© FASEB. Serine proteinase inhibitor, clade E, member 2 (SERPINE2), is a cell- and extracellular matrix-associated inhibitor of thrombin. Although SERPINE2 is a candidate susceptibility gene for chronic obstructive pulmonary disease, the physiologic role of this protease inhibitor in lung development and homeostasis is unknown. We observed spontaneous monocytic-cell infiltration in the lungs of Serpine2-deficient (SE2-/-) mice, beginning at or before the time of lung maturity, which resulted in lesions that resembled bronchus-associated lymphoid tissue (BALT). The initiation of lymphocyte accumulation in the lungs of SE2-/- mice involved the excessive expression of chemokines, cytokines, and adhesion molecules that are essential for BALT induction, organization, and maintenance. BALT-like lesion formation in the lungs of SE2-/- mice was also associated with a significant increase in the activation of thrombin, a recognized target of SE2, and excess stimulation of NF-κB, a major regulator of chemokine expression and inflammation. Finally, systemic delivery of thrombin rapidly stimulated lung chemokine expression in vivo. These data uncover a novel mechanism whereby loss of serine protease inhibition leads to lung lymphocyte accumulation.