It may be a respiratory disease, but COVID-19 impacts more than just the lungs. A whole host of vital organs can be affected, including the kidneys, and a recent study has finally worked out how.

SARS-CoV-2 – the virus that causes COVID-19 – can infect and directly damage the kidneys by causing scarring, according to research in the journal Cell Stem Cell. The resulting scar tissue could have long-term impacts on kidney function, the authors say.

While the ability of the virus to bind and infect kidney cells is old news, the finer details of what exactly causes damage have until now eluded scientists. This latest finding provides another “piece of the puzzle,” lead author Jitske Jansen of Radboud University Medical Center said in a statement.

In 2020, a study found that up to 50 percent of hospitalized COVID-19 patients experienced renal failure, with up to 10 percent requiring dialysis. Last year, a study of almost 90,000 COVID-19 survivors found them to be at greater risk of kidney damage than people who had never had COVID.

“Our work shows kidney scarring in COVID-19 patients, which provides an explanation why the virus might cause kidney functional decline as demonstrated in other studies,” co-author Katharina Reimer of RWTH Aachen Uniklinik said.

The team collected tissue samples from 62 COVID-19 patients admitted to the Intensive Care Unit (ICU). When compared to the control groups – ICU patients with a non-COVID-related lung infection and a group of healthy people – the tissue from COVID-19 patients showed substantially more scarring.

To identify whether the tissue damage was directly caused by the virus, as opposed to inflammation or other systemic effects, the team created “mini kidneys”. Also called “organoids”, they were cultured in the lab from stem cells, containing many different types of kidney cells, with the notable exception of immune cells. Once infected with SARS-CoV-2, the mini kidneys, like the COVID-19 patients’ tissues, became scarred. The team also found evidence of signaling molecules known to be involved in the scarring process.

“In our study, we thoroughly investigated the causal damaging effects of the Coronavirus in the kidneys. [We] show that the virus directly causes cell damage, independent of the immune system,” Jansen added.

All of this strongly suggests that the virus itself is directly responsible for the observed tissue damage.

Kidney fibrosis (scarring) can have serious long-term health implications, Reimer said, adding that further research is needed.

“Long-term follow-up studies will provide further insights into kidney-related pathologies caused by SARS-CoV-2,” she concluded.