of the cytoplasmic domain responsible for endocytosis and ultimately test the functional effects of inhibiting this process.