Renal macrophages have different subsets and occupy different microenvironments

Macrophages are immune cells that engulf and digest pathogens, cancer cells, or cell debris
.
The kidneys, like other tissues in the body, contain kidney macrophages (KRMs)
from birth.
These KRMs protect the kidneys from infection or damage and help maintain tissue health
by phagocytosis of debris or dead kidney cells.

In other organs, the location of macrophages affects their function
.
Now, Dr.
James George of the University of Alabama at Birmingham and his colleagues report for the first time that mouse kidneys contain seven different KRM populations, which are located in a spatially discrete microenvironment, and that each subpopulation has a unique transcriptome signature — a measure of which genes are active, suggesting that they have different functions
.

George said: "KRMs are unknown until stratification in the kidneys
to a specific area.
" The spatial location of macrophages affects their function in other tissues, such as the lungs, spleen, and liver, and shapes their response
to immune challenges.
Although many disease states are known to be associated with KRM and have great therapeutic promise for populations, the successful design and implementation of this strategy is limited
by our current understanding of KRM regulation and injury response as a function of time.
”

The UAB study, published in the journal JCI Insight, is an application of spatial transcriptomics and Natural Methods named it the best method
for 2020.

George, co-corresponding author Anupam Agarwal, M.
D.
, and colleagues at UAB tracked these KRMs in normal kidneys and in experiments that restricted blood flow for 19 minutes to damage kidneys
.
This acute kidney injury can lead to chronic kidney disease, so understanding of changes in the KRM subset after injury is an important part
of the mouse kidney KRM map.
Such an atlas will serve as a reference point for future studies of the role of macrophage systems in both normal and damaged kidneys
.

The injured kidneys were examined
12 hours, 1 day, 6 days and 28 days after the injury, respectively.

George said: "After being insulted, we tracked these subpopulations because they appeared to migrate throughout the tissue, suggesting that these cells may be moving
in response to injury.
" Macrophages have the ability to move, similar to amoebae
.

After 28 days of injury, most of the 3 macrophage subsets returned to the location found before the injury, but the 4 subpopulations remained scattered throughout
the kidneys.
"So," George said, "our data support the long-hypothesized immune system disorder after acute kidney injury, which may be a major factor
in increasing the risk of chronic kidney disease after an acute kidney injury event.
" ”

Both human kidneys have more than 1 million renal elements each
.
The kidney is the tiny function of the kidney, which removes fluid from the bloodstream and then returns most of it to the bloodstream while retaining the waste urine
flowing through the ureter to the bladder.
Different parts of the kidney have different functions, and the researchers found that different populations of macrophages are associated
with different parts of the kidney element.

The study first performed single-cell RNA sequencing
on 58304 KRMs isolated from entire mouse kidneys.
Through their analysis of 3,000 variable genes, they identified 7 major distinct subpopulations with unique transcriptome characteristics — messenger RNAs transcribed from the active gene
.

Differentially expressed genes in 6 gene clusters indicate at least one specific function
.
For example, George said, "The most important gene ontology terms in clusters 1, 3, and 6 are related
to antibacterial, antiviral, and antifungal responses.
" Cluster 2 contains terms related to iron reaction, phagocytosis, and wound healing, indicating that it is related
to homeostasis function.
Clusters of 0 and 4 mapped to very few terms, but the analysis included tumor necrosis factor and apoptosis
.
”

"These different genome mappings suggest that each cluster performs a different transcriptional program, which may be a function of where each cluster is located
.
"

The location
of the aggregation can be found by placing the kidney flakes on a Visium space gene expression microscope slide of about a quarter square inch.
The technique in the Visium system allows researchers to locate the position
of each subpopulation in kidney anatomy based on their transcriptome characteristics.

Two tumor clusters of a normal kidney are located in the renal cortex, the outer region
of the kidney.
Four are located in the medulla (the area below the cortex) and one is located in the nipple (the central area of the kidney).

An example of the importance of position is the coordinated position of three subgroups to protect the kidneys from infection
.
George said: "The transcriptome and location of clusters 1, 3 and 6 describe a strategic immune barrier from the ureter, which is the most common source of
kidney infection.
"

Importantly, the transcriptome profile of KRM was continuously altered 28 days after injury – many KRM subpopulations no longer express their original characteristics and exist in new locations
.
George said: "Given the continued disruption of transcription and spatial distribution after acute injury, KRMs may affect the transition
to chronic kidney disease.
" "A single event of acute kidney injury can significantly increase the risk of developing chronic kidney disease, although the mechanism of this transition is unclear
.
"

At UAB, George is a professor in the Department of Surgery and Agarwal is a professor
in the Department of Nephrology at the Faculty of Medicine.
The co-first authors of the study are Matthew D.
Zhang and Elise N.
Erman of UAB Surgery, "The resident macrophage subpopulation occupies a unique microenvironment in the kidneys.
"

In addition to George, Agarwal, Zhang and Erman, other authors include Kyle H.
Moore, Jeremie M.
Lever, Jennifer R.
LaFontaine, and Rafay Karim, UAB Surgery; Li Zhang and Bradley K.
Yoder, Department of Cellular, Developmental and Integrative Biology, UAB; along with Gelare Ghajar-Rahimi, Shanrun Liu and Zhengqin Yang
from the UAB Medical Department.

Support from the National Institutes of Health DK079337, DK59600, DK118932, GM-008361 and AI007051; and the American Heart Association to fund 906401 and 827257
.

At UAB, George serves as UAB's Chair of Cardiovascular Surgery Research and Agarwal is interim dean
of the Marnix E.
Heersink School of Medicine.
Surgery, medicine and cellular, developmental and integrative biology are the faculties
of the Heske Medical School.