In vitro effect of lung surfactant on alveolar macrophage defence mechanisms against Cryptococcus neoformans

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.

Surfactant protein D increases phagocytosis of hypocapsular Cryptococcus neoformans by murine macrophages and enhances fungal survival

Cryptococcus neoformans is a facultative intracellular opportunistic pathogen and the leading cause of fungal meningitis in humans. In the absence of a protective cellular immune response, the inhalation of C. neoformans cells or spores results in pulmonary infection. C. neoformans cells produce a polysaccharide capsule composed predominantly of glucuronoxylomannan, which constitutes approximately 90% of the capsular material. In the lungs, surfactant protein A (SP-A) and SP-D contribute to immune defense by facilitating the aggregation, uptake, and killing of many microorganisms by phagocytic cells. We hypothesized that SP-D plays a role in C. neoformans pathogenesis by binding to and enhancing the phagocytosis of the yeast. Here, the abilities of SP-D to bind to and facilitate the phagocytosis and survival of the wild-type encapsulated strain H99 and the cap59Delta mutant hypocapsular strain are assessed. SP-D binding to cap59Delta mutant cells was approximately sixfold greater than binding to wild-type cells. SP-D enhanced the phagocytosis of cap59Delta cells by approximately fourfold in vitro. To investigate SP-D binding in vivo, SP-D(-/-) mice were intranasally inoculated with Alexa Fluor 488-labeled cap59Delta or H99 cells. By confocal microscopy, a greater number of phagocytosed C. neoformans cells in wild-type mice than in SP-D(-/-) mice was observed, consistent with in vitro data. Interestingly, SP-D protected C. neoformans cells against macrophage-mediated defense mechanisms in vitro, as demonstrated by an analysis of fungal viability using a CFU assay. These findings provide evidence that C. neoformans subverts host defense mechanisms involving surfactant, establishing a novel virulence paradigm that may be targeted for therapy.

Immune response and immunotherapy to Cryptococcus infections

Cryptococcus neoformans is a ubiquitous fungus that can cause lifethreatening infections during immunosuppressive states such as acquired immunodeficiency syndrome (AIDS) and after bone marrow transplantation (BMT). Infected individuals normally succumb to meningitis and meningoencephalitis caused by dissemination of C. neoformans to the brain. In this review, we analyze the current understanding of the interaction between host immune response and C. neoformans as well as the current state of immunotherapeutic strategies for treating cryptococcosis.

Aggregates of ultrafine particles impair phagocytosis of microorganisms by human alveolar macrophages

We investigated whether exposure of alveolar macrophages to aggregates of ultrafine carbon particles affected subsequent phagocytosis of microorganisms. Human alveolar macrophages were obtained by bronchoalveolar lavage and exposed to aggregates of ultrafine carbon particles or diesel exhaust particles (DEP) for 20 h before measurements of phagocytosis. The particle loads were estimated to be comparable to those of air pollution exposure with established health effects in humans. Phagocytotic activity was measured as attachment and ingestion of four different test particles (amorphous silica particles, yeast cells from Candida albicans, and Cryptococcus neoformans opsonized with specific IgG or fresh serum) that bind to scavenger, mannose, Fc, and complement receptors, respectively. Carbon preloading significantly impaired the attachment and ingestion process (P<0.01) for all particles, except for yeast cells from C. neoformans opsonized with specific IgG. On the average, the accumulated attachment decreased by 30% and the ingested fraction decreased by 10%. Loading of alveolar macrophages with either aggregates of ultrafine DEP or carbon particles impaired the phagocytosis of silica test particles in a similar way. Exposure of human alveolar macrophages to aggregates of carbon or DEP, in concentrations relevant to human environmental exposures, caused significant impairment of phagocytosis of silica particles and microorganisms. The inhibitory effect on particle phagocytosis mediated by four different receptors suggests that air pollution particles cause a general inhibition of macrophage phagocytosis. Such an effect may contribute to increased susceptibility to infections and, for example, result in more exacerbations of asthma and chronic obstructive pulmonary disease.

Coccidioides posadasii infection alters the expression of pulmonary surfactant proteins (SP)-A and SP-D

BACKGROUND

Coccidioidomycosis or Valley Fever is caused by Coccidioides in Southwest US and Central America. Primary pulmonary infection is initiated by inhalation of air-borne arthroconidia. Since, lung is the first organ that encounters arthroconidia, different components of the pulmonary innate immune system may be involved in the regulation of host defense. Pulmonary surfactant proteins (SP)-A and SP-D have been recognized to play an important role in binding and phagocytosis of various microorganisms, but their roles in Coccidioides infection are not known.

METHODS

In this study, we studied the changes in amounts of pulmonary SP-A, SP-D and phospholipid in murine model of Coccidioides posadasii infection, and binding of SP-A and SP-D to Coccidioidal antigens. Mice were challenged intranasally with a lethal dose of C. posadasii (n = 30 arthroconidia) and bronchoalveolar lavage fluid (BALF) samples were collected on day 10, post infection. In another group of animals, mice were immunized with protective formalin killed spherule (FKS) vaccine prior to infection. The concentrations of BALF SP-A, SP-D, total phospholipid were measured using enzyme linked immunosorbent assay and biochemical assays.

RESULTS

We found that in lavage fluid samples of C. posadasii infected mice, the concentrations of total phospholipid, SP-A and SP-D were 17 % (SEM 3.5, p < 0.001), 38 % (SEM 5.8, p < 0.001) and 4 % (SEM 1.3, p < 0.001) of those in lavage fluid samples of non-infected control mice, respectively. However, the concentrations of SP-A and SP-D remained unchanged in BALF samples of C. posadasii protected mice after immunization with FKS vaccine. Also, we found that both SP-A and SP-D bind to Coccidiodal antigens.

CONCLUSION

Our results suggest that the C. posadasii infection perturbs the pulmonary SP-A, SP-D, and phospholipids, potentially enabling the disease progression and promoting fungal dissemination.

Experimental and calculated parameters on particle phagocytosis by alveolar macrophages

Phagocytosis of three types of fluorescein-labeled test particles by rat alveolar macrophages (AM) were studied: spherical silica (3.2 microm), heat-killed Candida albicans (3.8 microm), and heat-killed Cryptococcus neoformans (6.1 microm) opsonized with specific IgG. These particles should attach to scavenger, mannose, and Fc receptors, respectively. Both control AM and AM pretreated for 20 h with interferon-gamma (12.5 or 50 U/ml) were studied. The sum of the number of attached and ingested particles per AM (accumulated attachment) was used as a measure of the attachment process, and the number of ingested particles per AM divided by the accumulated attachment (ingested fraction) was used as a measure of the ingestion process. The average ingestion time (IT), which is also a measure of the ingestion process, was calculated from the experimental data. The ingestion process was independent of the attachment process. IT increased with the time of observation. This is explained by the fact that IT determined from observation times shorter than the whole distribution of IT for a certain particle results in a shorter IT than the real average IT. C. albicans (mannose receptor) had the fastest ingestion process, C. neoformans opsonized with specific IgG (Fc receptor) had ingestion that was nearly as fast, and the silica particles (scavenger receptors) had the slowest ingestion process. Treatment with interferon-gamma markedly impaired the attachment process for all three types of particles (and three types of receptors) but clearly impaired the ingestion process only for silica particles (scavenger receptors).

Treatment of experimental Cryptococcus neoformans infection in newborn rabbits by airway instillation of specific antibody and surfactant

Cryptococcosis in AIDS patients has a slow response to antifungal chemotherapy, and passive antibody treatment has thus been considered as an adjunct. Polyclonal anticryptococcal IgG dissolved in a suspension of modified natural surfactant was given intratracheally to near-term rabbits. Killing of Cryptococcus neoformans within the lungs was determined by counting the colony forming units (cfu). After 5 h a significant decrease in cfu was observed in rabbits treated with the IgG-surfactant mixture compared with control animals receiving saline. In conjunction with conventional therapy, the combined treatment of IgG-surfactant given by bronchoscopy might be used in high-risk patients to enhance killing of the yeast within the lungs.