Phagosome maturation of the professional phagocytes after ingestion of microbial pathogens, characterized by phagosomal acidification and phagosome/lysosome fusion, is a critical step in the killing and degradation of the internalized mTOR inhibitor pathogens and thus plays a key role in innate immunity against microbial infection [23-25]. We first measured phasosomal pH in infant macrophages and observed a substantially delayed and reduced phagosomal acidification in infant macrophages compared with adult macrophages after ingestion of either S. aureus or S. typhimurium. Consistent with

the defective phagosomal acidification, infant macrophages also exhibited severely impaired phagolysosome fusion in response

to both gram-positive and C59 wnt in vivo gram-negative bacterial challenges, as revealed by the impaired colocalization of either S. aureus-FITC or E. coli-FITC with LysoTraker red-labeled lysosomes in infant macrophages compared with adult macrophages. These data indicate that infant macrophages exhibit a defect in phagosome maturation into the late lysosomal stage. Collectively, our results reveal the deficiency of infant mice in their innate phagocyte-associated antimicrobial functions in response to bacterial infection, which is characterized by diminished PMN in vitro chemotaxis and in Interleukin-2 receptor vivo recruitment into the infections site, and impaired macrophage phagosome maturation and bactericidal activity. These defective innate immunity-mediated antimicrobial responses render infant mice more susceptible to microbial

sepsis. Two- and eight-week-old infant and adult C57BL/6 mice were purchased from Harlan (Oxon, U.K.) and maintained in the University Biological Services Unit, University College Cork / National University of Ireland. Mice were housed in barrier cages under controlled environmental conditions (12/12 h of light/dark cycle, 55% ± 5% humidity, 23°C) and had free access to standard laboratory chow and water. Animals were fasted 12 h before experiments and allowed water ad libitum. All animal procedures were carried out in the University Biological Services Unit under a license from the Department of Health (Republic of Ireland). All animal studies were conducted with ethical approval granted from the University College Cork Ethics Committee. Gram-positive S. aureus and gram-negative S. typhimurium were obtained from American Type Culture Collection (ATCC, Manassas, VA, USA) and the National University of Ireland Culture Collection, respectively. Bacteria were cultured at 37°C in trypticase soy broth (Merck, Darmstadt, Germany), harvested at the mid-logarithmic growth phase, washed twice, and resuspended in PBS for in vitro and in vivo use.