Temporal changes in outer surface proteins A and C of the lyme disease-associated spirochete, Borrelia burgdorferi, during the chain of infection in ticks and mice

The Lyme disease-associated spirochete, Borrelia burgdorferi, is maintained in enzootic cycles involving Ixodes ticks and small, mammals. Previous stu dies demonstrated that B. Burgdorferi expresses outer surface protein A (Os pA) but not OspC when residing in the midgut of unfed ticks. However, after ticks feed on blood, some spirochetes stop making OspA and express OspC, O ur current work examined the timing and frequency of OspA and OspC expressi on by B. burgdorferi in infected Ixodes scapularis nymphs as they fed on un infected mice and in uninfected I. scapularis larvae and nymphs as they fir st acquired spirochetes from infected mite. Smears of midguts from previous ly infected ticks were prepared at 12- or 24-h intervals following attachme nt through repletion at 96 h, and spirochetes were stained for immunofluore scence for detection of antibodies to OspA and OspC. As shown previously, p rior to feeding spirochetes in nymphs expressed OspA but not OspC. During n ymphal feeding, however, the proportion of spirochetes expressing OspA decr eased, while spirochetes expressing OspC became detectable. In fact, spiroc hetes rapidly began to express OspC, with the greatest proportion of spiroc hetes having this protein at 48 h of attachment and then with the proportio n decreasing significantly by the time that the ticks had completed feeding . In vitro cultivation of the spirochete at different temperatures showed O spC to be most abundant when the spirochetes were grown at 37 degrees C. Ye t, the synthesis of this protein waned with continuous passage at this temp erature. Immunofluorescence staining of spirochetes in smears of midguts fr om larvae and nymphs still attached or having completed feeding on infected mice demonstrated that OspA but not OspC was produced by these spirochetes recently acquired from mice, Therefore, the temporal synthesis of OspC by spirochetes only in feeding ticks that were infected prior to the blood mea l suggests that this surface protein is involved in transmission from tick to mammal but not from mammal to tick.