Induction of bronchus-associated lymphoid tissue is an early life adaptation for promoting human B cell immunity
Infants and young children are more susceptible to common respiratory pathogens compared to adults, but can fare better against novel pathogens like SARS-CoV-2. The mechanisms by which infants and young children mount effective responses to respiratory pathogens are unknown. Here, we demonstrate through study of lungs and lung-associated lymph nodes (LLN) from infant and pediatric organ donors, aged 0-13 years, that bronchus-associated lymphoid tissue (BALT), develops in lungs during the first year of life. BALT structures, consisting of B cell follicles and T cell zones, increase in numbers in the early years, and subsequently decrease over childhood coincident with accumulation of memory T cells in the lung. Early life BALT contains germinal centers and supports B cell differentiation, clonal expansion, somatic hypermutation, and immunoglobulin class switching. High dimensional flow cytometry reveals seeding of lungs by newly formed B cells (transitional cells) during infancy coincident with the timing of maximal BALT formation. We further demonstrate increased lung-localized B cell responses during respiratory virus infection in infants. Together, our findings provide novel evidence for BALT as an early life adaptation for mobilizing in situ immune protection to the diverse respiratory challenges during this formative life stage.