Fate of Histoplasma capsulatum in guinea pig polymorphonuclear leukocytes

Diagnosis of histoplasmosis by antigen detection based upon experience at the histoplasmosis reference laboratory

Histoplasmosis is a common infection in endemic regions of North and Latin America, causing a broad spectrum of clinical findings. The diagnosis may be missed or delayed because histoplasmosis is not considered in the differential. A battery of serologic and mycologic tests may be used for the diagnosis, but each has advantages and limitations. Antigen detection may be particularly helpful for making a rapid diagnosis in patients with more extensive infection. The purpose of this review is to provide a comprehensive discussion of the role of antigen detection in the diagnosis of histoplasmosis, to provide the clinician and laboratory worker with a fuller understanding of the benefits and limitations of this useful laboratory method. This report is based soley upon the experience at the Histoplasmosis Reference Laboratory, and can not be used in interpretation of results of Histoplasma antigen testing done at other laboratories.

Human neutrophil-mediated fungistasis against Histoplasma capsulatum. Localization of fungistatic activity to the azurophil granules

Human neutrophils (PMN) demonstrated potent fungistatic activity against Histoplasma capsulatum (Hc) yeasts in a sensitive microassay that quantifies the growth of yeasts by the incorporation of [3H]leucine. At a PMN:yeast ratio of 1:2, PMN inhibited the growth of yeasts by 37%. Maximum inhibition of 85% to 95% was achieved at a PMN/yeast ratio of 10:1 to 50:1. Opsonization of the yeasts in fresh or heat-inactivated serum was required for PMN-mediated fungistasis, but ingestion of the yeasts was not required. Recognition and phagocytosis of opsonized yeasts was via PMN complement receptor (CR) type 1 (CR1), CR3, and FcRIII (CD16). PMN fungistatic activity was evident by 2 h, was maximum at 24 h, and persisted up to 5 d. In contrast, yeasts multiplied within monocytes to a greater extent than in culture medium alone. PMN from three patients with chronic granulomatous disease (CGD) inhibited the growth of Hc yeasts by an average of 97%, compared with 86% in three normal controls. Furthermore, preincubation of PMN with the lysosomotropic agent NH4Cl inhibited fungistatic activity in a concentration-dependent manner. Finally, experiments with subcellular fractions of PMN demonstrated that the principal component of the fungistatic activity of PMN was localized in the azurophil granules. These data demonstrate that human PMN possess potent fungistatic activity against Hc yeasts and further show that fungistasis is mediated by antimicrobial agents contained in the azurophil granules.

Histoplasma variation and adaptive strategies for parasitism: new perspectives on histoplasmosis

This review summarizes the biology of Histoplasma capsulatum in relation to a wide variety of corresponding pathologies in histoplasmosis. Features of these disease syndromes can be explained in part by natural variations within the fungal population and adaptations made by individual organisms to specific environments. H. capsulatum grows as mycelia and conidia in the soil; once inhaled, the organism undergoes a dramatic morphological and physiological conversion to a yeast form. The yeasts proliferate within the phagolysosomes of macrophages, using a variety of specific strategies for intracellular survival. Even avirulent strains or variants are able to avoid being killed by macrophages and instead establish inapparent or persistent infections. The ingested avirulent organisms assume enlarged shapes similar in appearance to those seen in histological sections of tissues from patients with histoplasmosis. Respiratory tract epithelial cells also appear to play a role in persistence: within them yeasts undergo phenotypic switching akin to the phase variation observed in other pathogens. This particular change involves the loss or modification of cell wall alpha-(1,3)-glucan, which is also correlated with the spontaneous appearance of avirulent variants. The repertoire of adaptive responses and natural variations within this species probably evolved from the need to adjust to a wide range of dynamic environments. In combination with the immune status of the host, these characteristics of H. capsulatum appear to influence the epidemiology, extent, and persistence of histoplasmosis.

Resistance of Histoplasma capsulatum to killing by human neutrophils. Evasion of oxidative burst and lysosomal-fusion products

The basis for resistance of yeast form of Histoplasma capsulatum to antifungal activity of human neutrophils was studied. In limiting dilution assays and short term coculture assays human neutrophils were ineffective in killing H. capsulatum whereas Candida albicans was readily killed. By contrast, in a cell free hydrogen peroxide-peroxidase-halide system H. capsulatum was as sensitive to killing as C. albicans. Moreover, lysate of human neutrophils effectively substituted for horse-radish peroxidase in a cell free system for killing H. capsulatum. H. capsulatum elicited significant products of the oxidative burst in human neutrophils as detected by luminol-enhanced chemiluminescence. However, the response was two-fold less (p less than 0.05) than that induced by C. albicans. Transmission electron microscopy studies showed that phagosome-lysosome fusion took place when neutrophils phagocytosed C. albicans or H. capsulatum. Taken together, these findings indicate that, even though H. capsulatum elicits an oxidative burst and phagosome-lysosome fusion within the phagosome, it is capable of evading damage in short term assays.

Evaluation of phagolysosome fusion in acridine orange stained macrophages infected with Histoplasma capsulatum

A phagosome-lysosome (PL) fusion was performed in vitro using peritoneal cells from normal BALB/c mice and the J774.2 macrophage cell line infected with the yeast phase of the fungus Histoplasma capsulatum at ratios of 5 x 10(5), 5 x 10(6) or 1 x 10(7) yeasts per 1 x 10(6) macrophages, and phagocytosis was allowed to proceed for 5, 30 and 60 min. Macrophage lysosomes were pre-labelled with acridine orange and the cells were challenged with the parasite. Fusion was evaluated by fluorescence microscopy and the number of macrophages with stained yeast cells was scored. The phagolysosome fusion frequency (PLFF) was calculated by subtracting the specific fusions of infected macrophages from the non-specific fusions of uninfected macrophages and normalizing the total number of bound yeasts. The PLFF was determined using different doses and strains of H. capsulatum. Results showed that PLFF in infected macrophages depends on the infection dose. Inhibition of PL fusion was detected mainly at a high infection ratio (1 x 10(7) yeasts/1 x 10(6) macrophages), and PL fusion varied with phagocytosis time. No significant differences were observed in the fusions when different Histoplasma strains were used. Results with J774.2 cells were similar to peritoneal cells, indicating that both methodology and fusion calculations employed were useful, in spite of the heterogeneity of macrophage populations used.

Histoplasma capsulatum fails to trigger release of superoxide from macrophages

The yeast form of the dimorphic fungus Histoplasma capsulatum survives within macrophages after phagocytosis. To do so, it must avoid, inhibit, or resist a variety of toxic oxygen metabolites. Using ferricytochrome c reduction to assay superoxide release, we examined the response of mouse macrophages to the yeast form of various H. capsulatum strains. Doses of zymosan as low as 20 particles per macrophage elicited superoxide, whereas H. capsulatum failed to induce superoxide even at 160 yeast cells per macrophage. This phenomenon was observed with two virulent strains of H. capsulatum (G217B and G186A) and with an avirulent variant of G186A. Over a 15- to 150-min observation period, zymosan stimulated increasing reduction of ferricytochrome c, but H. capsulatum did not. When added concurrently with zymosan, H. capsulatum had no effect on superoxide production. Therefore, H. capsulatum was unable either to inactivate the oxygen radical or inhibit host cell superoxide response to other competent stimuli. Enzymatically generated superoxide reduced ferricytochrome c even in the presence of H. capsulatum, again implying that the organism does not readily inactivate superoxide. This experiment also demonstrated that the yeast did not interfere with the assay used. Thus, rather than inhibiting superoxide generation or inactivating the anion, H. capsulatum yeast cells appear to avoid the toxic effects of superoxide by failing to trigger its release.

In vitro susceptibility of fungi to killing by neutrophil granulocytes discriminates between primary pathogenicity and opportunism

Pathogenic fungi, according to their propensity to cause infection of apparently normal individuals, can be grouped into either primary pathogens (e.g., Coccidioides, Histoplasma, Paracoccidioides, Blastomyces, and Sporothrix) or opportunists (e.g., Candida, Mucoraceae, Aspergillus spp., Petriellidium, and Trichosporon). There is, however, no unifying concept explaining the difference between the virulence of the two fungal categories. Previously we have speculated that neutrophils are the common denominator of the high natural resistance to opportunistic fungi. Accordingly, we then compared the susceptibility to killing by neutrophil granulocytes of Histoplasma, Blastomyces, Paracoccidioides, and Sporothrix with that of 14 opportunistic fungi. We found the four virulent dimorphic yeasts, in contrast to opportunistic fungi, to be resistant to killing by neutrophils. Virulent dimorphic yeasts were ingested by neutrophils, and triggered a respiratory burst comparably to opportunists but were less susceptible to hydrogen peroxide, suggesting that differences in the susceptibility to microbicidal products of leukocytes may explain the difference in virulence.

Studies on the catalase of Histoplasma capsulatum

Factors which control the levels of catalase within yeast cells of Histoplasma capsulatum were studied. Only a small fraction of the total catalase activity could be detected in whole cells. The bulk of the activity was revealed in cell-free extracts or in cells permeabilized with acetone. The formation of the enzyme was regulated by glucose and by oxygen. There were large, consistent differences in the levels of catalase among strains of H. capsulatum. The sensitivity of the strains to H2O2 toxicity also varied remarkably. Peroxidase activity could not be detected in cell-free extracts of the strains. Resistance to H2O2 did not correspond to levels of catalase. There was no obvious correlation of H2O2 sensitivity and virulence among the strains.

Primary acute histoplasmosis in guinea pigs exposed to aerosolized Histoplasma capsulatum

Guinea pigs were examined as a possible in vivo model for human histoplasmosis. Guinea pigs were exposed to an aerosol of viable microconidia and mycelial fragments of Histoplasma capsulatum generated in a Henderson apparatus. Colonization and infection of the lungs occurred, with subsequent involvement of the regional lymph nodes and reticuloendothelial organs. Cultural recovery of the fungus from the nasopharynx and bronchoalveoli was initially high, but decreased with time. The mean number of colonies recovered from the lungs gradually increased, reaching a peak at 2 weeks, with involvement of the regional lymph nodes. Extrathoracic dissemination to the liver and spleen occurred in only a few animals. After 4 weeks, all tissues except the cervical and tracheobronchial lymph nodes were culturally negative; all specimens were culturally negative after 8 weeks. Disappearance of H. capsulatum from tissues appeared to correlate inversely with the development of hypersensitivity as measured by skin test reactivity. Histopathological studies supported cultural results and were similar to those described for primary human and canine histoplasmosis.

Comparative sensitivity of Histoplasma capsulatum conidiospores and blastospores to oxidative antifungal systems

The comparative sensitivity of blastospores and conidiospores of Histoplasma capsulatum to hydrogen peroxide, to hydrogen peroxide and halides, and to a combination of hydrogen peroxide and halide with the enzyme myeloperoxidase was studied. Blastospores of different strains of H. capsulatum varied in their sensitivity to hydrogen peroxide. This variation correlated with the amount of catalase in cell-free extracts from the strains. Blastospores and conidiospores of a single isolate were about equally susceptible to hydrogen peroxide, but this sensitivity could obviously vary with the catalase content of the two types of spores. Halides augmented the antifungal activity of hydrogen peroxide for both types of spores. Iodide was far more efficient in this regard than was chloride. A crude granule lysate from guinea pig polymorphonuclear leukocytes was quite inhibitory to blastospore but not to conidiospore germination. A study of the myeloperoxidase activity of such preparations against blastospores was thus precluded. A sample of a very highly purified human myeloperoxidase functioned in the presence of hydrogen peroxide and either iodide or chloride to prevent germination of both blastospores and conidiospores. The preparation had no toxicity for spores apart from its interaction with hydrogen peroxide and halides.