A specific immune system proteinase, granzyme A, was shown to promote arthritic inflammation in mice infected with chikungunya virus (CHIKV), in an Australian study reported in the Feb. 16, 2017, edition of PLOS Pathogens.
The study, by researchers at the QIMR Berghofer Medical Research Institute in Brisbane, also suggested granzyme A could serve as a potential target for new drugs to treat CHIKV infection and related viral arthritides in humans.
An alphavirus transmitted by mosquitoes, the most recent CHIKV epidemic began in 2004 and continues to spread, causing millions of cases globally. While it is rarely fatal, CHIKV infection can cause severe, chronic polyarthritis and/or polyarthralgia.
At present, no particularly effective CHIKV treatment or licensed vaccine is available for human use. Although standard anti-inflammatory drugs such as paracetamol/acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) can provide relief from rheumatic symptoms, they are often not particularly effective.
“NSAIDS and paracetamol do provide relief but this is often inadequate,” said lead researcher Andreas Suhrbier, professor and leader of the Inflammation Biology Group at the QIMR Berghofer Medical Research Institute. “This disease can last for many months and occasionally for over a year,” he told BioWorld Asia.
To aid research into potential new anti-inflammatory treatments, Suhrbier and his team exploited an adult wild-type mouse model of CHIKV infection and diseases that had previously been developed by the group.
Using RNA sequencing (RNA-Seq) to examine mouse inflammatory responses to infection in detail, they found that genes activated in the mouse model closely mirrored genes known to be activated in infected humans, validating the model.
“Mouse models are always questionable regarding their validity, in particular whether they really represent what happens in humans,” explained Suhrbier. “Using RNA-Seq, we got a complete picture of all the genes activated during infection and disease in mice. As these matched well with what is seen in humans, this provides some level of validation of the model; in other words, what we find in mice is likely to be true in humans.”
Particularly prominent in the RNA-Seq analysis was the up-regulation of a number of granzymes, serine proteinases secreted by cytoplasmic granules within the immune system’s natural killer cells. Granzymes were originally thought to be involved in killing virally infected cells or other target cells via apoptosis.
However, the physiological role of those molecules is far from certain, noted Suhrbier. “The textbooks will tell you they are involved in killing target cells. While this is true for granzyme B, it is perhaps not the case for granzymes A and K.”
Indeed, there is an emerging consensus, which is supported by the findings of the new PLOS Pathogens study, that some granzymes, in particular types A and K, play an important role in promoting inflammation.
“There is a growing body of evidence suggesting their physiological role is associated with the promotion of inflammation. This body of evidence now includes our paper, with very clear results on this issue,” noted Suhrbier.
“Granzyme A levels are elevated in the circulation after many infections,” he added. “Previously the textbooks would argue that this is simply left over from the killing of infected cells, although now it might be more appropriate to view this as a circulating active pro-inflammatory mediator.”
The researchers further showed that when infected with CHIKV, knockout mice missing the granzyme A gene experienced dramatically less foot swelling and arthritis. Treating mice with a granzyme A inhibitor also significantly reduced foot swelling and arthritis.
“Up to 80 percent less swelling was seen in the granzyme A gene knockout mice vs. controls,” said Suhrbier, adding that “this is an impressive finding, as it shows that granzyme A really drives arthritis.”
That contention was further supported by their use of an enzyme-linked immunosorbent assay (ELISA), which found significantly elevated granzyme A levels in blood samples taken from macaque monkeys infected with CHIKV, as well as from human CHIKV patients.
Together, those findings suggest that granzyme A could serve as a potential drug target for anti-inflammatory treatments for CHIKV arthritis and possibly for other inflammatory diseases, although it remains unknown how granzyme A mediates its pro-inflammatory activities.
“We have one clue, which is that granzyme A may activate protease-activated receptors (PARs). PARs are members of the G protein-coupled receptor family, which have emerged to be some of the most druggable targets in the human genome, so this clearly offers some new hope for treatment options,” said Suhrbier.
Further research will be needed to explore that potential and determine to which extent those new research findings can be extended from mice to humans.
“We are also now keen to ask a number of questions,” noted Suhrbier. “These include in what other infectious pathologies is granzyme A a major player – for instance, dengue; how does it mediate its pro-inflammatory activities, and are PAR antagonists suitable for treating alphaviral arthritides or other infectious pathologies?”