Chlamydia muridarum Lung Infection in Infants Alters Hematopoietic Cells to Promote Allergic Airway Disease in Mice

Background: Viral and bacterial respiratory tract infections in early-life are linked to the development of allergic airway inflammation and asthma. However, the mechanisms involved are not well understood. We have previously shown that neonatal and infant, but not adult, chlamydial lung infections in mice permanently alter inflammatory phenotype and physiology to increase the severity of allergic airway disease by increasing lung interleukin (IL)-13 expression, mucus hypersecretion and airway hyper-responsiveness. This occurred through different mechanisms with infection at different ages. Neonatal infection suppressed inflammatory responses but enhanced systemic dendritic cell: T-cell IL-13 release and induced permanent alterations in lung structure (i.e., increased the size of alveoli). Infant infection enhanced inflammatory responses but had no effect on lung structure. Here we investigated the role of hematopoietic cells in these processes using bone marrow chimera studies. Methodology/Principal Findings: Neonatal (<24-hours-old), infant (3-weeks-old) and adult (6-weeks-old) mice were infected with C. muridarum. Nine weeks after infection bone marrow was collected and transferred into recipient age-matched irradiated naive mice. Allergic airway disease was induced (8 weeks after adoptive transfer) by sensitization and challenge with ovalbumin. Reconstitution of irradiated naive mice with bone marrow from mice infected as neonates resulted in the suppression of the hallmark features of allergic airway disease including mucus hyper-secretion and airway hyper-responsiveness, which was associated with decreased IL-13 levels in the lung. In stark contrast, reconstitution with bone marrow from mice infected as infants increased the severity of allergic airway disease by increasing T helper type-2 cell cytokine release (IL-5 and IL-13), mucus hyper-secretion, airway hyper-responsiveness and IL-13 levels in the lung. Reconstitution with bone marrow from infected adult mice had no effects. Conclusions: These results suggest that an infant chlamydial lung infection results in long lasting alterations in hematopoietic cells that increases the severity of allergic airway disease in later-life.