Phenotype of the anti-Rickettsia CD8(+) T cell response suggests cellular correlates of protection for the assessment of novel antigens.

The obligately intracellular bacteria Rickettsia infect endothelial cells and cause systemic febrile diseases that are potentially lethal. No vaccines are currently available and current knowledge of the effective immune response is limited. Natural and experimental rickettsial infections provide strong and cross-protective cellular immunity if the infected individual survives the acute infection. Although resistance to rickettsial infections is attributed to the induction of antigen-specific T cells, particularly CD8(+) T cells, the identification and validation of correlates of protective cellular immunity against rickettsial infections, an important step toward vaccine validation, remains a gap in this field. Here, we show that after a primary challenge with Rickettsia typhi in the C3H mouse model, the peak of anti-Rickettsia CD8(+) T cell-mediated responses occurs 7 days post-infection (dpi), which coincides with the beginning of rickettsial clearance. At this time point, both effector-type and memory-type CD8(+) T cells are present, suggesting that 7 dpi is a valid time point for the assessment of CD8(+) T cell responses of mice previously immunized with protective antigens. Based on our results, we suggest four correlates of cellular protection for the assessment of protective rickettsial antigens: (1) production of IFN-γ by antigen-experienced CD3(+)CD8(+)CD44(high) cells, (2) production of Granzyme B by CD27(low)CD43(low) antigen-experienced CD8(+) T cells, (3) generation of memory-type CD8(+) T cells [Memory Precursor Effector Cells (MPECs), as well as CD127(high)CD43(low), and CD27(high)CD43(low) CD8(+) T cells], and (4) generation of effector-like memory CD8(+) T cells (CD27(low)CD43(low)). We propose that these correlates could be useful for the general assessment of the quality of the CD8(+) T cell immune response induced by novel antigens with potential use in a vaccine against Rickettsia.

Discovery of a protective Rickettsia prowazekii antigen recognized by CD8+ T cells, RP884, using an in vivo screening platform.

Rickettsia prowazekii has been tested for biological warfare due to the high mortality that it produces after aerosol transmission of very low numbers of rickettsiae. Epidemic typhus, the infection caused by these obligately intracellular bacteria, continues to be a threat because it is difficult to diagnose due to initial non-specific symptoms and the lack of commercial diagnostic tests that are sensitive and specific during the initial clinical presentation. A vaccine to prevent epidemic typhus would constitute an effective deterrent to the weaponization of R. prowazekii; however, an effective and safe vaccine is not currently available. Due to the cytoplasmic niche of Rickettsia, CD8(+) T-cells are critical effectors of immunity; however, the identification of antigens recognized by these cells has not been systematically addressed. To help close this gap, we designed an antigen discovery strategy that uses cell-based vaccination with antigen presenting cells expressing microbe's proteins targeted to the MHC class I presentation pathway. We report the use of this method to discover a protective T-cell rickettsial antigen, RP884, among a test subset of rickettsial proteins.