Pathogenetic and prognostic roles of bloodborne fibrocytes in asthma

Bloodborne fibrocytes are cells mobilized from the bone marrow, which express surface antigens commonly ascribed to hematopoietic progenitors and have phenotypic and functional characteristics similar to those of immature mesenchymal cells. They exhibit predominant proinflammatory or profibrotic activities at tissue sites, depending on the host's response to environmental insults and on the characteristics of the cell infiltrate and cytokine milieu. In patients with allergic asthma, fibrocytes egress from the bone marrow and are recruited into the airways after every allergen exposure and during viral infections. Recruited fibrocytes amplify the inflammatory responses driven by T helper type 2 lymphokines and favor viral replication and further inflammation on respiratory virus infections. Persistently elevated blood fibrocyte counts and persisting airway fibrocytosis are present in patients with chronically undertreated or corticosteroid-insensitive asthma, and are linked to an enhanced risk of adverse outcomes because of the major involvement of fibrocytes in the development of structural abnormalities that lead to chronic airflow obstruction in these patients. Consequently, blood fibrocyte count is an emerging biomarker of asthma control and disease progression and its clinical applicability as a new outcome measure deserves further evaluation in large clinical trials.