The Influenza-Induced Pulmonary Inflammatory Exudate in Susceptible Tpl2-Deficient Mice Is Dictated by Type I IFN Signaling

TPL2 kinase activity is required for Il1b transcription during LPS priming but dispensable for NLRP3 inflammasome activation

The NLRP3 inflammasome complex is an important mechanism for regulating the release of pro-inflammatory cytokines, IL-1β and IL-18, in response to harmful pathogens. Overproduction of pro-inflammatory cytokines has been linked to cryopyrin-associated periodic syndrome, arthritis, and other inflammatory conditions. It has been previously shown that tumor progression locus 2, a serine-threonine kinase, promotes IL-1β synthesis in response to LPS stimulation; however, whether TPL2 kinase activity is required during inflammasome priming to promote Il1b mRNA transcription and/or during inflammasome activation for IL-1β secretion remained unknown. In addition, whether elevated type I interferons, a consequence of either Tpl2 genetic ablation or inhibition of TPL2 kinase activity, decreases IL-1β expression or inflammasome function has not been explored. Using LPS-stimulated primary murine bone marrow-derived macrophages, we determined that TPL2 kinase activity is required for transcription of Il1b, but not Nlrp3, Il18, caspase-1 (Casp1), or gasdermin-D (Gsdmd) during inflammasome priming. Both Casp1 and Gsdmd mRNA synthesis decreased in the absence of type I interferon signaling, evidence of crosstalk between type I interferons and the inflammasome. Our results demonstrate that TPL2 kinase activity is differentially required for the expression of inflammasome precursor cytokines and components but is dispensable for inflammasome activation. These data provide the foundation for the further exploration of TPL2 kinase inhibitor as a potential therapeutic in inflammatory diseases.

Aspartyl peptidase May1 induces host inflammatory response by altering cell wall composition in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans causes cryptococcal meningoencephalitis, a disease that kills more than 180,000 people annually. Contributing to its success as a fungal pathogen is its cell wall surrounded by a capsule. When the cryptococcal cell wall is compromised, exposed pathogen-associated molecular pattern molecules (PAMPs) could trigger host recognition and initiate attack against this fungus. Thus, cell wall composition and structure are tightly regulated. The cryptococcal cell wall is unusual in that chitosan, the acetylated form of chitin, is predominant over chitin and is essential for virulence. Recently, it was shown that acidic pH weakens the cell wall and increases exposure of PAMPs partly due to decreased chitosan levels. However, the molecular mechanism responsible for the cell wall remodeling in acidic pH is unknown. In this study, by screening for genes involved in cryptococcal tolerance to high levels of CO2, we serendipitously discovered that the aspartyl peptidase May1 contributes to cryptococcal sensitivity to high levels of CO2 due to acidification of unbuffered media. Overexpression of MAY1 increases the cryptococcal cell size and elevates PAMP exposure, causing a hyper-inflammatory response in the host while MAY1 deletion does the opposite. We discovered that May1 weakens the cell wall and reduces the chitosan level, partly due to its involvement in the degradation of Chs3, the sole chitin synthase that supplies chitin to be converted to chitosan. Consistently, overexpression of CHS3 largely rescues the phenotype of MAY1oe in acidic media. Collectively, we demonstrate that May1 remodels the cryptococcal cell wall in acidic pH by reducing chitosan levels through its influence on Chs3.

IMPORTANCE

The fungal cell wall is a dynamic structure, monitoring and responding to internal and external stimuli. It provides a formidable armor to the fungus. However, in a weakened state, the cell wall also triggers host immune attack when PAMPs, including glucan, chitin, and mannoproteins, are exposed. In this work, we found that the aspartyl peptidase May1 impairs the cell wall of Cryptococcus neoformans and increases the exposure of PAMPs in the acidic environment by reducing the chitosan level. Under acidic conditions, May1 is involved in the degradation of the chitin synthase Chs3, which supplies chitin to be deacetylated to chitosan. Consistently, the severe deficiency of chitosan in acidic pH can be rescued by overexpressing CHS3. These findings improve our understanding of cell wall remodeling and reveal a potential target to compromise the cell wall integrity in this important fungal pathogen.

Interferon lambda in respiratory viral infection: immunomodulatory functions and antiviral effects in epithelium

Type III interferon (IFN-λ), a new member of the IFN family, was initially considered to possess antiviral functions similar to those of type I interferon, both of which are induced via the JAK/STAT pathway. Nevertheless, recent findings demonstrated that IFN-λ exerts a nonredundant antiviral function at the mucosal surface, preferentially produced in epithelial cells in contrast to type I interferon, and its function cannot be replaced by type I interferon. This review summarizes recent studies showing that IFN-λ inhibits the spread of viruses from the cell surface to the body. Further studies have found that the role of IFN-λ is not only limited to the abovementioned functions, but it can also can exert direct and/or indirect effects on immune cells in virus-induced inflammation. This review focuses on the antiviral activity of IFN-λ in the mucosal epithelial cells and its action on immune cells and summarizes the pathways by which IFN-λ exerts its action and differentiates it from other interferons in terms of mechanism. Finally, we conclude that IFN-λ is a potent epidermal antiviral factor that enhances the respiratory mucosal immune response and has excellent therapeutic potential in combating respiratory viral infections.