Despite 30 years of study, male-to-female transmission of the virus that causes AIDS is not fully understood. Even less is known about infection risk within an aging population.

Given the global AIDS epidemic among heterosexuals as well as longer lifespans for those with AIDS, a better understanding of HIV transmission and the impact of aging on the disease progression is crucial, says Fred Krebs, an associate professor in microbiology and immunology at Drexel. He has proposed exploring the effect of age on factors in seminal fluid that modulate the female reproductive tract’s immune response. That in turn could alter the risk of HIV infection.

“It’s an area that has not been studied much at all, in any population, at any age,” he says.

Until recently, it was thought that the static medium of semen deposited the virus and infected cells into the vaginal environment, from where the disease spread systemically. In fact, semen contains numerous active factors, such as cytokines and chemokines, that prepare women for reproduction by changing the immune response in the female reproductive tract to allow for the foreign antigen, semen.

“The immune system is dialed back to create an environment of tolerance,” Krebs says. While good for conception, this process could increase the risk of HIV infection, he says.

In addition, mouse models have shown that the introduction of seminal fluid into the female reproductive tract results in inflammation, which causes the recruitment of immune cells to the area.

“The immune response in the female reproductive tract is really a double-edged sword,” Krebs says. “It could increase risk of transmission. It also could decrease risk of transmission.” How? More immune cells could fight off an HIV infection, or they could heighten risk by offering more targets for infection.

Aging, it is suspected, further impacts transmission scenarios. It’s an important aspect to explore for a number of reasons, Krebs says.

AIDS is no longer a death sentence within 10 years of infection because of effective combination antiretroviral therapies. American society encourages and values sexuality among older people—increasing the risk of transmission among this aged population. Finally, immunosenescence, a natural process in which the immune system deteriorates over time, likely affects the transmission risk of HIV.

In the case of immunosenescence, the body becomes “less efficient in fighting off disease and pathogens and less efficient at controlling infection that has already taken root,” Krebs says.