Differential Location and Distribution of Hepatic Immune Cells

Abstract

The liver is one of the main organs in the body, performing several metabolic and immunological functions that are indispensable to the organism. The liver is strategically positioned in the abdominal cavity between the intestine and the systemic circulation. Due to its location, the liver is continually exposed to nutritional insults, microbiota products from the intestinal tract, and to toxic substances. Hepatocytes are the major functional constituents of the hepatic lobes, and perform most of the liver's secretory and synthesizing functions, although another important cell population sustains the vitality of the organ: the hepatic immune cells. Liver immune cells play a fundamental role in host immune responses and exquisite mechanisms are necessary to govern the density and the location of the different hepatic leukocytes. Here we discuss the location of these pivotal cells within the different liver compartments, and how their frequency and tissular location can dictate the fate of liver immune responses.

Keywords: hepatic leukocytes; hepatocytes; immune response; liver.

Figure 1

The hepatic immune cells. Schematic representation showing hepatic cells and their location. The liver harbors a large population of immune cells. Dendritic cells (CX3CR1⁺ cells) can be found in the subcapsular space and surround large vessels, as the centrilobular vein and the portal triade vessels (portal vein and hepatic artery). Kupffer cells are the liver resident macrophages and constitute the largest population of hepatic immune cells. They can be found within sinusoids, in contact with endothelial cells. Neutrophils, B lymphocytes, T lymphocytes, and NK cells eventually circulate in the sinusoids.

Figure 2

Immune system ontogeny. The colonization of immune cells in the liver occurs in three waves. The first wave originates in blood islets within the yolk sac on embryonic day E7.0. These cells are transient, being replaced by the cells of the second and third waves. The second wave also begins in the yolk sac as erythromyeloid progenitors (EMPs). The third wave gives rise to hematopoietic stem cells (HSCs) from the hemogenic endothelium at the aorta/gonada/mesonephros (AGM) region. EMPs and HSCs seed in the liver and this organ becomes the main place of hematopoiesis in the embryo. From the liver, the immune cells colonize other organs in the body, including the bone marrow, which replaces the hematopoietic function of the liver at the end of gestation and becomes the hematopoietic organ of the adult.

Figure 3

In vivo visualization of distinct morphological aspects of liver phagocytes. (A) Intravital imaging of CX3CR1^gfp/wt reporter mice under homeostatic condition revealed large hepatic cell populations with different spatial distribution: CX3CR1⁺ cells (in green), which is not expressed by liver macrophages and are suggestive of a dendritic cell population, and Kupffer cells stained with anti-F4/80 antibody (in red), merged with DAPI (in blue), a marker for DNA used to identify the nucleus of the cells in the field. Images through the hepatic surface showed that CX3CR1⁺ cells are abundant and uniformly distributed, unlike Kupffer cells; (B) a higher magnification of image showing the dendritic morphology of dendritic cells in the capsule; (C) transversal section of frozen tissue evidencing the fluorescence of hepatic parenchyma. There are CX3CR1⁺ cells around a large vessel (arrow), with a more circular form and less dendrites; (D) numerous Kupffer cells within the liver parenchyma, evidenced by a distinct morphology. Scale bars in (A), 120 μm. Scale bars in (B–D), 26 μm. All images were acquired using an inverted Nikon Eclipse Ti coupled to an A1 scanning head with no modifications. All animal studies were approved by the Animal Care and Use Committee at Universidade Federal de Minas Gerais, Brazil (CEUA 147/2016).

Figure 4

Distribution of hepatic phagocytes within liver intravascular and extravascular compartments. Intravital microscopy of CX3CR1^gfp/wt reporter mice enables the visualization of liver phagocytes in both intravascular and extravascular compartments. (A) Kupffer cells (in red) are observed in an intravascular focus. They are located inside the sinusoids, here stained with andi-CD31 PECAM-1 (in blue). Their location facilitates the capture of circulating bacterial products or intact bacteria by these cells. To image the extravascular cells only, the images were obtained in the capsule focus of the liver. There is an exclusively extravascular population of CX3CR1⁺ cells (in green) that inhabit the liver surface; (B,C) 3-dimensional reconstructions of liver compartments evidencing the intravascular population (KCs in red) and the extravascular population (DCs in green). The CX3CR1⁺ cells are found especially underneath the liver capsule and are rarely distributed in the parenchyma; (D) CX3CR1⁺ cells underneath the liver capsule and around a large vessel (arrow). Scale bars in (A), 120, 64, 36 μm. Scale bars in (D), 26 μm. All images were acquired using an inverted Nikon Eclipse Ti coupled to an A1 scanning head with no modifications. All animal studies were approved by the Animal Care and Use Committee at Universidade Federal de Minas Gerais, Brazil (CEUA 147/2016).

Figure 5

Accumulation of neutrophils in the hepatic sinusoids during acute liver injury. (A) Overdose of acetaminophen resulted in acute liver injury. Neutrophils, here stained with anti-LyCG PE (in red), are attracted to lesion sites in the liver, characterized by necrotic areas due to hepatocytes’ death, and stained with Sytox (bright green areas); (B) A higher resolution showing the neutrophils within the sinusoids. Scale bar in (A), 120 μm. Scale bar in (B), 30 μm. All images were acquired using an inverted Nikon Eclipse Ti coupled to an A1 scanning head with no modifications. All animal studies were approved by the Animal Care and Use Committee at Universidade Federal de Minas Gerais, Brazil (CEUA 147/2016).