PPARγ is essential for the development of bone marrow erythroblastic island macrophages and splenic red pulp macrophages

Abstract

Tissue-resident macrophages play a crucial role in maintaining homeostasis. Macrophage progenitors migrate to tissues perinatally, where environmental cues shape their identity and unique functions. Here, we show that the absence of PPARγ affects neonatal development and VCAM-1 expression of splenic iron-recycling red pulp macrophages (RPMs) and bone marrow erythroblastic island macrophages (EIMs). Transcriptome analysis of the few remaining Pparg-deficient RPM-like and EIM-like cells suggests that PPARγ is required for RPM and EIM identity, cell cycling, migration, and localization, but not function in mature RPMs. Notably, Spi-C, another transcription factor implicated in RPM development, was not essential for neonatal expansion of RPMs, even though the transcriptome of Spic-deficient RPMs was strongly affected and indicated a loss of identity. Similarities shared by Pparg- and Spic-deficient RPM-like cells allowed us to identify pathways that rely on both factors. PPARγ and Spi-C collaborate in inducing transcriptional changes, including VCAM-1 and integrin αD expression, which could be required for progenitor retention in the tissue, allowing access to niche-related signals that finalize differentiation.

The transcription factor EGR2 is indispensable for tissue-specific imprinting of alveolar macrophages in health and tissue repair

“…We focussed on EGR2, which is part of a family of early growth response (EGR) transcription factors, comprising EGR1-4, as Egr2 appeared to be expressed in a particularly selective manner by alveolar macrophages ( Figure 1D ) when compared with other tissue macrophages at mRNA ( Figure 1E ) and protein level ( Figure 1F, G & Supplementary Figure 2A ). In contrast, while highly expressed by alveolar macrophages, Pparg was also expressed at a high level by splenic red pulp macrophages ( Figure 1E ), consistent with previous reports (12, 13) . Next, we performed analogous analysis of EGR2 expression across a variety of human macrophage populations from scRNA-seq data sets within the Human Cell Atlas.…”

“…We did not detect differences in the proliferative activity of Egr2- sufficient and - deficient alveolar macrophages ( Figure 2G ). Importantly and consistent with the lack of EGR2 expression by other tissue resident macrophages, we saw no effect on the cell number and expression of signature markers by resident macrophages in other tissues, including in the spleen where macrophages share a dependence on PPAR-γ (12, 13) ( Supplementary Figure 3B, C ). Thus, these data demonstrate that while EGR2 expression is dispensable for alveolar macrophages survival and self-maintenance, it is indispensable for imprinting key phenotypic features of the cells in the healthy lung.…”

“…Publicly available datasets were downloaded from the COVID-19 Cell Atlas (13–15) to perform in silico analysis of EGR2 expression in human tissue macrophages. Data were pre-processed and merged using the Seurat R package (V3; R version 4.0.2) following standard methods.…”

“…Therefore, it remains unclear if or how these factors control the tissue-specific identity and function of alveolar macrophages. Moreover, while considered the ‘master transcription factor’ of alveolar macrophages, PPAR-γ has been implicated in the control of other tissue macrophages, including splenic red pulp macrophages (12, 13) , and thus, the transcriptional network responsible for conferring specificity upon alveolar macrophage differentiation remains unclear. Finally, if and how additional transcriptional regulators are involved in regulating these processes in the context of inflammation and repair is largely unexplored.…”

Macrophage‐fibroblast circuits in the spleen

“…RPM express high levels of α9 integrin, a known ligand of various extracellular matrix proteins such as fibronectin and tenascin‐C, which are abundant in the spleen 62,139‐141 . Transcriptional analyses revealed that developing RPM upregulate αD integrin, possibly contributing to their retention within the RP 142 . As introduced, RPM also express Spi C, which potently induces VCAM‐1 in these macrophages 51 .…”