Observations on tissue cultures of mouse peritoneal exudates inoculated with Histoplasma capsulatum

Conserved factors Ryp2 and Ryp3 control cell morphology and infectious spore formation in the fungal pathogen Histoplasma capsulatum

The human fungal pathogen Histoplasma capsulatum grows in a sporulating filamentous form in the soil and, after inhalation of infectious spores, converts to a pathogenic yeast form inside host macrophages in response to temperature. Here we report the identification of two genes (RYP2 and RYP3) required for yeast-phase growth. Ryp2 and Ryp3 are homologous to each other and to the Velvet A family of regulatory proteins in Aspergillus species and other filamentous fungi. Wild-type H. capsulatum grows as filaments at room temperature and as yeast cells at 37 degrees C, but ryp2 and ryp3 mutants constitutively grow as filaments independent of temperature. RYP2 and RYP3 transcripts accumulate to higher levels at 37 degrees C than at room temperature. This differential expression is similar to the previously identified RYP1 transcript, which encodes a transcriptional regulator required for the yeast-phase expression program. Ryp1 associates with the upstream region of RYP2, and each of the three RYP genes is required for the differential expression of the others at 37 degrees C. In addition to responding to the elevated temperature of the mammalian host, RYP2 and RYP3 are essential for viable spore production and regulation of sporulation at room temperature. This regulatory function is strikingly similar to the role of the Aspergillus Velvet A protein family in spore development in response to light, with the notable distinction that the H. capsulatum circuit responds to temperature.

Conversion in strains of Histoplasma capsulatum

Pine, Leo (Duke University, Durham, N. C.) and Robert E. Webster. Conversion in strains of Histoplasma capsulatum. J. Bacteriol. 83:149-157. 1962-Photographs of mycelial elements taken at various time intervals from a single strain of Histoplasma capsulatum have shown the direct conversion of microconidia to yeast cells by polar and nonpolar budding, the formation of yeast cells by budding of hyphal cells, and the probable conversion of hyphal cells to yeast phase by monilial chain formation. In addition, studies of fixed-slide preparations of a second strain have indicated the formation of stalked yeast cells arising in a manner similar to that observed for microconidia formation but, apparently, by-passing the microconidial stage. These observations are discussed briefly in regard to temperature-induced changes in cell-wall synthesis during conversion.

Some factors which affect the initiation of growth of Cryptococcus neoformans

Howard, Dexter H. (University of California, Los Angeles). Some factors which affect the initiation of growth of Cryptococcus neoformans. J. Bacteriol. 82:430-435. 1961.-The observation that a strain of Cryptococcus neoformans failed to grow in a medium containing normal human serum led to an investigation of some of the physiological factors controlling initiation of growth by this fungus. The data presented show that the growth of C. neoformans is markedly inhibited in media with initial pH values slightly above neutrality. The growth of four strains of the fungus was completely suppressed in peptone broth at pH values above 8. The growth of two strains was partially inhibited at a pH of 7.3. Two other strains grew well at 7.3, but were partially inhibited at pH 7.5. The growth in media at pH values which partially inhibited multiplication was granular in appearance. Microscopically the granules were composed of clumps of yeast cells. The possibility is suggested that pH may exert a portion of its effect on the growth of C. neoformans by influencing those factors which affect the stability of the cells in suspension. At a temperature of 25 C, growth of the fungus was initiated in media with a pH which suppressed growth at a temperature of 37 C. Under the experimental conditions employed in these studies, proteose peptone no. 3 (Difco) serves as both a carbon and a nitrogen source. However, glucose, which is assimilated by C. neoformans in contrast to lactose, permitted the growth of the fungus at pH values which suppressed growth in the absence of the sugar. Since weakly buffered media, containing filtered, normal human serum, have an alkaline reaction higher than that shown to suppress the growth of C. neoformans, it was concluded that this factor was primarily responsible for the failure of the fungus to reproduce in tissue culture media. However, it is also possible to show that even under optimal pH conditions normal human serum exerts an anticryptococcal effect. This activity of human serum was relatively heat-stable and was expressed only in high concentrations of serum. Serum adsorbed with viable cells of C. neoformans also inhibited the growth of the fungus, but a partially purified globulin fraction of serum was not inhibitory. The data are discussed in relation to the work of other investigators.

INTRACELLULAR GROWTH OF HISTOPLASMA CAPSULATUM

Howard, Dexter H. (University of California, Los Angeles). Intracellular growth of Histoplasma capsulatum. J. Bacteriol. 89:518-523. 1965.-Under a variety of experimental conditions, the generation time of Histoplasma capsulatum in mammalian histiocytes was remarkably constant. The rate of intracellular growth of five different isolates of H. capsulatum was very similar. The generation time of the fungus within guinea pig cells was nearly the same as it was within mouse cells under comparable conditions. Preliminary exposure of yeast cells to specific antibodies and complement did not affect intracellular proliferation of the fungus during the first 24 hr after phagocytosis by mouse histiocytes. The rate of growth of the fungus within cells from immunized mice and guinea pigs was comparable to that in cells from normal animals. This latter result was not altered by preliminary exposure of the fungus to hyperimmune serum and complement.

INTRACELLULAR BEHAVIOR OF HISTOPLASMA CAPSULATUM

Howard, Dexter H. (University of California, Los Angeles). Intracellular behavior of Histoplasma capsulatum. J. Bacteriol. 87:33-38. 1964.-The tinctorial characteristics and intracellular generation time of Histoplasma capsulatum within mouse histiocytes maintained in tissue culture were studied. The appearance of viable and nonviable cells of H. capsulatum was observed after staining with the May Grunwald-Giemsa, Methenamine Silver, and Periodic acid-Schiff (PAS) procedures. Death of the fungus, either induced by heat or occurring naturally in saline suspension, caused a loss of nuclear differentiation. This loss was not accompanied by changes in the PAS or Silver staining capacities of the cell walls of the fungus. There was no tinctorial evidence for intracellular death of H. capsulatum in normal mouse histiocytes. The intracellular generation time of the fungus was estimated to be 10.3 +/- 1.5 hr, and was not affected by the age of the histiocytes. The generation time was approximately the same in histiocytes from Webster-Swiss and A/J mice.

[Passive transfer of immunity against Clonorchis sinensis by peritoneal exudate cells in mice]

This study was undertaken to evaluate the role of peritoneal exudate cells in the transfer of immunity against the liver fluke, Clonorchis sinensis in the inbred BALB/c mice. Ten donor mice were divided into 2 groups. One group consisted of 5 mice was infected orally with 20 metacercariae of C. sinensis, and the other group was injected intraperitoneally with 20 excysted larvae. Thirty days after immunization, the peritoneal exudate cells were obtained from the donor mice. Twenty recipient mice were divided into 4 equal groups for the purpose of primary immunization. The mice of Group I were injected intraperitoneally with 2 x 10(6) peritoneal exudate cells of the donor mice infected orally, those of Group III were injected intraperitoneally with 2 x 10(6) peritoneal exudate cells of the donor mice injected intraperitoneally. Those of Group II were injected orally with 20 metacercariae of C. sinensis. The group IV mice served as controls. Four days after the primary immunization all recipient mice were challenged orally with 20 metacercariae of C. sinensis, and then killed 30 days after the challenging infection. When the peritoneal exudate cells were injected into the recipient mice, pronounced reduction in eggs per gram of the feces was found in the mice of Group I and Group II, but no reduction in those of Group III. In the worm burdens of C. sinensis, the number of flukes found in the mice of Group II was only significantly less than those in the control group(IV). In addition the number of plaque forming cells per spleen in the mice of Group II was found larger than those in Group I. It is likely that donor peritoneal exudate cells transferred to the recipients might result in the production of relative immunity.

Electron microscopy of yeastlike cell development from the microconidium of Histoplasma capsulatum

Fine details of the sequential morphological events occurring during transition of microconidia (spores less than 5 micrometer in diameter) to the yeastlike phase of Histoplasma capsulatum as seen in ultrathin section are described and illustrated by electron micrographs. Masses of microconidia were obtained when the fungas was grown on a garden soil extract medium. Spores were incubated under in vitro environmental conditions conducive for phase transition (an enriched medium at 37 degrees C). Within 48 h of incubation, the microconidia either germinated to give rise to a short mycelium or the germ tube process became a yeast mother cell without further extension. The wall of the yeast mother cell was thin and smooth, and its cytoplasmic content was ultrastructurally complex, consisting of numerous lipid bodies, vacuoles, glycogen-like deposits, and membrane systems. Within 96 h, the mother cell underwent multipolar budding to form simultaneously linear hyphal and/or ovate yeastlike daughter cells. During the transition, new cell wall materials of the germ tube, the mother cell, and yeastlike daughter cells arose by blastic action from the innermost layer(s) of the wall of the precursor form. Lomasome-like vesicles were often seen in association with areas of new cell wall formation. After organellar migration into and septation of the daughter cells, the yeast mother cell's cytoplasmic content underwent marked degenerative changes.

Investigation on dimorphism of Blastomyces dermatitidis by agar-implantation method

By the agar-implantation developed by the authors the process of conversion on Blastomyces dermatitidis from mycelial phase to yeast phase was observed. First of all slide cultures of the fungus were prepared at room temperature. Upon confirmation of good hyphal growth, a cover glass was removed and a part of medium was cut out in a square of about 3 mm a side. After mice were laparotomied, each agar block cut out was implanted in the peritoneal cavity of mouse. The mice implanted with the agar blocks were killed, two each, every day for 14 days, and thereafter at intervals of a week for 2 months. Therefore, the implanted agar blocks were all recovered. They were examined directly by a light microscope with histopathological and electron microscopic examinations carried out at the same time. Within the peritoneal cavity of mouse, the intercalary and terminal chlamydospores were formed from hyphae. These subsequently swelled to become yeastlike cells and proliferated thereafter by budding.

Further studies on the intracellular behavior of Torulopsis glabrata

The growth of Torulopsis glabrata was inhibited in glucose-peptone broth containing 10 to 20% normal human serum. Addition of iron to the medium diminshed the fungistatic effect. The intracellular growth of T. glabrata was remarkably restricted within mouse macrophages maintained in vitro, but this growth restriction was not caused by the limitation of iron imposed by the serum in the medium. The intracellular growth of T. glabrata within a very small percentage of the macrophages was not obviously related to the failure of lysosomal fusion to the phagosomes in those cells. The studies did not permit definite conclusions regarding the viability of the inhibited yeasts, but results suggested that a large portion of them survived. Potentially misleading artifacts of the technique for assessment of the intracellular behavior of the fungus were detected and are discussed.

Effect of temperature on the intracellular growth of Histoplasma capsulatum

The rate and form of growth of Histoplasma capsulatum within histiocytes derived from homothermic and poikilothermic animals, and incubated at 25, 30, and 37 C, are described. The generation time of the fungus in mouse cells incubated at 37 and 25 C was 11 and 24 hr, respectively. Blastospore formation was progressively retarded in cells at 25 C, and this retardation was accompanied by germination of some of the blastospores. The generation time of the fungus in mouse cells incubated at 30 C was the same as it was at 37 C. Germ tube formation was not a prominent feature of intracellular growth at 30 C. The rate of growth of H. capsulatum within frog histiocytes at 30 and 25 C was slower than it was in mouse cells at the same temperatures. Some loss of frog histiocytes in cultures incubated at 37 C prevented accurate estimation of the rate of growth of the fungus at this temperature. Growth of H. capsulatum in frog histiocytes kept at 25 C was progressively retarded, and the retardation was accompanied by germination of the yeasts. Yeast-phase growth predominated in fish histiocytes incubated at 30 C, whereas germ tubes were formed within such cells incubated at 25 C. Cell survival of fish histiocytes was relatively poor in culture, and no estimates of rate of growth of the fungus within these cells were made.