Diversity of B Cell Populations and Ig Repertoire in Human Lungs.

The human lung carries a unique microbiome adapted to the air-filled, mucous-lined environment, the presence of which requires an immune system capable of recognizing harmful populations while preventing reactions toward commensals. B cells in the lung play a key role in pulmonary immunity, generating Ag-specific Abs, as well as cytokine secretion for immune activation and regulation. In this study, we compared B cell subsets in human lungs versus circulating cells by analyzing patient-paired lung and blood samples. We found a significantly smaller pool of CD19+, CD20+ B cells in the lung relative to the blood. CD27+, IgD-, class-switched memory B cells (Bmems) composed a larger proportion of the pool of pulmonary B cells. The residency marker CD69 was also significantly higher in the lung. We also sequenced the Ig V region genes (IgVRGs) of class-switched Bmems that do, or do not, express CD69. We observed the IgVRGs of pulmonary Bmems to be as heavily mutated from the unmutated common ancestor as those in circulation. Furthermore, we found progenies within a quasi-clone can gain or lose CD69 expression, regardless of whether the parent clone expressed the residency marker. Overall, our results show that despite its vascularized nature, human lungs carry a unique proportion of B cell subsets. The IgVRGs of pulmonary Bmems are as diverse as those in blood, and progenies of Bmems retain the ability to gain or lose residency.