Immunoregulatory Role of NK Cells in Tissue Inflammation and Regeneration

NK cells represent an important first line of defense against viral infection and cancer and are also involved in tissue homeostasis. Studies of NK cell activation in the last decade have revealed that they are able to respond to the inflammatory stimuli evoked by tissue damage and contribute to both progression and resolution of diseases. Exacerbation of the inflammatory response through interactions between immune effector cells facilitates the progression of non-alcoholic fatty liver disease (NAFLD) into steatosis, cirrhosis, and hepatocellular carcinoma (HCC). When hepatic damage is incurred, macrophage activation is crucial for initiating cross talk with neighboring cells present in the liver, including hepatocytes and NK cells, and the importance of this interaction in shaping the immune response in liver disease is increasingly recognized. Inflicted structural damage can be in part regenerated via the process of self-limiting fibrosis, though persistent hepatic damage will lead to chronic fibrosis and loss of tissue organization and function. The cytotoxic activity of NK cells plays an important role in inducing hepatic stellate cell apoptosis and thus curtailing the progression of fibrosis. Alternatively, in some diseases, such as HCC, NK cells may become dysregulated, promoting an immunosuppressive state where tumors are able to escape immune surveillance. This review describes the current understanding of the contributions of NK cells to tissue inflammation and metabolic liver diseases and the ongoing effort to develop therapeutics that target the immunoregulatory function of NK cells.

Altered lipid metabolism influences course of hepatitis C virus infection and hepatocyte cytokine production (VIR1P.1011)

Hepatitis C virus (HCV) is remarkably efficient in establishing persistent infection leading to the development of chronic inflammatory liver diseases. Lipids play a central role in regulating inflammatory responses and are necessary for the replication and assembly of HCV. Fatty acids are the building blocks of lipids and are synthesized via a rate-limiting step catalyzed by the enzyme acetyl-CoA carboxylase (ACC). To determine the role of lipid synthesis in regulating innate inflammatory responses and HCV biogenesis, we examined the effect of ACC inhibition on HCV replication and cytokine production. Inhibition of ACC decreases intracellular viral RNA in Huh7.5.1 cells infected with JFH-1, a genotype 2a strain of HCV. Surprisingly, ACC inhibition slightly decreases IL-1β transcription and results in comparable production of IL-8, IL-13, and IP-10 in HCV-infected hepatocytes relative to vehicle controls. Further studies using electron and fluorescent microscopy demonstrate a reduction in the number of lipid droplets, which was accompanied by an accumulation of viral protein. These data highlight the paradoxical immune response of HCV-infected hepatocytes, where the continued accumulation of viral antigen fails to stimulate synthesis of inflammatory cytokines. The potential role of lipids in dampening immune responses in HCV-infected hepatocytes may provide an important link between host factors driving infection and suppression of immune responses in chronic infection.

Extracellular hepatitis C virus core protein activates STAT3 in human monocytes/macrophages/dendritic cells via an IL-6 autocrine pathway

Hepatitis C virus (HCV) infection is highly efficient in the establishment of persistent infection, which leads to the development of chronic liver disease and hepatocellular carcinoma. Impaired T cell responses with reduced IFN-γ production have been reported to be associated with persistent HCV infection. Extracellular HCV core is a viral factor known to cause HCV-induced T cell impairment via its suppressive effect on the activation and induction of pro-inflammatory responses by antigen-presenting cells (APCs). The activation of STAT proteins has been reported to regulate the inflammatory responses and differentiation of APCs. To further characterize the molecular basis for the regulation of APC function by extracellular HCV core, we examined the ability of extracellular HCV core to activate STAT family members (STAT1, -2, -3, -5, and -6). In this study, we report the activation of STAT3 on human monocytes, macrophages, and dendritic cells following treatment with extracellular HCV core as well as treatment with a gC1qR agonistic monoclonal antibody. Importantly, HCV core-induced STAT3 activation is dependent on the activation of the PI3K/Akt pathway. In addition, the production of multifunctional cytokine IL-6 is essential for HCV core-induced STAT3 activation. These results suggest that HCV core-induced STAT3 activation plays a critical role in the alteration of inflammatory responses by APCs, leading to impaired anti-viral T cell responses during HCV infection.

HCV core/gC1qR ligation activates STAT3 in human innate immune cells via an IL-6 autocrine pathway. (37.23)

Hepatitis C virus (HCV) infection in humans is highly efficient in the establishment of persistent infection. Persistent HCV infection can lead to the development of chronic liver disease and hepatocellular carcinoma. T cells in persistently infected individuals exhibit impaired function, and fail to clear virus. HCV core protein secreted from infected hepatocytes, has been reported to inhibit T cell function by inhibiting the pro-inflammatory response of APCs (i.e. monocytes, macrophages, and dendritic cells). To further characterize the molecular basis for APC regulation by extracellular HCV core, we examined the ability of HCV core to induce STAT3 activation. STAT3 plays a pivotal role in regulating the inflammatory response and differentiation of APCs. In this study we report the activation of STAT3 following ligation of HCV core with complement receptor, gC1qR, as well as with a gC1qR agonistic monoclonal antibody. HCV core-mediated STAT3 activation is dependant on the activation of the PI3K/AKT pathway. Additionally, transcription and translation are required for gC1qR mediated STAT3 activation. Lastly, we show that multi-functional cytokine IL-6 is essential for HCV core-induced STAT3 activation. These results suggest that HCV core-mediated STAT3 activation may contribute to alteration of inflammatory responses by monocytes, macrophages, and dendritic cells, leading to impaired T cell responses during HCV infection.

HCV Core/gC1qR Interaction Regulates LPS-Mediated MAPK Activation (134.66)

Hepatitis C Virus (HCV) is a dangerous human pathogen that is able to establish chronic infection in its host. Studies of the immune response to the virus reveal a weak and severely delayed T and B cell response. HCV core protein is the first viral protein made following infection within hepatocytes. In addition to forming the viral capsid, HCV core protein is secreted from infected hepatocytes and is present at high levels in the peripheral blood of infected individuals. Previous data from our laboratory has shown that extracellular HCV core protein binds complement receptor gC1qR. This interaction results in the suppression of LPS-mediated IL-12 production in a PI3K dependant manner in human monocytes. Here we show that signaling through the gC1qR results in the suppression of LPS-mediated MAPK activation in a PI3K-dependent fashion. Further, we show that gC1qR-mediated signaling activates AKT, and that AKT is able to phosphorylate GSK-3β and MAPKKK family protein ASK1 at serines 9 and 83 respectively. Phosphorylation at these sites results in reduced activity of both kinases. Interestingly, both GSK-3β and ASK1 are important for LPS-mediated IL-12 production. These results suggest that HCV core protein binds gC1qR on the surface of human monocytes resulting in the activation of the PI3K/AKT pathway. AKT then inhibits LPS-mediated GSK-3β and ASK1 activation resulting in impaired MAPK activation and IL-12 production. NIH Grant RO1AI057591 supported this research.

HCV core/gC1q Receptor ligation transduce a signal across the plasma membrane possibly by beta1 integrin complex. (133.51)

Hepatitis C virus (HCV) infection in humans is associated with viral persistence, which leads to the development of liver diseases. HCV core protein has been reported to be detectable in bloodstream of HCV-infected patients, and contains the immunomodulatory function. We have previously demonstrated that the interaction of HCV core with the complement receptor gC1qR inhibits the inflammatory response, particularly the TLR4-mediated production of IL-12 in monocytes. The inhibition of IL-12 production depends on the activation of the PI-3 kinase pathway. However, it is still unknown how this receptor anchors at the cell surface to transduce intracellular signaling since it has no transmembrane or GPI domain. To determine the protein(s) involved in the gC1qR complex responsible for anchoring gC1qR, we examined the association of gC1qR with β1 integrin (a putative transmembrane partner). Here, we show by performing GST-gC1qR pull down assays on human cell lysats, that β1 integrin specifically interacts with gC1qR. Moreover, we examine the cell surface localization of gC1qR and β1 integrin by immunofluorescence microscopy after THP-1stimulation by HCV core. Our data demonstrate that HCV core enhance the formation of gC1qR/β1 integrin complexes on HCV core-treated monocytes. These results suggest that β1 integrin may play the role of anchoring protein to gC1qR on the cell surface.

NIH grant RO1AI057591

A pathway for phagosome maturation during engulfment of apoptotic cells

Removal of apoptotic cells is critical for the physiological well-being of the organism and defects in corpse removal have been linked to disease states. Genes regulating corpse recognition and internalization have been identified, but few molecules involved in the processing of internalized corpses are known. Through a combination of targeted and unbiased reverse genetic screens in Caenorhabditis elegans, and studies in mammalian cells, we have identified genes required for maturation of apoptotic-cell-containing phagosomes. We have further ordered these candidates, which include the GTPases RAB-5 and RAB-7 and the HOPS complex, into a coherent linear pathway for the maturation of apoptotic cells within phagosomes. In depth analysis of two additional candidate genes, the phosphatidylinositol 3 kinase (PI(3)K) vps-34 (A001762) and dyn-1/dynamin, showed an accumulation of internalized, but undegraded, corpses within abnormal Rab5-negative phagosomes. We ordered these candidates in our pathway, with DYN-1 functioning upstream of VPS-34 in the recruitment and/or retention of RAB-5 to the phagosome. Finally, we have also identified a previously undescribed biochemical complex containing Vps34, dynamin and Rab5(GDP), thus providing a mechanism for Rab5 recruitment to the nascent phagosome.