A University of Alabama at Birmingham (UAB)-led research team has demonstrated a potential preventive treatment for Crohn’s disease, both in a mouse model and through the use of immune-reactive T cells from patients with the inflammatory bowel disease.
The researchers, headed by professor of medicine Charles O. Elson, MD, used a triple-punch treatment to remove T memory (Tm) cells and increase the number of T regulatory, or Treg, cells. This strategy prevented colitis in a T cell transfer mouse model, and similar inhibitory effects were also seen on immune-reactive CD4+ T cells isolated from Crohn’s disease patient blood samples.
These results, Elson said, support a potential immunotherapy to prevent or ameliorate inflammatory bowel disease. Reporting on their studies in Science Immunology (“CD4+ T cell activation and concomitant mTOR metabolic inhibition can ablate microbiota-specific memory cells and prevent colitis”), the team acknowledged that further studies will be needed to understand how and why the triple-punch treatment works.
Nevertheless, they concluded, “This study demonstrates an amelioration of immune-mediated inflammatory disease through peripheral ablation of CD4+ Tm cells.” They further suggest that the approach represents “… a promising clinical application for preventing or ameliorating the course of IBD.”
The prevalence of inflammatory bowel disease is greater than 0.3% in developed countries, and is on the rise in developing countries, the authors wrote. “Its major forms, Crohn’s disease and ulcerative colitis, are associated with substantial morbidity and larger medical care costs.” Inflammatory bowel diseases result from over-activation of the immune response against gut microbes in genetically susceptible hosts. One specific microbial antigen that causes this over-reaction by short-lived T effector cells is flagellin, the protein-subunit of bacterial flagella, the long tail-like structures that give some bacteria their motility.
One group of immunodominant flagellins are those from the Lachnospiraceae family, including CBir1. More than half of Crohn’s disease patients have elevated serological reactivity to CBir1 and related flagellins, “… which is accompanied by a more complicated clinical course,” the team continued.
Unlike the short-lived T effector cells that help to actively fight infections, T memory cells serve as sentinels that remember a previous encounter with flagellins. These cells are long-lived and quiescent, with a low level of metabolism. If reactivated by a fresh encounter with flagellin antigens, they undergo a profound metabolic transition and quickly expand into large numbers of pathogenic T effector cells. This metabolic switch is controlled by the signaling protein, mTOR, located in the Tm cell.
Activation of mTOR is necessary for T cell expansion, making it an inescapable metabolic checkpoint to create activated Tm cells. It is also the checkpoint for T naïve cells that are encountering flagellin for the first time. Elson and colleagues hypothesized that activation of CD4+ Tm or T naïve cells by flagellin antigens, while at the same time shutting down the metabolic checkpoint through the use of mTOR inhibition, would result in the death, or an absence of, the normal immune response to an antigen, a phenomenon known as anergy.
These effects comprise two parts of the triple-punch treatment, with the third being induction of Treg cells. The activation was prompted by a synthetic peptide that had multiple repeats of one CBir1 epitope. Such a peptide can selectively stimulate memory cells without activating an innate immune response.
To shut down the metabolic checkpoint, the UAB researchers used two existing drugs, rapamycin and metformin. Rapamycin directly inhibits mTOR, while metformin adds to that inhibition by activating a kinase called AMPK that negatively regulates mTOR activity. “In this study, with both in vitro and in vivo murine experiments, we showed that naïve CD4+ T cells mainly engage the mTOR pathway after activation and rapamycin is sufficient to inhibit this process, whereas a combination of rapamycin and AMPK activator metformin is needed for optimal inhibition of murine CD4+ TM cells upon antigen reencounter,” they wrote.
Elson has called the treatment cell activation with concomitant metabolic checkpoint inhibition, or CAMCI. In their reported experiments, the team showed that parenteral application of CAMCI in mice successfully targeted microbiota flagellin-specific CD4+ T cells, leading to significant antigen-specific CD4+ T cell death, impaired development, and impaired reactivation of CD4+ memory responses, and substantial induction of a CD4+ Treg cell response. The treatment approach prevented colitis in the mouse model, and had similar inhibitory effects on microbiota-flagellin-specific CD4+ T cells isolated from patients with Crohn’s disease.
“These results indicate that parenteral activation of microbiota-specific CD4+ T cells with concomitant metabolic inhibition is an effective way to ablate pathogenic CD4+ TM cells and to induce Treg cells that provide antigen-specific and bystander suppression, supporting a potential immunotherapy to prevent or ameliorate IBD,” the authors concluded.
For a potential future treatment of patients with Crohn’s disease, targeting only a single flagellin is unlikely to have much effect, Elson noted. “Instead, we anticipate the future use of a synthetic multi-epitope peptide containing multiple CD4-positive T cell flagellin epitopes to target many microbiota-flagellin-reactive CD4-positive Tm cells,” he commented. “Depending on the serologic or CD4-positive T cell response to certain microbiota antigens, this CAMCI approach could be tailored to individuals with different combinations of epitopes as a personalized immunotherapy.”
Elson envisions this CAMCI approach as an intermittent pulse therapy to maintain remission in patients with Crohn’s disease. And ” … with autoantigen epitopes better studied in the future,” the authors noted, ” … this approach could be expanded to treat other inflammatory or autoimmune diseases such as type 1 diabetes or multiple sclerosis.”