Non-classical tissue monocytes and two functionally distinct populations of interstitial macrophages populate the mouse lung

Resident tissue macrophages (RTM) can fulfill various tasks during development, homeostasis, inflammation and repair. In the lung, non-alveolar RTM, called interstitial macrophages (IM), importantly contribute to tissue homeostasis but remain little characterized. Here we show, using single-cell RNA-sequencing (scRNA-seq), two phenotypically distinct subpopulations of long-lived monocyte-derived IM, i.e. CD206⁺ and CD206⁻IM, as well as a discrete population of extravasating CD64⁺CD16.2⁺ monocytes. CD206⁺ IM are peribronchial self-maintaining RTM that constitutively produce high levels of chemokines and immunosuppressive cytokines. Conversely, CD206⁻IM preferentially populate the alveolar interstitium and exhibit features of antigen-presenting cells. In addition, our data support that CD64⁺CD16.2⁺ monocytes arise from intravascular Ly-6C^lo patrolling monocytes that enter the tissue at steady-state to become putative precursors of CD206⁻IM. This study expands our knowledge about the complexity of lung IM and reveals an ontogenic pathway for one IM subset, an important step for elaborating future macrophage-targeted therapies.