Bhlhe40 mediates tissue-specific control of macrophage proliferation in homeostasis and type 2 immunity

Abstract

Most tissue-resident macrophage populations develop during embryogenesis, self-renew in the steady-state and expand during type 2 immunity. Whether shared mechanisms regulate the proliferation of macrophages in homeostasis and disease is unclear. Here we found that the transcription factor Bhlhe40 was required in a cell-intrinsic manner for the self-renewal and maintenance of large peritoneal macrophages (LPMs), but not that of other tissue-resident macrophages. Bhlhe40 was necessary for the proliferation, but not the polarization, of LPMs in response to the cytokine IL-4. During infection with the helminth Heligmosomoides polygyrus bakeri, Bhlhe40 was required for cell cycling of LPMs. Bhlhe40 repressed the expression of genes encoding the transcription factors c-Maf and Mafb and directly promoted expression of transcripts encoding cell cycle-related proteins to enable the proliferation of LPMs. In LPMs, Bhlhe40 bound to genomic sites co-bound by the macrophage lineage-determining factor PU.1 and to unique sites, including Maf and loci encoding cell cycle-related proteins. Our findings demonstrate a tissue-specific control mechanism that regulates the proliferation of resident macrophages in homeostasis and type 2 immunity.

Bhlhe40 and Bhlhe41 transcription factors regulate alveolar macrophage self‐renewal and identity

“…These experiments revealed the downregulation of many proliferation‐associated genes in DKO AMs (Fig EV2B). A similar downregulation was observed in DKO peritoneal macrophages (Fig EV2B), confirming the recently published observations made for Bhlhe40‐deficient peritoneal macrophages (Jarjour et al , ). It was previously suggested that the low expression of the transcription factors Maf and Mafb, which function as negative regulators of macrophage proliferation (Aziz et al , ), contributes to the self‐renewal capacity of AMs (Soucie et al , ).…”

“…These results are consistent with the redundancy between Bhlhe40 and Bhlhe41 observed in other systems (Rossner et al, 2008;Shahmoradi et al, 2015;Kreslavsky et al, 2017). In contrast, the Bhlhe40 deficiency was solely responsible for the competitive disadvantage in peritoneal macrophages (Fig EV1C and D), in line with a recent report (Jarjour et al, 2019). Full-body irradiation can cause damage to lung tissue that could potentially alter properties of the resident phagocytes.…”

“…The cell‐surface phenotype, frequency and numbers of alveolar, peritoneal and red pulp macrophages, as well as microglia and Kupffer cells, were normal in DKO mice compared to WT mice (Figs C and D, and EV1A). It was recently reported that Bhlhe40‐deficient mice have a mildly reduced peritoneal macrophage compartment (Jarjour et al , ), however, when we assessed numbers of peritoneal macrophages in Bhlhe40 single KO mice we did not observe a significant change in numbers of these cells compared to WT mice (Fig EV1B). As Jarjour et al reported that the phenotype becomes much more pronounced upon induction of type 2 immune responses, it is conceivable that the discrepancy between our observations reflects differences in the environments of animal facilities, resulting in exposure of mice to different spectra of commensals and/or pathogens.…”

“…We next aimed to identify the mechanisms underlying the competitive disadvantage of the DKO macrophages. As a recent report attributed competitive disadvantage of Bhlhe40 −/− peritoneal macrophages to their impaired proliferation (a phenotype that required induction of the type 2 immune response to be clearly detected; Jarjour et al , ), we focused our analysis on AMs. DKO AMs did not show an increase in apoptosis (data not shown), but in mixed BM chimeras exhibited reduced proliferation, as judged by a twofold decrease in EdU incorporation (Fig A).…”

“…Bhlhe40 and Bhlhe41 were required to maintain a normal rate of the self‐renewal‐associated proliferation in AMs, which was likely achieved through indirect repression of Maf and Mafb . Repression of Maf and Mafb by Bhlhe40 is also thought to regulate self‐renewal of peritoneal macrophages (Jarjour et al , ), suggesting that this mechanism is conserved for both macrophage populations. The self‐renewal defect of Bhlhe40/Bhlhe41‐deficient AMs resulted in a severe competitive disadvantage of these cells evident both in mixed BM chimeras and in experiments with intranasal transfers of AM precursors into neonatal recipients.…”