PROJECT ABSTRACT While the microbiota has emerged as an important aspect of mammalian health, the vast majority of current studies have focused on the bacterial component of the microbiota, despite the presence of fungi, archaea and viruses. The virome has recently just begun to become characterized and while there are eukaryotic viruses present within a healthy individual, the overwhelming majority of the viruses on the human body are bacteria infecting viruses called bacteriophages (phages). Changes in these phage communities have been reported in animal and humans during a variety of diseases including inflammatory bowel disease and obesity, however we known very little about these populations during the healthy state. Our recent work has demonstrated that bacteriophages, despite being unable to actively infect mammalian cells, are capable of inducing immune system development. These data suggest that bacteriophages represent an unappreciated microbe that is capable of shaping immune system responses. Recent studies on the bacterial component of the microbiota, have revealed that identifying what bacteria the immune system reacts to, can provide insight into specific bacteria that influence disease, novel biomarkers and unique antigens that can influence natural immune system development. However, this has yet to be performed with the virome. Antibodies, including IgA and IgG, and are highly abundant at mucosal surfaces and sera, respectively and are known to have high reactivity to commensal bacteria. We present data within this application that both sera and mucosal antibodies have high reactivity to commensal bacteriophages in both humans and mice. Based on this, we propose to characterize and identify the commensal viruses that are targeted by IgA and IgG. We will propose three distinct aims. Aim 1 will catalog the mucosal IgA/IgG reactivity against commensal viruses from stool samples obtained from healthy humans. To do this, we have optimized a strategy to purify and sort viral particles allowing us to capture IgA bound DNA and RNA viral particles. As sera antibody reactivity can often reveal unique commensal organisms of interest, Aim 2, will characterize the sera reactivity against the gut resident viral communities using paired sera and stool samples from humans. Finally, to understand how antibodies can influence these human associated viral communities, Aim 3 will utilize gnotobiotic mice that lack adaptive immunity and IgA and colonized with healthy human microbiota to understand how viral communities are controlled by mammalian immunity. These studies will be the first to catalog how the human immune system reacts to commensal viruses of healthy human intestine and will provide a rich resource of data that has the potential to identify novel viruses of interest and establish paradigms how the immune system establishes homeostasis with the resident viral community.

PROJECT NARRATIVE Understanding how the immune system is able to interact with the organisms that live on our body has revealed unique immune-microbe interactions that are now known to influence mammalian health. This has yet to be done with our resident viral community. Our proposal will be the first to catalog and identify what commensal viruses interact with antibodies in humans, which has the potential to identify novel viruses that are relevant to human health and will facilitate future mechanistic studies.