Cell Properties of Lung Tissue-Resident Macrophages Propagated by Co-Culture with Lung Fibroblastic Cells from C57BL/6 and BALB/c Mice

Tissue-resident macrophages (Mo) originating from foetal precursors are maintained by self-renewal under tissue/organ-specific microenvironments (niches). We recently developed a simple propagation method applicable to tissue-resident Mo by co-culturing. Here, we examined the properties of lung tissue-resident Mo propagated by co-culturing with lung interstitial cells. The intracardially and intratracheally perfused lung from BALB/c and C57BL/6 mice could minimise the contamination of alveolar Mo and lung monocytes. Lung tissue-resident Mo could be largely propagated under standard culture media along with the propagation of lung interstitial cells demonstrating a fibroblastic morphology. Propagated lung Mo showed characteristic expression properties for Mo/monocyte markers: high expressions of CD11b, CD64 and CD206; substantial expressions of Mertk; and negative expressions of Ly6C, MHC II and Siglec-F. These properties fit with those of lung interstitial Mo of a certain population that can undergo self-renewal. Propagated fibroblastic cells by co-culturing with lung Mo possessed niche properties such as Csf1 and Tgfb1 expression. Propagated lung Mo from both the mouse types were polarised to an M2 phenotype highly expressing arginase 1 without M2 inducer treatment, whereas the M1 inducers significantly increased the iNOS-positive cell percentages in C57BL/6 mice relative to those in BALB/c mice. This is the first study to demonstrate fundamental properties of lung tissue-resident Mo propagated by co-culturing. Propagated lung Mo showing features of lung interstitial Mo can serve as an indispensable tool for investigating SARS-CoV-2 diseases, although lung interstitial Mo have gained little attention in terms of their involvement in SARS-CoV-2 disease pathology, in contrast to alveolar and recruited Mo.