Fetal liver monocytes seed the lungs between embryonic day 14.5 and birth.²  These cells then differentiate into prealveolar macrophages and acquire PPAR-γ expression through GM-CSFR signaling.³  These prealveolar macrophages then translocate and engraft into the alveolar space in the first week of life, where they develop into terminally differentiated alveolar macrophages. In LPL-deficient mice, the translocation and the engraftment into the alveolar space are impaired (compare left and right panel), resulting in a severe reduction in number of alveolar macrophages and in an increased susceptibility to pneumococcal infection.