Research into the causes of type 1 diabetes (T1D) often focuses on the autoimmune response, whereby the immune system destroys pancreatic islet beta cells that produce insulin. A newly reported study by scientists at the University of Chicago looked instead at the role of beta cells in triggering autoimmunity, and found that knocking out a proinflammatory gene in the beta cells of mice genetically predisposed to develop T1D preserved beta cell mass and protected the animals from developing diabetes.
The research findings also raise the possibility that new drugs that increase levels of PD-L1 on beta cells might be used to block the immune system from destroying beta cells and prevent T1D from developing in at-risk or early-onset patients.
“The immune system doesn’t just decide one day that it’s going to attack your beta cells,” said Raghavendra Mirmira, MD, PhD, professor of medicine and director of the Diabetes Translational Research Center at the University of Chicago. “Our thinking was that the beta cell itself has somehow fundamentally altered itself to invite that immunity. When we got rid of this gene, the beta cells no longer signaled to the immune system and the immune onslaught was completely suppressed, even though we didn’t touch the immune system. That tells us that there is a complex dialogue between beta cells and immune cells, and if you intervene in that dialogue, you can prevent diabetes.”
Mirmira and colleagues reported on their study in Cell Reports, in a paper titled, “Proinflammatory signaling in islet ß cells propagates invasion of pathogenic immune cells in autoimmune diabetes,” in which they concluded, “Our results support the contention that inflammatory signaling in cells promotes autoimmunity during type 1 diabetes progression.”
T1D is an autoimmune disease that results in a loss of insulin-secreting beta cells in pancreatic islets, the authors explained. Reduction of the beta cell population leads to an inability to maintain glucose homeostasis and so people with T1D need lifelong insulin therapy.
The traditional perspective, derived largely from studies in nonobese diabetic mice (NOD), has held that T1D results from the loss of immune tolerance to beta cell proteins, and so disease susceptibility is largely linked to features ascribed to the immune system, the team continued. This perspective has been supported by the observation that treating NOD mice using immune suppressants prevented or reversed diabetes. “Some of these interventions have also held true in human T1D,” the researchers noted, “although no study of immune suppressants or tolerance-inducing agents has completely prevented or reversed the disease.”
More recently, it’s been suggested that it might be the beta cells that represent the “center of a dialogue with immune cells,” such that proteins expressed or secreted by beta cells ultimately determine the pathogenicity of immune cells, the investigators continued.
In 2012, Sarah Tersey, PhD, research associate professor at the University of Chicago and co-senior author of the study, led a project that was among the first to suggest that the beta cell might be a central player in the development of T1D. “This allows us to understand the underlying mechanisms leading to the development of type 1 diabetes,” Tersey said. “This has been a huge, changing part of the field where we focus more on the role of beta cells and not just autoimmunity.”
For the newly reported study, the team used genetic tools to knock out or delete a gene called Alox15 in NOD mice that are genetically predisposed to developing T1D. “… we deleted the proinflammatory gene encoding 12/15-lipoxygenase (Alox15) in beta cells of nonobese diabetic mice at a pre-diabetic time point when islet inflammation is a feature.” The 12/15-lipoxygenase (12/15-LOX) enzyme encoded by Alox15 is known to be involved in processes that produce inflammation in beta cells. “Alox15 is also expressed in beta cells and is characteristically elevated in islets of humans at risk for T1D and in residual insulin-containing islets of individuals with established T1D,” the scientists pointed out. “… prior observations led us to hypothesize that 12/15-LOX in islets might govern an inflammatory cascade that propagates crosstalk between b cells and immune cells.”
The team’s newly reported study is the result of a long-term collaboration that began when Mirmira and several members of his lab were at Indiana University. Jerry Nadler, MD, dean of the School of Medicine and professor of medicine and pharmacology at New York Medical College discovered the role of the 12/15-Lipoxygenase enzyme, and Maureen Gannon, PhD, professor of medicine, cell and developmental biology, and molecular physiology and biophysics at Vanderbilt University, provided a strain of mice that was used in the study, which allowed for the knockout of the Alox15 gene when given the drug tamoxifen.
The results of their experiments confirmed that deleting Alox15 in the experimental mice preserved beta cells, reduced the number of immune T cells infiltrating the islet environment, and prevented T1D from developing in both male and female animals. The mice also showed increased expression of the gene encoding a protein called PD-L1 that suppresses autoimmunity. “Mice lacking Alox15 in beta cells exhibit an increase in a population of beta cells expressing the gene encoding the protein programmed death ligand 1 (PDL1), which engages receptors on immune cells to suppress autoimmunity,” they wrote. “Our findings show that in the absence of Alox15, there is increased generation of PD-L1 on beta cells that may account, at least in part, for the cellular crosstalk that leads to suppression of insulitis and effector T cell subtypes and the protection from T1D.”
The study has interesting connections to cancer treatments that harness the immune system to fight tumors. Cancer cells often express the PD-L1 protein to suppress the immune system and evade the body’s defenses. Checkpoint inhibitors target this protein, by inhibiting or removing the PD-L1 brakes and effectively unleashing the immune system to attack tumors. In the study in Cell Reports, increased PD-L1 in the knockout mice served its intended purpose by preventing the immune system from attacking the beta cells.
Tersey and colleagues then tested a drug designated ML355 that inhibits the 12/15-Lipoxygenase enzyme, on human beta cells. They found that this drug increased levels of PD-L1, suggesting that it could interrupt the autoimmune response and prevent diabetes from developing. Ideally, the drug would be given either to individuals who are at high risk of T1D because of family history and show early signs of developing T1D, or to patients shortly after a T1D diagnosis, and so before too much damage has been done to the pancreas. Mirmira and his team are taking the first steps to start clinical trials to test a possible treatment using ML355.
“This study certainly suggests that inhibiting the enzyme in humans can increase levels of PD-L1, which is very promising,” Mirmira said. “With beta cell targeted therapeutics, we believe that as long as the disease hasn’t progressed to the point that there’s massive destruction of beta cells, you can catch an individual before that process starts and prevent the disease progression altogether.”