Mannan-binding lectin reduces CpG DNA-induced inflammatory cytokine production by human monocytes

Polymorphisms in the MBL2 gene are associated with the plasma levels of MBL and the cytokines IL-6 and TNF-α in severe COVID-19

Introduction

Mannose-binding lectin (MBL) promotes opsonization, favoring phagocytosis and activation of the complement system in response to different microorganisms, and may influence the synthesis of inflammatory cytokines. This study investigated the association of MBL2 gene polymorphisms with the plasma levels of MBL and inflammatory cytokines in COVID-19.

Methods

Blood samples from 385 individuals (208 with acute COVID-19 and 117 post-COVID-19) were subjected to real-time PCR genotyping. Plasma measurements of MBL and cytokines were performed by enzyme-linked immunosorbent assay and flow cytometry, respectively.

Results

The frequencies of the polymorphic MBL2 genotype (OO) and allele (O) were higher in patients with severe COVID-19 (p< 0.05). The polymorphic genotypes (AO and OO) were associated with lower MBL levels (p< 0.05). IL-6 and TNF-α were higher in patients with low MBL and severe COVID-19 (p< 0.05). No association of polymorphisms, MBL levels, or cytokine levels with long COVID was observed.

Discussion

The results suggest that, besides MBL2 polymorphisms promoting a reduction in MBL levels and therefore in its function, they may also contribute to the development of a more intense inflammatory process responsible for the severity of COVID-19.

MBL Binding with AhR Controls Th17 Immunity in Silicosis-Associated Lung Inflammation and Fibrosis

Objective

Mannan-binding lectin (MBL), a soluble pattern recognition molecule of the innate immune system, is primarily synthesized in the liver and secreted into the circulation. Low serum level of MBL has been reported to be related to an increased risk of lung diseases. Herein, we aimed to investigate the function of MBL in silicosis-associated pulmonary inflammation.

Methods

Serum collected from silicosis patients was tested for correlation between serum MBL levels and Th17 immunity. In vitro studies were performed to further demonstrated the effect of MBL on Th17 polarization. Silica was intratracheally injected in wild type (WT) or MBL-deficient (MBL^–/–) mice to induce silicosis-associated lung inflammation and fibrosis. Th17 response was evaluated to explore the effect of MBL on silicosis in vivo.

Results

Silicosis patients with high serum MBL levels displayed ameliorative lung function. We demonstrated that serum MBL levels negatively correlated to Th17 cell frequency in silicosis patients. MBL protein markedly reduced expression of IL-17 but enhanced expression of Foxp3 in CD4⁺ T cells in vitro when subjected to Th17 or Treg polarizing conditions, respectively. The presence of MBL during Th17 cell polarization significantly limited aryl hydrocarbon receptor (AhR) expression and suppressed the signal transducer and activator of transcription 3 (STAT3) phosphorylation. Treatment with the AhR antagonist abolished the effect of MBL on Th17 response. Strikingly, MBL directly bound to AhR and affected its nuclear translocation. Furthermore, MBL^–/– mice displayed elevated Th17 cell levels compared with WT mice in response to the silica challenge. The CD4⁺ T lymphocytes from silica-administrated MBL^–/– mice exhibited more AhR expression than the wild-type counterparts.

Conclusion

Our study suggested that MBL limited the Th17 immunity via controlling the AhR/STAT3 pathway, thus providing new insight into silicosis and other inflammatory diseases in patients with MBL deficiency.

The Control of Intestinal Inflammation: A Major Objective in the Research of Probiotic Strains as Alternatives to Antibiotic Growth Promoters in Poultry

The reduction of antimicrobial resistance is a major challenge for the scientific community. In a few decades, infections by resistant bacteria are forecasted to be the main cause of death in the world. The withdrawal of antibiotics as growth promoters and their preventive use in animal production is essential to avoid these resistances, but this may impair productivity and health due to the increase in gut inflammation. This reduction in productivity aggravates the problem of increasing meat demand in developing countries and limits the availability of raw materials. Probiotics are promising products to address this challenge due to their beneficial effects on microbiota composition, mucosal barrier integrity, and immune system to control inflammation. Although many modes of action have been demonstrated, the scientific community is not able to describe the specific effects that a probiotic should induce on the host to maximize both productivity and animal health. First, it may be necessary to define what are the innate immune pathways acting in the gut that optimize productivity and health and to then investigate which probiotic strain is able to induce the specific effect needed. This review describes several gaps in the knowledge of host-microbiota-pathogen interaction and the related mechanisms involved in the inflammatory response not demonstrated yet in poultry.

Mannan-Binding Lectin Regulates Inflammatory Cytokine Production, Proliferation, and Cytotoxicity of Human Peripheral Natural Killer Cells

Natural killer (NK) cells represent the founding members of innate lymphoid cells (ILC) and play critical roles in inflammation and the immune response. NK cell effector functions are regulated and fine-tuned by various immune modulators. Mannan (or mannose)-binding lectin (MBL), a soluble C-type lectin, is traditionally recognized as an initiator of the complement pathway. Recently, it is also considered as an immunomodulator by its interaction with kinds of immune cells. However, the effect of MBL on NK cell function remains unexplored. In this study, we found that human plasma MBL could interact directly with peripheral NK cells partially via its collagen-like region (CLR). This MBL binding markedly suppressed the interleukin-2- (IL-2-) induced inflammatory cytokine tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ) production but increased the IL-10 production in NK cells. In addition, the expression of activation surface markers such as CD25 and CD69 declined after MBL treatment. Also, MBL impaired the proliferation and lymphokine-activated killing (LAK) of NK cells. Moreover, we demonstrated that MBL inhibited IL-2-induced signal transducers and activators of transcription 5 (STAT5) activation in NK cells. In conclusion, we have uncovered a far unknown regulatory role of MBL on NK cells, a new clue that could be important in the immunomodulatory networks of immune responses.

Mannan-Binding Lectin Suppresses Peptidoglycan-Induced TLR2 Activation and Inflammatory Responses

Peptidoglycan (PGN), as the major components of the bacterial cell wall, is known to cause excessive proinflammatory cytokine production. Toll-like receptor 2 (TLR2) is abundantly expressed on immune cells and has been shown to be involved in PGN-induced signaling. Although more and more evidences have indicated that PGN is recognized by TLR2, the role of TLR2 PGN recognition is controversial. Mannan-binding lectin (MBL), a plasma C-type lectin, plays a key role in innate immunity. More and more evidences show that MBL could suppress the amplification of inflammatory signals. Whether MBL can alter PGN-elicited cellular responses through TLR2 in macrophages is still unknown, and possible mechanism underlying it should be investigated. In this study, we found that MBL significantly attenuated PGN-induced inflammatory cytokine production, including TNF-α and IL-6, in PMA-stimulated THP-1 cells at both mRNA and protein levels. The expression of TLR2 was strongly induced by PGN stimulation. Furthermore, the administration of TLR2-neutralized antibody effectively suppressed PGN-induced TNF-α and IL-6 expression. These results supplied the evidence that PGN from Saccharomyces cerevisiae could be recognized by TLR2. In addition, we also found that MBL decreased PGN-induced TLR2 expression and suppressed TLR2-mediated downstream signaling, including the phosphorylation of IκBα, nuclear translocation of NF-κBp65, and phosphorylation of MAPK p38 and ERK1/2. Administration of MBL alone did not have an effect on the expression of TLR2. Finally, our data showed that PGN-mediated immune responses were more severely suppressed by preincubation with MBL and indicated that MBL can combine with both TLR2 and PGN to block the inflammation cytokine expression induced by PGN. All these data suggest that MBL could downregulate inflammation by modulating PGN/TLR2 signaling pathways. This study supports an important role for MBL in immune regulation and signaling pathways involved in inflammatory responses.

Mannan-binding lectin deficiency exacerbates sterile liver injury in mice through enhancing hepatic neutrophil recruitment

Noninfectious liver injury, including the effects of drugs and diet, is a major cause of liver diseases worldwide. The innate inflammatory response to hepatocyte death plays a crucial role in the outcome of liver injury. Mannan-binding lectin (MBL) is a pattern recognition molecule of the innate immune system, which is primarily produced by liver. MBL deficiency occurs with high frequency in the population and is reported associated with predisposition to infectious diseases. We here observed that genetic MBL ablation strongly sensitizes mice to sterile liver injury induced by carbon tetrachloride (CCl₄ ). Aggravated liver damage was shown in CCl₄ -administrated MBL^-/- mice, as evidenced by severe hepatocyte death, elevated serum alanine aminotransferase and lactate dehydrogenase activity, and enhanced production of inflammatory cytokines. Mechanistic studies established that MBL deficiency caused increased chemokine CXCL2 production from liver macrophages upon CCl₄ stimulation, thereby promoting the hepatic recruitment of neutrophils and subsequent liver damage. Furthermore, MBL-mediated protection from CCl₄ -induced liver injury was validated by administration of an MBL-expressing liver-specific adeno-associated virus, which effectively ameliorated the hepatic damage in CCl4-treated MBL^-/- mice. We propose that MBL may be exploited as a new therapeutic approach in the treatment of chemical-induced sterile liver injury in patients with MBL deficiency.

Mannan-binding lectin suppresses growth of hepatocellular carcinoma by regulating hepatic stellate cell activation via the ERK/COX-2/PGE2 pathway

Mannan binding lectin (MBL), initially known to activate the complement lectin pathway and defend against infection, was recently shown to be potentially involved in the development of several types of cancer; however, its exact role in cancers, especially its effect on tumor microenvironment remain largely unknown. Here, using a murine hepatocellular carcinoma (HCC) model, we showed that MBL was a component of liver microenvironment and MBL-deficient (MBL^-/-) mice exhibited an enhanced tumor growth compared with wild-type (WT) mice. This phenomenon was associated with elevation of myeloid derived suppressed cells (MDSCs) in tumor tissue of MBL^-/- mice. MBL deficiency also resulted in an increase of activated hepatic stellate cells (HSCs), which showed enhanced cyclooxygenase-2 (COX-2) expression and prostaglandin E2 (PGE₂) production. Pharmacological inhibition of COX-2 in vivo partially abrogated the MBL deficiency-promoted tumor growth and MDSC accumulation. Mechanistic studies revealed that MBL could interact directly with HSCs and inhibit HCC-induced HSCs activation via downregulating the extracellular signal-regulated kinase (ERK)/COX-2/PGE₂ signaling pathway. Furthermore, MBL-mediated suppression of HCC is validated by administration of MBL-expressing, liver-specific adeno-associated virus (AAV), which significantly inhibited HCC progression in MBL^-/- mice. Taken together, these data reveal that MBL may impact on tumor development by shaping the tumor microenvironment via its interaction with the local stromal cells, and also suggests its potential therapeutic use for the treatment of HCC.

Integration of Innate Immune Signaling

The last decades of research in innate immunology have revealed a multitude of sensing receptors that evaluate the presence of microorganisms or cellular damage in tissues. In the context of a complex tissue, many such sensing events occur simultaneously. Thus, the downstream pathways need to be integrated to launch an appropriate cellular response, to tailor the magnitude of the reaction to the inciting event, and to terminate it in a manner that avoids immunopathology. Here, we provide a conceptual overview of the crosstalk between innate immune receptors in the initiation of a concerted immune reaction to microbial and endogenous triggers. We classify the known interactions into categories of communication and provide examples of their importance in pathogenic infection.