We have examined the effect of the virulent Leptospira interrogans strain Teramo, serotype icterohemorrhagiae, on the adherence of human neutrophilic polymorphonuclear leukocytes (PMN) to cultured human umbilical vein endothelial cells (HEC). Selective pretreatment of HEC with intact or sonicated leptospires caused a dose- and time-dependent increase of HEC-PMN adhesion (13.2% +/- 2.5% adherence to untreated HEC versus 46.3% +/- 5.6% adherence to HEC pretreated for 4 h with 10(8) intact leptospires per ml [mean +/- standard error of six experiments; P < 0.001]). In contrast, selective leptospire pretreatment of PMN or the addition of leptospires during the adherence assay did not alter HEC-PMN adherence. Leptospire induction of endothelial-cell adhesiveness occurred without detectable HEC damage and was prevented by RNA and protein synthesis inhibitors and by monoclonal antibodies to the CD11/CD18 adhesion complex of neutrophils and to the endothelial-leukocyte adhesion molecule 1 (ELAM-1) of endothelial cells. Similar results were obtained with pretreatment of HEC with interleukin-1 or with the lipopolysaccharide (LPS) of the gram-negative bacterium Escherichia coli. The possibility that contamination by the LPS of gram-negative bacteria could be involved in the induction of HEC adhesiveness was ruled out by the observation that the LPS inhibitor polymyxin B, which abolished the proadhesive effect of E. coli LPS, was ineffective in inhibiting leptospire- as well as interleukin-1-induced adherence. Similarly, leptospire LPSs seemed to have no role in the increase of endothelial-cell adhesiveness, since pretreatment of HEC with a leptospire LPS extract (phenol-water method) or with a leptospire total lipid extract failed to induce the proadhesive phenotype for neutrophils. Instead, peptidoglycans extracted from our leptospires actively stimulated the endothelial proadhesive activity for neutrophils (16.5% +/- 2.1% adherence to untreated HEC versus 51.2% +/- 2.9% adherence to HEC pretreated for 4 h with 1 microgram of peptidoglycan per ml; [mean +/- standard error of four experiments; P < 0.001]). This peptidoglycan-induced activity was inhibited by monoclonal antibodies to the CD11/CD18 adhesion complex and to ELAM-1 but not by polymyxin B. We conclude that peptidoglycans from pathogenic leptospires are among the molecules that can directly activate vascular endothelial cells to increase their adhesiveness for neutrophilic granulocytes. These observations may contribute to a better understanding of the mechanisms whereby non-gram-negative bacteria modulate the local and systemic inflammatory reaction.