Specific messenger RNA expression for signal transduction molecules by lipopolysaccharide in intestinal macrophages

Distinct infection process of SARS-CoV-2 in human bronchial epithelial cell lines

Coronavirus disease 2019, caused by severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2), leads to a series of clinical symptoms of respiratory and pulmonary inflammatory reactions via unknown pathologic mechanisms related to the viral infection process in tracheal or bronchial epithelial cells. Investigation of this viral infection in the human bronchial epithelial cell line (16HBE) suggests that SARS‐CoV‐2 can enter these cells through interaction between its membrane‐localized S protein with the angiotensin‐converting enzyme 2 molecule on the host cell membrane. Further observation indicates distinct viral replication with a dynamic and moderate increase, whereby viral replication does not lead to a specific cytopathic effect but maintains a continuous release of progeny virions from infected cells. Although messenger RNA expression of various innate immune signaling molecules is altered in the cells, transcription of interferons‐α (IFN‐α), IFN‐β, and IFN‐γ is unchanged. Furthermore, expression of some interleukins (IL) related to inflammatory reactions, such as IL‐6, IL‐2, and IL‐8, is maintained at low levels, whereas that of ILs involved in immune regulation is upregulated. Interestingly, IL‐22, an IL that functions mainly in tissue repair, shows very high expression. Collectively, these data suggest a distinct infection process for this virus in respiratory epithelial cells, which may be linked to its clinicopathological mechanism.

SARS‐CoV‐2 does not lead to a specific cytopathic effect in 16HBE cells, but maintains continuous release of progeny virions from infected cells. During infection, IL‐22, an interleukin that functions mainly in tissue repair, showed very high expression.

Involvement of nitric oxide with activation of Toll-like receptor 4 signaling in mice with dextran sodium sulfate-induced colitis

Ulcerative colitis is an inflammatory bowel disease characterized by acute inflammation, ulceration, and bleeding of the colonic mucosa. Its cause remains unknown. Increases in adhesion molecules in vascular endothelium, and activated neutrophils releasing injurious molecules such as reactive oxygen species, are reportedly associated with the pathogenesis of dextran sodium sulfate (DSS)-induced colitis. Nitric oxide (NO) production derived from inducible NO synthase (iNOS) via activation of nuclear factor κB (NF-κB) has been reported. It is also reported that stimulation of Toll-like receptor 4 (TLR4) by lipopolysaccharide can activate NF-κB. In this study, we investigated the involvement of NO production in activation of the TLR4/NF-κB signaling pathway in mice with DSS-induced colitis. The addition of 5% DSS to the drinking water of male ICR mice resulted in increases in TLR4 protein in colon tissue and NF-κB p65 subunit in the nuclear fraction on day 3, increases in colonic tumor necrosis factor-α on day 4, and increases in P-selectin, intercellular adhesion molecule-1, NO2(-)/NO3(-), and nitrotyrosine in colonic mucosa on day 5. These activated inflammatory mediators and pathology of colitis were completely suppressed by treatment with a NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, as well as an iNOS inhibitor, aminoguanidine. Conversely, a NO-releasing compound, NOC-18, increased TLR4 levels and nuclear translocation of NF-κB p65 and exacerbated mucosal damage induced by DSS challenge. These data suggest that increases in TLR4 expression induced by drinking DSS-treated water might be directly or indirectly associated with NO overproduction.

Toll-like receptor 4-dependent adjuvant activity of Kakkon-to extract exists in the high molecular weight polysaccharide fraction

Kakkon-to, a traditional herbal medicine (Kampo formula), has been used historically in China and Japan for the treatment of infectious diseases such as influenza and the common cold. However, the biological mechanism of its therapeutic action has not yet been elucidated. In this study, we investigated the immunological function of Kakkon-to and found that the high molecular weight fraction of the extract activated macrophages in vitro. This fraction was found to be composed primarily of saccharides and in vitro intensively stimulated mouse peritoneal macrophages that produce Th1 inflammatory cytokines such as tumor necrosis factor α (TNFalpha), interleukin-1beta (IL-1beta), interferon-gamma (IFN-gamma), and interleukin-6 (IL-6). The fraction did not activate macrophages from C3H/HeJ lacking Toll-like receptor 4 (TLR4) or MyD88-deficient mice, indicating that macrophage activation by the fraction was mediated by TLR4. The route of administration of the fraction into mice regulated the kinetics of TNFalpha production in immune organs. Intravenous administration induced TNFalpha production in the four target organs of spleen, liver, lung, and Peyer’s patch; however, the most abundant production occurred in the liver and peaked at 30-60 min post administration. Peritoneal administration induced similar kinetics but the most abundant production occurred in the spleen. In contrast, oral administration induced TNFalpha production in the liver, lung, and Peyer’s patch, but not in the spleen. Although liver and lung are TNFalpha-abundant organs, production peaks in these organs occurred later than in Peyer’s patch. We also found that the fraction induced antibody production as an adjuvant against a specific antigen ovalbumin (OVA) when administered simultaneously and subcutaneously in a dose-dependent manner. Interestingly, the fraction induced IgG-class antibody in response to low doses of the antigen, which induced only IgM-class antibody when administered alone, suggesting that the fraction induces a class switch of immunoglobulin as an adjuvant in vivo. The high molecular weight fraction of Kakkon-to extract could be applicable as a potent immunostimulating drug and adjuvant.

Mucosal macrophages and the regulation of immune responses in the intestine

The healthy intestinal mucosa is home to one of the largest populations of macrophages (mvarphi) in the body [Lee SH, Starkey PM, Gordon S. Quantitative analysis of total macrophage content in adult mouse tissues. Immunochemical studies with monoclonal antibody F4/80. J Exp Med 1985;161:475-89], yet little is known about their function. Resident mvarphi in the large and small intestine are distinct from other mvarphi populations in the body, with regards to both their functional properties and surface phenotype. They respond in an unconventional manner to inflammatory stimuli, with little upregulation of proteins involved in antigen presentation and T cell co-stimulation, and no production of pro-inflammatory cytokines. This suggests that under resting conditions, intestinal mvarphi may be conditioned to be anti-inflammatory in response to local stimuli such as commensal bacteria. In contrast, during inflammation, intestinal mvarphi exhibit increased bactericidal and inflammatory abilities, promote protective immunity and/or mediate pathology. Thus the status of this cell may be the key to understanding how the intestine maintains a balance between being able to generate protective immunity against pathogens, but still prevent pathological inflammation under normal conditions. In this review, we discuss the current knowledge of intestinal mvarphi biology, and highlight the different levels of immunoregulation which influence these cells, with particular focus on innate pathogen recognition receptor (PRR) function and responsiveness to microbial stimuli.