Kalpathi R. Seshan

Gastrointestinal and systemic candidosis in immunocompromised mice

Oral-intragastric inoculation of 6-day-old outbred Crl:CFW(SW) BR mice with Candida albicans can lead to colonization of the gastrointestinal (GI) tract. We have shown that in the absence of an immunocompromising treatment, Candida is primarily localized in the stomach and intestines of mice at 20 days post-inoculation. Cultures of homogenates of the esophagus of most animals tested, and homogenates of the liver, lungs, spleen and kidneys of all animals tested, proved negative for C. albicans. Previous histological examinations of the GI tract of these colonized, non-immunocompromised mice showed hyphal elements associated with the stratified, squamous epithelium of the stomach in the region of the cardial-atrium fold. In this study, mice were immunocompromised by intraperitoneal injection of cyclophosphamide and cortisone acetate 11-14 days after oral-intragastric challenge with C. albicans and then sacrificed 20 days post-challenge. A high density of invasive hyphae was observed in the same, cardial-atrium region of the stomach of these animals. Cultures of the homogenized stomach showed a 100-fold increase in colony forming units (c.f.u.) of C. albicans compared with stomach homogenates of infected but non-immunocompromised controls. In addition, homogenates of the esophagus and selected body organs of most immunocompromised mice examined were positive for C. albicans by plate culture. When the immunocompromising drug treatment was delayed 3-5 weeks after oral-intragastric challenge, proliferation of C. albicans in the stomach and intestines was still evident, although fewer mice showed systemic spread and lower numbers of c.f.u. were recovered from body organ homogenates. Abscesses which contained both C. albicans hyphae and yeast cells were frequently observed in the liver and occasionally in the lungs and kidneys of immunocompromised mice sacrificed 20 days post-inoculation. The frequent occurrence of abscesses in the liver simulates a clinical variant of this mycosis, referred to as focal hepatic candidosis, which has been recognized with increasing frequency in immunocompromised patients. We suggest that the animal model described here may be particularly useful both for exploring methods which may prevent dissemination of C. albicans from localized foci of colonization in the GI tract after exposure of the host to immunocompromising drugs, and for testing the efficacy of anti-Candida drugs in clearance of the pathogen from body organs with established fungal abscesses.

Gastrointestinal candidiasis: histopathology of Candida-host interactions in a murine model

Candida albicans is a commonly occurring member of the indigenous microflora of the human gastrointestinal (GI) tract. The fungus may be acquired as a gut component at an early age, even by the foetus during its passage through the birth canal. C. albicans has apparently evolved as a successful GI commensal of humans. The significance of this fungus is that it is an opportunistic pathogen which takes advantage of the immunocompromised condition of patients. Such a condition may be due to an underlying primary disease or other microbial infection that leads to immune cell dysfunctions. C. albicans colonization of the gut poses a potential threat to the health of the individual, therefore, if that person becomes immunocompromised as in the case of patients with acquired immunodeficiency syndrome (AIDS). A murine model of GI candidiasis is described in this review, which demonstrates many of the aspects of fungal-host interactions that occur in the human neonate, immunocompetent juvenile and adult. Infant mice (5–6 d old) were inoculated with yeast by the oral-intragastric route. The immediate effects of this challenge were the ‘persorption’ of yeast cells across the bowel wall into the bloodstream, which resulted in systemic infection, as well as the adherence of large numbers of C. albicans to the GI mucosa. The systemic infections, which primarily involved the liver, were cleared in the mice over a period of approximately 96 h post-inoculation, while colonization of the stomach by C. albicans persisted for at least 4 wk. The duration of gut colonization after oral challenge at infancy is at least partly dependent on the strains of C. albicans and mouse employed. The infant mouse model permitted studies of the histopathology of long-term Candida colonization of the GI tract, combined with investigations of the nature of host mucosal immune and non-immune cell response to chronic yeast infection. These features of the murine model of GI candidiasis are the focus of this review.

The Role of Candida albicans NOT5 in Virulence Depends upon Diverse Host Factors In Vivo

ABSTRACT We previously identified Candida albicans Not5p as an immunogenic protein expressed during oropharyngeal candidiasis (OPC). In this study, we demonstrate that C. albicans NOT5 reverses the growth defects of a Saccharomyces cerevisiae not5 mutant strain at 37°C, suggesting that the genes share at least some functional equivalence. We implicate C. albicans NOT5 in the pathogenesis of disseminated candidiasis (DC) induced by intravenous infection among neutropenic and nonimmunosuppressed mice, as well as in that of OPC in mice immunosuppressed with corticosteroids. We find no role in virulence, however, among neutropenic and corticosteroid-suppressed mice with DC resulting from gastrointestinal translocation, nor do we implicate the gene in vulvovaginal candidiasis among mice in pseudoestrus. These findings suggest that the role of NOT5 in virulence depends on the specific in vivo environment and is influenced by diverse factors such as tissue site, portal of entry, and the status of host defenses. NOT5 is necessary for normal adherence to colonic and cervical epithelial cells in vitro, demonstrating that such assays cannot fully replicate disease processes in vivo. Lastly, antibody responses against Not5p do not differ in the sera of patients with OPC, patients with DC, and healthy controls, suggesting that the protein is associated with both commensalism and the pathogenesis of disease.

Factors Regulating Morphogenesis in Coccidioides Immitis

The parasitic cycle of Coccidioides immitis is unique among the human systemic fungal pathogens. However, at the level of cell wall biosynthesis and modification C. immitis demonstrates features which are shared by other fungal pathogens. Three distinct events in the morphogenesis of parasitic cells, or spherules, of Coccidioides are examined in this study. These include the diametric growth phase of round cells (young spherules), spherule segmentation, and endosporulation. Three enzymatic products of developing spherules have been suggested to participate in regulation of these successive morphogenetic stages of C. immitis. Preliminary evidence is presented that a s-1,3-endoglucanase contributes to plasticization of the round cell wall, and consequently plays a role in diametric expansion of young spherules. Results of earlier studies of a 34 kDa proteinase are reviewed which suggest that this wall-associated enzyme may function in development of the segmentation wall. Data from recent studies of a 100 kDa chitinase are presented which suggest that this enzyme participates in endosporulation of the parasitic cells. Morphogenetic studies of Coccidioides may lead to the identification of regulatory factors which are common to other pathogenic fungi, and therby, to the characterization of molecular targets for development of future antifungal reagents.

Cell Wall-Associated Proteinases of Coccidioides Immitis

Coccidioides immitis is the causative agent of a human respiratory disease known as coccidioidomycosis (’cocci’ or valley fever), which is characterized by a multiplicity of clinical forms. Approximately 100,000 cases of this fungal disease are diagnosed annually in the United States, primarily on the basis of serological tests (Cole et al., 1990). Results of clinical studies of coccidioidomycosis have suggested that underlying immunological illnesses are not prerequisite to infection. On the other hand, immunocompromised patients such as those diagnosed with acquired immunodeficiency syndrome (AIDS) who have previously lived or currently reside in endemic areas have been reported to contract symptomatic coccidioidomycosis at an unpredicted high annual frequency (27%; Bronnimann et al., 1987). A large percentage of these cases may represent reactivation of an old infection. C. immitis is apparently able to persist in body organs in the absence of overt illness. The nature of this persistent form of the pathogen in host tissue is not known.