Objective
The recent FDA approval of the T cell-targeting drug, teplizumab highlights the potential for drugs that modify immune system function (immunotherapies) in preventing or treating type 1 diabetes (T1D). Unfortunately, teplizumab-mediated protection against diabetes is transient and the majority of people treated with this drug still go on to develop diabetes. Our overarching goal is to complete the fundamental research necessary to continue expanding the arsenal of drugs available to treat T1D.
Islet autoantibodies are the most powerful biomarkers for predicting T1D risk and are thought to arise when T and B lymphocytes “talk” to each other in special immune structures called germinal centers. B lymphocytes “talk” to CD4+ T lymphocytes, which in turn “tell” CD8+ T lymphocytes to attack and destroy islet beta cells. The protein, Bcl6 plays a key role in supporting germinal center T and B lymphocyte development; without it, neither of these “highly trained” immune cell types form. Our recent data from one of our unique mouse models of T1D suggest that genetically deleting Bcl6 in CD4+ T cells completely protects against diabetes development. In other words, our data suggest blocking Bcl6 function stops the “rabble-rouser” B and CD4+ T lymphocytes from talking to each other, which in turn prevents them from sending CD8+ T cells on a mission to kill beta cells. Our specific objectives are therefore:
1) Test the ability of Bcl6 pharmacologic inhibition to disrupt autoimmune germinal centers and prevent diabetes.
2) Determine the impact of deleting Bcl6 in CD4+ cells on the pathogenic functions of anti-insulin T and B lymphocytes in T1D.
Background Rationale
Type 1 diabetes (T1D) occurs when the immune system mistakenly launches an attack against beta cells in pancreatic islets. The origins of this misguided autoimmune assault against self-tissues are still not entirely clear. We know that T and B lymphocytes are two types of immune cells that cooperate with each other to cause destruction of pancreatic beta cells. One outcome of this nefarious T and B lymphocyte cooperation is that islet autoantibodies are produced, which are among the best predictive biomarkers of T1D. Blocking communication between these self-reactive T and B lymphocytes is thus a major therapeutic goal. Studying these immune cells in humans is complicated by the inaccessibility of the pancreas. Mouse models allow us to study how immune cells behave at the site of attack and have taught us much of what we know about how T1D occurs. Preclinical studies in mice also allow us to vet ideas to ensure only the most promising candidates move on to human clinical trials. This proposal will therefore harness the power of mouse models to advance our understanding of immune pathways we can disrupt in order to block T1D.
Protective immune responses evolve following infection or vaccination to be better, faster, and stronger by continuing to “train and retain” the very best T and B lymphocytes. This “elite immune system training” happens in specialized immune structures called germinal centers. This germinal center “training” process is helpful when you’re fighting an infection but is dangerous when you’re inappropriately fighting self in autoimmune disease. Our data suggest that germinal centers help to unleash islet-reactive T and B lymphocytes to promote beta cell attack in T1D. We found that disrupting the protein, Bcl6, in CD4+ T cells blocks germinal center T and B lymphocyte formation and completely protects against T1D in a mouse model. Importantly, blocking Bcl6 does not completely block protective immune responses; antibody can still form, enabling pathogen clearance. Teplizumab is a recently FDA-approved drug that delays T1D onset for an average of two years by broadly disrupting T cell function. T1D prevention via teplizumab is unfortunately not durable, in part because it is given as a single course to limit chronic immunosuppression. Whereas teplizumab targets the majority of T cells, targeting Bcl6 will only disrupt the function of a small proportion of T and B lymphocytes, and should thus be less immunosuppressive. T1D varies widely across individuals; onset can range from infant to adulthood, likely driven at least in part by differences in the autoimmune response that develops. Increasing the arsenal of different immunotherapies available to treat T1D could allow for greater personalization of therapy in the future.
We developed a unique insulin autoimmunity-centric mouse model of T1D in which mice develop accelerated onset of diabetes compared to the conventional T1D mouse model. In addition to building rationale for Bcl6 as a new therapeutic target in T1D, we will complete studies using this novel, highly aggressive T1D mouse model to zero in on how the development of insulin autoimmunity is influenced by the ability of anti-insulin T and B lymphocytes to engage one another in germinal centers. These studies will therefore perform foundational studies to support Bcl6 as a new therapeutic target and increase our knowledge about the requirements for anti-insulin T and B lymphocyte pathogenic function in T1D.
Description of Project
Our immune systems are designed to protect us against germs such as viruses and bacteria. T and B lymphocytes are specialized immune cells that target specific germs to keep us healthy. Unfortunately, in autoimmune diseases like type 1 diabetes (T1D), T and B lymphocytes inappropriately direct their attack at self-tissues such as pancreatic beta cells. This immune cell attack on beta cells can occur for years or even decades before symptoms are noticed and T1D is diagnosed in the clinic. The good news is this long pre-symptomatic period provides an opportunity to intervene to control this inappropriate beta cell-directed immune response before T1D develops. For example, teplizumab is a recently FDA-approved drug which suppresses T lymphocytes to delay T1D onset in at-risk individuals. This diabetes protection is unfortunately only transient and works by causing immune suppression against both self (good) and germs (bad). Such broad immune suppression is particularly problematic in children, as they have not yet had the opportunity to form immune memory against the common germs they are likely to encounter throughout life. We are therefore focused on identifying more targeted therapies to adequately control the dangerous, beta cell-reactive lymphocytes while preserving protective immune responses against germs.
Features of a protective immune response against a particular germ include “specificity” and “memory”. “Specificity” is how the immune system launches an attack against a specific germ, and not your own tissues. “Memory” is how your immune system remembers a germ so it can fight it better and faster the next time it’s encountered. Top-notch immune responses typically come from germinal center reactions, which enhance both specificity and memory. These germinal centers are where specialized immune cells, T and B lymphocytes, communicate with each other to help each other strengthen their attack against the germ. We have evidence to suggest that germinal center reactions are also important contributors to T1D development. Specifically, we find that genetically deleting the protein, Bcl6 in CD4+ T cells prevents germinal center T and B lymphocyte formation and completely protects against T1D in a mouse model. To support translation of this finding to people, this proposal will perform the necessary preclinical studies to determine the efficacy and mechanism of action for a Bcl6 inhibitor in preventing T1D. Importantly, this inhibitor can disrupt germinal center reactions without blocking protective immune responses. Primary immune responses are retained even when Bcl6 function is blocked, which means this drug will likely be less immunosuppressive than drugs like teplizumab. This distinction is especially important in children, as the immune memory they have developed is much less complete than an adult.
Overall, this research will clarify the requirement for germinal centers in driving T1D to teach us about how the immune response against beta cells develops and will generate the foundational knowledge necessary to support translation of Bcl6 inhibitors to clinical trials in the future.
Anticipated Outcome
To our knowledge, we are the first to provide evidence for Bcl6 as a new target in type 1 diabetes (T1D) using mouse models in which Bcl6 is genetically deleted. To support future translation of these findings to the clinic, these studies will capitalize on unique resources and team strengths including 1) new preclinical mouse models that allow careful dissection of Bcl6 contributions to autoimmunity directed against a key T1D autoantigen, insulin and 2) nearly twenty years of research experience dedicated to enhancing our understanding how autoimmunity develops in T1D. We therefore anticipate the proposed experiments will enable the following outcomes:
Aim 1 experiments will evaluate the ability of the small molecule Bcl6 inhibitor, FX1, to disrupt T1D in a classic mouse model of this disease. These studies will determine whether FX1 can prevent early, pathogenic immune changes and downstream development of diabetes.
Aim 2 experiments will take advantage of several unique genetic mouse models we recently developed to mechanistically dissect T1D autoimmunity. For example, insulin autoantigen-specific B lymphocytes are difficult to study in people or mice at risk for T1D. This is because they are typically present at low numbers. To overcome this challenge, we will use our genetically modified mouse model that reliably has an easily detectable population of insulin autoantigen-specific B lymphocytes. If you’ll forgive a Star Wars analogy, we will use these unique mouse models to determine the impact of disrupting Bcl6 function on preventing insulin autoantigen-specific B lymphocytes from “turning to the dark side”. We will also monitor downstream changes in islet infiltration by B lymphocytes and other immune cells and ultimately diabetes development.
Relevance to T1D
Clinical type 1 diabetes (T1D) develops as a result of immune system attack on pancreatic beta cells. This immune attack is orchestrated by T and B lymphocytes which are specialized immune cells. Within general T and B lymphocytes, there are “elite” subsets that are intimately associated with generating protective immune responses. These “elite” subsets depend on Bcl6 for their transition into “lean, mean, fighting machines”. Our preliminary studies using genetically deficient mice identify Bcl6 as a promising target for disrupting the pathologic immune processes that cause beta cell death and eventual diabetes. The proposed experiments will determine whether the Bcl6 inhibitor, FX1, has similar efficacy in preventing T1D, and will define the roles that Bcl6-dependent “elite” lymphocyte subsets play in driving T1D.
The recent approval of teplizumab highlights the power that immunotherapy holds for T1D prevention and treatment. Translation of new, more selective drugs like Bcl6 inhibitors to the clinic might prolong the transient protection conferred by teplizumab or might serve as a less immunosuppressive alternative. Dr. Bonami (PI) is a Type 1 Diabetes TrialNet Investigator at Vanderbilt and leads A) mechanistic studies using mouse models and B) translational studies in people at risk for T1D. This cross-disciplinary experience positions our team perfectly for the “bench to bedside” research required to most rapidly translate this and other promising new immunotherapies to the clinic.
Translation of Bcl6 inhibitors to the clinic could help:
People at risk for T1D. Bcl6 inhibitors could delay or prevent diabetes onset in people at risk for T1D by stopping beta cell attack in its tracks.
People with established T1D. Insulin production could be normalized through successful beta cell transplant or regeneration in people with long-standing T1D. Beta cell replacement therapy alone would however likely fail over time, as the autoimmune response that destroyed beta cells initially would likely also attack and destroy nascent beta cells. Bcl6 inhibitors could be used to ensure durable protection against subsequent autoimmune attack.