The effect of monocyte-derived macrophages on the growth of Rickettsia conorii in permissive cells.

We examined whether monocyte-derived macrophages (MdM) incubated with rickettsia-infected HEp-2 or BGM cells a) affect R. conorii (Boutonneuse fever) growth, and b) secrete TNF and IL-1 alpha. BGM and HEp-2 cells were infected with R. conorii at multiplicities of infection (MOI) of 1-0.01. After 2 hr of adsorption, the cells were washed and MdM were added at an effector to target ratio of between 3 and 5. At 2, 24, 48, and 96 hr post-infection (p.i.) cells were scraped off; cell-free medium was collected and TNF and IL-1 levels were determined by ELISA and RIA, respectively. MdM caused a 50-70% reduction in the yield of R. conorii in HEp-2 cells as compared to the control (infected HEp-2 cells incubated without MdM). This reduction was more pronounced at MOI 0.1 and 0.01, than at MOI 1. In contrast, no reduction in the rickettsial yield was observed in the BGM cells incubated with MdM. TNF and IL-1 alpha levels in the cell-free medium from infected HEp-2 cells incubated with MdM were higher (2-5 fold) than those from infected BGM cells incubated with MdM. These data suggest the possibility that, and the mechanisms whereby, MdM may modulate rickettsia replication in vivo.