MORPHOLOGY AND DEVELOPMENT OF ANAPLASMA-MARGINALE (RICKETTSIALES, ANAPLASMATACEAE) IN CULTURED IXODES-SCAPULARIS (ACARI, IXODIDAE) CELLS

Anaplasma marginale Theiler, a tick-borne rickettsial pathogen of catt le, was recently propagated in a continuous tick cell line, IDE8, deri ved from embryonic Ixodes scapularis Say. Cell monolayers were infecte d briefly with a high multiplicity of infection to synchronize rickett sial development and allow for description of the invasion, developmen t and release of A. marginale from the cultured cells. Sequential samp les were collected, fixed, and processed for er;amination with light a nd electron microscopy. A. marginale entered host cells by an endocyto tic process and remained within a vacuolar membrane throughout develop ment. After entry, the dense form of A. marginale transformed into the vegetative or reticulated form that multiplied by binary fission, for ming large colonies of rickettsiae. The reticulated form subsequently transformed into the dense form of A. marginale, which was released fr om cells and survived extracellularly. The dense forms were eventually released From the cultured cells by a process in which the inclusion membrane fused with the host cell membrane. Release of A. marginale wa s effected without the loss of host cell cytoplasm. In subsequent cell cycles, A. marginale reinfected cultured cells resulting in the devel opment of multiple colonies per cell and eventual host cell destructio n. Small vesicles were abundant within the colonies and appeared to fo rm from individual rickettsiae. Development of A. marginale in IDE8 ce lls was similar to that described in naturally infected Dermacentor so p. ticks. However, destruction of cells by A. marginale as seen in vit ro was not observed in naturally infected ticks. An understanding of t he developmental cycle of a. marginale in cultured cells may provide i nsight into rickettsial development in its tick host and provide a bas is for studying pathogen-host cell interaction in vitro.