The success of antiretroviral therapy has resulted in HIV becoming a manageable disease, but people living with HIV often suffer from chronic inflammation. The results of a study by researchers at George Washington University may now explain why chronic inflammation can occur in these individuals, and why this inflammation may not be resolved even after suppression or eradication of HIV in the body.
The study showed how the HIV protein Nef permanently alters immune cells in a way that causes them to overreact to other pathogens. The in vitro experiments found that introducing the Nef protein to immune cells resulted in the expression of pro-inflammatory genes that remain switched on, even when the HIV protein was no longer in the cells. According to the researchers, this “immunologic memory” of the original HIV infection is why people living with HIV are susceptible to prolonged inflammation, putting them at greater risk for developing cardiovascular disease and other comorbidities that can impact on longevity and quality of live.
“This research highlights the importance of physicians and patients recognizing that suppressing or even eliminating HIV does not eliminate the risk of these dangerous comorbidities,” said Michael Bukrinsky, PhD, professor of microbiology, immunology, and tropical medicine at GW’s School of Medicine and Health Science. “Patients and their doctors should still discuss ways to reduce inflammation and researchers should continue pursuing potential therapeutic targets that can reduce inflammation and co-morbidities in HIV-infected patients.” Bukrinsky is lead author of the team’s published paper in Cell Reports, which is titled “Extracellular vesicles carrying HIV-1NEF induce long-term hyperreactivity of myeloid cells.”
Combination antiretroviral therapy (cART) has dramatically altered the course and prognosis of HIV infection, changing it from a fatal condition to a manageable chronic disease, the authors wrote. But people living with HIV (PLWHs), even those with undetectable viral load, are at an increased risk of comorbidities, including cardiovascular disease, and neurocognitive dysfunction (HIV-associated neurocognitive disorder, or HAND). As the team noted, these comorbidities all feature persistent low grade inflammation as a feature underlying their pathogenesis. “PLWHs show a persistent increase in inflammatory markers and chronic immune activation.”
One potential contributor to the observed long-term immune activation might be a “legacy effect,” wherebt exposure to a pathogen produces effects that last long after the pathogen is eliminated. “One mechanism of this phenomenon is “trained immunity,” the researchers noted. “This paradigm … provides an elegant explanation for the long-known phenomenon of immunologic memory associated with the innate immune responses.” The researchers’ newly reported study was undertaken “to investigate whether exposure to Nef, the key pathogenic factor of HIV, leaves a memory in monocyte-derived macrophages that may contribute to increased inflammatory responses to subsequent stimulation.”
For their study the scientists isolated human primary monocytes in vitro and exposed them to extracellular vesicles carrying the HIV protein Nef (exNef). The amount of Nef introduced to the cells was similar to the amount found in about half of HIV-infected people who are taking antiretrovirals and who have an undetectable HIV load. After exposure, the monocytes were allowed to differentiate into monocyte-derived macrophages (MDMs).
The researchers subsequently introduced a bacterial toxin to generate an immune response from the Nef-exposed MDM cells. They found that compared with cells that were not exposed to the HIV protein, the Nef-exposed cells produced an elevated level of inflammatory cytokines. When the team compared the genes of the Nef-exposed cells with the genes of the cells not exposed to Nef, they identified proinflammatory genes that were in a ready-to-be-expressed status as a result of the Nef exposure. “In conclusion, we demonstrated that EVs carrying HIV protein Nef trigger long-lasting changes in inflammatory responses of the host by mechanisms consistent with trained immunity,” the scientists pointed out. “Our findings demonstrate exNef-induced changes in chromatin composition of MDMs associated with enhanced secretion of two key inflammatory cytokines, TNF-α and IL-6, and increased expression of other pro-inflammatory genes.”
So while the half-life of circulating monocytes is generally relatively short (several days), the new studies indicate that exNef may train the myeloid progenitor cells, to provide provide a long-lasting memory. This suggests that even completely curing HIV infection may still leave infected individuals hyperresponsive to inflammatory stimuli. “This may provide a protective effect for acute infections but may put them at risk for inflammation-associated diseases if chronic stimuli are encountered,” the team stated. According to Bukrinsky, the findings in this study could help explain why certain comorbidities persist following other viral infections, including COVID-19.
“We’ve seen this pro-inflammatory immunologic memory reported with other pathogenic agents and often referred to as ‘trained immunity,’” Bukrinsky explained. “While this ‘trained immunity’ evolved as a beneficial immune process to protect against new infections, in certain cases it may lead to pathological outcomes. The ultimate effect depends on the length of this memory, and extended memory may underlie long-lived inflammatory conditions like we see in HIV infection or long COVID.” As the authors concluded, “In conclusion, we demonstrated that EVs carrying HIV protein Nef trigger long-lasting changes in inflammatory responses of the host by mechanisms consistent with trained immunity.”