Chlamydia wears a cloak, but it might not be invisible forever

Chlamydia is the most frequently reported infectious disease in Australia, with almost 97,000 diagnoses in Australia each year. Although often asymptomatic, it can cause serious health issues such as infertility, ectopic pregnancy and chronic pain.

Part of the reason chlamydia is so prevalent is because the bacteria that causes it, Chlamydia trachomatis, is highly adept at evading the body’s immune system, enabling an infection to last for months. Exactly how it does this is the focus of a new study from Duke University in the US, which found a protein that might stop the body’s cells from marking the bacteria for destruction once it invaded the cell.

Like a wolf in sheep’s clothing, pathogenic bacteria ‘cloak’ themselves in a piece of the target cell’s membrane which can then float around within the cell, undetected.

A mouse
Mice have their own type of chlamydia. Credit: Dr T J Martin/Getty Images

Chlamydia is able to do this so well that, even when primed for the bacteria using an immune stimulant, the human cell cultures are still somehow unable to see it. “We said, there’s the pathogen”, says Jörn Coers, associate professor of molecular genetics and microbiology and co-author of the study. “Our defence system should see it. Why does it not see it?”

The researchers investigated the interaction between human cell cultures and mouse chlamydia. Unlike human chlamydia, human cells were able to identify and destroy the mouse chlamydia easily. “Humans don’t get mouse Chlamydia because it evolved with mice and human Chlamydia evolved with humans,” said Coers. “So there’s this really fine-tuned adaptation that the pathogen has undergone.”

“This common ancestor may go back as far as when humans and rodents basically split from each other. This is a long time for the bacteria to really fine-tune their interactions with their host species.”

Read more: First chlamydia vaccine could be a jab in the nose

Chlamydia cells with genetic mutations on right panel
A wild type Chlamydia (green) surrounded by the GarD protein (red) that cloaks it from detection inside human cells. Right: Chlamydia with GarD knocked out (green) are enveloped by the antimicrobial proteins ubiquitin (yellow) and RNF213 (magenta). Credit: Stephen C. Walsh/Duke University

When examining the differences the researchers noticed a particular protein in the human form, called GarD (gamma resistance determinant) that appeared to be able to block the ability of another protein called RNF213 or ‘mysterin’ to notice and mark the chlamydia for destruction.

“RNF213 is basically the eyes of the immune system,” Coers said. So, by blinding mysterin, the human form of chlamydia is able to sneak around without being noticed.

When researchers mutated the gene responsible for GarD, the bacteria became much more vulnerable to extermination by the human cell.

Although this is an important step forward for understanding how chlamydia and other similar pathogens are able to evade the body’s immune systems, however, it is still not well understood how mysterin is able to detect pathogens, nor how GarD manages to blind it.

These questions will need to be solved before the power of this discovery can be harnessed for targeted therapies.

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