Host defenses in experimental scrub typhus: role of cellular immunity in heterologous protection

Abstract

The relative contributions of cellular and humoral immunity in scrub typhus infections were studied in inbred mice employing paired strains of Rickettsia tsutsugamushi differing in virulence. An infectious dose (100 ID50) of the less virulent Gilliam strain resulted in heterologous immune protection against an otherwise lethal challenge (1000 LD50) of the virulent Karp strain. Partial heterologous protection against lethal Karp challenge was observed in animals preimmunized with the Gilliam strain as early as 3 days prior to challenge; complete protection against illness and death existed in animals immunized at least 7 days prior to challenge. In the heterologous protection provided by prior Gilliam infection, the role of humoral immunity was not of primary importance for the following reasons: significant levels of complement-fixing antibody against R. tsutsugamushi were not detectable until long after animals were solidly immune; antibody eventually appearing after Gilliam immunization exhibited a consistently low complement-fixing titer against the immunizing homologous (Gilliam) strain and contained no detectable activity against the heterologous challenge (Karp) strain; and passive transfer of large quantities of serum from Gilliam immune mice, themselves immune to Karp challenge, failed to protect recipients against a similar challenge. Protection was afforded by the passive transfer of serum containing antibody against Karp, suggesting a major role for antibody in protection against homologous infection. This heterologous challenge system was particularly useful because it minimized the role of humoral immunity, at least early in the course of infection, and allowed a definitive examination of the cellular response. Cell-mediated immunity played a major role in the heterologous protection observed after Gilliam immunization, as evidenced by the significant protection against Karp challenge afforded by the passive transfer of spleen cells from animals immunized with Gilliam 7-63 days previously. Of the immune spleen cells, only those which were nonadherent, presumably lymphocytes, were capable of transferring passive heterologous protection. This protective effect of nonadherent cells could be ablated by depleting the cell population of thymus-derived (T) cells with anti-.theta. serum and complement prior to transfer but not by use of anti-immunoglobulin serum and complement, which selectively removes bone marrow-derived (B) cells. The cell in immune spleens capable of conferring heterologous protection was probably a T lymphocyte.