Objective:
Candida albicans is the first fungal agent related to nosocomial infections and the fourth causative agent of bloodstream infections. Despite the use of antifungals, mortality rates are still high (50%). Moreover, the use of antifungals leads to toxicity, and, on the other hand, resistances to the most used drugs are emerging. Therefore, there is an urgent need to find alternative treatment options. In this sense, monoclonal antibodies (mAbs) have shown to play a protective role in animal models. Our group has developed a mAb (Ca37) directed against a metabolic protein of C. albicans and has characterized the effect of this mAb on yeast growth.
Methods:
The in vitro effect of Ca37 was assessed by two different methods: firstly, by incubating C. albicans in presence of the mAb in a rich nutrient medium, measuring the turbidity every 10 min for 30 h at 37°C. Secondly, by incubation of the yeast with the mAb in DPBS buffer for 18 h at 37°C, measuring the growth inhibition by colony formation units (CFU) on Sabouraud agar plates. Next, the role of the mAb in combination with the most commonly used antifungals, amphotericin B and fluconazole, was studied by calculating the minimal inhibitory concentration (MIC) following the EUCAST guideline. Finally, the effect of the mAb was studied in vivo, using the Galleria mellonella model, by treating the yeast with the mAb before being injected into the larvae. G. mellonella were incubated at 37°C and survival was measured for 7 days.
Results:
Regarding the in vitro data, the mAb concentration showing the highest effect, 10 μg/ml, induced an inhibition of 50 to 90% of the yeast incubated in buffer, while an isotype control did not produce any effect. In presence of a nutrient rich medium, which stimulates active yeast growth, the mAb was not as effective, showing a growth reduction of 50% only in one out of four strains. Regarding MIC measurements, the inhibitory value was reduced to half in presence of the mAb in case of both amphotericin B and fluconazole. Finally, the growth inhibition showed in vitro was validated in vivo, being observed an increase in mean larval survival of 3 days in presence of the antibody, similar to the increase in survival induced by fluconazole treatment.
Conclusion:
The results presented here show that Ca37 mAb in vitro is able to inhibit C. albicans growth and to reduce the necessary dose of antifungals. More interestingly, in an animal model the survival was increased, suggesting that the usefulness of this antibody as an alternative treatment should be more deeply studied and that this metabolic protein might be considered as an attractive new target against C. albicans infections.
Financial support: AA, IB and LM-S are recipients of a PhD grant of the Basque Government.


6 / Ca37 monoclonal antibody inhibits Candida albicans growth in vitro and in vivo