[Disseminated pulmonary infection due to Cryptococcus neoformans in a non immunocompromised patient]

Systemic Approach to Virulence Gene Network Analysis for Gaining New Insight into Cryptococcal Virulence

Cryptococcus neoformans is pathogenic yeast, responsible for highly lethal infections in compromised patients around the globe. C. neoformans typically initiates infections in mammalian lung tissue and subsequently disseminates to the central nervous system where it causes significant pathologies. Virulence genes of C. neoformans are being characterized at an increasing rate, however, we are far from a comprehensive understanding of their roles and genetic interactions. Some of these reported virulence genes are scattered throughout different databases, while others are not yet included. This study gathered and analyzed 150 reported virulence associated factors (VAFs) of C. neoformans. Using the web resource STRING database, our study identified different interactions between the total VAFs and those involved specifically in lung and brain infections and identified a new strain specific virulence gene, SHO1, involved in the mitogen-activated protein kinase signaling pathway. As predicted by our analysis, SHO1 expression enhanced C. neoformans virulence in a mouse model of pulmonary infection, contributing to enhanced non-protective immune Th2 bias and progressively enhancing fungal growth in the infected lungs. Sequence analysis indicated 77.4% (116) of total studied VAFs are soluble proteins, and 22.7% (34) are transmembrane proteins. Motifs involved in regulation and signaling such as protein kinases and transcription factors are highly enriched in Cryptococcus VAFs. Altogether, this study represents a pioneering effort in analysis of the virulence composite network of C. neoformans using a systems biology approach.

Macrophage M1/M2 polarization dynamically adapts to changes in cytokine microenvironments in Cryptococcus neoformans infection

The outcome of cryptococcal pneumonia correlates with local macrophage polarization status, as M1 and M2 polarization marks protective and nonprotective responses, respectively. Overall, pulmonary macrophage polarization status changes over time during a cryptococcal infection. This could have been caused by repolarization of individual macrophages or by a replacement of M2-polarized cells by new M1-polarized cells. To explore the ability of macrophages to change between polarization states, we conducted a series of experiments using in vitro macrophages. Coculture of macrophages with Cryptococcus neoformans resulted in development of a weak M1-like phenotype, with modestly increased inducible nitric oxide synthase (iNOS) but lacking interleukin 6 (IL-6) induction. The C. neoformans-induced M1-like polarization state was plastic, as macrophages stimulated first with C. neoformans and then with gamma interferon (IFN-γ) or IL-4 expressed mRNA polarization patterns similar to those stimulated with cytokines alone. To further evaluate macrophage polarization plasticity, cytokine stimulatory conditions were established which fully polarized macrophages. IFN-γ and IL-4 stimulation differentially induced complete M1 and M2 polarization, defined by differential expression of marker mRNA panels, surface marker expression, and tumor necrosis factor alpha (TNF-α) protein production. Switching IFN-γ- to IL-4-stimulating conditions, and vice versa, resulted in uniform changes in profiles of polarization marker genes consistent with the most recent cytokine environment. Furthermore, the ability of sequentially stimulated macrophages to inhibit C. neoformans reflected the most recent polarizing condition, independent of previous polarization. Collectively, these data indicate that M1/M2 macrophage polarization phenotypes are highly plastic to external signals, and interventions which therapeutically repolarize macrophages could be beneficial for treatment of cryptococcosis.

Chemokine receptor 2-mediated accumulation of fungicidal exudate macrophages in mice that clear cryptococcal lung infection

Clearance of pulmonary infection with the fungal pathogen Cryptococcus neoformans is associated with the accumulation and activation of lung macrophages. However, the phenotype of these macrophages and the mechanisms contributing to their accumulation are not well-defined. In this study, we used an established murine model of cryptococcal lung infection and flow cytometric analysis to identify alveolar macrophages (AMs) and the recently described exudate macrophages (ExMs). Exudate macrophages are distinguished from AMs by their strong expression of CD11b and major histocompatibility complex class II and modest expression of costimulatory molecules. Exudate macrophages substantially outnumber AMs during the effector phase of the immune response; and accumulation of ExMs, but not AMs, was chemokine receptor 2 (CCR2) dependent and attributable to the recruitment and subsequent differentiation of Ly-6C(high) monocytes originating from the bone marrow and possibly the spleen. Peak ExM accumulation in wild-type (CCR2(+/+)) mice coincided with maximal lung expression of mRNA for inducible nitric oxide synthase and correlated with the known onset of cryptococcal clearance in this strain of mice. Exudate macrophages purified from infected lungs displayed a classically activated effector phenotype characterized by cryptococcal-enhanced production of inducible nitric oxide synthase and tumor necrosis factor α. Cryptococcal killing by bone marrow-derived ExMs was CCR2 independent and superior to that of AMs. We conclude that clearance of cryptococcal lung infection requires the CCR2-mediated massive accumulation of fungicidal ExMs derived from circulating Ly-6C(high) monocytes.

Mechanisms of cryptococcal virulence and persistence

Cryptococcus neoformans is an environmental yeast that is a leading cause of fatal mycosis in AIDS patients and a major cause of meningoencephalitis and CNS-related mortality around the globe. Although C. neoformans infection is mostly a manifestation of immune deficiency, up to 25% of cases reported in the USA occur in patients without recognizable immune defects, indicating that C. neoformans can develop mechanisms that allow it to evade immune defenses and persist in noncompromised hosts. This article discusses mechanisms and routes of infection and the most important elements of host response as well as the mechanisms that promote cryptococcal survival within the host. Metabolic adaptation to physiological host conditions and the mechanisms limiting immune recognition, interfering with phagocytosis and extending intracellular survival of C. neoformans are highlighted. We describe the mechanisms by which C. neoformans can alter adaptive host responses, especially cell-mediated immunity, which is required for clearance of this microbe. We also review cryptococcal strategies of survival in the CNS and briefly discuss adaptations developing in response to medical treatment.

Dual roles of CD40 on microbial containment and the development of immunopathology in response to persistent fungal infection in the lung

Persistent pulmonary infection with Cryptococcus neoformans in C57BL/6 mice results in chronic inflammation that is characterized by an injurious Th2 immune response. In this study, we performed a comparative analysis of cryptococcal infection in wild-type versus CD40-deficient mice (in a C57BL/6 genetic background) to define two important roles of CD40 in the modulation of fungal clearance as well as Th2-mediated immunopathology. First, CD40 promoted microanatomic containment of the organism within the lung tissue. This protective effect was associated with: i) a late reduction in fungal burden within the lung; ii) a late accumulation of lung leukocytes, including macrophages, CD4+ T cells, and CD8+ T cells; iii) both early and late production of tumor necrosis factor-α and interferon-γ by lung leukocytes; and iv) early IFN-γ production at the site of T cell priming in the regional lymph nodes. In the absence of CD40, systemic cryptococcal dissemination was increased, and mice died of central nervous system infection. Second, CD40 promoted pathological changes in the airways, including intraluminal mucus production and subepithelial collagen deposition, but did not alter eosinophil recruitment or the alternative activation of lung macrophages. Collectively, these results demonstrate that CD40 helps limit progressive cryptococcal growth in the lung and protects against lethal central nervous system dissemination. CD40 also promotes some, but not all, elements of Th2-mediated immunopathology in response to persistent fungal infection in the lung.

Accumulation of CD11b+ lung dendritic cells in response to fungal infection results from the CCR2-mediated recruitment and differentiation of Ly-6Chigh monocytes

Pulmonary clearance of the encapsulated yeast Cryptococcus neoformans is associated with the CCR2-mediated accumulation of lung dendritic cells (DC) and the development of a T1 adaptive immune response. The objective of this study was to identify the circulating DC precursor(s) responsible for this large increase in lung DC numbers. An established murine model was used to evaluate putative DC precursors in the blood, bone marrow, and lungs of CCR2(+/+) mice and CCR2(-/-) mice throughout a time course following infection with C. neoformans. Results demonstrate that numbers of Ly-6C(high) monocytes increased in parallel in the peripheral blood and lungs of CCR(+/+) mice, whereas CD11c(+) MHC class II(+) pre-DC were 10-fold less prevalent in the peripheral blood and did not differ between the two strains. Accumulation of Ly-6C(high) monocytes correlated with a substantial increase in the numbers of CD11b(+) DC in the lungs of infected CCR2(+/+) mice. Comparative phenotypic analysis of lung cells recovered in vivo suggests that Ly-6C(high) monocytes differentiate into CD11b(+) DC in the lung; differentiation is associated with up-regulation of costimulatory molecules and decreased Ly-6C expression. Furthermore, in vitro experiments confirmed that Ly-6C(high) monocytes differentiate into CD11b(+) DC. Accumulation of Ly-6C(high) monocytes and CD11b(+) DC was not attributable to their proliferation in situ. We conclude that the CCR2-mediated accumulation of CD11b(+) DC in the lungs of Cryptococcus-infected mice is primarily attributable to the continuous recruitment and differentiation of Ly-6C(high) monocytes.

Robust Th1 and Th17 immunity supports pulmonary clearance but cannot prevent systemic dissemination of highly virulent Cryptococcus neoformans H99

The present study dissected the role of a Th2 bias in pathogenesis of Cryptococcus neoformans H99 infection by comparing inhalational H99 infections in wild-type BALB/c and IL-4/IL-13 double knockout mice. H99-infected wild-type mice showed all major hallmarks of Th2 but not Th1/Th17 immunity in the lungs and lung-associated lymph nodes. In contrast, the IL-4/13(-/-) mice developed robust hallmarks of Th1 and Th17 but not Th2 polarization. The IL-4/IL-13 deletion prevented pulmonary eosinophilia, goblet cell metaplasia in the airways and resulted in elevated serum IgE, and a switch from alternative to classical activation of macrophages. The development of a robust Th1/Th17 response and classical activation of macrophages resulted in significant containment of H99 in the lungs of IL-4/13(-/-) mice compared with unopposed growth of H99 in the lungs of wild-type mice. However, IL-4/13(-/-) mice showed only 1-week longer survival compared with wild-type mice. The comparison of brain and spleen cryptococcal loads at weeks 2, 3, and 4 postinfection revealed that the systemic dissemination in IL-4/13(-/-) mice occurred with an approximate 1-week delay but subsequently progressed with similar rate as in the wild-type mice. Furthermore, wild-type and IL-4/13(-/-) mice developed equivalently severe meningitis/encephalitis at the time of death. These data indicate that the Th2 immune bias is a crucial mechanism for pulmonary virulence of H99, whereas other mechanisms are largely responsible for its central nervous system tropism and systemic dissemination.

Cryptococcal urease promotes the accumulation of immature dendritic cells and a non-protective T2 immune response within the lung

Urease, a major virulence factor for Cryptococcus neoformans, promotes lethal meningitis/encephalitis in mice. The effect of urease within the lung, the primary site of most invasive fungal infections, is unknown. An established model of murine infection that utilizes either urease-producing (wt and ure1::URE1) or urease-deficient (ure1) strains (H99) of C. neoformans was used to characterize fungal clearance and the resultant immune response evoked by these strains within the lung. Results indicate that mice infected with urease-producing strains of C. neoformans demonstrate a 100-fold increase in fungal burden beginning 2 weeks post-infection (as compared with mice infected with urease-deficient organisms). Infection with urease-producing C. neoformans was associated with a highly polarized T2 immune response as evidenced by increases in the following: 1) pulmonary eosinophils, 2) serum IgE levels, 3) T2 cytokines (interleukin-4, -13, and -4 to interferon-gamma ratio), and 4) alternatively activated macrophages. Furthermore, the percentage and total numbers of immature dendritic cells within the lung-associated lymph nodes was markedly increased in mice infected with urease-producing C. neoformans. Collectively, these data define cryptococcal urease as a pulmonary virulence factor that promotes immature dendritic cell accumulation and a potent, yet non-protective, T2 immune response. These findings provide new insights into mechanisms by which microbial factors contribute to the immunopathology associated with invasive fungal disease.

Inheritance of immune polarization patterns is linked to resistance versus susceptibility to Cryptococcus neoformans in a mouse model

Genetic background variation between inbred strains accounts for different levels of susceptibility to Cryptococcus neoformans in the mouse infection model. To elucidate the inheritance of immunophenotypic traits and their associations with clearance outcomes during cryptococcal infection, we compared C57BL/6, BALB/c, and their first-generation hybrid, CB6F1 (F1), mice. Mice from each group were infected with C. neoformans (10(4) CFU) and analyzed at weekly intervals over a 6-week period. BALB/c mice progressively cleared the cryptococcal infection in the lungs and showed a Th1-skewed immune response: a Th1-shifted cytokine profile, modest lung pathology, and no significant elevation in the systemic immunoglobulin E (IgE) level. In contrast, C57BL/6 mice developed a chronic infection with a Th2-skewed immune response: a Th2-shifted cytokine profile, pulmonary eosinophilia, severe lung pathology, elevated serum IgE, fungemia, and cryptococcal dissemination in the central nervous system. F1 mice demonstrated intermediate resistance to C. neoformans, with a stronger resemblance to the immunophenotype of the resistant (BALB/c) mice. F1 mice also demonstrated enhanced pulmonary recruitment of lymphocytes, especially CD8(+) T cells, in comparison to both parental strains, suggesting positive heterosis. We conclude that the inheritance of traits responsible for early cytokine induction in the infected lungs and dendritic-cell maturation/activation status in draining nodes is responsible for the intermediate immune response polarization and clearance outcome observed initially in the lungs of F1 mice. The enhanced pulmonary lymphocyte recruitment could be responsible for a gradual shutdown of the undesirable Th2 arm of the immune response and subsequently improved anticryptococcal resistance in F1 mice.

Fungal Infections

The incidence of invasive fungal infections has increased dramatically over the past two decades, mostly due to an increase in the number of immunocompromised patients.1–4 Patients who undergo chemotherapy for a variety of diseases, patients with organ transplants, and patients with the acquired immune deficiency syndrome have contributed most to the increase in fungal infections.5 The actual incidence of invasive fungal infections in transplant patients ranges from 15% to 25% in bone marrow transplant recipients to 5% to 42% in solid organ transplant recipients.6,7 The most frequently encountered are Aspergillus species, followed by Cryptococcus and Candida species. Fungal infections are also associated with a higher mortality than either bacterial or viral infections in these patient populations. This is because of the limited number of available therapies, dose-limiting toxicities of the antifungal drugs, fewer symptoms due to lack of inflammatory response, and the lack of sensitive tests to aid in the diagnosis of invasive fungal infections.1 A study of patients with fungal infections admitted to a university-affiliated hospital indicated that community-acquired infections are becoming a serious problem; 67% of the 140 patients had community-acquired fungal pneumonia.8