Effect of immune serum on infectivity of Rickettsia tsutsugamushi

Hyperimmune antirickettsial serum was shown to prevent the attachment/penetration stage of Rickettsia tsutsugamushi infection of suspended chicken cells. The extent of the inhibition depended on the serum concentration but not on the presence of complement. The neutralizing activity was reduced by prior adsorption of immune serum with staphylococcal protein A or with intact rickettsiae but was not affected by adsorption with target cells. In the neutralization tests, there was no cross-reactivity between the Karp and Gilliam strains of R. tsutsugamushi. Incubation of rickettsiae with immune serum did not alter their capacity to metabolize glutamate nor grossly damage the permeability barrier function of their cytoplasmic membranes. Although the assay method had the capacity to detect some aggregated infectious organisms, none were found in immune serum-treated suspensions. It was concluded that immune serum may inhibit rickettsial infection by blocking a surface component(s) whose function is necessary for attachment to and/or penetration of target cells.