Chimeric Allergen Receptor Treg Cells Suppress Allergic Asthma in Mice

Genetically engineered CAR T cells expressing artificial receptor proteins are increasingly used in the clinic to boost the immune system’s response against leukemias and other cancers. Researchers at Lausanne University Hospital and University of Lausanne, and at Center for Human Immunology Lausanne, have now adapted this approach to suppress the immune system’s response to a common birch pollen allergen. The investigators developed regulatory T cells (Tregs) armed with chimeric allergen receptors (CAlleR Tregs), which in tests reduced or preventing asthma symptoms in mice sensitized to the allergen. The team suggests that their technique could eventually be used to treat a wide variety of allergies in humans.

“Our study provides proof-of-concept and preclinical evidence that CAlleR Tregs redirected against a birch pollen allergen can downmodulate birch pollen–induced allergic asthma,” said study lead Yannick D. Muller, MD, PhD, an associate professor at Lausanne University Hospital and the University of Lausanne.

Muller and colleagues reported on their study in Journal of Experimental Medicine (JEM) in a paper titled “Chimeric allergen receptor regulatory T cells suppress birch pollen allergic airway inflammation,” concluding “These findings unveil a novel mechanism for targeting soluble antigens and highlight the potential of CAlleR Tregs to prevent and treat severe allergies.”

Asthma affects over 300 million people worldwide, around 60% of whom suffer from allergic asthma. Allergens trigger an immune response in a patient’s airways, causing inflammation, excessive mucus production, and difficulty in breathing. “Allergic asthma is driven by an exacerbated type 2 immune response, characterized by the overproduction of IL-4, IL-5, and IL-13 by Th2 cells,” the authors explained. These cytokines trigger events that ultimately result in airway hyperresponsiveness, and airway mucus plugging, which is the primary cause of death in asthma.

Allergen immunotherapy (AIT), which involves the administration of gradually increasing doses of allergen, is the only treatment that addresses the underlying cause of asthma. Yet, it is not recommended for patients with severe asthma, representing the most vulnerable population at greatest risk of asthma-related morbidities and mortality. “This highlights the need for new, safe, and durable treatments for restoring allergen tolerance in severe allergic asthma,” Muller commented.

Regulatory T cells (Tregs) are immune cells that can dampen the body’s immune responses and prevent excessive inflammation. Tregs are being investigated as potential therapies for a variety of inflammatory and autoimmune disorders. “Importantly, Tregs can be expanded ex vivo and reinfused with multiple clinical trials evaluating their potential in autoimmune and inflammatory disorders,” the team continued. “However, Treg therapy has shown only limited efficacy, which has been mostly attributed to the lack of antigen specificity.”

Muller and colleagues wondered whether they could boost the therapeutic potential of Tregs by genetically engineering them to express receptor proteins that recognize specific allergens. This approach is analogous to the CAR T cell method that is now commonly used to treat cancers: cytotoxic T cells are engineered to express chimeric antigen receptors that specifically recognize proteins on the surface of cancer cells, directing the immune system to attack and kill the tumor cells.

A leading cause of allergic asthma is birch tree pollen, to which 8–16% of the European population are sensitive. The birch allergen Bet v1 is the most abundant allergenic protein, the authors commented. And while AIT for birch pollen–associated rhinitis and asthma has been shown to be effective, it is contraindicated for patients with severe and uncontrolled asthma. “This highlights the unmet need for new, safe, and durable treatments for restoring allergen tolerance in severe allergic asthma.”

Muller’s team constructed chimeric allergen receptors (CAlleRs) that specifically recognize the Bev v1 component of birch tree pollen. These CAlleRs were based on antibodies isolated from a birch-allergic patient, linked to protein signaling domains that can activate Treg cells. “We identified and characterized four novel anti–birch-specific antibodies and generated single-chain variable fragments (scFvs) fused to a CD28-ζ signaling domain,” the investigators noted.

Exposure to the birch pollen allergen, when stabilized by noncompetitive antibodies, boosted the suppressive activity of Tregs expressing these CAlleRs. The researchers found that simultaneous binding by a CAlleR and a non-competing antibody promotes receptor–allergen cross-linking underlying a novel mechanism to induce T cell activation by soluble antigens. “This mechanism opens new avenues not only for rewiring synthetic receptors against any soluble antigens including autoantigens for therapeutic intervention but also for delineating a more global pathway for antigen cross-presentation in allergies.”

Muller and colleagues injected these CAlleR-expressing Tregs into mice that were already allergic to birch pollen. When these treated animals were re-exposed to birch pollen, they showed decreased signs of allergic inflammation, reduced mucus production, and increased lung function. Next, the researchers injected CAlleR-expressing Tregs into mice that had never been exposed to birch pollen. When these animals were subsequently exposed to pollen, they failed to develop any asthma symptoms.

The team concluded, “These findings unveil a novel mechanism for targeting soluble antigens and highlight the potential of CAlleR Tregs to prevent and treat severe allergies.”  Muller added, “Future studies should evaluate the persistence and stability of CAlleR Tregs over time and define the optimal modalities for implementing such a therapeutic approach.” CAlleRs could also be developed that specifically suppress the immune response to other common allergens, including house dust mites or certain food allergens.” Future work should evaluate whether such approach could also be suitable to restore tolerance against food allergies,” the investigators said.

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