INVASION OF VERO CELLS AND INDUCTION OF APOPTOSIS IN MACROPHAGES BY PATHOGENIC LEPTOSPIRA-INTERROGANS ARE CORRELATED WITH VIRULENCE

Interactions of virulent Leptospira interrogans serovar icterohaemorrh agiae strain Verdun with Vero cells (African green monkey kidney fibro blasts) and a monocyte-macrophage-like cell line (J774A.1) were assaye d by a double-fluorescence immunolabelling method. Infectivity profile s were investigated according to (i) the duration of contact between l eptospires and eukaryotic cells and (ii) the number of in vitro passag es after primary isolation from lethally infected guinea pigs. Compara tive experiments were conducted with the corresponding high-passage av irulent variant and the saprophytic leptospire Leptospira biflexa Pato c I. In Vero cells, virulent leptospires were quickly internalized fro m 20 min postinfection, whereas avirulent and saprophytic strains rema ined extracellularly located. In addition, the virulent strain demonst rated an ability to actively invade the monocyte-macrophage-like J774A .1 cells during the early stages of contact and to induce programmed c ell death, as shown by the detection of oligonucleosomes in a quantita tive sandwich enzyme immunoassay. In both cellular systems, subsequent in vitro subcultures demonstrated a progressive decrease of the invas iveness, pointing out the necessity of using primocultures of Leptospi ra for virulence studies. Invasiveness of virulent leptospires was sig nificantly inhibited with monodansylcadaverine, indicating that intern alization was dependent on receptor-mediated endocytosis. Invasion of epithelial cells and induction of apoptosis in macrophages may be rela ted to the pathogenicity of Leptospira, and both could contribute to i ts ability to survive in the host and to escape from the immune respon se.