Macrophage heterogeneity in human secondary lymphoid organs

Macrophages play a major role in immune responses and in the maintenance of tissue homeostasis. Most of our knowledge of macrophage biology comes from mouse studies, which have revealed the existence of distinct subpopulations differing in origin, life cycle and tissue-specific features, as well as the influence of tissue perturbations on macrophage heterogeneity. Here, we review the evidence supporting the conservation of these principles in human macrophages, along with the methodological approaches employed, including omics analyses of macrophages from clinical samples, human transplantation studies and studies of monogenic mutations. We also focus on the macrophage populations that are present in human secondary lymphoid organs-namely the spleen, lymph nodes, and tonsils. While some organ-specific features are shared with their mouse counterparts, we highlight important differences between mouse and human lymphoid tissue macrophages in their functional specialization.