Applying advanced understanding of CTLA-4 function to optimise therapies for autoimmunity

While the immune system protects us from infection by viruses and bacteria, the weapons used to destroy such pathogens can also attack our own bodies resulting in autoimmune diseases. Collectively, these diseases are relatively common and include type 1 diabetes, rheumatoid arthritis and multiple sclerosis along with numerous other conditions. Genetic evidence suggests that these diseases are regulated by common pathways.

We are trying to understand how a key mechanism, involving a protein called CTLA-4, works to control the immune system. People with a faulty CTLA-4 gene have poorly controlled immune systems and can develop multiple autoimmune diseases. However, our understanding of exactly what CTLA-4 does is still limited. In this proposal we will increase our knowledge of how CTLA-4 works and test ways to improve its function so we can develop better treatments for autoimmune diseases.

CTLA-4 works together with 3 other molecules (CD28, CD80, CD86) that are expressed on cells of the immune system, forming a type of thermostat for controlling immune activity. Preventing CTLA-4 function "turns up" the immune response and has generated a major breakthrough in cancer therapy, where the immune system can be used to attack cancer. However, the side effects of this treatment are autoimmune responses that can damage the body. We wish to develop a better understanding of how we might enhance CTLA-4 function and "turn down" the immune response to treat autoimmune conditions.

Both CD28 and CTLA-4 are expressed on T cells, specialised white blood cells that play a key role in triggering immune responses. CD28 promotes T cell activation and without it immune responses are feeble. On the other hand, CTLA-4 regulates T cell responses, to prevent them getting out of hand. A precise understanding of how CTLA-4 works with its partners would enable us to change the strength of immune responses as desired, for example increasing immune responses against tumours or decreasing unwanted immune responses in autoimmune conditions.

The challenge we face is that the pathway is complex. Although CD28 and CTLA-4 have opposing functions, they share the same binding partners, CD80 and CD86. In our previous work we found that CTLA-4 behaves in an unusual way, essentially "eating" CD80 and CD86 so they cannot promote immune responses via CD28. This "eating" behaviour has been difficult to study, but with previous funding we have now generated novel research tools that allow us to visualise it in a different way. These tools will be used in the present proposal to allow us to see for the first time how CTLA-4 captures its ligands at different places in the body, both under normal conditions and during the course of an autoimmune disease.

We have also discovered key differences in how the two binding partners, CD80 and CD86, affect CTLA-4 behaviour during the "eating" process. We now plan to use this understanding to test ways to improve CTLA-4 function and enhance immune suppression. We will also investigate how the upregulation of CD86 and CD80 is controlled, looking at how cells communicate between each other to collectively reach a decision. This cooperation between cells may affect CTLA-4 function and could explain why regulation sometimes fails. Lastly, soluble CTLA-4 (abatacept) is used clinically in rheumatoid arthritis but performs rather poorly in other autoimmune diseases. Our work has generated new ideas on how to overcome this limitation and we will test these with the goal of generating more effective immune suppression.

Overall, this programme will help us to develop better therapies for autoimmune conditions, at a time when these diseases are affecting more and more individuals.

An essential control point for T cell activation is the interplay between the T cell proteins CD28 and CTLA-4 that confer positive and negative regulation respectively. This costimulatory checkpoint operates by shared binding to two ligands, CD80 and CD86, and involves the capture of the ligands by CTLA-4 by a process of transendocytosis (TE).

We have made substantial progress in understanding this process using reductionist systems in vitro and have recently established that the fate of CTLA-4 molecules differs depending whether CD80 or CD86 is captured. However connecting our detailed analysis of CTLA-4 TE in vitro to the process of ligand capture in vivo presents a challenge. In this programme, we will use newly generated mouse strains expressing mCherry-tagged costimulatory ligands to overcome this problem, allowing us to pinpoint cells performing TE in vivo. We will define the characteristics of cells engaged in TE in vivo and establish where and when this process operates to regulate autoimmunity. Building on recent findings, we will test strategies to improve CTLA-4 TE and enhance regulation.

We will also explore how CTLA-4 function is countered by ligand upregulation. Emerging evidence supports a role for quorum-sensing in CD86 upregulation, indicating that cell populations can coordinately upregulate this molecule if a threshold number of cells is triggered. We will establish the implications of this for CTLA-4 function and whether it serves as a switch to overcome regulation and licence T cell activation. Finally, we will test new approaches to improve the effectiveness of soluble CTLA-4 therapy. The latter has proved disappointing in type 1 diabetes trials, but our recent work suggests new opportunities to overcome this problem. Together this programme will generate new insights into a crucial immune checkpoint and will support the generation of new immunotherapy approaches for use in autoimmune disease.