Factors affecting the adherence of Candida albicans to human buccal epithelial cells in human immunodeficiency virus infection

The genotypes and virulence attributes of C. albicans isolates from oral leukoplakia

Background

There is a debate as to whether some types of oral leucoplakias (OL) are caused by Candida species, and whether they contribute to the malignant transformation, associated with a minority of such lesions. As no detailed population analysis of yeast isolates from OL is available, we evaluated the virulence attributes, and genotypes of 35 C. albicans from OL, and compared their genotypes with 18 oral isolates from healthy individuals.

Material and Methods

The virulence traits evaluated were esterase, phospholipase, proteinase, haemolysin and coagulase production, and phenotypic switching activity, and yeast adherence and biofilm formation. DNA from OL and control yeasts were evaluated for A, B or C genotype status.

Results

Phospholipase, proteinase, and coagulase activity and biofilm formation was observed in 80%, 66%, 97 % and 77 % of the isolates, respectively. Phenotypic switching was detected in 8.6%, while heamolytic, and esterase activity and adherence were noted in all isolates.

Conclusions

The genotype A was predominant amongst both the OL and control groups. Due to the small sample size of our study a larger investigation to define the role of candidal virulent attributes in the pathogenicity of OL is warranted, and the current data should serve as a basis until then.

Key words:C. albicans, oral cavity, leukoplakia, virulence factors, genotyping.

Oral candidal carriage correlates with CD4+ cell count but not with HIV and highly active antiretroviral therapy status

AIM

The occurrence of oropharyngeal candidiasis (OPC) may be influenced by oral candidal carriage (OCC). Although OPC is strongly associated with low CD4⁺ cell count (400-700 cells/mm³ ) and a lack of highly active antiretroviral therapy (HAART), the effect of these two parameters on OCC is debatable. We investigated the oral candidal carriage, species diversity, antifungal susceptibility and the association of OCC with CD4⁺ cell count and HAART.

METHODS

Oral candidal isolates from 120 HIV⁺ patients (60 receiving and 60 not receiving HAART) and 60 healthy controls were quantified, and their species determined using standard culture and biochemical methods, followed by antifungal susceptibility testing using the agar dilution method.

RESULTS

The OCC was significantly higher in HIV⁺ patients; Candida albicans was the most frequently isolated species in both groups, followed by Candida tropicalis. Candidal density carriage correlated significantly with CD4⁺ cell count, but not with HIV and HAART status. Among the isolates from HIV⁺ patients, 35.4% showed reduced susceptibility to fluconazole.

CONCLUSION

HIV status results in significantly elevated rates of OCC C albicans remains the predominant pathogen, although other species are emerging rapidly. Resistance to fluconazole is on the rise, and more efficient treatment strategies need to be implemented.

Experimental superficial candidiasis on tissue models

Candida species are common pathogens causing superficial mycoses primarily affecting the mucosa and the skin in humans. Crucial steps during pathogenesis of superficial candidiasis comprise fungal adhesion, colonisation and subsequent penetration of the respective tissues. Exploring these pathological events and perhaps fungal and tissue responses towards drug treatment is imperative in the management of this infection. Unfortunately, pathological biopsies of superficial candidiasis do not exhibit the early changes in the host-pathogen interaction as the tissues are already invaded by the fungi. In vivo experimental assessments of pathological processes of superficial candidiasis are also limited because of the difficulties in providing reproducible and comparable conditions in the host environment. Conversely, in vitro models have helped studying fungal-host interactions under more defined and controlled conditions. Some common in vitro models used to simulate superficial candidiasis are chick chorioallantoic membrane, mucosal explants and single layer or multiple layer cell cultures. Interestingly, these experimental approaches share advantages as well as disadvantages when compared with in vivo conditions. Hence, this review intends to discuss about the experimental superficial candidiasis produced in various tissue models and their advantages as well as disadvantages with a particular reference to further improvement of validity and reliability of such experiments.

(B1) Candida and mycotic infections

Oral candidiasis (OC) is the most common mucosal manifestation of HIV infection. This workshop examined OC and other mycoses associated with HIV infection. Historically, blood CD4 cell numbers were the primary prognosticator for the development of OC. However, a study that statistically evaluated the predictive role of HIV viral load vs. CD4 cell counts revealed viral load to be a stronger predictor for OC. The role of biofilms and antifungal resistance in recalcitrant OC is unclear at present. In general, micro-organisms including yeasts in biofilms are more resistant to antifungals than their planktonic counterparts. When the remaining organisms are eliminated, the few resistant organisms may not be problematic, because they are present in low numbers. Unusual exotic mycoses in HIV-infected patients are more common in patients from the developing than the developed world. These infections may be recurrent and recalcitrant to therapy, be present in multiple and uncommon sites, increase with the progression of HIV disease, and may play a role similar to that of the more common mycoses. Typing and subtyping of yeasts are probably not critical to the clinical management of candidiasis caused by Candida albicans and non-albicans strains, including C. dubliniensis, because it is responsive to antifungal therapy. C. glabrata is probably the only exception. The presence of oral thrush in infants younger than 6 months of age is associated with an increased post-natal transmission risk of HIV infection. Thus, perinatal retroviral therapy should be combined with the treatment of oral thrush to prevent the post-natal acquisition of HIV.

Current trends of HIV disease of the mouth

HIV infection affects residents of all countries of the world, but the greater majority of affected individuals reside in the developing world. In the past decade there have been substantial changes in the management of HIV disease, particularly the introduction of highly active antiretroviral therapy (HAART). Such agents have reduced significantly the morbidity and mortality associated with HIV disease, however, they are not available for most HIV-infected individuals in the developing world. There is now considerable understanding of the molecular epidemiology, transmission and therapy of the common opportunistic oral infections of HIV disease, and as a consequence of improved anti-HIV strategies, the frequency and severity of oral disease associated with HIV infection have reduced considerably, although HAART may predispose to human papilloma virus infection of the mouth and potentially increase the risk of later oral squamous cell carcinoma. Despite advances in clinical care the majority of individuals with HIV disease worldwide will continue to develop oral disease, as they are resident in the developing world and do not have ready access to even simple therapies.

Evaluation of the roles of four Candida albicans genes in virulence by using gene disruption strains that express URA3 from the native locus

Reintroducing URA3 to its native locus in Candida albicans not5, not3, bur2, and kel1 disruption mutants enabled us to directly compare strains with control strain CAI-12. We showed that URA3 position affected orotidine 5'-monophosphate decarboxylase activity, hyphal morphogenesis, adherence, and mortality in murine disseminated candidiasis. After URA3 was reintroduced to its native locus, only NOT5 could be conclusively ascribed a role in virulence.

Adhesion and cell-surface-hydrophobicity of sequentially isolated genetic isotypes of Candida albicans in an HIV-infected Southern Chinese cohort. Adharenz und Zelloberflachen-Hydrophobizitat von sequentiell isolierten genetischen Isotypen von Candida albicans aus einer HIV-infizierten sudchinesischen Kohorte

Objectives of the study were to investigate the variability in yeast adhesion and cell-surface-hydrophobicity (CSH) during human immunodeficiency virus (HIV) disease progression, using a total of 60 sequential Candida albicans isolated from oral rinse samples of seven HIV-infected individuals with (4) and without (3) clinical symptoms of oropharyngeal candidosis. Significant differences in the adhesion to buccal epithelial cells (BECs) during sequential visits were observed for all genetic isotypes in five of the seven individuals and three isotypes belonging to the sixth individual. A single isotype of patient HK1 and another of HK4 (genotype I) demonstrated significant variations in their CSH during sequential visits whereas no such differences were noted for the remaining genotypes. On Spearman correlation analysis an isotype from HK1 demonstrated a significant increased adherence to BECs and CSH during HIV disease progression whereas no such correlation was noted for the remaining isotypes studied. No significant differences in adherence to BECs or CSH values were observed between the symptomatic oral candidosis and the asymptomatic carrier group. Further, on regression analysis only the single isotype of HK1 demonstrated a significant positive correlation between adherence to BECs and CSH whereas no such correlation was observed when all tested Candida isolates were pooled and evaluated as a single, large group.

Biofilm-forming ability of Candida albicans is unlikely to contribute to high levels of oral yeast carriage in cases of human immunodeficiency virus infection

An increased prevalence of candidal carriage and oral candidiasis is common in cases of human immunodeficiency virus (HIV) infection, and the reasons for this may include the enhanced ability of colonizing yeasts to produce biofilms on mucosal surfaces. The aim of the present study was therefore to examine the differences, if any, in the biofilm-forming abilities of 26 Candida albicans yeast isolates from HIV-infected individuals and 20 isolates from HIV-free individuals, as this attribute of yeast isolates from patients with HIV disease has not been examined before. Biofilm formation in microtiter plate wells was quantitatively determined by both the 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-2H-tetrazolium hydroxide (XTT) reduction method and the crystal violet method. Although candidal biofilm formation could be quantitatively evaluated by either technique, the better reproducibility (P < 0.05) of the XTT reduction assay compared with that of the crystal violet method led us to conclude that the former is more reliable. There were no significant quantitative differences in biofilm formation between C. albicans isolates from HIV-infected patients and isolates from HIV-free individuals during in vitro incubation in a multiwell culture system over a period of 66 h. Three of eight host factors in the HIV-infected group were found to be associated with candidal biofilm formation. Thus, yeasts isolated from older individuals and those with higher CD4-cell counts exhibited decreased biofilm formation, while the findings for yeasts from individuals receiving zidovudine showed the reverse (P < 0.05 for all comparison). Our data indicate that attributes other than biofilm formation may contribute to the increased oral yeast carriage rates in cases of HIV infection.

HIV-1 and its transmembrane protein gp41 bind to different Candida species modulating adhesion

Oral candidiasis in HIV-1-infected individuals is widely believed to be triggered by the acquired T-lymphocyte immunodeficiency. Recently, binding of the HIV-1 envelope protein gp160 and its subunit gp41, and also of the whole virus itself, to Candida albicans has been shown. The present study shows that, in addition to C. albicans, HIV-1 gp41 also binds to yeast and hyphal forms of Candida dubliniensis, a species which is closely related to C. albicans, and to Candida tropicalis but not to Candida krusei, Candida glabrata or Saccharomyces cerevisiae. The previous finding that gp41 binding to C. albicans augments fungal virulence in vitro is supported by the observation that the yeast showed an enhanced adhesion to HIV-infected H9 cells in comparison to uninfected cells. In line with these results soluble gp41 itself reduced binding of C. albicans to both endothelial and epithelial cell lines, confirming a dominant role of the gp41 binding moiety on the surface of Candida for adhesion. Surface-associated secreted aspartic proteinases (Saps) play an important role in candidial adhesion, but are not likely to be involved in the interaction as gp41 binding to the C. albicans parental wild-type strain was comparable to that of three different isogenic Sap deletion mutants. Furthermore, gp41 binding to the yeast killer toxin-susceptible C. albicans strain 10S was not inhibitable by an anti-YKT receptor antibody. In conclusion, HIV-1 interacts with different clinically important Candida spp., and may thereby affect the outcome of the respective fungal infection.

Identification of Candida albicans genes induced during thrush offers insight into pathogenesis

Candida albicans causes a wide spectrum of diseases, ranging from mucocutaneous infections like oral thrush to disseminated candidiasis. Screening for C. albicans genes expressed within infected hosts might advance understanding of candidal pathogenesis, but is impractical using existing techniques. In this study, we used an antibody-based strategy to identify C. albicans genes expressed during thrush. We adsorbed sera from HIV-infected patients with thrush against candidal cells grown in vitro and screened a C. albicans genomic expression library. We identified 10 genes encoding immunogenic antigens and used reverse transcription-polymerase chain reaction to verify that they were induced within thrush pseudomembranes recovered from a patient. The in vivo induced genes are involved in diverse functions, including regulation of yeast-hyphal morphogenesis, adhesion to host cells, nutrient uptake, phospholipid biosynthesis and amino acid catabolism. Four genes encode known virulence determinants (HWP1, CST20, CPP1 and RBF1). Another gene, LPD1, for which a role in candidal pathogenesis is unknown, encodes a protein homologous to a bacterial virulence determinant. Most importantly, disruption of CaNOT5, a newly identified gene, conferred defects in morphogenesis, decreased adherence to human buccal epithelial cells and attenuated mortality during murine disseminated candidiasis, proving that our strategy can identify genes encoding novel virulence determinants.

Fungal infections associated with HIV infection

Oral candidiasis is perhaps the commonest infection seen in HIV disease. The aim of this workshop was to provide a sketch of the multifarious aspects of the disease from a global perspective. To this end the panellists addressed issues such as the virulence of Candida, emergence of antifungal resistance, management of candidiasis and other exotic, oral mycotic diseases. An all-pervasive theme was the dramatic differences in the management of fungal infections consequential to the availability (or the lack) of anti-HIV drugs in the developed and the developing world. Further, the social stigmata associated with the HIV disease in many developing regions in Africa and Asia appears to modify the therapeutic strategies. Additionally, the lesser-known regional variations in the disease manifestations and therapeutic approaches were stark. Further work is direly needed to address these issues.