Macrophage LC3-associated phagocytosis is an immune defense against <i>Streptococcus pneumoniae</i> that diminishes with host aging.

Abstract

<i>Streptococcus pneumoniae</i> is a leading cause of pneumonia and invasive disease, particularly, in the elderly. <i>S. pneumoniae</i> lung infection of aged mice is associated with high bacterial burdens and detrimental inflammatory responses. Macrophages can clear microorganisms and modulate inflammation through two distinct lysosomal trafficking pathways that involve 1A/1B-light chain 3 (LC3)-marked organelles, canonical autophagy, and LC3-associated phagocytosis (LAP). The <i>S. pneumoniae</i> pore-forming toxin pneumolysin (PLY) triggers an autophagic response in nonphagocytic cells, but the role of LAP in macrophage defense against <i>S. pneumoniae</i> or in age-related susceptibility to infection is unexplored. We found that infection of murine bone-marrow-derived macrophages (BMDMs) by PLY-producing <i>S. pneumoniae</i> triggered Atg5- and Atg7-dependent recruitment of LC3 to <i>S. pneumoniae</i>-containing vesicles. The association of LC3 with <i>S. pneumoniae</i>-containing phagosomes required components specific for LAP, such as Rubicon and the NADPH oxidase, but not factors, such as Ulk1, FIP200, or Atg14, required specifically for canonical autophagy. In addition, <i>S. pneumoniae</i> was sequestered within single-membrane compartments indicative of LAP. Importantly, compared to BMDMs from young (2-mo-old) mice, BMDMs from aged (20- to 22-mo-old) mice infected with <i>S. pneumoniae</i> were not only deficient in LAP and bacterial killing, but also produced higher levels of proinflammatory cytokines. Inhibition of LAP enhanced <i>S. pneumoniae</i> survival and cytokine responses in BMDMs from young but not aged mice. Thus, LAP is an important innate immune defense employed by BMDMs to control <i>S. pneumoniae</i> infection and concomitant inflammation, one that diminishes with age and may contribute to age-related susceptibility to this important pathogen.