Coordinated Chemokine Expression Defines Macrophage Subsets Across Tissues

Tissue-resident macrophages in the lung comprising alveolar and interstitial macrophages (IMs) display a high degree of heterogeneity. In general, macrophage heterogeneity is thought to arise from various forms of activation that are heavily confounded by the recruitment of monocytes to the tissue-resident macrophage pool. To better understand the functional heterogeneity of IMs in the lung, we profiled the transcription of resident CD206 ^hi and CD206 ^lo IMs under steady-state and inflammatory conditions, excluding recruited macrophages. Rather than observing conventional in vitro M1 and M2 activation states, we identified seven chemokine-expressing IM subsets: IMck1 ( Ccl2, Ccl7, Ccl12, and some Cxcl14 ), IMck2-4 ( Ccl3, Ccl4, Ccl5, Cxcl1, Cxcl2, and Cxcl3 ), IMck5 ( Ccl8 ), IMck6 ( Ccl6 and Ccl9 ), IMck7 ( Cxcl9 and Cxcl10 ), IMck8 ( Cxcl13 ), and IMck9 ( Ccl24 ), which were found in steady-state or induced by acute inflammation. Beyond the mouse lung, similar coordinated chemokine signatures were observed in macrophages and monocytes from other tissues and across species. Although all IMs expressed Pf4 (CXCL4), mainly CD206 ^hi IMs were selectively depleted in Pf4 ^Cre R26 ^EYFP-DTR mice. Loss of CD206 ^hi IMs resulted in significantly reduced inflammatory cell influx in allergen- and infection-driven models, as well as significantly diminished tertiary lymphoid formation and subsequent accumulation of GL7 ⁺ germinal center B cells. Overall, our study highlights a division of labor among interstitial macrophages, reflected by the coordinated production of chemokines to control inflammatory cell influx and organize tertiary lymphoid tissue architecture.