Beyond blocking HIV: How tenofovir shapes the immune system

The development of the Pre-Exposure Prophylaxis (PrEP) regimen transformed HIV prevention, giving people at risk of human immunodeficiency virus (HIV) infection a powerful new tool to protect their health. When taken as prescribed, PrEP is highly effective at preventing HIV infection by blocking the virus from establishing itself after exposure.

PrEP is a combination of antiretroviral medications that interfere with HIV replication. One commonly used component of PrEP is tenofovir, a nucleotide analog reverse transcriptase inhibitor (NRTI) that prevents the virus from copying its genetic material. Together with other antiretroviral medications, tenofovir has helped transform HIV from a life-threatening infection into a manageable chronic condition and has provided millions of people with greater control over their health.

But medications often have effects beyond their intended targets. New research from Dr. Florian Hladik’s group in the Vaccine and Infectious Disease Division reveals an unexpected biological activity of tenofovir: its ability to stimulate immune signaling pathways in mucosal tissues, even in the absence of HIV infection.

The study, recently published in Microbiology Spectrum, represents the third and final installment in a series of investigations examining how tenofovir influences the immune environment of mucosal tissues. While the researchers focused on topical 1% tenofovir gel, they also compared their findings with previous studies of oral tenofovir. Both routes of administration produced similar activation of type I/III interferon pathways, suggesting that these immune effects are an intrinsic property of the drug rather than the method of delivery.

“This is the last of a trio of papers, confirming and extending our prior findings from 2016 and 2020 that tenofovir causes immune activation, even in the absence of HIV infection,” says Hladik.

The mucosal surfaces of the body serve as critical barriers against infection. They are also among the first places where HIV can encounter susceptible cells after exposure. Understanding how preventive medications influence these tissues may provide important insights into both HIV prevention and long-term immune health.

In their previous work, Hladik’s team found that short-term exposure to topical tenofovir caused a broad range of changes within mucosal tissues. In this latest study, the researchers investigated which of these effects persisted after longer-term use.

“In our 2016 paper, we found that short-term topical tenofovir use causes a wide range of changes in the mucosa,” says Hladik. “Here we show that many of these effects wane with longer-term use, except for the type I/III interferon-stimulating activity.”

Interferons are signaling molecules that coordinate antiviral immune responses. When a virus is detected, interferon pathways help alert nearby cells and prepare the immune system to respond. However, prolonged activation of these pathways can also contribute to chronic inflammation. The researchers found that tenofovir continued to stimulate type I and type III interferon pathways in mucosal tissues, even after many of the other early changes associated with treatment had subsided. Whether this sustained signaling contributes to long-term immune activation in people taking antiretroviral therapy remains an important question for future research.

“Taken together, our research provides unequivocal proof that tenofovir has interferon pathway-stimulating activity,” says Hladik.

The researchers analyzed clinical specimens collected from studies conducted through the Microbicide Trials Network, as well as from a clinical study initiated and conducted by Hladik’s group in Seattle. Their findings demonstrate that tenofovir’s activity extends beyond directly blocking HIV replication, revealing a previously underappreciated interaction between antiretroviral medications and the immune system.

The findings also raise important questions about the long-term effects of NRTI-containing therapies used by people living with HIV. While the researchers have not yet demonstrated that similar interferon stimulation occurs in people receiving antiretroviral therapy (ART), this remains an important area of investigation.

“Our findings imply that a similar interferon-inducing effect by tenofovir or other NRTI class drugs also occurs in people living with HIV who take ART. However, we have not proven this yet,” says Hladik.

For people living with HIV, ART is a lifelong treatment that is essential for maintaining viral suppression and preventing disease progression. As people live longer with HIV, researchers are increasingly interested in understanding how long-term immune activation contributes to health outcomes and whether therapies can be optimized to minimize unnecessary inflammation. Over time, persistent inflammation has been linked to an increased risk of cardiovascular disease, neurocognitive disorders, and other age-related comorbidities. Understanding whether individual drugs contribute to this process could help researchers further improve lifelong HIV treatment.

“In the long run, it is important to optimize antiretroviral treatment, which people living with HIV must take lifelong, so that it does not contribute to chronic immune activation,” says Hladik.

Which is to say, the goal is not to replace today's highly effective HIV therapies, but to better understand how they interact with the immune system. With many antiretroviral medications now available, future treatment strategies may preserve powerful antiviral protection while minimizing unnecessary immune activation.

To help reach that goal, Hladik's group is developing new human cell-based models to study—and ultimately identify ways to prevent—chronic immune activation caused by antiretroviral medications. By uncovering how these therapies shape immune biology, the researchers hope to continue improving long-term treatment for people living with HIV.

For those interested in learning more about the underlying mechanisms of tenofovir’s effects on immunity and potential strategies to mitigate it, Dr. Hladik and colleagues have also just published an open-access review article in the American Journal of Reproductive Immunology.  

Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium Member Dr. Florian Hladik contributed to this research.

The spotlighted research was funded by the National Institutes of Health, the Microbicide Trials Network, the Eunice Kennedy Shriver National Institute of Child Health and Human Development, and the National Institute of Mental Health.

Hughes SM, Calienes FL, Levy CN, Pandey U, Gornalusse GG, Cranston RD, Lama JR, Pickett J, Brand RM, Hendrix CW, Marzinke MA, Dai JY, Balar B, Justman JE, Nair G, Berard AR, Birse K, Noel-Romas L, Mayer KH, Stekler JD, Mackelprang R, Burgener AD, McGowan I, Cameron CM, Cameron MJ, Woodrow KA, Hladik F. 2026. Mucosal tenofovir 1% gel stimulates cell proliferation and type I/III interferon pathways. Microbiology Spectrum. https://doi.org/10.1128/spectrum.01680-25.