The testes contain two populations of tissue macrophages, interstitial and peritubular; among other functions, they are involved in maintaining immunological privilege, but their developmental relationships are unknown. In the Journal of Experimental Medicine, Sieweke and colleagues observe that beyond the histological localization and morphology of these cells, the markers CD64 and MHC class II can be used to reliably distinguish interstitial macrophages from peritubular macrophages. Expression analysis shows that both populations have an anti-inflammatory tissue-macrophage-like signature, with slow turnover and a long lifespan. However, fate-mapping and adoptive-transfer approaches show that the two populations are developmentally distinct: interstitial macrophages are initially entirely embryonically derived, whereas peritubular macrophages appear only postnatally and are bone marrow derived. As the in situ self-renewal ability of interstitial macrophages diminishes post-natally, there is a degree of input from bone marrow precursor cells, which gives them a mixed origin.

J. Exp. Med. (7 August 2017) doi:10.1084/jem.20170829