Immunity view of liver macrophages when healthy and sick

Written by | Sister Xian

The liver is responsible for many important physiological processes in the body, including metabolism, detoxification, and immunity, and the importance of its function is closely related
to the complexity of its structure 。 The liver consists of repeated hexagonal lobules, where a portal vein (which transports nutrient-rich blood from the intestines, spleen, and stomach to the liver), a hepatic artery (transports oxygenated blood from the heart to the liver), and a bile duct composed of bile duct cells (special epithelial cells) together form a portal vein tripty, and all blood entering the liver then passes through the hepatic blood sinuses (a dynamic microvascular structure, The nutrient exchange between the blood and the perisinus space known as the Dijli's cavity) flows from the portal triad to the central vein (Figure 1
).

Hepatocytes are the metabolic energy banks of the liver, located between the hepatic blood sinuses, responsible for metabolic and detoxification functions depending on their position in the hepatic blood sinuses, and given the difference in blood content at different distances from the central vein, these hepatocytes are metabolically partitioned, meaning that they exhibit functional heterogeneity in their position within the leaflets
。 Among them, macrophages were found throughout the liver structure, and it was once thought that liver macrophages consisted of only a single population of resident Kupffer cells (KCs), but single-cell and spatial (proteome) genomics techniques have revolutionized the way people think about cell heterogeneity, allowing unprecedented resolution to assess the heterogeneity of macrophages in the liver, their specific markers, and their unique location
.
KC cells are present in the hepatic blood sinuses and are the best location to sample and respond to pathogens that enter the liver through the bloodstream, in close contact
with hepatic sinus endothelial cells (LSECs).
In addition, a significant portion of KC's cells extend into the Dystic cavity and come into close contact
with astrocytes and hepatocytes.
In addition to KC, there are small amounts of other macrophages and dendritic cells in the central venous and portal triad (Figure 1
).
It is now well known that even in the steady state, mouse and human livers have multiple subsets of macrophages, and many more are recruited in disease settings
.

Figure 1

These findings at this stage have led to a rethinking of the role of macrophages in liver homeostasis and disease, however, there are still limitations in current research and techniques on macrophages in the liver, which limit the understanding of their definition, state and function, on September 13, 2022, Charlotte L.
Scott and Martin Guilliams from Ghent University in Belgium published Liver macrophages online on Immunity The opinion piece in health and disease provides a detailed overview of the definitions of the different liver macrophage populations currently known, markers that can be used to identify them, how they integrate in the liver, and their function
in health and disease.
While highlighting future issues to be solved, it is noted that identifying conserved genetic signatures within these cells across species and diseases will undoubtedly help to correctly identify specific macrophage subpopulations and gain more specific tools to track and study the function of
these cells.

Kupffer cells, resident macrophages of the liver

KC cells were discovered in 1876 and identified as macrophages 20 years later
.
As one of the oldest immune cells known to man, there is still relatively little understanding of these cells, especially their function
.
One reason for this is the inability to distinguish KC from other liver macrophages because they all express universal macrophage markers
including CD64, F4/80, and MerTK.
In recent years, however, researchers have made considerable progress
in this area.
CLEC4F is confirmed as a specific marker of KC in mice, but it is only expressed late in KC development, making it difficult to identify cells that are developing into KC, such as monocyte-derived KC (moKC) recognition in diseased
livers.
Moreover, CLEC4F is not conservative among humans, so the identification of real human KCs is still lacking
.
In addition, other surface receptors on mouse KCs have also been identified, enabling them to further differentiate them from other liver macrophages, including CLEC2 and CD206, where CLEC2 is a very early marker of KC that is continuously expressed throughout its life cycle, making it a very useful marker
for identifying moKC before expressing CLEC4F.

Current studies using single-cell or monocytic RNA sequencing (sc/snRNA-seq) to analyze healthy liver cells promise to reveal the true identity of human KCs, but the results of existing studies vary
.
By comparing humans and mice, core genetic traits specific to KC, including CD5L, VSIG4, CD163, FOLR2, MARCO, and SLC40A1, have recently been proposed that can be used to identify a single homogeneous population
of KC in the livers of 7 different species.
Pooling the "KC signatures" of different existing sc/snRNA-seq data can reveal a consistent cell population suggesting that it may be a useful feature
for the identification of KC.
In any case, although further research is currently needed to reach a consensus on the homogeneity of KC in mouse and human livers, it is certain that KC can eventually be distinguished from
other macrophages by utilizing a combination of KC-specific surface markers and genes.

Other macrophage populations in a healthy liver

In addition to KC, there are other identified macrophage populations
in the liver.
One class is hepatic envelope macrophages (LCMs)
present in the hepatic envelope matrix below the liver surface.
In mouse livers, LCM is derived from monocytes and accumulates at weaning, expressing general macrophage markers but not KC markers
.
However, the characteristics of these cells in humans are still unknown
.
While the features of LCMs in mouse livers have been relatively clear, their transcriptional signatures are not unique in macrophages because they are shared
with the macrophage subsets present in the central vein.
In addition, there is a subset of macrophages near the bile ducts in healthy mouse and human livers, which have the same transcription profile between different species and are similar to macrophages known as lipid-associated macrophages (LAMs) in obese adipose tissue and fatty liver, hence the name bile duct LAM (BD-LAM
).
In human liver, BD-LAM can be identified in CITE-seq analysis based on the expression of CD14, CD11a, CD26, CD141, and CD9, but there is a lack of useful markers in mouse livers
.

Thus, although KCs make up the major macrophage population in a healthy liver, they coexist with at least two other macrophage subpopulations
.

Integration niche: KC function is connected by integration and intercellular loops

In addition to its typical role as phagocytes in tissue homeostasis and immunomonitoring, KC also expresses many genes that are not involved in immune defense or are expressed by resident macrophages in other tissues, suggesting that they may be involved in achieving liver-specific functions such as iron and cholesterol homeostasis
.
However, studies have shown that certain key genes involved in iron and cholesterol homeostasis in the human liver are expressed only by specific cells, suggesting that hepatocytes, LSECs, stellate cells, and KCs worked
together as an integrated niche throughout evolution.
This broadens the concept of macrophage niches, not just a local supply
of growth factors.
Based on this, the authors propose a more modular view - hepatic blood sinuses are constructed from repeated 4 cells, including KC, stellate cells, LSECs, and hepatocytes, and in order to emphasize that the functional characteristics of each cell are controlled by a combined cell-cell loop, the term "integrated niche"
is proposed.
The close interweaving of transcriptome properties will ensure that the division of labor between the four cells that make up the integrated niche is correct and coordinated, but this also limits their plasticity
.
All in all, the strong interconnected properties of integrating niches have an important impact on the development of liver organoids, as the genetic integration of different regulatory procedures means that organoids
containing hepatocytes, LSEC, stellate cells, and KCs must be constructed before these cells can gain in vivo function.
In addition, the integrated niche also has an important impact on the understanding of liver disease, because long-term damage may destroy the integrated niche, which in turn may destabilize
the entire liver ecosystem.

Kupffer cells and recruited macrophages in the diseased liver

A common feature of liver disease is the reduction or disappearance of resident KC groups (Figure 2), including non-alcoholic fatty liver disease (NAFLD), nonalcoholic steatohepatitis (NASH), viral hepatitis, acute liver injury, bacterial infections, cirrhosis, and hepatocellular carcinoma (HCC
).
Unlike steady-state conditions, resident KC also fails to proliferate after NASH or Listeria monocytogenes
infection.
It has been suggested that KC can mediate inflammation or regression
by becoming a plastic cell that responds to different stimuli.
But the authors argue that integrating niches greatly limits the plasticity of KC, so the function of KC plasticity may be greatly overestimated by studying liver macrophages without correctly distinguishing between true KC and recruited macrophages
.
Therefore, in any case, the way KC is activated is a question that needs to be answered, and it is worth noting that in many disease models, KC is correctly distinguished from recruited macrophages, and changes in transcription procedures are observed, and these studies also report similar KC activation profiles
.
In addition to studying how KC is activated, the correlation between this activation and KC's involvement in disease pathogenesis has yet to be studied
.

Figure 2

Liver disease has also been linked to the recruitment of monocyte-derived macrophages, but their fate is not well understood
.
The presence of macrophage populations of different monocyte-derived macrophages during disease raises two questions: (1) whether these cells play different roles in the diseased liver, and (2) whether these macrophage subpopulations represent different cell types or activation states
.
To this end, the authors point out that different subsets of known monocyte-derived macrophages, such as moKC and LAM
, need to be considered first.
Studies have found that in two distinct models of liver disease, NAFLD/NASH and acetaminophen overdose (APAP), monocytes have been shown to localize specific areas of injury and differentiate into LAM (Figure 2
).
In NASH, these cells replace KCs that die in areas of steatosis and fibrosis, while the few KCs that are lost from areas with less steatosis/fibrosis are replaced
by moKCs.
In addition to KC death, LAM-like KC phenotypes were also reported in NASH's fibrosis model, suggesting that before they died, KCs may have sensed the same signal as infiltrated LAM, leading to a mixed LAM-like KC phenotype
.
Similarly, LAM-like KCs are present around the damaged area of the APAP central vein, and in addition, LAM develops around the central vein and may interact with activated fibroblasts, necrotic hepatocytes, and other recruited cells such as neutrophils or cDCs
.
This also suggests that LAM can play a protective role
by facilitating tissue repair.
Still, due to the lack of technical tools, the role
of different subsets of liver macrophages recruited in diseased livers is unclear.
The limited evidence collected so far suggests that newly recruited cells respond differently to injury compared to resident KC, re-emphasizing the importance of distinguishing between these populations
.

In summary, in a steady-state liver, KC is not the only population of macrophages; In the diseased liver, there are other populations of macrophages recruited, including LAM
that is recruited to the lesion region.
This heterogeneity underscores the need to describe markers for these cell populations and prompts the field to reconsider known general-purpose macrophage markers or tools
for distinguishing KCs.
All in all, although recent research has revealed the heterogeneity of liver macrophages, this understanding inevitably raises more questions
.
After identifying these cell populations, the next step will be to accurately distinguish these cell populations to understand their unique functional contributions in health and disease as well as regulatory mechanisms, so as to obtain specific functional protocols to improve liver function, induce liver regeneration, and benefit patient health
.

Original link:

https://doi.
org/10.
1016/j.
immuni.
2022.
08.
002

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