Evidence for finely-regulated asynchronous growth of Toxoplasma gondii cysts based on data-driven model selection.
PLoS Comput Biol 2013;
9:e1003283. [PMID:
24244117 PMCID:
PMC3828147 DOI:
10.1371/journal.pcbi.1003283]
[Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2013] [Accepted: 08/30/2013] [Indexed: 11/19/2022] Open
Abstract
Toxoplasma gondii establishes a chronic infection by forming cysts preferentially in the brain. This chronic infection is one of the most common parasitic infections in humans and can be reactivated to develop life-threatening toxoplasmic encephalitis in immunocompromised patients. Host-pathogen interactions during the chronic infection include growth of the cysts and their removal by both natural rupture and elimination by the immune system. Analyzing these interactions is important for understanding the pathogenesis of this common infection. We developed a differential equation framework of cyst growth and employed Akaike Information Criteria (AIC) to determine the growth and removal functions that best describe the distribution of cyst sizes measured from the brains of chronically infected mice. The AIC strongly support models in which T. gondii cysts grow at a constant rate such that the per capita growth rate of the parasite is inversely proportional to the number of parasites within a cyst, suggesting finely-regulated asynchronous replication of the parasites. Our analyses were also able to reject the models where cyst removal rate increases linearly or quadratically in association with increase in cyst size. The modeling and analysis framework may provide a useful tool for understanding the pathogenesis of infections with other cyst producing parasites.
A large portion of people worldwide are chronically infected with T. gondii. Chronic infection with this parasite is characterized by formation of tissue cysts. Bradyzoites slowly replicate within cysts during the chronic stage of infection leading to a corresponding increase in cyst size. Cysts occasionally rupture and release bradyzoites that invade nearby host cells and convert into tachyzoites which can quickly proliferate. Tissue cysts can also be targeted by immune T cells and phagocytes for removal. We developed a differential equation model to investigate the cumulative effects of unknown growth and removal functions on the cyst-size distribution. We then used the AIC to select models that best fit experimental cyst size distribution data obtained from the brains of chronically infected mice. The results suggest that the within-cyst growth of bradyzoites is finely-regulated asynchronous such that the per capita growth rate is inversely proportional to the number of bradyzoites. While it may be surprising that the bradyzoites replicate in a way analogous to a factory producing a product, there may be factors such as nutrient availability, resource allocation, immune response, and other stress factors that may limit replication in cysts.
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