Insights on the Functional Role of Beta-Glucans in Fungal Immunity Using Receptor-Deficient Mouse Models

Nucleic acid sensing Toll-like receptors 3 and 9 play complementary roles in the development of bacteremia after nasal colonization associated with influenza co-infection

Streptococcus pneumoniae can cause mortality in infant, elderly, and immunocompromised individuals owing to invasion of bacteria to the lungs, the brain, and the blood. In building strategies against invasive infections, it is important to achieve greater understanding of how the pneumococci are able to survive in the host. Toll-like receptors (TLRs), critically important components in the innate immune system, have roles in various stages of the development of infectious diseases. Endosomal TLRs recognize nucleic acids of the pathogen, but the impact on the pneumococcal diseases of immune responses from signaling them remains unclear. To investigate their role in nasal colonization and invasive disease with/without influenza co-infection, we established a mouse model of invasive pneumococcal diseases directly developing from nasal colonization. TLR9 KO mice had bacteremia more frequently than wildtype in the pneumococcal mono-infection model, while the occurrence of bacteremia was higher among TLR3 KO mice after infection with influenza in advance of pneumococcal inoculation. All TLR KO strains showed poorer survival than wildtype after the mice had bacteremia. The specific and protective role of TLR3 and TLR9 was shown in developing bacteremia with/without influenza co-infection respectively, and all nucleic sensing TLRs would contribute equally to protecting sepsis after bacteremia.

Blocking DCIR mitigates colitis and prevents colorectal tumors by enhancing the GM-CSF-STAT5 pathway

Dendritic cell immunoreceptor (DCIR; Clec4a2), a member of the C-type lectin receptor family, plays important roles in homeostasis of the immune and bone systems. However, the intestinal role of this molecule is unclear. Here, we show that dextran sodium sulfate (DSS)-induced colitis and azoxymethane-DSS-induced intestinal tumors are reduced in Clec4a2^-/- mice independently of intestinal microbiota. STAT5 phosphorylation and expression of Csf2 and tight junction genes are enhanced, while Il17a and Cxcl2 are suppressed in the Clec4a2^-/- mouse colon, which exhibits reduced infiltration of neutrophils and myeloid-derived suppressor cells. Granulocyte-macrophage colony-stimulating factor (GM-CSF) administration ameliorates DSS colitis associated with reduced Il17a and enhanced tight junction gene expression, whereas anti-GM-CSF exacerbates symptoms. Furthermore, anti-NA2, a ligand for DCIR, ameliorates colitis and prevents colorectal tumors. These observations indicate that blocking DCIR signaling ameliorates colitis and suppresses colonic tumors, suggesting DCIR as a possible target for the treatment of these diseases.

Yeast β-glucan increases etoposide sensitivity in lung cancer cell line A549 by suppressing Nrf2 via the non-canonical NF-κB pathway

Etoposide is regarded as one of the main standard cytotoxic drugs for lung cancer. However, mutations in Keap1, the main regulator of nuclear factor erythroid 2-related factor 2 (Nrf2), are often detected in lung cancer and lead to chemoresistance. Since the aberrant activation of Nrf2 enhances drug resistance, the suppression of the Nrf2 pathway is a promising therapeutic strategy for lung cancer. We herein used the human lung adenocarcinoma cell line A549 because it harbors a Keap1 loss-of-function mutation. A treatment with β-glucan, a major component of the fungal cell wall, reduced Nrf2 protein levels, down-regulated the expression of CYP3A5, UGT1A1, and MDR1, and increased etoposide sensitivity in A549 cells. Furthermore, the ephrin type-A receptor 2 (EphA2) receptor was important for the recognition and biological activity of β-glucan in A549 cells. EphA2 signaling includes nuclear factor kappa B (NF-κB), STAT3, and p38 mitogen-activated protein kinase (MAPK). However, treatment of cells with stattic (STAT3 inhibitor) or SB203580 (p38 MAPK inhibitor) did not diminish the effects of β-glucan. In contrast, knockdown of RelB abolished the effects of β-glucan, suggesting the involvement of the non-canonical NF-κB pathway. The β-glucan effects were also attenuated by the knockdown of WDR23. The β-glucan treatment and RelB overexpression induced the expression of CUL4A, which increased WDR23 ligase activity and promoted the subsequent depletion of Nrf2. These results revealed a novel property of β-glucan as a resistance-modifying agent in addition to its widely reported immunomodulatory effects for lung cancer therapy via the EphA2-RelB-CUL4A-Nrf2 axis. Significance Statement Chemotherapeutic resistance remains a major obstacle in cancer therapy despite extensive efforts to elucidate the underlying molecular mechanisms and overcome multidrug resistance. The present study revealed a novel resistance-modifying property of β-glucan, thereby expanding our knowledge on the beneficial roles of β-glucan and providing an alternative strategy to prevent drug resistance by cancer. The present results provide evidence for the involvement of a novel mode of NF-κB and Nrf2 crosstalk in the drug resistance phenotype.

The C-type lectin receptor Clec1A plays an important role in the development of experimental autoimmune encephalomyelitis by enhancing antigen presenting ability of dendritic cells and inducing inflammatory cytokine IL-17

Clec1A, a member of C-type lectin receptor family, has a carbohydrate recognition domain in its extracellular region, but no known signaling motif in the cytoplasmic domain. Clec1a is highly expressed in endothelial cells and weakly in dendritic cells. Although this molecule was reported to play an important role in the host defense against Aspergillus fumigatus by recognizing 1,8-dihydroxynaphthalene-melanin on the fungal surface, the roles of this molecule in un-infected animals remain to be elucidated. In this study, we found that Clec1a^−/− mice develop milder symptoms upon induction of experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. The maximum disease score was significantly lower, and demyelination and inflammation of the spinal cord were much milder in Clec1a^−/− mice compared to wild-type mice. No abnormality was detected in the immune cell composition in the draining lymph nodes and spleen on day 10 and 16 after EAE induction. Recall memory T cell proliferation after restimulation with myelin oligodendrocyte glycoprotein peptide (MOG_35–55) in vitro was decreased in Clec1a^−/− mice, and antigen presenting ability of Clec1a^−/− dendritic cells was impaired. Interestingly, RNA-Seq and RT-qPCR analyses clearly showed that the expression of inflammatory cytokines including Il17a, Il6 and Il1b was greatly decreased in Clec1a^−/− mice after induction of EAE, suggesting that this reduced cytokine production is responsible for the amelioration of EAE in Clec1a^−/− mice. These observations suggest a novel function of Clec1A in the immune system.

β-Glucan Induces Distinct and Protective Innate Immune Memory in Differentiated Macrophages

Bacterial infections are a common and deadly threat to vulnerable patients. Alternative strategies to fight infection are needed. β-Glucan, an immunomodulator derived from the fungal cell wall, provokes resistance to infection by inducing trained immunity, a phenomenon that persists for weeks to months. Given the durability of trained immunity, it is unclear which leukocyte populations sustain this effect. Macrophages have a life span that surpasses the duration of trained immunity. Thus, we sought to define the contribution of differentiated macrophages to trained immunity. Our results show that β-glucan protects mice from Pseudomonas aeruginosa infection by augmenting recruitment of innate leukocytes to the site of infection and facilitating local clearance of bacteria, an effect that persists for more than 7 d. Adoptive transfer of macrophages, trained using β-glucan, into naive mice conferred a comparable level of protection. Trained mouse bone marrow-derived macrophages assumed an antimicrobial phenotype characterized by enhanced phagocytosis and reactive oxygen species production in parallel with sustained enhancements in glycolytic and oxidative metabolism, increased mitochondrial mass, and membrane potential. β-Glucan induced broad transcriptomic changes in macrophages consistent with early activation of the inflammatory response, followed by sustained alterations in transcripts associated with metabolism, cellular differentiation, and antimicrobial function. Trained macrophages constitutively secreted CCL chemokines and robustly produced proinflammatory cytokines and chemokines in response to LPS challenge. Induction of the trained phenotype was independent of the classic β-glucan receptors Dectin-1 and TLR-2. These findings provide evidence that β-glucan induces enhanced protection from infection by driving trained immunity in macrophages.

Synthesis and Study of Antifungal Properties of New Cationic Beta-Glucan Derivatives

The interaction of positively charged polymers (polycations) with a biological membrane is considered to be the cause of the frequently observed toxicity of these macromolecules. If it is possible to obtain polymers with a predominantly negative effect on bacterial and fungal cells, such systems would have great potential in the treatment of infectious diseases, especially now when reports indicate the growing risk of fungal co-infections in COVID-19 patients. We describe in this article cationic derivatives of natural beta-glucan polymers obtained by reacting the polysaccharide isolated from Saccharomyces boulardii (SB) and Cetraria islandica (CI) with glycidyl trimethyl ammonium chloride (GTMAC). Two synthesis strategies were applied to optimize the product yield. Fungal diseases particularly affect low-income countries, hence the emphasis on the simplicity of the synthesis of such drugs so they can be produced without outside help. The three structures obtained showed selective anti-mycotic properties (against, i.e., Scopulariopsis brevicaulis, Aspergillus brasiliensis, and Fusarium solani), and their toxicity established using fibroblast 3T3-L1 cell line was negligible in a wide range of concentrations. For one of the polymers (SB derivative), using in vivo model of Aspergillus brasiliensis infection in Galleria mellonella insect model, we confirmed the promising results obtained in the preliminary study.