Group B streptococci induce cyclooxygenase-2 expression by human monocytes

Introduction: GBS infections are associated with a high degree of infant morbidity. GBS type III are the most virulent. The exact mechanism leading to pathogenesis is unknown. Cyclooxygenase is the enzyme involved in the conversion of arachidonic acid to prostaglandins and thromboxane. Type 1 (COX-1) is constitutively present in most cells and is responsible for prostaglandin production involved in cellular homeostasis. Type 2 (COX-2) is not present in normal resting cells but is inducible under a variety of inflammatory conditions. This study was undertaken to identify the effect that GBS type III has on COX activity in human leukocytes and to determine the regulatory mechanisms involved. Methods: Freshly isolated human monocytes were obtained from healthy adult donors. Monocytes were exposed to type III GBS, LPS or buffer under different pre-incubation conditions. Exogenous arachidonic acid was added to the cell preparations following agonist exposure and the supernatant assayed for prostaglandin E₂ (PGE₂) by enzyme immunoassay. The cells were lysed and protein was isolated from the cell preparation. COX protein was assayed by Western blot analysis. Results: COX-1 protein expression did not change from baseline when monocytes were exposed to GBS. Monocytes exposed to GBS expressed COX-2 protein in both a time- and concentration dependent manner. COX-2 was not present in freshly isolated cells, nor cells exposed to buffer alone, but was detected within 4 hours following exposure to GBS. COX-2 protein expression peaked around eighteen hours following exposure to GBS. This pattern of COX-2 expression was very similar to that observed for human monocytes exposed to LPS. COX-2 protein expression was paralleled by prostaglandin E₂ secretion. Anti-TNF-α pre-incubation did not effect COX-2 protein expression when monocytes were subsequently exposed to either GBS or LPS. Pre-incubation with either anti-inflammatory cytokine IL-4 or IL-10 resulted in marked attenuation of COX-2 protein expression when monocytes were subsequently exposed to either GBS or LPS. Conclusion: We are the first to report that exposure of a gram positive bacteria to human monocytes results in induction of COX-2. In addition, the COX-2 protein is functional, converting arachidonic acid to prostaglandin E2. The signal transduction pathway leading to COX-2 expression is likely very similar to the pathway resulting from exposure to LPS, as both the time-course of expression and effect of anti-inflammatory cytokines are comparable. These data suggest that modification of eicosanoid metabolism plays an important role in the pathogenesis of GBS sepsis.