Structural, antigenic, and biological characterization of Rickettsia tsutsugamushi proteins

Abstract

Rickettsia tsutsugamushi, the etiologic agent of scrub typhus, is a small, obligately intracellular bacterium. Several antigenically distinct strains exist. Monoclonal antibodies (Mab) to four Rickettsia tsutsugamushi proteins, Sta56, Sta58, Sta47 and Sta110, were developed and used to probe (1) their subcellular location, (2) structural and antigenic properties of proteins from eight prototype strains and several recent isolates from Thailand, (3) biological relevance, and (4) the fine antigenic structure of one protein. The subcellular location of these proteins was examined by several methods, immunoelectron microscopy (IEM), immunoprecipitation and rickettsial cellular fractionation. Sta56, Sta47 and Sta110 were present in the cell envelope. Sta58 was also found in the cell envelope but was predominantly in the cytoplasmic fraction. We have confirmed and expanded upon the characterization of Sta56 as a source of rickettsial variation and have also shown considerable heterogeneity to exist in Sta110. Sta58 and Sta47 were largely conserved in electrophoretic mobility and antigenicity, by comparison. Six recent isolates from Thailand were found to differ from eight prototype strains and from each other as five distinct SDS-PAGE protein profiles and antigenicity patterns were obtained. Mab to two immunodominant outer membrane proteins, Sta56 and Sta47, neutralized rickettsial entry into cultured Vero cells and partially protected mice from a lethal dose of rickettsiae. Because of the demonstrated involvement of Sta47 in the neutralization of infectivity, its conserved nature and its reactivity with human immune sera, the fine antigenic structure of Sta47 was examined with overlapping synthetic peptides of the deduced amino acid sequence and Mab and human convalescent sera. Six immunodominant regions were identified with the human sera and binding sites for three cross-reactive Mab (two known to bind surface epitopes, one not tested) were localized onto the Sta47 sequence. Epitopes important in cellular invasion or mouse infectivity could not be mapped by this method. Thus, we have shown the feasibility of pursuing the recombinant Sta47 or a mixture of peptides for use in vaccine and diagnostics development.