Direct Inhibition of T-Cell Responses by the Cryptococcus Capsular Polysaccharide Glucuronoxylomannan

The major virulence factor of the pathogenic fungi Cryptococcus neoformans and C. gattii is the capsule. Glucuronoxylomannan (GXM), the major component of the capsule, is a high-molecular-weight polysaccharide that is shed during cryptococcosis and can persist in patients after successful antifungal therapy. Due to the importance of T cells in the anticryptococcal response, we studied the effect of GXM on the ability of dendritic cells (DCs) to initiate a T-cell response. GXM inhibited the activation of cryptococcal mannoprotein-specific hybridoma T cells and the proliferation of OVA-specific OT-II T cells when murine bone marrow-derived DCs were used as antigen-presenting cells. Inhibition of OT-II T-cell proliferation was observed when either OVA protein or OVA323-339 peptide was used as antigen, indicating GXM did not merely prevent antigen uptake or processing. We found that DCs internalize GXM progressively over time; however, the suppressive effect did not require DCs, as GXM directly inhibited T-cell proliferation induced by anti-CD3 antibody, concanavalin A, or phorbol-12-myristate-13-acetate/ionomycin. Analysis of T-cell viability revealed that the reduced proliferation in the presence of GXM was not the result of increased cell death. GXM isolated from each of the four major cryptococcal serotypes inhibited the proliferation of human peripheral blood mononuclear cells stimulated with tetanus toxoid. Thus, we have defined a new mechanism by which GXM can impart virulence: direct inhibition of T-cell proliferation. In patients with cryptococcosis, this could impair optimal cell-mediated immune responses, thereby contributing to the persistence of cryptococcal infections. SynopsisInfections due to the pathogenic yeast Cryptococcus are a significant cause of morbidity and mortality in persons with impaired T-cell functions, particularly those with AIDS. The major virulence factor of Cryptococcus is its capsule, which is composed primarily of the polysaccharide glucuronoxylomannan (GXM). The capsule not only surrounds the organism but also is shed during cryptococcosis. GXM is taken up by macrophages in vitro and in vivo; however, little is known about the interaction between GXM and dendritic cells, which are the most potent cells capable of activating T cells. Because of the importance of T cells in the anticryptococcal response, the authors investigated the effect of GXM on the ability of dendritic cells to initiate a T-cell response. They found the polysaccharide was internalized by dendritic cells and inhibited antigen-specific T-cell responses. Furthermore, GXM had a direct, inhibitory effect on T-cell proliferation, independent of the effect on dendritic cells. These findings may help explain the persistence of cryptococcal infections and suggest that GXM could be therapeutic in situations where suppression of T-cell responses is desired.National Institutes of Health (R01 AI25780, R01 AI066087, R01 AI37532

Cooperative Stimulation of Dendritic Cells by Cryptococcus neoformans Mannoproteins and CpG Oligodeoxynucleotides

While mannosylation targets antigens to mannose receptors on dendritic cells (DC), the resultant immune response is suboptimal. We hypothesized that the addition of toll-like receptor (TLR) ligands would enhance the DC response to mannosylated antigens. Cryptococcus neoformans mannoproteins (MP) synergized with CpG-containing oligodeoxynucleotides to stimulate enhanced production of proinflammatory cytokines and chemokines from murine conventional and plasmacytoid DC. Synergistic stimulation required the interaction of mannose residues on MP with the macrophage mannose receptor (MR), CD206. Moreover, synergy with MP was observed with other TLR ligands, including tripalmitoylated lipopeptide (Pam3CSK4), polyinosine-polycytidylic acid (pI:C), and imiquimod. Finally, CpG enhanced MP-specific MHC II-restricted CD4+ T-cell responses by a mechanism dependent upon DC expression of CD206 and TLR9. These data suggest a rationale for vaccination strategies that combine mannosylated antigens with TLR ligands and imply that immune responses to naturally mannosylated antigens on pathogens may be greatly augmented if TLR and MR are cooperatively stimulated.National Institutes of Health (RO1 AI25780, RO1 AI37532, K08 AI 53542

Journal Club discussion of Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study

This presentation presents the following journal article about COVID-19 for analysis and discussion: Zhou F et al. Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study. Lancet. 2020 Mar 28;395(10229):1054-1062. doi: 10.1016/S0140-6736(20)30566-3

The Case for Adopting the Species Complex Nomenclature for the Etiologic Agents of Cryptococcosis

Cryptococcosis is a potentially lethal disease of humans/animals caused by Cryptococcus neoformans and Cryptococcus gattii. Distinction between the two species is based on phenotypic and genotypic characteristics. Recently, it was proposed that C. neoformans be divided into two species and C. gattii into five species based on a phylogenetic analysis of 115 isolates. While this proposal adds to the knowledge about the genetic diversity and population structure of cryptococcosis agents, the published genotypes of 2,606 strains have already revealed more genetic diversity than is encompassed by seven species. Naming every clade as a separate species at this juncture will lead to continuing nomenclatural instability. In the absence of biological differences between clades and no consensus about how DNA sequence alone can delineate a species, we recommend using Cryptococcus neoformans species complex and C. gattii species complex as a practical intermediate step, rather than creating more species. This strategy recognizes genetic diversity without creating confusion

Interplay of anisotropy in shape and interactions in charged platelet suspensions

Motivated by the intriguing phase behavior of charged colloidal platelets, we investigate the structure and dynamics of charged repulsive disks by means of Monte-Carlo simulations. The electrostatic interactions are taken into account through an effective two-body potential, obtained within the non-linear Poisson-Boltzmann formalism, which has the form of anisotropic screened Coulomb potential. Recently, we showed that the original intrinsic anisotropy of the electrostatic potential in competition with excluded volume effects leads to a rich phase behavior that not only includes various liquid-crsytalline phases but also predicts the existence of novel structures composed of alternating nematic-antinematic sheets. Here, we examine the structural and dynamical signatures of each of the observed structures for both translational and rotational degrees of freedom. Finally, we discuss the influence of effective charge value and our results in relation to experimental findings on charged platelet suspensions.Comment: 22 pages, 17 figure

The Case for Adopting the Species Complex Nomenclature for the Etiologic Agents of Cryptococcosis

Cryptococcosis is a potentially lethal disease of humans/animals caused by Cryptococcus neoformans and Cryptococcus gattii. Distinction between the two species is based on phenotypic and genotypic characteristics. Recently, it was proposed that C. neoformans be divided into two species and C. gattii into five species based on a phylogenetic analysis of 115 isolates. While this proposal adds to the knowledge about the genetic diversity and population structure of cryptococcosis agents, the published genotypes of 2,606 strains have already revealed more genetic diversity than is encompassed by seven species. Naming every clade as a separate species at this juncture will lead to continuing nomenclatural instability. In the absence of biological differences between clades and no consensus about how DNA sequence alone can delineate a species, we recommend using Cryptococcus neoformans species complex and C. gattii species complex as a practical intermediate step, rather than creating more species. This strategy recognizes genetic diversity without creating confusion

Innate sensing of chitin and chitosan

Chitin is the second most common polysaccharide found in nature. It is present in crustacean shells, insect exoskeletons, parasitic nematode eggs and gut linings, and in the cell wall of fungi. The deacetylated derivative of chitin, chitosan, is less common but is particularly evident in certain species of fungi, such as Cryptococcus, and the cyst wall of Entamoeba. How mammals sense and respond to these polymers is not well understood, and conflicting reports on their immunological activity have led to some controversy. Despite this, promising translational applications that exploit the unique properties of chitin and chitosan are being developed