Nonopsonic Binding of Type III Group B Streptococci (III GBS) to Human Neutrophils (PMN) Induces IL-8 Release Mediated by the p38 MAP Kinase Pathway

Abstract 1564 Neonatal Infectious Disease Platform, Monday, 5/3

Nonopsonic interactions between microorganisms and phagocytic cell are important to host defense We have shown previously that III GBS bind nonopsonically to PMN in a stable and inoculum-dependent manner, at a maximum of 83 ± 5 % PMN participation. The mitogen activated protein kinase (MAPK) cascades p38 and extracellular signal-regulated kinases 1/2 (ERK 1/2) have been shown to play a central role in regulating PMN function in response to a variety of extracellular stimuli. We hypothesized that the nonopsonic association of III GBS with PMN is sufficient to activate these intracellular signaling pathways. PMN (10⁷/ml) were stimulated with heat killed III GBS (10⁸cfu/ml) in a serum free system for various time intervals. Western blot analysis using a rabbit anti-phosphorylated MAPK antibody was employed to detect activated p38 and ERK 1/2. Incubation of III GBS with PMN induced the activation of both p38 and ERK 1/2 that was noted after 5 min of incubation, peaked in intensity at 15 min and began to diminish at 25 min. The nonopsonic interaction of III GBS with PMN also resulted in the elaboration of a significant amount of interleukin-8 (IL-8), measured by ELISA to be 645 ± 92 pg/ml compared to PMN alone (86 ± 9 pg/ml); p=0.009, mean ± SEM of 4 experiments. Treatment of PMN with the p38 MAPK specific inhibitor SB 203580 resulted in a dose-dependent reduction of IL-8 release in response to III GBS (46 ± 10 % at 0.1 µM, p = 0.04; 70 ± 7 % at 5 µM, p = 0.006; and 84 ± 4 % at 20 µM of SB 203580, p = 0.003 as compared to III GBS with no inhibitor). Inhibition of PMN ERK 1/2 using the specific inhibitor PD 98059 resulted in a 20 % reduction of IL-8 release at a concentration of 10 µM and maximal reduction of 29 ± 6 % at a concentration of 50 µM. Thus, the nonopsonic association of III GBS with PMN results in the release of the potent chemotactic cytokine IL-8 through induction of the p38 MAPK pathway. The MAPK, ERK 1/2, although activated by this interaction does not appear to mediate IL-8 release. We postulate that this initial inflammatory response is an important arm of innate immunity serving to protect the nonimmune host from invasive III GBS infection.