Host-fungal pathogen interactions: A study of Candida albicans and mammalian macrophage and epithelial cells at the transcriptional level

Abstract

It is estimated that fungal infections kill greater than 1.6 million people annually, a number that is comparable to the number of deaths associated with tuberculosis. Candida species are the fourth leading cause of hospital-acquired blood infections and Candida albicans is the most common cause of these fungal blood infections (known as candidemia). Immunocompromised individuals, such as those who have HIV/AIDS, those undergoing chemotherapy treatments, or those on broad-spectrum antibiotics, are most likely to develop candidemia. Candidemia is associated with a 20-40% mortality rate. However, when patient treatment for candidemia is delayed for over 48 hours, associated mortality rates increase to 78%. Blood infections can disseminate Candida albicans throughout the body, eventually leading to infection in vital organs like the liver, kidney and brain. Optimal patient outcomes are achieved if antifungal therapy is given within 12 hours after a blood sample is obtained for culture and testing. However, current blood tests cannot reliably detect Candida this early and thus antifungals are not routinely given to patients in this time frame.\n\nCounterintuitively, it is believed that some fungi, like many bacteria, are non-harmful residents in small intestines of most adults and this hypothesis is supported by the fact that the most common fungal species in the human gut is Candida albicans. However, intestinal overgrowth of C. albicans is linked to Crohn's disease, and disease-causing forms of C. albicans can arise from commensal strains that once resided in the patient's gastrointestinal tract. The specific molecular mechanisms by which C. albicans interacts with host immune cells versus intestinal cells, and those that trigger Candida pathogenicity remain unknown. Many strains of Candida albicans have developed resistance to azoles, the major class of drugs used to treat both superficial and systemic infections. In order to develop new treatments, we must better understand host-fungal pathogen biology to determine novel antifungal targets or therapeutics to fight fungal infections at the early stages of infection. In this work, we developed a novel tool that allows us to measure which genes are important to both the host and Candida albicans simultaneously, in specific infection states. We have applied this tool to measure gene expression in Candida albicans interacting with mammalian macrophages and small intestine epithelial cells– at both the population and single cell levels. \n\nWhen examining populations of sorted infection samples, we found that host immune cells both exposed to and infected with fungal cells exhibit similar expression patterns. In contrast, phagocytosed C. albicans exhibit unique expression patterns compared to those merely exposed to macrophages. We found that immune response genes in single, Candida infected macrophages exhibited bimodal expression patterns for some immune response genes. We also observed examples of expression bimodality in live Candida inside of single macrophages. Both Candida albicans and host small intestine epithelial cells demonstrate distinct patterns of expression when exposed to each other at the population level, compared to unexposed controls. However, the magnitude of these differences is dependent on the multiplicity of infection. Some expression programs overlapped with those observed in populations of Candida cells interacting with macrophages, with key differences. We also observed expression bimodality among epithelial cells infected with C. albicans. We believe the information obtained using this technique could be used when considering new antifungal or therapeutics targets; if uniform and high expression of particular in genes in Candida populations phagocytosed by macrophages or invading epithelial cells leads to high and early protein production, these proteins may be effective antifungal targets. Similarly, if some host immune response genes are not expressed in a population of Candida infected macrophages as uniformly and highly as expected, these genes or proteins could be target of a therapeutic for patients with Candida infections that are resistant to azoles.\