In a series of experiments, Brodsky and his collea

In a series of experiments, Brodsky and his colleagues used normal human blood serum and three subunits of the SARS-CoV-2 spike protein to discover exactly how the virus activates the APC, hijacks the immune system and endangers normal cells. They discovered that two of the subunits, called S1 and S2, are the components that bind the virus to heparan sulfate — setting off the APC cascade and blocking factor H from connecting with the sugar — and in turn, disabling the complement regulation by which factor H deters a misdirected immune response.

In turn, the researchers say, the resulting immune system response to chemicals released by the lysing of killed cells could be responsible for the organ damage and failures seen in severe cases of COVID-19.

Most notably, Brodsky says, the research team found by blocking another complement protein, known as factor D, which works immediately upstream in the pathway from factor H, they were able to stop the destructive chain of events triggered by SARS-CoV-2.

“When we added a small molecule that inhibits the function of factor D, the APC wasn’t activated by the virus spike proteins,” Brodsky says. “We believe that when the SARS-CoV-2 spike proteins bind to heparan sulfate, it triggers an increase in the complement-mediated killing of normal cells because factor H, a key regulator of the APC, can’t do its job.”

To better understand what happens, Brodsky says think of the APC like a car in motion.

“If the brakes are disabled, the gas pedal can be floored without restraint, very likely leading to a crash and destruction,” he explains. “The viral spike proteins disable the biological brakes, factor H, enabling the gas pedal, factor D, to accelerate the immune system and cause cell, tissue and organ devastation. Inhibit factor D, and the brakes can be reapplied and the immune system reset.”

Brodsky adds that cell death and organ damage from a misdirected APC associated with factor H suppression is already known to occur in several complement-related human diseases, including age-related macular degeneration, a leading cause of vision loss for people age 50 and older; and atypical hemolytic uremic syndrome (aHUS), a rare disease that causes clots to block blood flow to the kidneys.

Brodsky and his colleagues hope that their work will encourage more study into the potential use against COVID-19 of complement-inhibiting drugs already in the pipeline for other diseases.

“There are a number of these drugs that will be FDA-approved and in clinical practice within the next two years,” Brodsky says. “Perhaps one or more of these could be teamed with vaccines to help control the spread of COVID-19 and avoid future viral pandemics.”