86 Persistent Macrophage Depletion and Arrested Replenishment is Dependent on Carbon Nanotube type as Shown by Single Cell Transcriptomics

Abstract Inhalation of carbon nanomaterials (NMs) can cause severe health effects, including chronic inflammation and fibrosis. We aim for a better understanding of NM specific cellular perturbation patterns. Therefore, we exposed mice intratracheally to soot like carbon black, tangled double-walled (DWCNT) and rigid, multi-walled carbon nanotubes (MWCNT). All lungs underwent single cell RNA sequencing and BAL analysis at 12h, 6d and 28d. At the chosen doses, all NMs caused comparable airspace neutrophilia at 12h, which increased until d6 for CNTs and remained elevated until d28 for MWCNT. We observed a significant increase in total BAL macrophages, however, exclusively after 6d CNT instillation. This d6 peak coincided with the loss of F4/80+/CD11c+/CD11b- alveolar macrophages (AMs) for MWCNTs, and to a lesser extent DWCNTs, in lung FACS. In parallel, we noticed the pulmonary accumulation of a transitional macrophage population at d6 uniquely after MWCNT instillation, and not DWCNTs. According to RNA velocity analysis, blood monocytes differentiate into Lyve1-/CD163- interstitial macrophages and subsequently give rise to this transitional population which fails to replenish the CD11c+/SiglecFhigh AM pool. These cells show high similarity with the initiating monocyte-derived macrophage population necessary for bleomycin- and asbestos-induced lung fibrosis (Misharin et al.,JEM,2017.214(8)). The pro-fibrotic role of this population also has been demonstrated in context of SARsCoV2 infection-associated acute respiratory distress (Wendisch et al.,Cell,2021.184(26)). We summarize, that this transitional cell population are monocyte-derived AM precursors, which lack the ability to differentiate into mature AMs being the decisive mechanism for MWCNT induced, not resolving lung fibrosis.