Evasion of complement-mediated killing by pathogenic rickettsial species

描述（由申请人提供）：立克次体属革兰氏阴性¿-变形菌群很小(0.3-0.5 x 0.8-1.0 ？M)，专性细胞内生物。它们被分为两大类，斑疹热组（SFG）和斑疹伤寒组（TG），可以通过抗原性和细胞内基于肌动蛋白的运动性来区分。该属的成员负责严重的人类疾病和包括康氏R.和立克次体在内的几个物种，已被国家过敏和传染病研究所（NIAID）列为B类和C类优先病原体，并被疾病控制和预防中心（CDC）列为选择剂（立克次体），因为它们可能被用作生物恐怖主义的工具。

DESCRIPTION (provided by applicant): The Gram-negative ¿-proteobacteria of the genus Rickettsia are small (0.3-0.5 x 0.8-1.0 ?m), obligate intracellular organisms. They are categorized into two major groups, the Spotted Fever Group (SFG) and Typhus Group (TG), which can be distinguished by antigenicity and intracellular actin-based motility. Members of this genus are responsible for severe human diseases and several species including R. conorii and R. rickettsii, have been classified as Category B and C Priority Pathogens by the National Institute of Allergy and Infectious Diseases (NIAID) and as the Select Agents (R. rickettsii) by the Centers for Disease Control and Prevention (CDC) for their potential use as tools for biological terrorism. We have recently determined that in the absence of neutralizing antibodies, a model SFG rickettsial species, R. conorii the etiologic agent of Mediterranean spotted fever (MSF), is able to evade complement-mediated killing in human and murine serum. These results suggested that R. conorii and possibly other rickettsial species likely have evolved active mechanisms to evade the bactericidal effects of complement deposition as the pathogen disseminates in the bloodstream to target organs and tissues. We have identified two highly conserved rickettsial antigens, rOmpB/Sca5 and RC1281/Adr1, which are sufficient to mediate survival in ex-vivo human serum models of rickettsia bacteremia. We have demonstrated that R. conorii interact with human Factor H (fH), a fluid phase regulator of the alternative complement pathway and that rOmpB co-immunopreciptates with human fH, demonstrating its ability to serve as a bona fide fH binding protein (fHbp). We have also demonstrated that R. conorii survival is compromised when incubated in human serum depleted of fH suggesting that specific rickettsial interactions with serum components are critical to survival. Homologues to rOmpB and RC1281 exist in all pathogenic rickettsiae suggesting that the ability to actively perturb complement-mediated killing in the blood is a novel virulence attribute for this class of pathogens. The experiments outlined in this proposal will address the following research interests: i. The contribution of rickettsial fH binding proteins to serum resistance will be analyzed in vitr using surrogate expression systems in E. coli. We will initially determine the significance of rOmpB and RC1281 to serum resistance in vitro and will also determine the mechanisms underlying the evasion of complement-mediated killing. ii. The contribution of generated targeted insertion mutants in rompB/sca5 and rc1281 will be analyzed using in vitro and in vivo models of R. conorii and R. rickettsii infection.