The Liver Mediates Apoptotic Cell-Induced Immune Regulation

Tissue-Resident Macrophages Are Locally Programmed for Silent Clearance of Apoptotic Cells

Although apoptotic cells (ACs) contain nucleic acids that can be recognized by Toll-like receptors (TLRs), engulfment of ACs does not initiate inflammation in healthy organisms. Here we identified macrophage populations that continually engulf ACs in distinct tissues and found that these macrophages share characteristics compatible with immunologically silent clearance of ACs; such characteristics include high expression of AC recognition receptors, low expression of TLR9, and reduced TLR responsiveness to nucleic acids. Removal of the macrophages from tissues resulted in loss of many of these characteristics and the ability to generate inflammatory responses to AC-derived nucleic acids, suggesting that cues from the tissue microenvironment program macrophages for silent AC clearance. The transcription factors KLF2 and KLF4 control the expression of many genes within this AC clearance program. The coordinated expression of AC receptors with genes that limit responses to nucleic acids might ensure maintenance of homeostasis and thus represent a central feature of tissue macrophages.

O death where is thy sting? Immunologic tolerance to apoptotic self

In higher organisms, innate scavenging cells maintain physiologic homeostasis by removal of the billions of apoptotic cells generated on a daily basis. Apoptotic cell removal requires efficient recognition and uptake by professional and non-professional phagocytic cells, which are governed by an array of soluble and apoptotic cell-integral signals resulting in immunologically silent clearance. While apoptosis is associated with profound suppression of adaptive and innate inflammatory immunity, we have only begun to scratch the surface in understanding how immunologic tolerance to apoptotic self manifest at either the molecular or cellular level. In the last 10 years, data has emerged implicating professional phagocytes, most notably stromal macrophages and CD8α(+)CD103(+) dendritic cells, as critical in initiation of the regulatory cascade that will ultimately lead to long-term whole-animal immune tolerance. Importantly, recent work by our lab and others has shown that alterations in apoptotic cell perception by the innate immune system either by removal of critical phagocytic sentinels in secondary lymphoid organs or blockage of immunosuppressive pathways leads to pronounced inflammation with a breakdown of tolerance towards self. This challenges the paradigm that apoptotic cells are inherently immunosuppressive, suggesting that apoptotic cell tolerance is a "context-dependent" event.

Apoptotic cells attenuate fulminant hepatitis by priming Kupffer cells to produce interleukin-10 through membrane-bound TGF-β

The liver, a unique tolerogenic organ, is regarded as the site to trap and destroy aging erythrocytes and activated T cells. However, to date, the mechanisms for why the liver is tolerogenic and whether liver Kupffer cells (KC) are critical phagocytes for apoptotic cells (AC) contributing to the liver immunosuppression remain unclear. Here we report that KC is the main phagocyte for AC in the liver. Contact of AC inhibits proinflammatory cytokine but enhances anti-inflammatory cytokine production of KC in response to lipopolysaccharide (LPS) stimulation. Membrane-bound transforming growth factor (TGF)-β on AC is responsible for the increased production of interleukin (IL)-10 in KC through extracellular signal-regulated kinase (ERK) activation via the Smad3 pathway. Importantly, KC-derived IL-10 is critical for AC infusion-mediated protection of endotoxin-induced fulminant hepatitis through suppression of tumor necrosis factor (TNF)-α and nitric oxide (NO) production from KC and consequently attenuation of KC-mediated cytolysis of hepatocytes.

CONCLUSION

AC can be preferentially phagocytosed by KC in the liver, leading to attenuation of fulminant hepatitis through IL-10-mediated suppression of KC-derived inflammatory TNF-α and NO production. These findings demonstrate that priming of KC by AC may contribute to maintain liver immunosuppression, providing a new mechanistic explanation for how immune homeostasis is maintained in the liver.

Spleen migrating dendritic cells primed with CC531 colon cancer antigen and LPS - is it a method to compromise liver metastases?

The anti-tumor vaccination is burdened by low recruitment rate of intravenously administered in vitro primed DC in liver metastases and lack of supplying them continuously in large numbers. Therefore, it seemed rational to create a model of in vivo vaccination with specifically primed splenic DC and cytotoxic T lymphocytes being continuously supplied to the liver vascular bed. The question we raised was whether anti-tumor immunized splenic DC flowing to liver metastases could adhere to and be cytotoxic to tumor cells. We immunized rats with CC531 tumor cells and stimulated them with Escherichia coli LPS. Subsequently, spleen DC-enriched population was isolated, its activation by LPS, adherence to CC531 cells and cytotoxicity were measured. Spleen cells home to the liver reaching it via splenic vein. These cells can be retrieved by simple washout of liver sinusoids (liver sinusoidal washout cells - LSWC). Their adherence to and cytotoxicity against CC531 cells were evaluated. Moreover, in vitro adherence of splenic DC-enriched cells and LSWC to CC531 liver tumor sections was measured. We found that in vivo immunization of splenic population containing DC, NK cells and lymphocytes with CC531 cells and stimulation with LPS activated these cells but did not significantly increase the cytotoxicity against CC531 cells. There was also no increase in cytotoxicity of LSWC. Adhesion of splenic DC and LWSC to liver CC531 metastases on cryosections was higher than to the adjacent liver tissue. However, it was more expressed on tumor stromal than neoplastic cells. The level of splenic Treg cells down-regulating immune response was found only slightly increased after immunization. Taken together, in the model of in vivo immunization against CC531 cells, low level of spleen DC and spleen-derived LSWC cytotoxicity as well as adherence rate to tumor cells were observed. More effective methods of immunizing splenic DC overcoming the suppressive mechanisms should be looked for.