Changes in expression of the 44-kilodalton outer surface membrane antigen (p44 kD) for monitoring progression of infection and antimicrobial susceptibility of the human granulocytic ehrlichiosis (HGE) agent in HL-60 cells

Changes in human granulocytic ehrlichiosis (HGE)-specific major outer membr ane protein (p44 kD) were assayed by Western blot analysis in HL-60 cells i n vitro infected by the HGE agent. Time course study demonstrated that the expression of p44 preceded the rise in cell infection as determined by the presence of intracellular morulae. To test whether the expression of p44 ma y be suitable for evaluating the effects of antibiotics in vitro, three rec ent isolates of the HGE agent mere exposed to doxycycline and ampicillin du ring culture with HL-60 cells. Loss of infection concurrent with disappeara nce of the 44 kD protein was found with doxycycline treatment. In contrast, ampicillin treatment had no discernible effects on infection or 44 kD expr ession. There was excellent agreement between infection, as measured by mor ulae, and 44 kD expression (coefficient of correlation r = 0.97, p < 0.01). Following treatment with doxycycline, the 44 kD protein disappeared with a n estimated t(1/2) of similar to 24-30 h, which was considerably shorter th an a t(1/2) of >60 h calculated for loss of morulae. Measurement of p44 exp ression may be a more rapid and simple assay to determine antibiotic suscep tibility of the HGE agent in cell culture. Furthermore, it may be used to i ndicate the presence of infection before morulae are apparent, (C) 1999 Aca demic Press.