Functional defect of natural immune system in an apparent immunocompetent patient with pulmonary cryptococcosis

Cryptococcus infection with asymptomatic diffuse pulmonary disease in an immunocompetent patient: A case report

BACKGROUND

Cryptococcus presenting as an opportunistic pathogen mainly affects immunocompromised patients, but the disseminated form of infection is rare among immunocompetent populations. The partial radiographic characteristics of pulmonary cryptococcosis mimic lung carcinoma, leading to unnecessary open chest exploratory surgery, and the lack of a gold-standard noninvasive diagnostic increases the risk of misdiagnosis. Positron emission tomography/computed tomography (PET/CT), a sensitive method for distinguishing malignant tumors, coupled with cryptococcal latex agglutination test showing a high positive rate may overcome these issues.

CASE

A 36-year-old man presented for general examination, without health complaints. Routine CT showed multiple pulmonary nodules and a mass with high maximum standardized uptake value. Initially, we suspected primary malignancy with hematogenous metastasis. Although his routine fungal analysis had been negative, subsequent CT-guided percutaneous core needle biopsy and histopathology examination indicated a diagnosis of pulmonary cryptococcosis. Fluconazole (200 mg/d) antifungal drug treatment was initiated, and 1 mo later the pulmonary mass had reduced in size markedly (on chest CT scan) without any complications.

CONCLUSION

Serologic and PET/CT examinations may not rule out cryptococcosis, and percutaneous lung puncture is critical under all circumstances.

Animal Models of Cryptococcus neoformans in Identifying Immune Parameters Associated With Primary Infection and Reactivation of Latent Infection

Cryptococcus species are environmental fungal pathogens and the causative agents of cryptococcosis. Infection occurs upon inhalation of infectious particles, which proliferate in the lung causing a primary infection. From this primary lung infection, fungal cells can eventually disseminate to other organs, particularly the brain, causing lethal meningoencephalitis. However, in most cases, the primary infection resolves with the formation of a lung granuloma. Upon severe immunodeficiency, dormant cryptococcal cells will start proliferating in the lung granuloma and eventually will disseminate to the brain. Many investigators have sought to study the protective host immune response to this pathogen in search of host parameters that keep the proliferation of cryptococcal cells under control. The majority of the work assimilates research carried out using the primary infection animal model, mainly because a reactivation model has been available only very recently. This review will focus on anti-cryptococcal immunity in both the primary and reactivation models. An understanding of the differences in host immunity between the primary and reactivation models will help to define the key host parameters that control the infections and are important for the research and development of new therapeutic and vaccine strategies against cryptococcosis.

The Diverse Roles of Monocytes in Cryptococcosis

Monocytes are considered to play a central role in the pathogenesis of Cryptococcus neoformans infection. Monocytes and monocyte-derived macrophages and dendritic cells are key components for the control of infection, but paradoxically they can also contribute to detrimental host responses and may even support fungal proliferation and dissemination. Simultaneously, the C. neoformans polysaccharide capsule can impair the functions of monocytes. Although monocytes are often seen as simple precursor cells, they also function as independent immune effector cells. In this review, we summarize these monocyte-specific functions during cryptococcal infection and the influence of C. neoformans on monocyte responses. We also cover the most recent findings on the functional and phenotypic heterogeneity of monocytes and discuss how new advanced technologies provide a platform to address outstanding questions in the field.

Contribution of IL-1RI Signaling to Protection against Cryptococcus neoformans 52D in a Mouse Model of Infection

Interleukin-1 alpha (IL-1α) and interleukin-1 beta (IL-1β) are pro-inflammatory cytokines that are induced after Cryptococcus neoformans infection and activate the interleukin-1 receptor type I (IL-1RI). To establish the role of IL-1RI signaling in protection against cryptococcal infection, we analyzed wild-type (WT) and IL-1RI-deficient (IL-1RI^−/−) mice on the BALB/c background. IL-1RI^−/− mice had significantly reduced survival compared to WT mice after intratracheal challenge with C. neoformans 52D. Microbiological analysis showed a significant increase in the lung and brain fungal burden of IL-1RI^−/− compared to WT mice beginning at weeks 1 and 4 postinfection, respectively. Histopathology showed that IL-1RI^−/− mice exhibit greater airway epithelial mucus secretion and prominent eosinophilic crystals that were absent in WT mice. Susceptibility of IL-1RI^−/− mice was associated with significant induction of a Th2-biased immune response characterized by pulmonary eosinophilia, M2 macrophage polarization, and recruitment of CD4⁺ IL-13⁺ T cells. Expression of pro-inflammatory [IL-1α, IL-1β, TNFα, and monocyte chemoattractant protein 1 (MCP-1)], Th1-associated (IFNγ), and Th17-associated (IL-17A) cytokines was significantly reduced in IL-1RI^−/− lungs compared to WT. WT mice also had higher expression of KC/CXCL1 and sustained neutrophil recruitment to the lung; however, antibody-mediated depletion of these cells showed that they were dispensable for lung fungal clearance. In conclusion, our data indicate that IL-1RI signaling is required to activate a complex series of innate and adaptive immune responses that collectively enhance host defense and survival after C. neoformans 52D infection in BALB/c mice.

Role of dendritic cell-pathogen interactions in the immune response to pulmonary cryptococcal infection

This review discusses the unique contributions of dendritic cells (DCs) to T-cell priming and the generation of effective host defenses against Cryptococcus neoformans (C.neo) infection. We highlight DC subsets involved in the early and later stages of anticryptococcal immune responses, interactions between C.neo pathogen-associated molecular patterns and pattern recognition receptors expressed by DC, and the influence of DC on adaptive immunity. We emphasize recent studies in mouse models of cryptococcosis that illustrate the importance of DC-derived cytokines and costimulatory molecules and the potential role of DC epigenetic modifications that support maintenance of these signals throughout the immune response to C.neo. Lastly, we stipulate where these advances can be developed into new, immune-based therapeutics for treatment of this global pathogen.

Pulmonary Cryptococcosis in the Immunocompetent Patient-Many Questions, Some Answers

Background. There are no prospective data regarding the management of pulmonary cryptococcosis in the immunocompetent patient. Clinical guidelines recommend oral fluconazole for patients with mild to moderate symptoms and amphotericin B plus flucytosine followed by fluconazole for severe disease. It is unclear whether patients who have histological evidence of Cryptococcus neoformans but negative cultures will even respond to drug treatment. We evaluated and managed a patient whose presentation and course raised important questions regarding the significance of negative cultures, antifungal choices, duration of therapy, and resolution of clinical, serologic, and radiographic findings.

Methods. In addition to our experience, to answer these questions we reviewed available case reports and case series regarding immunocompetent patients with pulmonary cryptococcosis for the last 55 years using the following definitions: Definite - Clinical and/or radiographic findings of pulmonary infection and respiratory tract isolation of C. neoformans without other suspected etiologies; Probable - Clinical and radiographic findings of pulmonary infection, histopathologic evidence of C. neoformans, and negative fungal cultures with or without a positive cryptococcal polysaccharide antigen.

Results. Pulmonary cryptococcosis resolves in most patients with or without specific antifungal therapy. Clinical, radiographic, and serologic resolution is slow and may take years.

Conclusions. Persistently positive antigen titers are most common in untreated patients and may remain strongly positive despite complete or partial resolution of disease. Respiratory fungal cultures are often negative and may indicate nonviable organisms.

Sphingolipids as Regulators of the Phagocytic Response to Fungal Infections

Fungal infections pose a significant risk for the increasing population of individuals who are immunocompromised. Phagocytes play an important role in immune defense against fungal pathogens, but the interactions between host and fungi are still not well understood. Sphingolipids have been shown to play an important role in many cell functions, including the function of phagocytes. In this review, we discuss major findings that relate to the importance of sphingolipids in macrophage and neutrophil function and the role of macrophages and neutrophils in the most common types of fungal infections, as well as studies that have linked these three concepts to show the importance of sphingolipid signaling in immune response to fungal infections.

Development of an aerosol model of Cryptococcus reveals humidity as an important factor affecting the viability of Cryptococcus during aerosolization

Cryptococcus is an emerging global health threat that is annually responsible for over 1,000,000 infections and one third of all AIDS patient deaths. There is an ongoing outbreak of cryptococcosis in the western United States and Canada. Cryptococcosis is a disease resulting from the inhalation of the infectious propagules from the environment. The current and most frequently used animal infection models initiate infection via liquid suspension through intranasal instillation or intravenous injection. These models do not replicate the typically dry nature of aerosol exposure and may hinder our ability to decipher the initial events that lead to clearance or the establishment of infection. We have established a standardized aerosol model of murine infection for the human fungal pathogen Cryptococcus. Aerosolized cells were generated utilizing a Collison nebulizer in a whole-body Madison Chamber at different humidity conditions. The aerosols inside the chamber were sampled using a BioSampler to determine viable aerosol concentration and spray factor (ratio of viable aerosol concentration to total inoculum concentration). We have effectively delivered yeast and yeast-spore mixtures to the lungs of mice and observed the establishment of disease. We observed that growth conditions prior to exposure and humidity within the Madison Chamber during exposure can alter Cryptococcus survival and dose retained in mice.

Spinal cryptococcoma in an immunocompetent cat

This report describes an unusual case of primary cryptococcoma in the proximal thoracic spinal cord of an 11-year-old immunocompetent cat from a farm on which there were large numbers of pigeons. This animal was referred for examination with progressive paralysis and shown to be free from feline immunodeficiency virus, feline leukaemia virus, feline coronavirus and Toxoplasma gondii. It died 2 months later. At necropsy, the only lesion detected was a malacic area, 4cm in length, in the spinal cord. Histopathological examination of the spinal cord revealed severe granulomatous inflammation associated with large numbers of encapsulated yeast cells. In addition to the granulomatous host response, necrosis, digestion chambers, Gitter cells, spheroids and lymphocytic perivascular cuffs were features of the malacic areas. Immunohistochemistry confirmed the presence of Cryptococcus neoformans var. grubii yeast cells.

Enhanced innate immune responsiveness to pulmonary Cryptococcus neoformans infection is associated with resistance to progressive infection

Genetically regulated mechanisms of host defense against Cryptococcus neoformans infection are not well understood. In this study, pulmonary infection with the moderately virulent C. neoformans strain 24067 was used to compare the host resistance phenotype of C57BL/6J with that of inbred mouse strain SJL/J. At 7 days or later after infection, C57BL/6J mice exhibited a significantly greater fungal burden in the lungs than SJL/J mice. Characterization of the pulmonary innate immune response at 3 h after cryptococcal infection revealed that resistant SJL/J mice exhibited significantly higher neutrophilia, with elevated levels of inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) and keratinocyte-derived chemokine (KC)/CXCL1 in the airways, as well as increased whole-lung mRNA expression of chemokines KC/CXCL1, MIP-1alpha/CCL3, MIP-1beta/CCL4, MIP-2/CXCL2, and MCP-1/CCL2 and cytokines interleukin 1beta (IL-1beta) and IL-1Ra. At 7 and 14 days after infection, SJL/J mice maintained significantly higher levels of TNF-alpha and KC/CXCL1 in the airways and exhibited a Th1 response characterized by elevated levels of lung gamma interferon (IFN-gamma) and IL-12/IL-23p40, while C57BL/6J mice exhibited Th2 immunity as defined by eosinophilia and IL-4 production. Alveolar and resident peritoneal macrophages from SJL/J mice also secreted significantly greater amounts of TNF-alpha and KC/CXCL1 following in vitro stimulation with C. neoformans. Intracellular signaling analysis demonstrated that TNF-alpha and KC/CXCL1 production was regulated by NF-kappaB and phosphatidylinositol 3 kinase in both strains; however, SJL/J macrophages exhibited heightened and prolonged activation in response to C. neoformans infection compared to that of C57BL/6J. Taken together, these data demonstrate that an enhanced innate immune response against pulmonary C. neoformans infection in SJL/J mice is associated with natural resistance to progressive infection.

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.

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.