STUDIES OF PERSISTENT INFECTION BY CHLAMYDIA-TRACHOMATIS SEROVAR-K INTPA-DIFFERENTIATED U937 CELLS AND THE ROLE OF IFN-GAMMA

Inoculation of phorbol ester-differentiated U937 cells as a model for human macrophages with Chlamydia trachomatis of the urogenital serovar K resulted in a persistent infection, with maximal growth at day 7, u ntil day 10 post-infection. At these times inclusion bodies were prese nt in 0.5-2% of the cells. Typical inclusion bodies containing element ary bodies and reticulate bodies were observed by electron microscopy. Furthermore, single chlamydial particles resembling atypical elementa ry or intermediate bodies were identified in the cytoplasm in > 80% of the host cells. IFN-gamma exerts antichlamydial activity in epithelia l and fibroblastoid cells, but the infection of U937 cells by C. trach omatis was not affected by IFN-gamma. The activity of the tryptophan-d egrading enzyme indoleamine 2,3-dioxygenase (IDO) was not detected in untreated or in IFN-gamma-treated or chlamydiae-infected or mock-infec ted U937 cells. The presence of atypical persisting chlamydiae and the lack of IDO expression in U937 cells indicates that the development o f these atypical bacteria is independent from IFN-gamma-mediated trypt ophan deprivation and other IFN-gamma-mediated effects. Evaluation of persistently infected cells revealed that the expression of the chlamy dial major outer-membrane protein, heat-shock protein (hsp60) and lipo polysaccharide (LPS) antigens was not significantly altered in the cou rse of the culture. An intense staining of the LPS on the surface of t he host cells was demonstrated by immunofluorescence. The data show th at phorbol ester-differentiated U937 cells restrict chlamydial growth strongly but not completely through a mechanism distinct from IDO-medi ated tryptophan deprivation. The mechanisms of persistence of chlamydi ae in monocytes, which differ considerably from those described for ot her cells, require further investigation.