Vivaxin genes encode highly immunogenic non-variant antigens unique to the Trypanosoma vivax cell-surface

Trypanosoma vivax is a unicellular hemoparasite, and a principal cause of animal African trypanosomiasis (AAT), a vector-borne and potentially fatal disease of livestock across sub-Saharan Africa. Previously, we identified diverse T. vivax-specific genes that were predicted to encode cell surface proteins. Here, we examine the immune responses of naturally and experimentally infected hosts to many of these unique parasite antigens, to identify immunogens that could become vaccine candidates. Immunoprofiling of host serum showed that one particular family (Fam34) elicits a consistent IgG antibody response. This gene family, which we now call Vivaxin, encodes at least 124 transmembrane glycoproteins that display quite distinct expression profiles and patterns of genetic variation. We focused on one gene (viv-β8) that is among the most immunogenic and highly expressed but displays minimal polymorphism. VIVβ8 was localized across the cell body and flagellar membrane, suggesting that vivaxin is substantial family of novel surface proteins. Although vaccination of mice with VIVβ8 adjuvanted with Quil-A elicits a strong, balanced immune response and delays parasite proliferation in some animals, ultimately, it does not prevent disease. However, our phylogenetic analysis shows vivaxin includes other antigens shown to induce immunity against T. vivax. Thus, the introduction of vivaxin represents an important advance in our understanding of the T. vivax cell surface. Besides being a source of proven and promising vaccine antigens, the gene family is clearly an important component of the parasite glycocalyx, with potential to influence the host-parasite interaction.