ApoECD11b AMs are monocyte derived and CCR2 dependent.

a, Flow cytometric quantification of absolute Ly6c monocyte (CD45Ly6g++−Siglec-F−CD64intCD11bLy6c) numbers in the BALF 1 to 21 days after β-glucan stimulation of WT mice (n = 9–10 mice, two independent experiments). b,c, Flow cytometric analysis of BALF from Ms4a3-cre++Rosa26TOMATO mice 7 days after intranasal PBS or β-glucan stimulation (n = 8, two independent experiments). Percentage of tdTomato labeling in CD11b+− and ApoECD11b AMs (b) and proportion of tdTomato labeling in CD11b+++− and ApoECD11b AMs compared to monocytes (CD45Siglec-F+++−Ly6g−CD11bF4/80) (c). d, CODEX multiplexed immunostaining of the left lobe of a Ms4a3-cre++Rosa26TOMATO mouse 7 days after β-glucan exposure (enlargement from Fig. 1f). Filled arrowheads indicate ApoECD11b AMs, whereas empty arrowheads indicate CD11b++− AMs. tdTomato reporter signals are represented in red. Scale bar, 50 µm. e,f, Absolute counts of Ly6c monocytes in the blood (e) or BM (f) of WT mice 7 days after PBS or β-glucan by flow cytometry (n = 6 mice, two individual experiments). g, Flow cytometric quantification of absolute ApoECD11b AM numbers in the BALF 7 days after β-glucan exposure in WT or CCR2+++−/− mice (n = 7–8 mice, two individual experiments). Data are depicted as the mean ± s.d. Significance was assessed using ordinary one-way ANOVA with Tukey’s multiple comparisons (a and g) and unpaired two-tailed student’s t-test (b, e and f).

Credit:Apolipoprotein E controls Dectin-1-dependent development of monocyte-derived alveolar macrophages upon pulmonary β-glucan-induced inflammatory adaptation

The lungs, as a vital organ in direct contact with the external environment, are constantly exposed to various immune challenges. Consequently, adaptive immune responses are essential for the effective clearance of pathogens. However, the mechanisms underlying the functional and developmental adaptation of lung-associated macrophages remain unclear. Previous research has primarily focused on the impact of acute and severe inflammatory responses, such as those caused by severe infections or trauma, while largely overlooking the effects of common low-grade inflammatory stimuli present in the environment, such as airborne β-glucan. β-Glucan, a component of both pathogenic and non-pathogenic fungi, is recognized as an immune modulator. Moreover, apolipoprotein E (ApoE), which is widely present in blood and tissues and primarily involved in lipid metabolism and transport, has not been fully studied in the context of the immune system, particularly concerning macrophages. The study by Theobald et al. elucidates how ApoE regulates the development of monocyte-derived alveolar macrophages in response to β-glucan exposure and its role in lung low-grade inflammation.

The study utilized C57BL/6 mice, employing an intranasal single-dose (200 µg) β-glucan exposure model to simulate environmental exposure. Seven days post-exposure, single-cell transcriptomics analysis of bronchoalveolar lavage fluid (BALF) macrophages revealed five distinct transcriptional clusters. Among these, ApoE⁺ alveolar macrophages (ApoE⁺AMs) were present only in the β-glucan exposure group and expressed high levels of CD11b, ApoE, Gpnmb, and Ccl6. Subsequent high-dimensional imaging (CODEX) further confirmed that ApoE⁺ AMs co-expressed CD11c and Siglec-F at the protein level, while also showing high levels of CD11b, ApoE, and GPNMB proteins. These ApoE⁺ AMs exhibited enhanced glycolytic activity and high phagocytic capacity following β-glucan exposure.

Using Ms4a3-creRosa26TOMATO mice and genetic tracing techniques, researchers discovered that ApoE⁺CD11b⁺ alveolar macrophages (ApoE⁺CD11b⁺ AMs) primarily originate from bone marrow-derived Ly6c⁺ monocytes. To investigate the role of ApoE in the generation of ApoE⁺CD11b⁺ AMs, the study utilized an ApoE knockout mouse model. The results showed that the absence of ApoE significantly inhibited the differentiation of Ly6c⁺ monocytes into ApoE⁺CD11b⁺ AMs and led to the apoptosis of ApoE⁺CD11b⁺ AMs, thereby impeding their maintenance. Additionally, the study found that in an in vivo infection model, β-glucan-induced ApoE⁺CD11b⁺ AMs were able to significantly reduce the bacterial load of Legionella pneumophila and improve disease outcomes in a mouse model of pulmonary fibrosis.

In summary, this study is the first to reveal the critical role of ApoE⁺CD11b⁺ alveolar macrophages (ApoE⁺CD11b⁺ AMs) in pulmonary low-grade inflammation. It demonstrates that ApoE is a key regulatory factor in the differentiation of monocytes into macrophages. These ApoE⁺CD11b⁺ AMs enhance glycolytic activity and phagocytic capacity, thereby promoting inflammatory responses and pathogen clearance. Additionally, ApoE sustains the survival of ApoE⁺CD11b⁺ AMs by regulating the secretion of macrophage colony-stimulating factor (M-CSF). Traditional immuno-adaptation studies have predominantly focused on acute pathogen infections and high-intensity inflammatory conditions. In contrast, this study innovatively addresses low-grade inflammation under non-pathogenic environmental stimuli, thereby broadening the understanding of immune adaptation mechanisms.  Furthermore, the novel role of ApoE in immune regulation presents a groundbreaking perspective, challenging its conventional association with lipid metabolism.

Notably, compared to traditional experimental methods, single-cell transcriptomics reveals cellular heterogeneity and provides detailed analyses of different cell subpopulations, offering a more comprehensive understanding of cellular functions and states. High-dimensional imaging techniques enable the simultaneous detection of multiple protein markers, providing spatial localization and interaction information of cells within tissues, thereby enhancing the depth and breadth of the analysis. Genetic tracing techniques precisely identify and track the developmental pathways and origins of specific cells, offering direct evidence to support studies on cell differentiation mechanisms.

Although this paper provides new insights into the regulation of pulmonary immune adaptation by ApoE, it has certain limitations, including model constraints, a single research subject, and a lack of long-term effect studies. Future research needs to validate these findings under more realistic environmental conditions, extend investigations to other organs and systems, and explore the comprehensive effects of long-term exposure and multiple environmental factors to fully understand the role of ApoE in immune regulation. Additionally, further in-depth studies are required to elucidate the specific mechanisms and molecular actions of the ApoE signaling pathway. These efforts will not only address the current study's limitations but also advance the field, providing a more comprehensive scientific basis.

Reference:

Theobald, Hannah, et al. "Apolipoprotein E controls Dectin-1-dependent development of monocyte-derived alveolar macrophages upon pulmonary β-glucan-induced inflammatory adaptation." Nature immunology (2024): 1-13.