Acanthamoeba: A Microbial Factory for Human Pathogenesis

By CS

An evolutionary arms race between pathogens, and their hosts has been raging since the beginnings of life itself. Pathogens have evolved a vast number of mechanisms to out maneuver their host, while their host’s evolve further measures of resistance. This has resulted in a diverse range of strategies with varying impact on the two populations. Few of these strategies are more problematic than the ability of a pathogen to thrive intracellularly, thus hidden from the immune system of a human host. It is even more troublesome when intracellular microbes can survive phagocytosis by macrophages that have evolved to degrade pathogens through oxidative stress, antimicrobial peptides, and hydrolytic enzymes. All of which should easily kill the most hardened of microbes.

One microbe that has achieved this ability to survive in macrophages is Cryptococcus neoformans. On rare occasions this pathogen is introduced to a human host by inhalation of spores from environment. The odd part of this interaction is that C. neoformans thrives in its natural environment, and seems to gain very little from its interaction with a human host. So how does a microbe with no apparent need to be intracellular evolve the ability to invade an immune cell that it rarely comes in contact with?

Figure 1. Intracellular C. neoformans
6h post infection in murine macrophages
(top) and A. castellanii (bottom)

One theory is that environmentally acquired pathogens have had lots of practice. C. neoformans, for example, has been subject to predation for millions of years, and if you can’t beat them you might as well join them. The constant predation by protozoans, such as Acanthamoeba, provides a unique selective pressure that may have resulted in pathogens with the ability to survive in a harsh intracellular environment. Specifically, this type of predation may have resulted in virulence factors that allowed pathogens to thrive inside both macrophages and Acanthamoeba due to some key similarities that they both share. These include similarities in cell motility, cell structure, and the ability to phagocytose just about anything they encounter with few limitations.

Predation by Acanthamoeba castellanii in particular is often used to model phagocytosis by macrophages with a diverse range of human pathogens. A. castellanii is ubiquitous in nature, and is not that picky about its source of food. Acting like a microbial garbage disposal, it travels around engulfing anything it deems as a possible food source. This has resulted in numerous amoeba-resistant microbes, with a variety of different ways that they can evade the host’s degradation mechanisms. Some microbes prevent the fusion of the phagosome to the lysosome, which is full of digestive enzymes. Other microbes have developed ways to escape the phagosome all together, thus avoiding starvation by entering the nutrient rich cytoplasm. These mechanisms of intracellular resistance illustrate some of the ways pathogenic microbes can develop resistance to phagocytosis by macrophages, and thus curtail the host immune response.

A study published in Eukaryotic Cell found that predation by A. castellanii turned on many of the same genes in C. neoformans, as phagocytosis by macrophages¹. This suggested to many scientists that the phagocytic mechanism, and intracellular environment of A. castellanii and macrophages are very similar. Furthermore, it suggests that predation could be a driving force for the evolution and retention of virulence factors. This raises an interesting question as to whether predation by Acanthamoeba, and other protozoans, could be a proving ground for pathogens to produce virulent traits before moving on to human hosts. This idea has lead scientists to look to microbes that are known to thrive in Acanthamoeba for virulent traits that may suggest pathogenicity in humans. In doing this scientists can take a more proactive role in protecting the public from emerging pathogens.

The result of one recent investigation was the discovery of virulence factors produced by Parachlamydia, a putative human pathogen that is now thought to be an emerging agent of pneumonia². However, Parachlamydia appears to have co-evolved with amoebic species, because it is known to have had parasite specific proteins before ancestral protists, plants, and animals diverged³. This suggests that Parachlamydia may have required internalization by a host cell for proliferation long before Protozoa such as Acanthamoeba existed. While virulence factors specific to macrophages may have existed for quite sometime, increased pathogenicity could still arise through selective pressures such as improved transmission, or proliferation within the host.

One example of improved transmission is the case of Legionella pneumophila, the causative agent of legionnaires disease. Where in virulence factors are repressed during phagocytosis in order to allow the normal fusion of the digestive lysosome to the phagosome. After several rounds of replication the virulence factors are again expressed to allow the parasite to escape the host. The microbe then persists in the environment until such time as to be inhaled by a human host where it is phagocytosed, and proliferates in macrophages⁴. This method transmission would not be possible without the selective pressures created by predation via phagocytosis.

While the similarity between the intracellular conditions of Acanthamoeba and macrophage may be coincidental, the ability of microbes to persist intracellularly in both hosts is clearly not. For billions of years microbes have formed intracellular coping mechanisms with protozoa such as Acanthamoeba, and it has provided the microbes with a mechanism for evolution. In this way predators such as Acanthamoeba have become a microbial factory for human pathogenesis.

Definitions:

Phagocytosis – the cellular process of internalizing a particle or microbe by engulfing it.

Lysosome – compartment within the cell containing digestive enzymes.

Virulence Factor – molecules, or proteins made  by a pathogen that allow it to survive in its host. 

References:

1) http://www.cdc.gov/fungal/cryptococcosis-neoformans/statistics.html

1) Derengowski Lda S, Paes HC, Albuquerque P, Tavares AH, Fernandes L, Silva-Pereira I, Casadevall A. 2013. The transcriptional response of Cryptococcus neoformans to ingestion by Acanthamoeba castellanii and macrophages provides insights into the evolutionary adaptation to the mammalian host. Eukaryot. Cell vol. 12 no. 5 761-774.

2) Greub G (2009) Parachlamydia acanthamoebae, an emerging agent of pneumonia. Clin Microbiol Infec 15: 18–28.

3) Greub G, Raoult D. History of the ADP/ATP-translocase-encoding gene, a parasitism gene transferred from a Chlamydiales ancestor to plants 1 billion years ago. Appl Environ Microbiol. 2003;69(9):5530–5535. doi: 10.1128/AEM.69.9.5530-5535.2003.

4) Swanson MS, Hammer BK. 2000. Legionella pneumophila pathogenesis: a fateful journey from amoebae to macrophages. Annu. Rev. Microbiol. 54:567–613