TRAIL but not FasL and TNFα, regulates IL-33 expression in murine hepatocytes during acute hepatitis

The alarmin functions of high-mobility group box-1 and IL-33 in the pathogenesis of systemic lupus erythematosus

'Alarmins' are a group of endogenous proteins or molecules that are released from cells during cellular demise to alert the host innate immune system. Two of them, high-mobility group box-1 (HMGB1) and IL-33 shared many similarities of cellular localization, functions and involvement in various inflammatory diseases including systemic lupus erythematosus (SLE). The expressions of HMGB1 and IL-33, and their corresponding receptors RAGE (receptor for advanced glycation end products) and ST2, respectively, are substantially upregulated in patients with lupus nephritis (LN). This review highlights the emerging roles of alarmin proteins in various pathologies of LN, by focusing on classical HMGB1 and a newly discovered alarmin IL-33.

The IL-33/ST2 axis is associated with human visceral leishmaniasis and suppresses Th1 responses in the livers of BALB/c mice infected with Leishmania donovani

During visceral leishmaniasis, the control of hepatic parasite burden is mainly due to granuloma assembly in a microenvironment consisting of both Th1 and Th2 components. Using enzyme-linked immunosorbent assay (ELISA) dosages, quantitative PCR (qPCR), immunohistochemistry, and flow cytometry, we studied the role of interleukin-33 (IL-33), a recently described cytokine signaling through the ST2 receptor, during visceral leishmaniasis. We showed that a higher level of IL-33 was detected in the serum of patients with visceral leishmaniasis than in that from healthy donors and demonstrated the presence of IL-33(+) cells in a liver biopsy specimen from a patient. Similarly, in BALB/c mice experimentally infected with L. donovani, a higher level of IL-33 was detected in the serum, as well as the presence of IL-33(+) cells and ST2(+) cells in the mouse liver. In ST2(-/-) BALB/c mice, better control of the hepatic parasite burden and reduced hepatomegaly were observed. This was associated with strong induction of Th1 cytokines (gamma interferon [IFN-γ] and IL-12) compared to the level in wild-type (WT) mice and better recruitment of myeloid cells associated with strongly induced chemokines (CCL2 and CXCL2) and receptors (CCR2 and CXCR2). Conversely, BALB/c mice treated twice weekly with recombinant IL-33 showed a dramatically reduced induction of Th1 cytokines and delayed inhibition of monocyte and neutrophil recruitment in the liver, which was associated with reduced KC/CXCL1 and CXCR2 expression. Taken together, our results suggest that IL-33 could be a new deleterious regulator of the hepatic immune response against Leishmania donovani, via the repression of the Th1 response and myeloid cell recruitment.

IMPORTANCE

Visceral leishmaniasis is a life-threatening systemic disease due to the Leishmania protozoa L. infantum and L. donovani and is ranked by the World Health Organization as the second most important protozoan parasitic disease after malaria for its grave morbidity, high mortality, and global distribution. Leishmania parasites subvert the host's immune response to propagate to target organs, including the spleen, the bone marrow, and the liver. Control of hepatic parasite burdens depends on a delicate and poorly understood Th1/Th2 immune balance. To better understand this complex immune response, new cytokines are interesting targets for research studies. IL-33 is a newly described cytokine usually associated with Th2 response and involved in different diseases, including infectious diseases and hepatitis. Our results suggest that IL-33 could be a new factor of susceptibility and a potential prognostic marker during visceral leishmaniasis.

Pathogenic mouse hepatitis virus or poly(I:C) induce IL-33 in hepatocytes in murine models of hepatitis

The IL-33/ST2 axis is known to be involved in liver pathologies. Although, the IL-33 levels increased in sera of viral hepatitis patients in human, the cellular sources of IL-33 in viral hepatitis remained obscure. Therefore, we aimed to investigate the expression of IL-33 in murine fulminant hepatitis induced by a Toll like receptor (TLR3) viral mimetic, poly(I:C) or by pathogenic mouse hepatitis virus (L2-MHV3). The administration of poly(I:C) plus D-galactosamine (D-GalN) in mice led to acute liver injury associated with the induction of IL-33 expression in liver sinusoidal endothelial cells (LSEC) and vascular endothelial cells (VEC), while the administration of poly(I:C) alone led to hepatocyte specific IL-33 expression in addition to vascular IL-33 expression. The hepatocyte-specific IL-33 expression was down-regulated in NK-depleted poly(I:C) treated mice suggesting a partial regulation of IL-33 by NK cells. The CD1d KO (NKT deficient) mice showed hepatoprotection against poly(I:C)-induced hepatitis in association with increased number of IL-33 expressing hepatocytes in CD1d KO mice than WT controls. These results suggest that hepatocyte-specific IL-33 expression in poly(I:C) induced liver injury was partially dependent of NK cells and with limited role of NKT cells. In parallel, the L2-MHV3 infection in mice induced fulminant hepatitis associated with up-regulated IL-33 expression as well as pro-inflammatory cytokine microenvironment in liver. The LSEC and VEC expressed inducible expression of IL-33 following L2-MHV3 infection but the hepatocyte-specific IL-33 expression was only evident between 24 to 32h of post infection. In conclusion, the alarmin cytokine IL-33 was over-expressed during fulminant hepatitis in mice with LSEC, VEC and hepatocytes as potential sources of IL-33.

Interleukin-33: a mediator of inflammation targeting hematopoietic stem and progenitor cells and their progenies

Inflammation is defined as a physiological response initiated by a variety of conditions that cause insult to the body, such as infection and tissue injury. Inflammation is triggered by specialized receptors in the innate immune system, which recognize microbial components known as pathogen-associated molecular patterns or endogenous signals produced by damaged cells (damage-associated molecular patterns). IL-33 is a cytokine that is released predominantly at the epithelial barrier when it is exposed to pathogens, allergens, or injury-inducing stimuli. IL-33 target cells are various, ranging from hematopoietic stem and progenitor cells (HSPCs) and essentially all types of their progeny to many non-hematopoietic cells. The pleiotropic actions of IL-33 suggest that IL-33 is involved in every phase of the inflammatory process. In this review, we discuss recent advances in the understanding of how IL-33 orchestrates inflammatory responses by regulating HSPCs and innate immune cells.

IL-33 induces nuocytes and modulates liver injury in viral hepatitis

Molecules containing damage-associated molecular patterns play an important role in many pathogenic processes. In this study, our aim was to investigate the role of IL-33, a damage-associated molecular pattern molecule, in adenovirus (Ad)-induced liver inflammation. Ad-infected mice exhibited a steadily increased IL-33 and its receptor IL-1R-like 1 expression in the liver during the first week of infection. Treatment of exogenous IL-33 resulted in a great decrease in the serum alanine aminotransferase levels and the number of Councilman bodies in the liver. Attenuated liver injury by IL-33 correlated with an increase in T regulatory cells but with a decrease in macrophages, dendritic cells, and NK cells in the liver. IL-33 enhanced both type 1 (IL-2 and IFN-γ) and type 2 (IL-5 and IL-13) immune responses in infected mice. However, IL-33 inhibited TNF-α expression in hepatic T cells and macrophages, and significantly reduced TNF-α levels in the liver. We found that in addition to its direct effects, IL-33 strongly induced novel nuocytes in the livers and spleens of infected mice. When cocultured with nuocytes, hepatic T cells and macrophages expressed lower levels of TNF-α. The IL-33-treated mice also demonstrated a slight delay, but no significant impairment, in eliminating an intrahepatic infection with Ad. In conclusion, this study reveals that IL-33 acts as a potent immune stimulator and a hepatoprotective cytokine in acute viral hepatitis. Its direct immunoregulatory functions and ability to induce novel nuocytes further suggest to us that it may be a potentially promising therapeutic candidate for the management of viral hepatitis.