Identification of protective rickettsial antigens for vaccine development
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The genus Rickettsia contains some of the most lethal pathogens known to man, including Rickettsia prowazekii, a select agent that could be used as a biological weapon. Although Rickettsia spp. cause important diseases of public health importance and there is a need to develop countermeasures for biodefense, there are no commercially available vaccines. Resistance to rickettsial infections and cross-protective immunity between typhus and spotted fever group rickettsiae are attributed to the induction of antigen-specific T cells, particularly CD8+ T cells. Defining specific T-cell antigens and correlates of protective cellular immunity are critical steps towards vaccine development for Rickettsia. However, these are major gaps in this field that are impeding progress towards a vaccine. In this investigation, I developed and validated an in silico algorithm that allowed the identification of five novel R. prowazekii vaccine antigen candidates recognized by CD8+ T cells. The novel rickettsial vaccine candidate antigens, RP884, RP778, RP739, RP598, and RP403, protected mice against a lethal challenge with typhus group R. typhi and spotted fever group R. conorii, which demonstrates a level of cross-protective immunity against these molecularly distinct pathogenic rickettsial groups. Moreover, considering that surviving a natural rickettsial infection results in long-lived immunity, I characterized the primary and memory CD8+ T cell response after a rickettsial challenge using phenotypic markers for activation and measured effector molecules that could be used to validate the level of cellular immunity induced by novel antigens. Based on the studies presented herein, four correlates of protection against R. typhi infection in animals immunized with protective rickettsial antigens are proposed: 1) production of IFN- by antigen experienced CD3+CD8+CD44high cells, 2) production of Granzyme B by CD27lowCD43low antigen-experienced CD8+ T cells, 3) generation of memory-type CD8+ T cells [Memory Precursor Effector Cells (MPECs), as well as CD127highCD43low, and CD27highCD43low CD8+ T cells], and 4) generation of effector-like memory CD8+ T cells (CD27lowCD43low). Together, these findings validate a reverse vaccinology approach as a strategy to identify protective rickettsial antigens that induce cellular immunity. In addition, this work demonstrates the feasibility of developing a subunit vaccine that triggers T-cell-mediated cross-protection between phylogenetically distant Rickettsia spp. Finally, the proposed correlates could be useful for the validation and assessment of the quality of the CD8+ T cell responses induced by novel antigens with potential use in a vaccine against Rickettsia.