Similar to a burglar breaking a window to enter a home, Indiana University researchers have discovered a previously unknown process by which pathogens enter a cell by physical force, circumventing the body’s immune defenses that prevent infection.
The paper, published in the journal Proceedings of the National Academy of Sciences, introduces a potential game changer in the fight against intracellular pathogens responsible for causing devastating infectious diseases such as tuberculosis, malaria and chlamydia. These diseases are notoriously difficult to treat because the pathogens are protected inside the host cells.
“Using the parasite Toxoplasma As our representative pathogen, our work shows that some intracellular pathogens can exert physical forces upon entry into host cells, which then allow the pathogens to avoid degradation and survive intracellularly,” said the study’s lead author Yan Yu, a professor at the College of Arts. and Department of Chemical Sciences at IU Bloomington. “This work suggests that targeting pathogen motility may be a new way to fight infection inside cells.”
Normally, when an invading pathogen encounters a phagocyte — a type of white blood cell responsible for destroying bacteria, viruses and other types of foreign particles — it is captured and swallowed by the phagocyte. For pathogens that escape this process, it is commonly believed that these pathogens must release a “secret arsenal” to “paralyze” the degradation mechanisms in the cell.
However, Yu’s study shows that this common belief is not true. She and her colleagues have found that pathogens can avoid being swallowed inside the immune cell by exerting a “pushing force.” With this dynamic entry, pathogens are diverted to vacuoles that do not have the ability to disrupt these invaders. A vacuole is a structure intended for storage and digestion within a cell.
To conduct the research, Yu and his colleagues introduced the disease-causing parasite Toxoplasma in mouse-derived cells, observing their behaviors through a fluorescence microscope. These live parasites force their way into and thrive in the cells of the immune system.
The biggest challenge then was to determine whether the living parasite evaded immune defenses with unknown chemicals or simply through violence. To address this question, Yu and her team took an inventive approach: They created dormant parasites that cannot exert force or create chemicals. Unlike live parasites, these “zombie” parasites rapidly degraded in the cell.
The researchers then used magnetic tweezers to push the inactivated parasite into the immune cell to mimic the dynamic entry seen in live Toxoplasma. The inactivated parasite, now subjected to simulated forcible entry, avoided degradation, similar to its live counterpart. This suggests that the force of entry, not the chemicals, explains the pathogen’s survival, Yu said.
To manipulate the parasite’s movement in the second experiment, the researchers had to develop the “tweezer system” with magnetic nanoparticles. They also worked with a team at the University of Tennessee to develop computational models to simulate the behavior.
In addition, the researchers conducted the same experiments using yeast to confirm that the mechanism observed could be found in other infectious agents, not only Toxoplasma.
“This study clarifies the contribution of physical forces to immune evasion and highlights the importance of targeting pathogen movement to combat intracellular infections,” said Yu. “We hope that this work can ultimately contribute to new efforts to combat a variety of infections that are harmful to human health.”
Other IU researchers in the study were first author Zihan Zhang, as well as Jin Ou, Yanqi Yu and Qiong Zhou. Additional co-authors are Thomas K. Gaetjens and Steven M. Abel at the University of Tennessee. This work was supported by the National Institutes of Health and the National Science Foundation.