Microorganism Uses ‘Human’ Immune System Strategies

A microorganism – a social amoeba – using ‘human’ immune system strategies has been documented in a new paper published in Nature Communications.

immune cells attack

Two immune system strategies that have hitherto been documented in humans and higher animals only have now been discovered in a social amoeba, Dictyostelium discoideum, a unicellular organism thriving in the soils of temperate forests. Apparently, the amoeba has been using the combination of the two techniques for more than a billion years. This finding might lead to positive effects on the combat against genetic diseases impacting on immunity.

The innate immune system of humans fights infections in two main ways: phagocytosis, a process involving killing harmful foreign bodies inside the phagocyte, which is a cell of the immune system, and destroying the pathogens outside the cell, as in cases when they are too large to be engulfed. The social amoeba has been shown to have both abilities. Given the similarity, amoeba D. discoideum might be used in research pertaining to understanding and fighting genetic diseases affecting the immune system.

During phagocytosis, the foreign body is pulled into the phagocyte, and destroyed in its inside by reactive oxygen species (ozone, hydrogen peroxide, bleach) whose synthesis is mediated by enzyme NOX2. The other defense mechanism involves an outward movement of genetic material (DNA). Once expelled, the DNA turns into sticky and poisoned nets known as ‘neutrophil extracellular traps’ (NETs) which then get hold of the bacteria to ultimately destroy them.

Now, how about the social amoeba? When they form an aggregation of thousands of cells in conditions of food scarcity, they dedicate a certain proportion of cells to act as immune cells that protect the mass. The microorganisms will gather to form a ‘mini animal’ consisting of over 100,000 single cells. This aggregation is called a slug which transforms into a ‘fruiting body’ that includes a stalk (made of 20% of cells that have to self-sacrifice) on which sits a mass of spores. Dormant spores are able to live without food until they are dispersed to other areas where germination and feeding can occur. This slug has to protect itself: for this purpose, around 1% of the cells retain their phagocytic properties, and are called ‘sentinel’ cells.

The sentinel cells make up the primitive innate immune system of the slug. They have the same functions as human immune cells. They have been found to use phagocytosis as well as DNA nets to get rid of bacteria threatening the life of the slug.

This two-fold strategy was thought to have been developed in higher animals, something that unicellular organisms would not have had. However, these findings indicate that it has actually been actively present in microorganisms for a billion years, says author Thierry Soldati. This finding is expected to lead to a better understanding of diseases affecting the human immune system. Social amoeba D discoideum can be genetically modified to be used for experimenting on mechanisms of the innate immune system. It might even become a scientific model for such researches. It is hoped that this will pave the way to developing treatment for diseases of the immune system.

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