Eosinophilic asthma is a generally severe subtype of asthma.  It is commonly seen in people who develop asthma in adulthood, from 35-50 years of age, although it may occur in children.  Overall, five to ten percent of asthmatics suffer from a severe form that is difficult to adequately control, of which 50-60 percent is actually eosinophilic asthma.  The cause of eosinophilic asthma is unknown, although it may be related to genetics.  Patients with eosinophilic asthma do not typically have underlying allergies (e.g., pollens, dust mites, pet dander) that trigger asthma symptoms.

Eosinophilic comes from the Greek root-word philia for love, plus eosin, and refers to the staining of certain tissue cells, or organelles, after they have been washed with eosin, a dye.  Eosinophilic describes the appearance of cells and structures seen in histological sections that take up the staining dye eosin.  Eosinophils are one type of white blood cells originating from bone marrow and are not normally present in healthy airway and lung tissue.  In eosinophilic asthma, the number of eosinophils increases in the sputum, blood, and lung tissue.  Higher eosinophil levels usually indicate more severe asthma.  Interleukin 5 (IL5), secreting from bronchial epithelium immune cells, plays a crucial role in the development, maturation, prolonged survival, and release of eosinophils from bone marrow.  Eosinophils migrate and infiltrate tissues of the upper and lower airways, which may lead to sinus inflammation, nasal polyps, otitis media, and asthma.  Most people with persistent eosinophilic asthma depend on oral corticosteroids, which they often respond poorly to, even at the highest doses.  Furthermore, they frequently suffer from severe exacerbations of asthma symptoms.

In the past decade, breakthroughs in molecular biology and genetic engineering have led to a better understanding of chemical mediators in the inflammation of eosinophilic asthma.  We are now able to develop biologics to block the effects of IL5 and decrease eosinophils at the targeted tissues.  Different from conventional medications, biologic drugs are made from proteins derived from human DNA and “grown” through a sophisticated manufacturing process.

The Food and Drug Administration has approved biologic therapies targeting eosinophils for adults with eosinophilic asthma that cannot be adequately controlled with conventional medications:  Nucala (mepolizumab), Cinqair (reslizumab) and anti-IL5 receptor agent: Fasenra (benralizumab).  The beneficial effects of mepolizumab and resilizumab are related to the biologic ability to bind with high affinity to IL5 and block the interaction between IL5 and its receptor on the surface of the eosinophils.  Benralizumab directly binds IL5 receptors and affects IL5 actions.  These biologics decrease eosinophils and curb the inflammation of eosinophilic asthma, leading to fewer and/or less severe symptoms.  Although quality of life can be seriously impacted by severe eosinophilic asthma, these new biologic medications give good reason to be optimistic.