Comparison of surface proteins of Anaplasma marginale grown in tick cell culture, tick salivary glands, and cattle

Anaplasma marginale, a tick-borne rickettsial pathogen of cattle, infects b ovine erythrocytes, resulting in mild to severe hemolytic disease that caus es economic losses in domestic livestock worldwide. Recently, the Virginia isolate of A. marginale was propagated in a continuous tick cell Line, IDE8 , derived from embryonic Ixodes scapularis. Development of A. marginale in cell culture was morphologically similar to that described previously in ti cks. In order to evaluate the potential of the cell culture-derived organis ms for use in future research or as an antigen for serologic tests and vacc ines, the extent of structural conservation of the major surface proteins ( MSPs) between the cell culture-derived A. marginale and the bovine erythroc ytic stage, currently the source of A. marginale antigen, was determined. S tructural conservation on the tick salivary-gland stage was also examined. Monoclonal and monospecific antisera against MSPs 1 through 5, initially ch aracterized against erythrocyte stages, also reacted with A, marginale from cell culture and tick salivary glands, MSP1a among geographic A. marginale isolates is variable in size because of different numbers of a tandemly re peated 28- or 29-amino-acid peptide. The cell culture-derived A. marginale maintained the same-size MSP1a as that found on the Virginia Isolate of A. marginale in bovine erythrocytes and tick salivary glands. Although differe nces were observed in the polymorphic MSP2 antigen between culture and sali vary-gland stages, MSP2 did not appear to vary, by two-dimensional gel elec trophoresis, during continuous passage in culture, These data show that MSP s of erythrocyte-stage A. marginale are present on culture stages and may b e structurally conserved during continuous culture, The presence of all cur rent candidate diagnostic and vaccine antigens suggests that in vitro cultu res are a valuable source of rickettsiae for basic research and for the dev elopment of improved diagnostic reagents and vaccines against anaplasmosis.