Viral protein causing chronic inflammation in people living with HIV: study
JThanks to antiretrovirals, people living with HIV can lead long and relatively healthy lives. But even though the virus is nearly undetectable, people living with HIV are more likely to suffer from chronic inflammation than those without, which can put them at risk for several illnesses, including atherosclerosis and miscellaneous neurological disorders
Now, a new study published in Cell reports November 22 finds that a protein produced by HIV called Nef can cause long-lasting, chronic inflammation in a mouse model of HIV – and that inflammation can persist even if the virus is totally suppressed or even eliminated. This, according to the researchers behind the study, could explain why such inflammation occurs in people living with HIV.
“It’s a very nice paper,” says mike powell, a molecular biologist at Morehouse School of Medicine who was not involved in the study. “We thought Nef had been involved in this inflammation for some time now.”
According to the study, Nef could cause long-term genetic changes in immune cells, a phenomenon known as trained immunity. These changes “prime” the cells, so they are constantly “on alert”, as the study authors put it, and ready to mount a strong inflammatory response against future immune challenges.
The study researchers, a group from George Washington University, had already related Nef-containing exosomes – nanoscale lipid packets that transport small molecules from cell to cell – to inflammation and cholesterol buildup in immune cells. Yet they did not know how long the effect lasted.
So to interrogate this question, the researchers began by exposing human immune cells, specifically monocytes, to exosomes containing Nef. Then, after six days, the researchers introduced the cells to lipopolysaccharides (LPS), a substance commonly found on bacterial cell walls, to trigger an immune response.
The researchers found that exposure to Nef triggered several pro-inflammatory changes in immune cells. In response to LPS, cells exposed to Nef produced higher levels of inflammatory cytokines TNF-α and interleukin-6 (IL-6) than cells not exposed to Nef. And RNA-seq experiments showed that immune cells exposed to Nef upregulated genes involved in cytokines and inflammatory pathways, including several interleukin-coding genes. The cells also accumulated cholesterol, forming lipid rafts, which harbor antigen receptors capable of triggering inflammatory responses.
Nef appears to induce gene expression changes epigenetically by restructuring chromatin and exposing inflammation-related genes to the cell’s transcription machinery. However, the mechanism behind this restructuring remains unclear, according to the co-author of the study. Michael Boukrinskyimmunologist at George Washington University.
Burkinsky and his colleagues then tested whether Nef could modify immune cells in vivo by injecting mice with exosomes containing Nef. Then, after about a week, the scientists isolated the immune cells from the mice’s bone marrow and exposed the cells to LPS. Immune cells from Nef-injected mice produced higher levels of the cytokine TNF-α in response to LPS than those not exposed to Nef, similar to results observed in vitro. This effect, the researchers said, lasted for up to several months after the initial treatment, and Burkinsky says he doesn’t know how long the changes last beyond that time.
The researchers also implanted mice irradiated with bone marrow from animals injected with Nef and were able to see the same effect up to 11 weeks after implantation. This indicates that the interaction between Nef and immune cells, and not other cells, was responsible for the long-lasting immune training phenomenon.
“To our surprise, we found that this is a very, very long-term change,” Bukrinsky says. “The cell acquires a kind of memory after seeing the Nef protein.”
The researchers say this is consistent with scientists’ existing idea of trained immunity, as long-term genetic changes in immune cells resulted in altered responses to future unrelated immune challenges. And given that scientists exposed the cultured immune cells to less than half the concentration of Nef found in the blood of HIV-positive patients with undetectable levels of virus, the process could occur in people living with HIV, explains Bukrinsky. However, they did not perform any tests on human patients.
“It’s an excellent newspaper,” says Craig Bond, a molecular biologist at Morehouse School of Medicine who was not involved in the study. Bond says researchers are still finding exosomes in the blood of HIV-positive patients on antiretrovirals, so “I would actually go a step further and argue that these exosomes are causing this ‘ripple’ effect. . . it is clearly the process that is causing this inflammation.
This process could also occur in other viral infections, Bukrinsky says. “We don’t know which viruses” arouse trained immunity. “For example, it is not yet clear whether COVID induces this hyperresponsiveness or not.”
Next, the researchers want to describe the mechanism by which Nef induces trained immunity. “It is essential for [developing] therapeutic treatments.