Understanding hepatic macrophage activation by microbes and microbial components in vivo

Project Information

MACtivate

Grant agreement ID: 101027317

DOI

10.3030/101027317

Project closed

EC signature date 17 March 2021

Start date 1 April 2021

End date 31 March 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 166 320,00

EU contribution

€ 166 320,00

Coordinated by

VIB VZW
Belgium

Periodic Reporting for period 1 - MACtivate (Understanding hepatic macrophage activation by microbes and microbial components in vivo)

Reporting period: 2021-04-01 to 2023-03-31

Summary of the context and overall objectives of the project

Macrophages are specialized immune cells and are present in virtually all organs of the body. There, these cells carry out different organ-specific functions but also play an important role in defending the immune system. In the liver the tissue resident macrophages are termed Kupffer cells (KCs). These cells reside in the blood vessel of the liver called sinusoids in close contact with endothelial and stellate cells and are thus ideally positioned to recognize, capture and eliminate microbes and microbial components reaching the liver. Although, these cells have been proposed to exert critical function in systemic immune surveillance, little is known about the cellular and molecular mechanisms underpinning KC responses to microbial insult. During inflammation resident macrophages are often lost and monocyte-derived cells are recruited to the site of inflammation where these cells contribute to anti-microbial immunity and tissue repair. Recruited monocytes can differentiate into macrophages and display similar surface markers and expression profiles compared to resident macs which in the past has hampered the unambiguous identification of the different macrophage populations and thus our understanding of their specific functions during inflammation. However, the KC-specific markers identified recently now allow to ambiguously discriminate between the different macrophages present in the liver after microbe-induced inflammation. Focusing on KCs we have determined their fate, specific responses and functions during microbial insult (WP1). Preliminary data have shown that mice lacking a negative regulator of the inflammatory response specifically in KCs develop excessive systemic inflammation and show increased mortality upon exposure to the microbial component LPS. Here, we have investigated the cellular and molecular processes leading to severe inflammation in these mice (WP2). Ultimately, we have provided novel insight into the crosstalk between KCs and their cellular environment in the liver during microbial inflammation (WP3).

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

This project sought to investigate hepatic macrophage responses to microbes and microbial products focusing on the heterogeneity, fate, activation profile and tissue context of these cells. The objectives proposed were divided in three independent work packages (WP). In
WP1 we have profiled the different macrophage populations in the liver and have determined their fate, location, origin and specific responses during microbial insult (WP1). In WP2 we have focused on the Kupffer cells (KCs) and have investigated their specific function during microbial inflammation. Having analysed the specific response of KCs to microbial component, we have started to uncover the crosstalk between KC and their cellular in environment in WP3.
We could show that the number of resident KCs (resKCs) decreased upon exposure to the bacterial component lipopolysaccharide (LPS) which was accompanied by a concomitant increase of non-KC macrophages in the liver. While with time recruited macrophages were progressively lost from the liver, resident KC numbers recovered and reached homeostatic levels within a month after LPS challenge. Investigating the origin of these macrophages we demonstrated that the resident KCs present the liver after microbial insult were not monocyte-derived indicating that the remaining cells may have proliferated to repopulate the liver. Preliminary data showed that mice lacking an important regulatory factor specifically in KCs develop excessive systemic inflammation and display significantly decreased survival in response to LPS challenge. Here, we have identified the cellular and molecular mediators responsible for these excessive inflammatory responses. Furthermore, by using cell-specific knock-out mice we have been able to provide a better understanding of the crosstalk between KCs and endothelial cells.
We are currently preparing a manuscript to publish the findings obtained during this fellowship in a relevant journal in the coming months. In the context of his work the fellow has published a review article discussing the distinct macrophage populations present in the liver and their plasticity during hepatic inflammation (Zwicker, Bujko, Scott, Front Immunology 2021). Moreover, fellow has contributed as a co-author to a paper identifying several distinct macrophage populations and their niche in the mouse and human liver (Guilliams, Cell 2022).
Data generated during this fellowship was presented at two international conference and at departmental seminars to other experts in the field. Moreover, to reach a wider public audience the fellow has used his twitter account (@ChriZwi) to advertise publications and inform about activities of the host lab.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Kupffer cells (KCs) are specifically position in the bloodstream of the liver to rapidly recognize and capture microbes and microbial components entering the liver and thus these cells have been proposed to be critical for systemic immune surveillance. Despite this, the precise mechanisms at play and the specific role(s) of KCs and recruited macrophages in the context of systemic infection remain largely unknown. This paucity of knowledge stems largely from the difficulty to distinguishing between KCs and recruited macrophages. Typically, all liver macrophages were considered to be KCs which has led to significant confusion regarding their specific functions, with many studies suggesting often contradictory roles. However, the identification of Clec4F as a KC-specific marker and the development of mouse models to specifically deplete KCs or remove genes expressed by KCs has allowed to unambiguously identify KCs and study their specific (gene) function.
Using these Kupffer cell-specific tools and the technologies available in the host lab, we have determined the fate, origin and specific responses during systemic microbial inflammation. Moreover, we have demonstrated that KCs can respond rapidly to microbial components, however, this response needs to be tightly controlled. KCs failing to regulate these responses become hyperactivated which causes excessive systemic inflammation with detrimental consequences for the host. In this project we have provided novel insight into the cellular and molecular effector mechanism responsible for KC hyperactivation and have determined the molecular mediators contributing this excessive inflammatory response.
Systemic bacterial infections causing sepsis remain a significant challenge in the clinic due to the lack of highly effective treatment options. Thus, understanding the cellular and molecular mechanisms underpinning the KC response to microbes and microbial components, may allow us to develop novel and innovative macrophage-directed therapies to improve patient outcomes in systemic inflammatory conditions like sepsis in the future.

Group picture Scott lab