Vaccination with conserved TSA56 blocks accelerates bacterial clearance in a rhesus macaque scrub typhus model

Scrub typhus, caused by the intracellular bacterium Orientia tsutsugamushi , remains a major public health concern in endemic regions, and no licensed vaccine is currently available. Antigenic diversity of the major surface antigen TSA56 has hindered vaccine development. Here, we evaluated a recombinant TSA56 antigen containing conserved regions (cTSA56) and the autotransporter protein ScaA as candidate vaccines and identified an adjuvant formulation capable of inducing robust cellular immunity. Adjuvant screening in mice showed that the GLA-SE + QS21 formulation elicited strong polyfunctional T-cell responses and provided significant protection following challenge. Using this formulation, we compared cTSA56 and ScaA vaccines in rhesus macaques. Although both vaccines induced antigen-specific antibody and T-cell responses, cTSA56 vaccination generated stronger cellular immunity and accelerated reduction of bacteremia in peripheral blood after O. tsutsugamushi challenge, accompanied by reduced systemic inflammatory responses. These findings support conserved TSA56 combined with Th1-inducing adjuvants as a promising strategy for scrub typhus vaccine development.