Macrophage Chitinase and Lipoprotein Uptake Responses to Chitin, a Structural Component of the Fungal Cell Wall: Diagnostic and Biomedical Implications

Recipients of solid organs and hematopoietic cell transplants (HCT), acquired immunodeficiency syndrome patients, and burn victims are usually immunosuppressed for extended periods of time. Their prolonged immunosuppressive state is associated with a high risk of contracting invasive fungal infections (IFIs). Most IFIs advance rapidly and are often not diagnosed early enough for anti-fungal drugs to function with full efficacy; therefore, the majority of these infections lead to death.;In contrast to immunosuppressed patients, immunocompetent individuals are protected from fungal infections by their innate immune system, which readily recognizes and eliminates fungal invaders. Recognition of fungal cellular features by the immune system appears to be a key component of the human antifungal defense. An important component of the fungal cell wall is chitin, a polymer of N-acetylglucosamine. Chitin is the second most abundant biopolymer on earth, after cellulose, and it is found on fungal cell walls and exoskeletons of numerous organisms including parasitic worms (helminths) and arthropods. Although humans do not biosynthesize chitin, they do express chitin degrading enzymes, known as chitinases. There are two known human chitinases that have chitinolytic activity, chitotriosidase (CHIT-1) and acidic mammalian chitinase (AMCase). The functions of these chitinases are unknown, but they are thought to aid in the defense of chitin–containing pathogens. In fact, recombinant CHIT–1 may have anti-fungal properties both in vitro and in vivo, suggesting the possibility of a gene therapy approach. This research found that intracellular chitinase protein levels are elevated in response to in vitro stimulation by chitin or by the fungal pathogen Aspergillus fumigatus. .;The fact that chitinase levels can be elevated in response to fungal stimulation suggests the possibility of using host chitinase responses as a diagnostic tool. Chitinase activity has already been shown to be an important biomarker for Gaucher's disease. However, several polymorphisms in the CHIT-1 and AMCase genes are known to decrease chitinase activity. Thus, there are challenges to be overcome if chitinase responses were to be used in the diagnosis of fungal infections. This research has explored solutions to such challenges and has determined that regardless of a common CHIT-1 duplication which decreases chitinase activity, in HCT recipients suspected to have fungal infections, there is a correlation between chitinase activity and aspergillosis. Therefore, it is possible to utilize chitinase activity as a diagnostic tool to detect infections with A. fumigatus and potentially other fungi.;Interestingly, in studying host responses to chitin stimulation, we found that stimulation of human macrophages with chitin particles can induce cellular uptake of apolipoprotein (Apo) A1 from the growth medium. Apo A1 is an important protein involved in cholesterol transport. Some literature suggests that a fungal chitin mixture may aid in reducing atherosclerosis. Others have found that increased chitinase activity correlates with increased risk of atherosclerosis. We therefore analyze the origin and role of the Apo A1 protein, and discuss how chitin stimulation could be relevant to cardiovascular diseases.;Overall, this dissertation explores chitinase responses for the diagnosis of IFIs, and the involvement of chitin stimulation in apolipoprotein and cholesterol transport. Furthermore, this dissertation adds to the current knowledge of the mechanism of chitin recognition by host immune cells, and that of the regulation of host chitinases.