Induction of interleukin-6 mRNA in rat alveolar macrophages byin vitroexposure to bothCryptococcus neoformansand anti-C. neoformansantiserum

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.

Differential activation of peritoneal cells by subcutaneous treatment of rats with cryptococcal antigens

Previous studies in our laboratory have shown that the subcutaneous pretreatment of rats with heat-killed cells (HKC) of Cryptococcus neoformans emulsified in complete Freund adjuvant (CFA) promotes protective immunity against an intraperitoneal challenge with C. neoformans. In contrast, subcutaneous treatment with the capsular polysaccharide (PSC) emulsified in CFA exacerbates the cryptococcal infection. The purpose of this study was to analyze the mechanisms involved in these phenomena. Adherent peritoneal cells from rats treated with HKC-CFA showed upregulated ED2, CD80, and CD86 expression; an increase in the level of production of anticryptococcal metabolites; and the enhanced production of interleukin-12 (IL-12) in comparison with the findings for cells from rats treated with CFA-phosphate-buffered saline (PBS). Adherent peritoneal cells from rats treated with PSC-CFA, however, also presented upregulated ED2, CD80, and CD86 expression compared to the level of expression for peritoneal cells from controls, but these cells showed an increase in arginase activity and decreased levels of production of IL-12 and tumor necrosis factor (TNF) compared with the activity and levels of production by peritoneal cells from CFA-PBS-treated rats. In addition, treatment with HKC-CFA resulted in a rise in the phagocytic and anticryptococcal activities of adherent peritoneal cells compared to those for control rats. However, adherent peritoneal cells from rats treated with PSC-CFA presented a reduction in anticryptococcal activity in comparison with that for cells from animals treated with CFA-PBS. These results show the differential activation between adherent peritoneal cells from HKC-CFA- and PSC-CFA-treated rats, with this differential activation at the primary site of infection possibly being responsible, at least in part, for the phenomena of protection and exacerbation observed in our model.

Therapeutic efficacy of a conjugate vaccine containing a peptide mimotope of cryptococcal capsular polysaccharide glucuronoxylomannan

Vaccination with P13, a peptide mimotope of the cryptococcal capsular polysaccharide glucuronoxylomannan (GXM), has been shown to confer protection against a subsequent lethal Cryptococcus neoformans challenge. In this study, we sought to investigate whether P13-based vaccines could be effective in an already-established infection. To address this question, we developed a systemic chronic cryptococcal infection model. We vaccinated chronically infected mice with P13-protein conjugates and monitored their survival. Compared to the controls, the conjugates prolonged the survival of chronically infected mice. The degree of protection was a function of the mouse strain (BALB/c or C57BL/6), the carrier protein (tetanus toxoid or diphtheria toxoid), and the route of infection (intraperitoneal or intravenous). Serum GXM levels were correlated with the day of death, but the correlation was driven by the carrier protein and mouse strain. The passive transfer of heat-treated sera from P13 conjugate-vaccinated mice conferred protection to naïve BALB/c mice, indicating that antibody immunity could contribute to protection. The measurement of peripheral blood cytokine (gamma interferon [IFN-gamma], interleukin-10 [IL-10], and IL-6) gene expression showed that P13 conjugate-vaccinated BALB/c and C57BL/6 mice mounted a strong Th2 (IL-10)-like response relative to the Th1 (IFN-gamma)-like response, with the degree depending on the mouse strain and carrier protein. Taken together, our data suggest that a vaccine could hold promise in the setting of chronic cryptococcosis, and that vaccine efficacy could depend on immunomodulation and augmentation of the natural immune response of the host.

In vivo role of dendritic cells in a murine model of pulmonary cryptococcosis

Dendritic cells (DC) have been shown to phagocytose and kill Cryptococcus neoformans in vitro and are believed to be important for inducing protective immunity against this organism. Exposure to C. neoformans occurs mainly by inhalation, and in this study we examined the in vivo interactions of C. neoformans with DC in the lung. Fluorescently labeled live C. neoformans and heat-killed C. neoformans were administered intranasally to C57BL/6 mice. At specific times postinoculation, mice were sacrificed, and lungs were removed. Single-cell suspensions of lung cells were prepared, stained, and analyzed by microscopy and flow cytometry. Within 2 h postinoculation, fluorescently labeled C. neoformans had been internalized by DC, macrophages, and neutrophils in the mouse lung. Additionally, lung DC from mice infected for 7 days showed increased expression of the maturation markers CD80, CD86, and major histocompatibility complex class II. Finally, ex vivo incubation of lung DC from infected mice with Cryptococcus-specific T cells resulted in increased interleukin-2 production compared to the production by DC from naïve mice, suggesting that there was antigen-specific T-cell activation. This study demonstrated that DC in the lung are capable of phagocytosing Cryptococcus in vivo and presenting antigen to C. neoformans-specific T cells ex vivo, suggesting that these cells have roles in innate and adaptive pulmonary defenses against cryptococcosis.

Cryptococcus neoformans {alpha} strains preferentially disseminate to the central nervous system during coinfection

Cryptococcus neoformans is a fungal pathogen that has evolved over the past 40 million years into three distinct varieties or sibling species (gattii, grubii, and neoformans). Each variety manifests differences in epidemiology and disease, and var. grubii strains are responsible for the vast majority of human disease. In previous studies, alpha strains were more virulent than congenic a strains in var. neoformans, whereas var. grubii congenic a and alpha strains exhibited equivalent levels of virulence. Here the role of mating type in the virulence of var. grubii was further characterized in a panel of model systems. Congenic var. grubii a and alpha strains had equivalent survival rates when cultured with amoebae, nematodes, and macrophages. No difference in virulence was observed between a and alpha congenic strains in multiple inbred-mouse genetic backgrounds, and there was no difference in accumulations in the central nervous system (CNS) late in infection. In contrast, during coinfections, a and alpha strains are equivalent in peripheral tissues but alpha cells have an enhanced predilection to penetrate the CNS. These studies reveal the first virulence difference between congenic a and alpha strains in the most common pathogenic variety and suggest an explanation for the prevalence of alpha strains in clinical isolates.

Cytokine profiles of AIDS patients are similar to those of mice with disseminated Cryptococcus neoformans infection

Cryptococcosis is an hematogenously disseminated meningoencephalitis during which the relationship between the disease severity and the immune response remains unclear. We thus analyzed, by enzyme-linked immunosorbent assay, proinflammatory (tumor necrosis factor alpha [TNF-alpha] and interleukin-6 [IL-6]) and anti-inflammatory (IL-10) cytokine levels in plasma at the time of diagnosis in 51 AIDS patients with culture-proven cryptococcosis. We used a murine model to determine the correlation between cytokine levels and fungal burden in blood and tissues and the kinetics of the immune response and of the formation of cerebral lesions. In AIDS patients, plasma TNF-alpha and IL-10, but not IL-6, levels were significantly higher in the case of fungemia or disseminated infection than in their absence, whereas the presence of meningitis had no influence on these levels. In mice, none of these cytokines were detected within the first day after inoculation. Later on, TNF-alpha and IL-10, but not IL-6, levels in plasma correlated significantly with the fungal burden in the blood and spleen but not the brain. In the brain, cytokine levels were low compared to those in other compartments, and tissue lesions and a degree of infection similar to those observed in humans were seen, further suggesting the relevance of this experimental model. Thus, AIDS patients with cryptococcosis produce an immune response that reflects the dissemination but not the meningeal involvement. This murine model of disseminated cryptococcosis can be used to investigate the pathophysiology of cryptococcosis and new therapeutic approaches.