Relapsing Eosinophilia in a Severe Allergic Asthma Patient on Biological Therapy.

Background: Severe asthma often remains uncontrolled despite optimized inhaled treatment. The rise of biologic therapy in severe asthma represented a major advance for the disease management. However, correct phenotyping and monitoring of severe asthma patients is key to the success of targeted biologic therapy. Materials and Methods: We present the case of a 63-year-old female, never a smoker, diagnosed with asthma at the age of 45 and associated persistent mild rhinitis, without other notable comorbidities. She was prescribed medium-dose ICS/LABA, administered inconstantly in the first years after the diagnosis, with poor overall control of the disease. After several exacerbation episodes, treatment compliance improved, but the control of the disease remained poor despite adding an antileukotriene. In January 2019, she presented an exacerbation episode requiring treatment with oral corticosteroids (OCS) and she was afterwards put on high-dose ICS/LABA and continued the antileukotriene. She was referred for a skin allergy test, which revealed mild sensitization to Dermatophagoides pteronyssinus and farinae, with a total IgE level of 48.3 IU/mL. The blood eosinophil level was 270 cells/mm 3 . The lung function was variable, going from mild impairment to severe fixed obstruction during exacerbations. Despite optimized inhaled treatment, good adherence and inhaler technique, and allergen avoidance strategies, asthma control was not achieved, and she continued to experience severe episodes of exacerbation requiring OCS. Results: In October 2019, she was initiated on biologic therapy with omalizumab, which allowed asthma control to be achieved and maintained for 18 months, with preserved lung function, good symptom control, no exacerbations and slightly elevated blood eosinophil level (340-360 cells/mm 3 ). In April 2021, she started experiencing exacerbation episodes requiring OCS (three episodes within 6 months), with a progressive increase in blood eosinophil level (up to 710 cells/mm 3 ), and progressive deterioration of asthma control and lung function, despite continuation of previous therapy. A specific IgE test against Aspergillus was negative, and total IgE level was 122.4 IU/mL. In December 2021, the patient was switched from omalizumab to benralizumab. Asthma control was again achieved, lung function improved significantly and the patient did not experience any other exacerbation episodes up until today, which allowed for a reduction in ICS dose. Intriguingly, a relapsing eosinophilia was also noted under anti-IL5-R treatment prior to the dose administration, but with preserved asthma control. Conclusions: This case underscores the pivotal role of meticulous phenotyping in severe asthma management on one side, and careful monitoring of patient evolution and possible side effects of treatment on the other side. By showcasing how diverse inflammatory pathways can coexist within a single patient and impact treatment outcomes, it highlights the necessity of tailored biologic therapy for sustained control.