Regulation of cytokine expression in mice immunized with cryptococcal polysaccharide, a glucuronoxylomannan (GXM), associated with peritoneal antigen-presenting cells (APC): requirements for GXM, APC activation, and interleukin-12

Involvement of the capsular GalXM-induced IL-17 cytokine in the control of Cryptococcus neoformans infection

Cryptococcus neoformans is an opportunistic fungus that can cause lethal brain infections in immunosuppressed individuals. Infection usually occurs via the inhalation of a spore or desiccated yeast which can then disseminate from the lung to the brain and other tissues. Dissemination and disease is largely influence by the production of copious amounts of cryptococcal polysaccharides, both which are secreted to the extracellular environment or assembled into a thick capsule surrounding the cell body. There are two important polysaccharides: glucuronoxylomannan (GXM) and galactoxylomannan, also called as glucuronoxylomanogalactan (GXMGal or GalXM). Although GXM is more abundant, GalXM has a more potent modulatory effect. In the present study, we show that GalXM is a potent activator of murine dendritic cells, and when co-cultured with T cells, induces a Th17 cytokine response. We also demonstrated that treating mice with GalXM prior to infection with C. neoformans protects from infection, and this phenomenon is dependent on IL-6 and IL-17. These findings help us understand the immune biology of capsular polysaccharides in fungal pathogenesis.

The capsule of the fungal pathogen Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans has been studied extensively in recent decades and a large body of information is now available to the scientific community. Well-known aspects of the capsule include its structure, antigenic properties and its function as a virulence factor. The capsule is composed primarily of two polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), in addition to a smaller proportion of mannoproteins (MPs). Most of the studies on the composition of the capsule have focused on GXM, which comprises more than 90% of the capsule's polysaccharide mass. It is GalXM, however, that is of particular scientific interest because of its immunological properties. The molecular structure of these polysaccharides is very complex and has not yet been fully elucidated. Both GXM and GalXM are high molecular mass polymers with the mass of GXM equaling roughly 10 times that of GalXM. Recent findings suggest, however, that the actual molecular weight might be different to what it has traditionally been thought to be. In addition to their structural roles in the polysaccharide capsule, these molecules have been associated with many deleterious effects on the immune response. Capsular components are therefore considered key virulence determinants in C. neoformans, which has motivated their use in vaccines and made them targets for monoclonal antibody treatments. In this review, we will provide an update on the current knowledge of the C. neoformans capsule, covering aspects related to its structure, synthesis and particularly, its role as a virulence factor.

Cryptococcus neoformans activates bone marrow-derived conventional dendritic cells rather than plasmacytoid dendritic cells and down-regulates macrophages

Induction of IL-12 and IL-23 is essential for protective immunity against Cryptococcusneoformans. The contribution of dendritic cells vs. macrophages to IL-12/23 production in response to C. neoformans infection is unclear. Activation of conventional bone marrow-derived dendritic cells (BMDC), plasmacytoid BMDC, and bone marrow-derived macrophages (BMMPhi) was assessed by analyzing cytokine responses and the expression of MHC-II, CD86, and CD80 in each cell type. Cryptococcus neoformans induced the release of IL-12/23p40 by BMDC, but not by BMMPhi, in a TLR2- and TLR4-independent but MyD88-dependent manner. Conventional BMDC rather than plasmacytoid BMDC up-regulated MHC-II and CD86, while BMMPhi down-regulated MHC-II and CD86 in response to C. neoformans. The up-regulation of MHC-II and CD86 on BMDC required MyD88. Our data point to conventional DC as critical IL-12/23-producing antigen-presenting cells during cryptococcosis.

Macrophage activation by polysaccharide biological response modifier isolated from Aloe vera L. var. chinensis (Haw.) Berg

A mannose-rich polysaccharide biological response modifier (BRM), derived from Aloe vera L. var. chinensis (Haw.) Berg., was demonstrated to be a potent murine B- and T-cell stimulator in our previous study. We here report the stimulatory activity of PAC-I on murine peritoneal macrophage. The polysaccharide when injected into mice enhanced the migration of macrophages to the peritoneal cavity. Peritoneal macrophage when treated by PAC-I in vitro had increased expression of MHC-II and FcgammaR, and enhanced endocytosis, phagocytosis, nitric oxide production, TNF-alpha secretion and tumor cell cytotoxicity. The administration of PAC-I into allogeneic ICR mice stimulated systemic TNF-alpha production in a dose-dependent manner and prolonged the survival of tumor-bearing mice. PAC-I is thus a potent stimulator of murine macrophage and the in vitro observed tumoricidal properties of activated macrophage might account for the in vivo antitumor properties of PAC-I. Our research findings may have therapeutic implications in tumor immunotherapy.

Feasibility and prospects for a vaccine to prevent cryptococcosis

Cryptococcosis is a relatively common fungal disease caused by Cryptococcus neoformans that has high morbidity and mortality. Numerous studies have established the feasibility of enhancing host immunity to C neoformans in naive immunocompetent animal models by vaccination. Several antigens have been identified that appear to be suitable vaccine candidates. Induced immune responses can mediate protection through both humoral and cellular immunity. Hence, a vaccine against cryptococcosis in humans is probably feasible but there are significant obstacles to vaccine development that range from uncertainties about the pathogenesis of infection to economic considerations.

Role of interleukin-4 in resistance to Cryptococcus neoformans infection

The role of interleukin (IL)-4 in cryptococcal disease was studied in IL-4 knockout (IL-4KO) and wild-type (WT) mice infected with Cryptococcus neoformans isolates that vary widely in their virulence. Delayed-type hypersensitivity responses were reduced in IL-4KO mice following primary infection with either isolate. Splenic T helper 1 (Th1) cytokine responses were increased in the IL-4KO mice infected with the weakly virulent isolate (184A) but did not change during infection with the highly virulent isolate (NU-2). Th2 cytokine responses (IL-5, IL-10) were downregulated in the IL-4KO mice infected with either isolate. Survival after primary infection with either isolate was not influenced by the absence of IL-4. Fewer colony-forming units were found in the lungs of 184A-infected, IL-4KO mice as compared to WT mice, suggesting that some immunity had developed. IL-4KO mice, primed with small doses of cryptococcal antigen (CneF), had significantly enhanced delayed-type hypersensitivity responses after intravenous infection with 184A and were more resistant to infection compared with WT mice. Increased expression of IL-5 with decreased interferon-gamma contributed to the inability of primed WT mice to resist infection with 184A. Enhanced immunity in the primed IL-4KO mice was reflected in a more moderate increase in IL-5 and IL-10 with maintenance of interferon-gamma levels.

Glucuronoxylomannan of Cryptococcus neoformans exacerbates in vitro yeast cell growth by interleukin 10-dependent inhibition of CD4+ T lymphocyte responses

Glucuronoxylomannan (GXM), the major capsular polysaccharide of Cryptococcus neoformans, is the most important virulence factor of this fungus. We analyzed the molecular events related to protective immune responses against a non-encapsulated strain of C. neoformans, mediated by murine splenic CD4(+) T lymphocytes in vitro, and the impact of GXM addition upon these events. Both the lymphoproliferation of CD4(+) T cells and the control of fungus growth were dependent on B7 co-stimulation. Addition of GXM did not modify CD4(+) T cell proliferation, but exacerbated infection in cultures obtained from normal and infected hosts. GXM enhanced the secretion of IL-10 and IL-4, while it reduced the production of pro-inflammatory cytokines TNF-alpha and IFN-gamma. The blockade of IL-10 activity with neutralizing antibodies increased TNF-alpha production and reduced yeast cell growth. The findings suggest that GXM exacerbates infection by down-regulating cell-mediated protective immune response and that IL-10 is implicated in yeast evasion.

Roles for CD40, B7 and major histocompatibility complex in induction of enhanced immunity by cryptococcal polysaccharide-pulsed antigen-presenting cells

Immunization of mice with activated antigen-presenting cells (APC) pulsed ex vivo with cryptococcal capsular polysaccharide, a glucuronoxylomannan (GXM-APC) results in prolongation of survival and delayed-type hypersensitivity (DTH) responsiveness following infection with Cryptococcus neoformans (NU-2). GXM-APC has both non-specific and GXM-specific effects that influence the immune responses that develop in mice after infection with NU-2. Type 1 cytokine responses are augmented after immunization with APC alone, while GXM must be present for the vaccine to influence survival and DTH reactions. This investigation evaluated the role that major histocompatibility complex (MHC) and co-stimulatory molecules play in the non-specific and GXM-specific responses induced by GXM-APC. APC from CD40 knockout mice were as effective as wild-type APC for the induction of non-specific and GXM-specific responses. Blocking activity of B7-1 and B7-2 by treatment of immunized mice with monoclonal antibodies specific for these molecules just before and for 6 days following GXM-APC immunization decreased the splenic interferon-gamma response of mice subsequently infected with NU-2, but only in mice that were treated with both antibodies. These antibody treatments had no effect on DTH reactivity in similarly treated animals. MHC class I molecules were not involved in the antigen non-specific or GXM-specific activities of the vaccine. MHC class II molecules were not required for augmentation of type 1 cytokine responses but were needed for induction of the GXM-specific response that regulates the expression of DTH reactivity. This investigation has shown that an MHC class II-restricted, GXM-specific response is responsible for altering DTH responsiveness which is the correlate of immunity in this model.