As scientists continue to make progress in the fight against human immunodeficiency virus (HIV), a recent discovery suggests that certain other microbes may play a role in how the body responds to vaccination.

According to researchers at the California National Primate Research Center (CNPRC) at the University of California, Davis (UC Davis), microbes living in the rectum could alter the effectiveness of experimental HIV vaccines.

Evidence from human and animal studies with other vaccines suggests supplements containing the bacteria Lactobacillus can boost antibody production, while treatment with antibiotics can hamper beneficial immune responses, according to Smita Iyer, assistant professor at the UC Davis Center for Immunology and Infectious Diseases and School of Veterinary Medicine. 

Iyer and her team specifically sought to learn if microbes living in the rectum and vagina—sites of HIV transmission—interacted with an experimental HIV vaccine similar to the HVTN 111 vaccine currently in early stage clinical trials in humans. According to Iyer, a vaccine that produces antibodies at the mucosal membranes where infection takes place is thought to be crucial.

The team studied rectal and vaginal microbes from rhesus macaques before and after they were vaccinated. While vaginal microbes did not show much difference before and after vaccination, rectal microbes did, with certain bacteria decreasing after vaccination. 

Furthermore, the amounts of the common gut bacteria Lactobacillus and Clostridia in the rectum correlated positively with the immune response. Animals with high levels of either Lactobacillus or Clostridia made more antibodies to certain HIV proteins, the researchers found. Prevotella bacteria showed the opposite pattern: High levels of Prevotella were correlated with weaker immune responses.

It’s not clear what the mechanism could be for some bacteria to boost local immune responses in a specific site in the body, Iyer said. However, targeting these bacteria could help scientists get the best possible performance out of vaccines that do not induce a particularly strong immune response, as is the case with HIV vaccines.

The NPRCs are actively conducting HIV/AIDS research across the country. Discover more ways our scientists are making progress against this disease in the ongoing pursuit of a cure.