BACKGROUND: Resident microbiota maintain intestinal homeostasis by regulating digestion, metabolism, immune development, and providing protection from infection. Pioneering work has also shown that resident microbiota enhance the transmission of a limited number of viruses that target the intestinal tract. The intestinal tract is a major site of HIV infection. However, the effect of resident microbiota on HIV acquisition, replication, and pathogenesis in the intestinal tract is unknown due to the strict species tropism of HIV.
METHODS: Bone marrow/liver/thymus (BLT) humanized mice have been extensively utilized to study HIV acquisition, pathogenesis and prevention strategies in vivo. To examine the role of resident microbiota in HIV acquisition and infection, we constructed germ-free (GF) BLT mice and BLT mice colonized with resident intestinal microbiota. First, we rederived a GF strain of immune deficient mice by sterile embryo transfer. GF-BLT humanized mice were then surgically constructed in a sterile gnotobiotic surgical isolator. All GF mice were housed and experiments performed in sterile gnotobiotic isolators and the GF status of mice confirmed longitudinally. BLT mice colonized with resident microbiota were also constructed. We then evaluated HIV acquisition, replication, and pathogenesis in GF BLT mice and BLT mice colonized with resident microbiota following an oral or rectal HIV exposure. HIV-RNA levels were monitored longitudinally in the peripheral blood plasma of mice weekly by real-time PCR analysis. At necropsy, we measured the levels of HIV-DNA, HIV-RNA, and T cell activation blood and tissues.
RESULTS: Our results show that HIV acquisition, replication and pathogenesis in the intestinal tract is greatly enhanced in the presence of resident microbiota. HIV acquisition was 300% higher following an oral HIV challenge (P=0.013) and 200% higher following a rectal HIV challenge (P=0.0286) in the presence of resident microbiota. Mean plasma viral loads were up to 34-fold higher and cell-associated HIV-RNA levels in tissue were also over a 1000-fold higher in the presence of resident microbes. Furthermore, HIV-associated CD8⁺ T cell activation was higher in the intestinal tract in the presence of resident microbiota.
CONCLUSIONS: These data directly demonstrate for the first time that resident microbiota are a major driver of HIV acquisition, replication, and pathogenesis.