Emergence of CCL18- and IFI27-expressing macrophages in aging lungs, and potential roles for alveolar type 2 cell AGTR2 and macrophage BSG and CTSL expression in COVID-19
Aging lungs are associated with several lung diseases, including chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis and pneumonia. This latter age-associated complication has been particularly prominent in the recent COVID-19 pandemic. We have analyzed carefully selected normal lung samples from human subjects at different ages simultaneously using hashtag-labeled samples and single cell RNA-sequencing technology. We identify prominent changes in macrophage populations in lungs from older individuals, including an emerging populations of macrophages expressing IFI27 and other interferon-regulated genes. We also see increased gene expression of CCL18 and other chemokines in FABP4+/alveolar and SPP1+/interstitial macrophages. All three macrophage populations highly express BSG/CD147a putative SARS-CoV-2 receptor and CSTL, a gene associated with SARS-CoV-2 activation. ACE2, the SARS-CoV-2 receptor, and TMPSS2, a SARS-CoV2 protease activator, are both expressed highly on AT2 cells with increased expression on AT2 cells from older lungs. AGTR2, a recently proposed alternative receptor for SARS-CoV-2 was highly selectively and markedly upregulated by AT2 cells from aging lungs.  Thus, our data show increased lung expression of  ACE2, AGTR2 and TMPRSS2 by AT2 cells with aging. In addition, our data indicate a distinct molecular pathway for lung SARS-CoV-2 infection of lung macrophages. Macrophages may contribute to increased  COVID-19 severity in older patients by specific macrophage populations poised to contribute to the cytokine storm, in particular through enhanced interferon responses and interleukin-6 production. Overall design: Normal healthy donor lungs were, at the time of procurement, mechanically and enzymatically dissociated to generate single cell suspensions which were cryogenically stored in liquid nitrogen.  Histology was assessed and ten samples procured from individuals of varying ages (19 years old to 78 years old) were selected to be multiplexed together for single cell RNA-sequencing using BioLegend TotalSeq-C cell hashing antibodies and 10X Genomics' Feature Barcoding technology for the 5 Prime Immune Profiling chemistry.