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Article by Nicole Hachmann, Pamela Graney & Margaretha Morsink

Macrophages: Guardians of tissues everywhere

 

Source Publication: 

Coordinated chemokine expression defines macrophage subsets across tissues, Nature Immunology, 2024

Xin Li et al., Claudia Jakubzick Lab

Tissue-resident macrophages represent a diverse population of immune cells that can be found in nearly every tissue of the body. These cells are crucial for maintaining tissue balance (homeostasis), controlling inflammation, and even helping to repair damage. Although they are present in nearly all tissues, macrophages do not always behave the same way. Instead, they adapt their behavior in response to the dynamic signals present in their local microenvironment, taking on unique roles depending on the location and needs of the tissue. This leads to extraordinary diversity in their appearance (phenotype) and function, both within a single tissue and across different tissues. This makes them incredibly versatile but also hard to categorize.

 

Understanding these differences is crucial because macrophages play key roles in both health and disease. They help fight infections, but can also contribute to chronic inflammation and tissue damage. To better understand macrophages and their functions, the authors of this study used a technique called single-cell RNA sequencing - a method that examines the genetic activity of individual cells. Their goal was to map the diverse populations of macrophages in the lungs and understand their unique roles.

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What did these researchers do?

The authors of this paper tackled the challenge of studying the complexity of macrophages in the lungs, where these cells face a particularly diverse variety of signals and environments. Using single-cell RNA sequencing, they were able to identify and categorize lung macrophages into ten distinct groups. These groups were defined based on the chemical signals they produce, called chemokines, which help macrophages communicate with other immune cells and coordinate their actions.

 

Their findings revealed that these macrophage groups are not unique to the lungs - they also exist in other tissues, suggesting that these roles are conserved throughout the body. The study also explored how certain macrophage groups regulate the recruitment of immune cells during inflammation and help form specialized structures called tertiary lymphoid structures (TLS). These structures are important hubs for coordinating immune responses, especially during infections or chronic inflammation.

 

To test their findings, the researchers used mouse models. By removing specific macrophage groups from the lungs, they were able to observe reduced immune cell recruitment and fewer TLS formations. This confirmed that these macrophage subsets play key roles in orchestrating immune responses.

 

Why is this important?

Understanding the diversity of macrophages isn’t just about satisfying scientific curiosity, it has real-world implications. Macrophages are essential to our health, they fight infections, clean up cellular debris, and help repair tissues. When things go wrong, they can also drive chronic inflammation or contribute to diseases like asthma, pneumonia, and even cancer.

           

This study provides a framework for understanding macrophage diversity. By identifying their specific markers, researchers can recognize different macrophage groups and highlight their transcription factors (proteins that control gene activity) which could be used to regulate their functions. This knowledge advances our basic understanding of how macrophages work and opens doors for potential targeted therapies that could manipulate specific groups to improve outcomes in various diseases.

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How did the researchers do this?

The researchers began by collecting macrophages from mouse lungs under both normal conditions (as a baseline) and during inflammation (to observe changes) administration of lipopolysaccharide, a component of gram-negative bacteria, into the nasal cavity. To ensure accurate analysis, they used single-cell RNA sequencing to group cells based on their gene expression profiles. This approach allowed them to identify and exclude non-interstitial macrophage (IM) populations, such as alveolar macrophages (AMs) and circulating immune cells that may have entered the lung during inflammation.

 

Lung IMs were identified by specific protein markers present on the surface of the cells: they expressed CD64 and CD11b, with varying levels of CD206, and lacked Ly6c2, which helped differentiate them from other macrophages and monocytes. To further refine the groups, the researchers used Dpp4 (CD26) and C5ar1 (CD88) to separate CD88+ monocyte macrophages from CD26+ conventional dendritic cells (cDCs). They also employed markers like CD45−Siglec F−Ly6G−CD206+MHCII+ to distinguish IMs from recruited macrophages (recMacs). This careful combination of marker-based identification and sequencing ensured their analysis focused specifically on interstitial macrophages.

 

The researchers then used single-cell RNA sequencing to examine the genetic activity of individual macrophage cells. This analysis grouped the cells based on the chemokines they produce, revealing ten distinct subsets of IM macrophages. Each subset had unique roles and functions, shedding light on the diversity of these immune cells.

 

To confirm their findings, the team used specialized mouse models that allowed them to selectively remove specific macrophage subsets. By genetically modifying mice, they were able to deplete certain groups of interstitial macrophages (IMs) based on their expression of key markers, such as CD206. For example, they used a Pf4creR26EYFP+DTR mouse model, which enabled targeted depletion of CD206hi IMs through the administration of diphtheria toxin (DT). This precise removal allowed the researchers to directly observe how the absence of these macrophages affected immune responses in the lungs.

 

By studying these modified mice, the researchers demonstrated that specific IM subsets play critical roles in regulating the recruitment of immune cells during inflammation and the formation of TLS. Without these macrophages, immune cell recruitment was significantly reduced, and the development of TLS, which are essential for organized immune responses, was impaired. This approach not only confirmed the functional importance of these macrophage subsets but also highlighted the tools and markers that other researchers can use to study similar populations in different contexts.

NicoleMacrophage.png

Single-cell sequencing clusters the distribution of macrophages from naive, LPS treated, and post treatment with Gr1 antibody to identify different types of macrophages

What comes next?

While this study gives us a detailed map of lung macrophages, many questions are yet to be answered. For instance, how do these unique macrophage subsets function in humans, and how do they interact with other cells in disease settings? Future research should focus on translating these findings into human health and developing therapies that target specific macrophage subsets. These advances could help treat inflammatory diseases, infections, and even cancer.

© 2024 by Tissue Engineering Resource Center 

With support from the NIBIB.

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