In vitro infection models to study fungal-host interactions

Abstract

Fungal infections (mycoses) affect over a billion people per year. Approximately, two million of these infections are life-threatening, especially for patients with a compromised immune system. Fungi of the genera Aspergillus, Candida, Histoplasma and Cryptococcus are opportunistic pathogens that contribute to a substantial number of mycoses. To optimize the diagnosis and treatment of mycoses, we need to understand the complex fungal-host interplay during pathogenesis, the fungal attributes causing virulence and how the host resists infection via immunological defenses. In vitro models can be used to mimic fungal infections of various tissues and organs and the corresponding immune responses at near-physiological conditions. Furthermore, models can include fungal interactions with the host-microbiota to mimic the in vivo situation on skin and mucosal surfaces. This article reviews currently used in vitro models of fungal infections ranging from cell monolayers to microfluidic 3D organ-on-chip (OOC) platforms. We also discuss how OOC models can expand the toolbox for investigating interactions of fungi and their human hosts in the future.

Keywords: Aspergillus; Candida; Cryptococcus; Histoplasma; in vitro model; fungal–host interaction.

Figure 1.

(A)Evolution of in vitro models from low to high complexity. Culture dish: one cell type cultured in media. Transwell system: transwell inserts separate the culture area into an upper and lower compartment; cells are cultured under static conditions on a porous membrane allowing apical-basal polarization. Organoid: 3D miniature organ generated out of intestinal stem cells. Organ-on-chip (example): 3D lung on-chip model on a microfluidic biochip holding a porous membrane and two individually accessible channels with one inlet and outlet each; pulmonary epithelial cells are cultured in the upper compartment in an air–liquid interface; and endothelial cells in the lower compartment are perfused with cell culture medium enabling the removal of metabolites. Organoid-on-chip: maturation of organoids within a dynamic culture environment. Principle of a multi-organ-on-chip: interconnected organ-on-chip models of gut and liver, or gut and brain or other combinations of lung, intestine, liver, brain and/or kidneys. Such combinations can, for example, mimic certain steps of fungal dissemination throughout the body. The intestine and lung serve as primary infection sites. (B)Selected in vitro models to study host–fungal interactions. 3D reconstituted human oral (RHOE) or vaginal (RHVE) epithelium grown at an air–liquid interface. Central nervous system (CNS) co-culture model including microglia cells and astrocytes. Intestinal co-culture model including epithelial cells, goblet cells and bacteria. Circulatory model with perfused endothelial cells.

Figure 2.

Fungal–host interactions during fungal diseases that are mimicked by in vitro infection models discussed in this review. (A) Fungal–monocyte/macrophage interactions resulting in several effector mechanisms that contribute to immunity against fungal infections (ROS: reactive oxygen species). (B)C. neoformans and C. albicans can cross the BBB via transcytosis (I);C. neoformans can overcome the barrier paracellularly (II) or use macrophages as shuttles (macrophages as ‘Trojan horse’) (III). (C) In the lung, C. neoformans and H. capsulatum induce their own phagocytosis by innate immune cells; they can replicate intracellularly and use host cells as shuttles to reach the blood stream and subsequently escape (I and IV); evasion of C. neoformans via transcytosis (II) or crossing of C. neoformans through a compromised epithelium (III). Aspergillus spp. form hyphae, can invade endothelial cells and enter the bloodstream (V). (D)Candida spp. can escape the blood circulation after adhesion to endothelial cells (I). Candida spp. and A. fumigatus can be endocytosed (II); Candida spp. can also use fenestrated endothelium as an escape route (III) or use leukocytes as shuttles (IV). (E) In the oral cavity, C. albicans hyphae can actively penetrate the epithelium (I) and/or invade via induced endocytosis (II) or translocate paracellularly (III). (F) In the intestine, C. albicans can actively penetrate the epithelium by hyphal growth (I), translocate paracellularly (II), invade without damaging the host cell (III) or translocate via M cells by inducing endocytosis (IV). (G) In the vaginal tract, C. albicans hyphae can actively penetrate the epithelium (I) or invade via induced endocytosis (II), thereby attracting neutrophils.