Experimental Candida albicans infection in conventional mice and germfree rats

Gnotobiotics and the Microbiome

This chapter provides an overview of germfree (GF), gnotobiotic (GN), and defined flora (DF) laboratory rats, relating their history, traditional and modern derivation procedures, the anatomy and physiology, and their use in the study of mammalian host–microbiome relationships. Extensive literature on the nutrition and physiology of GF rats and the expanding library of immunological reagents have increased the research utility of GF, GN, or DF rats. Such rats have been extensively used in metabolic experiments as nucleus seed stocks for the production of disease-free animals and as tools for infectious disease studies, among others. The chapter also presents research applications of GF rats that are particularly suitable for testing candidate viral carcinogens since they are uniquely free of all known viruses, for pathology studies in the distinguishing of primary mediation lesions from those associated with infections, and the study of the biological effects of radiation.

Human serum inhibits adhesion and biofilm formation in Candida albicans

BACKGROUND

Candida albicans can form biofilms on intravenous catheters; this process plays a key role in the pathogenesis of catheter infections. This study evaluated the effect of human serum (HS) on C. albicans biofilm formation and the expression of adhesion-related genes in vitro. A C. albicans laboratory strain (ATCC90028) and three clinical strains were grown for 24 h in RPMI 1640 supplemented with HS or RPMI 1640 alone (as a control). The growth of biofilm cells of four strains was monitored by a Live Cell Movie Analyzer, and by XTT reduction assay. The expression of the adhesion-related genes BCR1, ALS1, ALS3, HWP1 and ECE1 was analyzed by RT-PCR at three time points (60 min, 90 min, and 24 h).

RESULTS

In the adhesion phase, C. albicans cells kept a Brownian movement in RPMI medium containing HS until a large number of germ tubes were formed. In the control group, C. albicans cells quickly adhered to the bottom of the reaction plate. Compared with RPMI 1640, medium supplemented with 3-50% HS caused a significant decrease in biofilm development (all p < 0.001). However, the presence of HS had no significant inhibitory effect on the pre-adhered biofilms (all p > 0.05). Biofilm formation was also inhibited by heat-inactivated and proteinase K pre-treated HS. The presence of 50% HS did not significantly affect the planktonic growth of C. albicans (p > 0.05). At three time points, HS inhibited expression of the ALS1 and ALS3 genes and promoted expression of the HWP1 and ECE1 genes. Significant up-regulation of BCR1 was observed only at the 90-min point.

CONCLUSIONS

Human serum reduces biofilm formation by inhibiting the adhesion of C. albicans cells. This response may be associated with the down-regulation of adhesion-related genes ALS1, ALS3 and BCR1. The inhibitory serum component is protease-resistant and heat stable.

Mouse intravenous challenge models and applications

Murine intravenous (IV) challenge models have been widely used in medical mycology to study fungal virulence, host responses, and antifungal efficacy. This chapter describes the well-characterised Candida albicans IV challenge model, where fungal cells are administered directly into the mouse bloodstream to initiate a systemic infection. The preparation of tissue samples from infected mice to allow evaluation of disease progression and host responses is also described.

Candidiasis--do we need to fight or to tolerate the Candida fungus?

Candidiases, infections caused by germination forms of the Candida fungus, represent a heterogeneous group of diseases from systemic infection, through mucocutaneous form, to vulvovaginal form. Although caused by one organism, each form is controlled by distinct host immune mechanisms. Phagocytosis by polymorphonuclears and macrophages is generally accepted as the host immune mechanism for Candida elimination. Phagocytes require proinflammatory cytokine stimulation which could be harmful and must be regulated during the course of infection by the activity of CD8+ and CD4+ T cells. In the vaginal tissue the phagocytes are inefficient and inflammation is generally an unwanted reaction because it could damage mucosal tissue and break the tolerance to common vagina antigens including the otherwise saprophyting Candida yeast. Recurrent form of vulvovaginal candidiasis is probably associated with breaking of such tolerance. Beside the phagocytosis, specific antibodies, complement, and mucosal epithelial cell comprise Candida eliminating immune mechanisms. They are regulated by CD4+ and CD8+ T cells which produce cytokines IL-12, IFN-gamma, IL-10, TGF-beta, etc. as the response to signals from dendritic cells specialized to sense actual Candida morphotypes. During the course of Candida infection proinflammatory signals (if initially necessary) are replaced successively by antiinflammatory signals. This balance is absolutely distinct during each candidiasis form and it is crucial to describe and understand the basic principles before designing new therapeutic and/or preventive approaches.

Evaluation of antifungal pharmacodynamic characteristics of AmBisome against Candida albicans

A liposomal formulation of Amphotericin B (AmBisome), with small unilamellar vesicles containing amphotericin B, shows characteristic pharmacokinetics as liposomes, and in consequence, has different pharmacological activity and toxicity from amphotericin B deoxycholate (Fungizone). In this study, we evaluated the antifungal pharmacodynamic characteristics of AmBisome against Candida albicans using the in vitro time-kill method and murine systemic infection model. A time-kill study indicated that the in vitro fungicidal activities of AmBisome and Fungizone against C. albicans ATCC 90029 increased with increasing drug concentration. For in vivo experiments, leucopenic mice were infected intravenously with the isolate 4 hr prior to the start of therapy. The infected mice were treated for 24 hr with twelve dosing regimens of AmBisome administered at 8-, 12-, 24-hr dosing intervals. Correlation analysis between the fungal burden in the kidney after 24 hr of therapy and each pharmacokinetic/pharmacodynamic parameter showed that the peak level/MIC ratio was the best predictive parameter of the in vivo outcome of AmBisome. These results suggest that AmBisome, as well as Fungizone, has concentration-dependent antifungal activity. Furthermore, since AmBisome can safely achieve higher concentrations in serum than Fungizone, AmBisome is thought to have superior potency to Fungizone against fungal infections.

Calcineurin is required for Candida albicans to survive calcium stress in serum

The calcium-activated protein phosphatase calcineurin plays a critical role in the virulence of Candida albicans. Previous studies demonstrated that calcineurin is not required for the yeast-hypha dimorphic transition, host cell adherence, or host cell injury, which are all established virulence attributes of this organism. Calcineurin is, however, essential for survival in serum and disseminated infection. Here we identify the component of serum that is toxic to calcineurin mutant cells. Proteins, peptides, lipids, and other hydrophobic components were all excluded as essential toxic elements. Upon testing of small molecules present in serum, we discovered that calcineurin protects cells from stress caused by the endogenous levels of calcium ions present in serum. These studies illustrate how calcineurin functions in a calcium homeostatic pathway that enables a common human commensal to survive passage through the hostile environment of the bloodstream to establish deep-seated infections and cause disease.

Temporal events in the intravenous challenge model for experimental Candida albicans infections in female mice

We characterized the intravenous (i.v.) challenge model for disseminated Candida albicans infection in female BALB/c and DBA/2 mice. Clearance of fungi from the bloodstream and appearance of fungi in tissues were measured at intervals after challenge with various doses of C. albicans. The wild-type isolate SC5314 and derived strains CAF2-1 and CAI-4 transformed with CIp10 were of equal virulence in the model. Variability in mouse survival times, kidney fungal burdens and cachexia was lowest when challenge inocula were within the range 10(4)-10(5) CFU g(-1) body weight in BALB/c mice, but brain fungal burdens and outcomes in DBA/2 mice were variable for all inocula tested. Critical times in the development of infections in optimally challenged BALB/c mice were at 5-10 h (bloodstream fully cleared of fungi), 24 h (start of exponential fungal growth in kidneys) and 48 h (50% of blood cultures become positive). Differential involvement of right and left kidneys occurred almost exclusively in mice challenged with <2 x 10(4) CFU g(-1). We conclude that the i.v. challenge model in female BALB/c mice is now sufficiently well characterized to permit more refined experimentation in future virulence studies with C. albicans mutants.

Comparative virulence of Candida albicans strains in CFW1 mice and Sprague-Dawley rats

To verify host-species specificities of virulence of Candida albicans in experimental systemic mycoses, 10 ATCC strains of Candida albicans, were compared for their virulence in CFW1 mice and Sprague-Dawley rats. Virulence was parallel in mice and rats, four strains were avirulent (ATCC 10231, 18804, 38245, 44831), one strain had an intermediate virulence (ATCC 32354), and five strains (ATCC 10261, 44373, 44505, 62342, 90028) were highly virulent in both host species. Infection doses of 2 x 10(6) CFU per mouse and 5 x 10(6) CFU per rat were comparable with respect to mortality of animals within 10 days; this represents a 4 : 1 ratio on the basis of body weight. In Sprague-Dawley rats haemorrhage occurred in infections with all virulent strains which was not observed in CFW1 mice.

Systemic candidiasis in Sprague-Dawley rats

A reproducible model of a generalized Candida albicans infection was established in rats to allow a precise evaluation of the efficacy of antifungal compounds. In contrast to the intravenous C. albicans model in mice, which serves as a primary model for in vivo efficacy studies of antimycotic compounds, the infectious process in Sprague-Dawley rats is more severely spread into organs other than the kidneys, such as brain, heart, liver, lung, retina and spleen. Apart from a severe granulomatous nephritis beginning 1 day after infection, we observed a severe pneumonitis 3 days after infection with a mass of extravasal erythrocytes in the interstitium and the alveolar space. In addition, multiple nodular lesions could be observed in the brain, heart, liver, retina and spleen on the first day after infection. Lethality was 100% within 1 week, the majority of deaths occurring from 5 to 7 days. Antifungal therapy with amphotericin B or fluconazole led to long-term survival over 4 months, which could not be achieved in mice.

Protection against murine disseminated candidiasis mediated by a Candida albicans-specific T-cell line

The role of T lymphocytes in disseminated candidiasis in a mouse model of irradiation-induced immunosuppression was investigated. A continuously cultured Candida albicans-specific T-cell line mediated protection of sublethally irradiated mice from disseminated candidiasis as measured by both the fungal load in the kidneys and mortality. These results are the first to demonstrate directly a role for antigen-specific T cells in the protective immune response against murine disseminated candidiasis.

Tumor necrosis factor and interleukin-6 in Candida albicans infection in normal and granulocytopenic mice

We administered a neutralizing monoclonal antibody to tumor necrosis factor (TNF) during infection with Candida albicans in normal and granulocytopenic mice. Mice were rendered granulocytopenic (less than 0.1 x 10(9) granulocytes per liter) with cyclophosphamide. Growth of C. albicans from the kidneys was significantly increased in normal mice treated with the antibody to TNF, compared with that in control mice, after 36 h (3.6 x 10(4) +/- 1.2 x 10(4) CFU per kidney versus 9.1 x 10(3) +/- 6.2 x 10(3) CFU per kidney; P less than 0.05) and after 72 h (3.7 x 10(6) +/- 2.7 x 10(6) CFU per kidney versus 2.3 x 10(4) +/- 1.3 x 10(4) CFU per kidney; P less than 0.01). In granulocytopenic mice, the antibody to TNF had no effect on the growth of C. albicans from the kidneys. Furthermore, our study showed that the cytokines TNF and interleukin-6 (IL-6) were produced in a dose-dependent manner during C. albicans infection. TNF was detectable between 6 and 60 h, with peak levels at 24 h. Both TNF and IL-6 levels were significantly higher in cyclophosphamide-treated mice than in normal mice. Heat-inactivated C. albicans induced a TNF response different from that induced by viable C. albicans, with an early peak occurring at 3 to 4 h and declining to non-detectable levels after 15 to 24 h. Peak levels of TNF obtained with heat-inactivated C. albicans were lower than those obtained with viable C. albicans. Our study demonstrates that TNF and IL-6 are produced systemically during C. albicans infection and suggests that TNF is essential for granulocyte antifungal activity in vivo.

Mannan estimation as a measure of the growth of Candida albicans

Determination of Candida albicans mannan levels by an enzyme-linked immunosorbent assay was compared with dry weight measurement and viable counting as a means to estimate growth of the fungus in broth cultures. Levels of extracted mannan showed good correlation with fungal biomass (dry weight) (r = 0.984) and, although biomass and mannan levels correlated with viable counts, the correlation was not as strong (r = 0.954-0.959). The technique offered an accurate alternative to viable counts in the brain and kidney when mannan estimation was used to follow the distribution of C. albicans in an experimental model of systemic candidosis in mice. Mannan levels were less useful in following the course of the infection in the liver and spleen where accumulation of free and serum-bound mannan was a problem. Mannan estimation would therefore seem to offer an accurate alternative to viable counts for determining the quantity of Candida both in vitro and in certain infected organs of experimental animals, where measurement of fungal biomass is not feasible and viable counts can be grossly inaccurate.

Pathogenicity of Trichosporon capitatum for normal and irradiated mice and the efficacy of miconazole on experimental systemic trichosporosis in mice

The authors describe the course of experimental trichosporosis in normal and X-irradiated ICR mice after i.v. inoculation with Trichosporon capitatum. The irradiated animals were considerably more sensitive to infection than normal animals. The LD50 challenge dose used was by approximately two orders lower (1 X 10(3) c.f.u. ml-1) in irradiated mice than in control animals. The histopathological examination of the internal organs of the infected mice demonstrated that the greatest tissue damage was associated with the kidneys, liver and spleen. However, the infectious agent was also found in heart, lungs and brain. The degree of impairment of the tissues was dependent on the inoculation dose and on the irradiation status. Miconazole (50 mg kg-1) was administered i.p. immediately after inoculation with Ts. capitatum and resulted in an alteration of infection and prolonged survival time. Miconazole was ineffective when challenge dose were used which produced 100% mortality (1 X 10(6) and 1 X 10(4) c.f.u. ml-1 for normal and irradiated mice, respectively). With the use of these doses also the course of infection was nearly identical both in the miconazole-treated and untreated animals.

Comparative pathogenicity of a wild-type strain and respiratory mutants of Candida albicans in mice

The pathogenicity of a parent wild-type strain and three respiratory mutants of Candida albicans was examined in intravenously infected mice. The wild-type strain K grew well in the kidney and caused severe candidosis, and the 21-day LD50 value was 7.2 x 10(6) cells/mouse. A mutant with a low rate of respiration (KRD-8) whose growth rate in vitro was somewhat lower than that of the wild type, produced germ tubes in vitro to the same extent as the wild-type strain and was associated with mortality rates similar to those of the wild-type strain. Two respiration-deficient (petite) mutants (KRD-19 and KRD-51), whose growth rates in vitro were far lower than that of the wild-type strain, could neither colonize the kidney nor cause fatal infection, even at a dose of 10(8) cells/mouse. Formation of germ tubes and hyphal growth in vitro of the petite mutants were less extensive than those of the wild-type strain or KRD-8. Extracellular proteinase was produced at pH 3.5 by the wild-type strain and by KRD-8 but not by the petite mutants. From these results, it is most likely that the nonlethality of infection by the petite mutants in mice results primarily from the low capacity of growth of these mutants, even though the inability of the petite mutants to produce extracellular proteinase may be also related to some extent to their avirulence.

Effect of Friend leukemia virus infection on susceptibility to Candida albicans

Previous studies have demonstrated that Friend leukemia virus (FLV) induces a profound immunosuppression in susceptible mice. The studies described in this report indicate that mice infected with FLV have an increased susceptibility to subsequent infection with the opportunistic pathogen Candida albicans, as measured by increased numbers of C. albicans CFU in the kidneys of FLV-infected mice relative to uninfected controls. Experiments in which the NB-tropic and N-tropic strains of FLV were used suggest that virus replication or the resulting virus burden may be important in the observed increased susceptibility to C. albicans. Since neutrophils are believed to be important in the response of mice to systemic Candida infections, the effect of FLV infection on neutrophil candidacidal activity was investigated. The percentage of neutrophils present in unfractionated Proteose Peptone-elicited peritoneal exudates of mice infected with FLV for 14 days was significantly lower than in uninfected control mice or mice infected with FLV for 6 or 10 days. When neutrophils from FLV-infected and control mice were purified, adjusted to equal concentrations, and tested for in vitro candidacidal activity, neutrophils from mice infected with FLV for 14 days were deficient in their ability to kill C. albicans relative to normal controls and mice infected with FLV for 6 or 10 days. Addition of normal mouse serum increased killing in all groups but did not restore candidacidal activity of neutrophils from mice infected with FLV for 14 days to levels of control neutrophils or neutrophils from mice infected for 6 or 10 days with the virus. These results suggest a defect in neutrophil function, at the later stages of FLV infection, involving in vitro candidacidal activity. In addition, neutrophils from FLV-infected mice may be deficient in in vivo chemotactic activity. These defects in neutrophil function could account, at least in part, for the observed increased susceptibility of FLV-infected mice to C. albicans.

The pathogenesis of acute systemic candidiasis in a susceptible inbred mouse strain

The histopathological features of the acute infection by Candida albicans in susceptible CBA/H mice, as well as its dissemination in various organs has been examined. The brain, in which the localization of blastospores is minimal, is the organ most severely affected. A significant deposition of blastospores occurs in lungs, liver, spleen and kidney, but of these organs, only in the kidneys is infection established. A transient myocarditis follows the minor blastospore deposition that occurs in the heart. Infection in the affected organs is characterized by abscess formation while in the brain the process is complicated by the development of hydrocephalus and a pachymeningitis that erodes the petrous temporal and involves the vestibular apparatus. There is also infection of the retinae with an accompanying endophthalmitis.

Systemic candidiasis in mice immunized with Candida albicans ribosomes

In view of our previous findings that vaccination of mice with Candida albicans ribosomes protects them against experimental systemic candidiasis, the aim of this study was to investigate the effect of this vaccination on the course of infection in immunized animals. Since the kidney is the major target in systemic candidal infection, we concentrated in this research on studying the histopathology and determining quantitatively the candidal colonization of this organ. The experiments were carried out at various time intervals after intravenous inoculation with live C. albicans. The colonization of kidneys in immunized mice was markedly lower than that in controls. The maximal difference in renal colonization between immunized and non immunized animals was observed when relatively low challenge doses were used. The inhibition of candidal multiplication in immunized mice seemed to be correlated to their increased resistance against lethal challenge, as expressed by a significantly higher survival rate. Histopathological changes and fungal elements were found in kidneys of control mice as early as 20 h post infection, while the kidneys of immunized mice did not seem affected by the disease. Moreover, even 3 days post infection, the kidneys of vaccinated animals still seemed normal. In conclusion, apparently the ribosomal vaccination leads to diminished colonization of the major site of infection in candidiasis, thus affording protection to the immunized animals against these infections.

Lysosomal enzyme activities in polymorphonuclear leukocytes, macrophages, serum, and spleen of conventional, germ-free, and antigen-free Minnesota miniature swine

The activities of lysozyme, myeloperoxidase, and elastase were lower in PMNs and AMs from GF and AF Minnesota miniature piglets than in the leukocytes from their CONV counterparts. In the spleen and serum of gnotobiotic piglets only the levels of lysozyme were slightly reduced. Substantially depressed activities of these LEs were found also in PMNs from precolostral piglets in comparison with PMNs from their CONV mother. The bisassociation of GF piglets with Enterococcus liquefaciens and Escherichia coli caused an increase of LE activities in their AMs, spleens, and sera. Fewer LEs were released after phygocytic stimulation with zymosan from PMNs of GF, AF, and precolostral piglets than from PMNs of CONV animals of the same age. These data suggest that the antigenic-microbial stimulation is important for the development of normal lysosomal enzyme activities in PMNs and AMs from gnotobiotic animals.

Absence of correlation between delayed-type hypersensitivity and protection in experimental systemic candidiasis in immunized mice

We found that in mice which had been immunized intraperitoneally with 2 x 10(8) heat-killed Candida albicans cells there was a striking temporal relationship between resistance to systemic challenge with 10(6) living C. albicans cells and a number of measurable cellular parameters of the host response. These included the emergence of delayed-type hypersensitivity and the development of granulocytosis. Since it had been shown in previous work that granulocytosis was associated with an increase in resistance when nonspecific immunostimulation was used, we performed experiments to induce delayed-type hypersensitivity without any measurable modification of the granulocyte population. Adoptive transfer of delayed-type hypersensitivity with spleen cells from immune and resistant donor mice did not produce any increase in resistance in normal recipients. When separate groups of mice were immunized intraperitoneally or subcutaneously with varying doses of heat-killed C. albicans, we found that doses of less than 10(8) cells did induce significant delayed-type hypersensitivity without any increase in granulocytosis. In such mice, as well as in animals pretreated with immunomodulators before immunization with heat-killed C. albicans, the presence of cell-mediated immunity, as measured by the delayed-type hypersensitivity test, was not associated with an increase in resistance against systemic candidiasis. On the contrary, the results suggested that cell-mediated immunity was associated with an increase in the susceptibility of these mice. The same effect on candidiasis susceptibility was observed when animals were immunized with heat-killed filamentous C. albicans.

Physiological and metabolic alterations accompanying systemic candidiasis in mice

Mice challenged intravenously with 10(6) viable Candida albicans died between 1 and 16 days after infection. Near the time of death, over 98% of the recoverable fungi came from the kidneys. Physiologically, animals were in renal failure near the time of death as evidenced by elevated blood urea nitrogen (BUN) and blood creatinine levels and a creatinine clearance rate which was about one-half normal. No abnormalities in liver glucogen and blood glucose levels were detectable. When mice were challenged with 4.5 X 10(6) viable C. albicans, they all died within 12 h. Near the time of death they had normal BUN values and were hyperglycemic. In mice receiving 4.5 X 10(6) heat-killed C. albicans, no deaths occurred and liver glycogen, blood glucose, and BUN levels all remained within a normal range and were different from responses to bacterial endotoxin. Cumulatively, the results demonstrate two distinct syndromes for the pathogenesis of experimental C. albicans infections. At the lower dose, mice were in renal failure associated with progressive renal infection. At the higher dose, renal failure was not observed. If a toxin was associated with death from the latter dose, it was not similar to bacterial endotoxin.

Comparison of the growth of virulent and attenuated strains of Candida albicans in the kidneys of normal and cortison-treated mice by chitin assay

Candida albicans strain 22114 was avirulent for mice compared with strain 19321 in that, administered intravenously, 10(6) blastospores of 22114 failed to kill whereas 10(6) blastospores of 19321 produced 100% mortality. Cortisone treatment rendered mice susceptible to killing by 22114. Chitin assay showed that cortisone stimulated the growth of both strains in the mouse kidney. Growth of 19321 was increased up to five-fold and 22114 up to forty-fold. The strains may have differential susceptibility to cortisone-sensitive host defences which control fungal growth in vivo.

Kinetics of lymphocyte transformation in mice immunized with viable avirulent forms of Cryptococcus neoformans

A murine model was developed to study the cell-mediated immune response of mice immunized with one of two live, avirulent forms of Cryptococcus neoformans: a nonencapsulated mutant and a thinly encapsulated pseudohyphal variant. A lymphocyte transformation assay was used to evaluate the cellular response of control and sensitized spleen cells after in vitro incubation with three merthiolate-killed whole-cell antigens of C. neoformans. An antigen-to-spleen cell ratio of 10:1 and 5 days of incubation of antigen-spleen cell mixtures were established as optimal conditions for maximum lymphocyte transformation. Maximum responses occurred from 2 to 3 weeks after the last of eight weekly intraperitoneal inoculations of C. neoformans. This assay provided an accurate, reproducible method of studying cell-mediated immunity to C. neoformans, and applications to the study of cryptococcal pathogenesis are proposed.

Candida albicans dimorphism and virulence: role of copper

Previously reported observations that Candida albicans grows in the yeast phase at 30C and the mycelial phase at 37 degrees C and that the former phase is more virulent than the latter were confirmed. A novel factor, copper, was discovered to suppress filamentation. Injection of copper into mice permitted the filamentous phase to be as virulent as the yeast phase. In subsequent studies on candidosis, copper assays should be performed on relevant body fluids to determine if there might be a correlation between elevated copper and heightened susceptibility to the fungus.

Biochemical examination of sera during systemic Candida infection in mice

Candida pathogenesis was examined by intravenous challenge of mice with either C. albicans or C. guilliermondii. Animals were moribund 12 h postchallenge with C. albicans and were found to have the greatest number of organisms in the heart and kidney, severe interstitial myocarditis, and elevated serum levels of blood urea nitrogen, creatine phosphokinase, serum glutamic oxalacetic transaminase, serum glutamic pyruvic transaminase, and lactic dehydrogenase. In contrast, challenge with C. guilliermondii resulted in a significantly lower renal census, no myocarditis, and no significant change in the concentration of these serum constituents. Challenge with nonviable C. albicans did not produce the effects observed with viable organisms. Moreover, challenge with filamentous C. albicans resulted in biochemical alterations of lower magnitude and in lower mortality rates. These results indicated that altered serum biochemistries were correlated with the histopathology of fatal Candida infection and that there were distinct differences with C. guilliermondii and the dimorphic phases of C. albicans.

Immunity to experimental renal candidiasis in rats

Germfree rats were found to be more susceptible to intravenous challenge with Candida albicans than were conventional rats. The resistance of conventional rats could be overcome by increasing the challenge dose. The resistance of the germfree rat was enhanced by vaccination with Formalin-killed C. albicans in complete Freund adjuvant, complete Freund adjuvant, or incomplete Freund adjuvant. These results, and histological evidence obtained from infected gnotobiotic rats, provided further information on the mechanism of resistance to the disseminated form of candidiasis.

Experimental murine candidiasis: pathological and immune responses to cutaneous inoculation with Candida albicans

Cutaneous infection of mice with Candida albicans elicited a predominantly acute inflammatory response, stimulated the production of precipitating antibodies, and conferred protection against subsequent intravenous challenge with the same organism. The acute inflammatory skin reaction seen after cutaneous infection suggested a predominantly humoral response to Candida. Animals infected cutaneously a second time with viable C. albicans developed larger skin lesions than animals infected only once, and the twice-infected animals were more resistant to an intravenous challenge as well. The cutaneous inoculation of mice with heat-killed C. albicans was less effective in stimulating antibody production, in eliciting the inflammatory response, and in inducing a protective response demonstrable by intravenous challenge with viable Candida. This model of experimental candidiasis represents a reproducible means of studying a protective immune response to the organism.