Antifungal activity of human monocytes alone and combined with caspofungin against Candida albicans biofilm and planktonic cells

Abstract number: 1733_125

Katragkou A., Simitsopoulou M., Dalakiouridou M., Diza-Mataftsi E., Tsantali C., Roilides E.

Objectives:Candida albicans (CA) forms biofilms (BF) on implanted medical devices, and most commonly on intravascular catheters. BF constitute a niche where CA is protected from innate immunity, an important component of which are monocytes (MNCs), and antifungal agents. Moreover, CA BF have shown resistance to a variety of antifungal agents but susceptibility to caspofungin (CAS). Because little is known about the effects of host defences against Candida BF, we aimed to study the antifungal activity of human MNCs alone or combined with CAS against CA BF in comparison to their planktonic (PL) counterparts.

Methods: CA-M61, a documented biofilm-producing CA intravascular catheter isolate, was used. PL cells were grown in YNB at 37°C overnight. BF were grown on silicone elastomer disks in 96-well plates at 37°C with shaking for 48 h. THP1 monocytic cell line was used as MNC source. MNCs (MNC/target ratios 1:1 and 5:1) and CAS (0.0625 and 0.015 mg/l) alone or in combination were incubated with mature BF and PL cells in RPMI-1640 with 10% fetal bovine serum at 37°C, 5% CO2 for 20 h. Plain BF and PL cells served as controls. Percent damage of BF and PL cells was then assessed by XTT colorimetric micro assay as reduction in the metabolic activity after incubation for 30 min at 450 nm with reference wavelength at 690 nm. Statistical analysis (n = 7) included ANOVA and post hoc analysis (P < 0.05).

Results: MNC-induced BF damage was significantly decreased compared to PL cells at 5:1 ratio (mean±SE, 31.5±3.5% vs 43.6±3%, p = 0.01) but not at 1:1 ratio. CAS-induced damage to BF was lower than that to PL cells at 0.0625 mg/l (57.7±4.1% vs 75±11%, p < 0.001). The damage induced by MNCs at 5:1 combined with CAS, at both concentrations, on BF was lower than that on PL cells (71.3±5.2% vs 85±6%, p = 0.001; 65.5±9.3% vs 82.3±6.5%, p = 0.008, respectively). While an additive effect between MNCs at 5:1 ratio and CAS at 0.015 mg/l was noted against PL cells (82.2±6.5% vs 43.6±3%, p < 0.001 and 82.2±6.5% vs 62.8±11%, p < 0.05), no significant collaboration between MNCs and CAS was observed against BF at any MNC/target ratio and any CAS concentration used.

Conclusions:C. albicans BF are more resistant than PL cells of CA to MNCs, to CAS and to the combination of MNCs with CAS. While MNCs and CAS exhibit an additive effect against PL, no significant collaboration between MNCs and CAS exists against BF. The mechanism(s) behind resistance of CA BF to host defences need to be determined.