Ef. Blouin et Km. Kocan, MORPHOLOGY AND DEVELOPMENT OF ANAPLASMA-MARGINALE (RICKETTSIALES, ANAPLASMATACEAE) IN CULTURED IXODES-SCAPULARIS (ACARI, IXODIDAE) CELLS, Journal of medical entomology, 35(5), 1998, pp. 788-797
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.