Final Report Summary - CD4INEAR (Defining the cellular interactions that control the retention of and tolerance induction in CD4 T cells at inflammatory sites) IntroductionOur immune system protects our bodies from dangerous pathogens that cause infectious diseases. CD4 T cells are one of the most important immune cells: they can specifically identify an invading pathogen and coordinate the most effective immune response against it. In some individuals, CD4 T cells mistakenly recognise the host's own proteins. These self or auto-reactive cells can cause damage to the host's tissue leading to autoimmune diseases such as rheumatoid arthritis (RA). In RA, which affects around 1% of the population, painful inflammation persists in the patient's joints leading to long-term disability and systemic inflammation can lead to a reduced lifespan.Treatments for many autoimmune conditions, including RA, involve suppressing the immune system. In the last twenty years, considerable progress has been made in RA therapies with a number of biological drugs acting on inflammatory mediators to reduce inflammation. However, many patients do not respond to current therapies and/or they experience only a limited improvement in their disease. Current treatments reduce disease symptoms rather than offer a permanent cure. Furthermore, this long-term suppression of the immune system can lead to an increase in opportunistic infections which can lead to serious infectious disease. There is, therefore, still considerable unmet need for new therapeutics for autoimmune conditions. A current major goal of the field is to cure autoimmunity by targeting the autoreactive CD4 T cells that coordinate the disease. These CD4 T cells have been previously activated, this means that they are trained to make rapid and large immune responses. Previous data from the Marie Curie fellow, Dr MacLeod, indicated that these previously activated or memory CD4 T cells were resistant to the induction of tolerance. A major unanswered question for the field is, therefore, whether and how these CD4 T cells can be turned off.Project ObjectivesThis project aimed to investigate the underlying biology that could provide important novel data to improve the treatment of autoimmune diseases. The project was split into two areas. The first addressed the molecular mediators that influence the persistence of CD4 T cells at sites of inflammation. Identification of these molecules could reveal novel targets to reduce the accumulation of CD4 T cells at inflamed sites. The second part of the project aimed to investigate whether CD4 T cells could be turned off or 'tolerised' in peripheral tissues. In many autoimmune conditions, the target tissue is a peripheral rather than lymphoid tissue. Little is known about how or even whether previously activated or memory CD4 T cells can be tolerised in peripheral tissues. Project resultsRegulation of CD4 T cell retentions in inflamed tissuesThe accumulation of immune cells in inflamed tissues is driven by both attractive factors that cause cell migration into inflamed sites, and retention factors that encourage them to stay. Many studies have focused on which molecules promote the migration of immune cells into inflamed sites. Much less is known about factors that cause the retention of immune cells at these sites. In this project we developed a novel experimental system to investigate retention signals in a mouse model of inflammation. We examined activated CD4 T cells at the site of inflammation and in the draining lymph node. We used techniques that enable us to quantify the numbers of activated T cells present and to determine their behaviour. Our studies, published in the peer-reviewed open access journal, Frontiers in Immunology (PMID: 29225602), showed that activated CD4 T cells were more likely to persist in inflamed compared to resting tissues. We identified a key molecule responsible for this increased retention of the activated CD4 T cells. Sphingosine-1-phospahate (S1P) is a lipid that regulates the migration of CD4 T cells out of lymphoid organs and may also provide survival signals. We showed that blocking the interaction between S1P and its receptor on the activated CD4 T cells led to a reduction in the persistence of CD4 T cells in the inflamed site. Similarly, signals through the receptor for S1P were sufficient to increase the persistence of activated CD4 T cells in resting tissues. These treatments did not alter the migration of the activated CD4 T cells from the inflamed site to the draining lymph node. We concluded, therefore, that inhibiting S1P receptor signalling in activated CD4 T cells leads to reduced survival of the cells. Importantly, we found that one of the key enzymes required to produce S1P is increased in the inflamed joints of patents with RA when compared to patients with the less inflammatory form of arthritis, osteoarthritis. Together, these studies suggest that increased S1P at inflamed tissues leads to an accumulation of CD4 T cells at inflamed tissues. Targeting this pathway may offer a novel local and targeted approach to reducing chronic inflammation.Tolerance in memory CD4 T cells in peripheral tissuesWe have used a mouse influenza virus infection model to generate a population of trackable CD4 T cells that reside within lymphoid organs and a peripheral tissue, the lung. We have exposed these memory CD4 T cells to a signal that induced tolerance in naïve T cells. This involves activating the CD4 T cells with a signal through their specific T cell receptor in absence of the accessory signals normally required to activate the T cell fully.As in our previous experiments published in PNAS 2014 (PMID: 24821788), we have found that memory CD4 T cells in all tissues are resistant to tolerance induction. The memory CD4 T cells exposed to the tolerance signal, continue to survive and produce inflammatory cytokines. However, these cells have a reduced ability to accumulate following a further reactivation in vivo. Together, these data suggest that while some T cell functions can be modified by tolerisation signals, important functions involved in the coordination of the inflammatory response continue. These data suggest that current strategies aimed at inducing tolerance in autoreactive T cells in patients must assess a range of different T cell functions to conclude whether or not the T cells have been fully tolerised.Potential ImpactsThis project has generated important novel data relevant to the treatment of autoimmune diseases. We have identified a role for S1P in the accumulation of activated CD4 T cells at inflamed tissues. Modulation of the S1P receptor is a current treatment for patients with the autoimmune disease, multiple sclerosis. In this condition, modulation of the S1P receptor is thought to prevent entry of activated CD4 T cells into the central nervous system. Our data suggest that modulation of the S1P receptor may have a second mode of action in T cells that have already been recruited to the inflamed tissue. These data may help clinicians to target the use of S1P receptor modulators to patients most likely to benefit from the treatment and/or to help scientists develop novel therapies that disrupt the accumulation of CD4 T cells at inflamed tissues.Our results in the second part of the project provide novel insights into treatment for autoimmune disease. These data are likely to form the basis for one-two publications that are due to be submitted to peer-review journal within the next year. Our data suggest that caution must be taken in the analysis of T cells from patients involved in trials designed to induce tolerance in autoreactive T cells. We found that previously activated CD4 T cells are resistant to tolerance induction, in particular they are still able to produce inflammatory cytokines. This suggests that they could continue to contribute to disease.
