Figure 3.

Pneumococcal clearance in the lung. Host defence in the lower respiratory tract is mediated by alveolar macrophages. (i) During early infection where the bacterial load is low, resident alveolar macrophages efficiently kill and phagocytise opsonised pneumococci in a quiescent manner, effectively preventing bacteria–dendritic cell interaction, and hence inhibiting initiation of T cell-mediated inflammatory responses. (ii) In situations where bacterial load exceeds the capability for macrophages to perform effective opsonophagocytosis, neutrophils are recruited following secretion of TNF-α by alveolar macrophages and/or IL-8 by epithelial cells. (iii) T cells are recruited following successful antigen presentation in the draining lymph nodes by pulmonary dendritic cells. These cells secrete IFN-γ which activates macrophages to kill internalised pneumococci and also promotes further TNF-α production by alveolar macrophages. (iv) Following clearance of pneumococci from the lungs, neutrophils, some macrophages and T cells undergo rapid apoptosis. Surviving T cells remain in the alveoli as resident effector memory cells.