International Journal of Medical Microbiology 305 (2015) 75–84 Contents lists available at ScienceDirect International Journal of Medical Microbiology j ourna l h o mepage: www.elsevier.com/locate/ijmm The Shigella flexneri OspB effector: an early immunomodulator Cecilia Ambrosi a,∗,1 , Monica Pompili a,1 , Daniela Scribano b , Dolores Limongi c , Andrea Petrucca d , Sonia Cannavacciuolo a , Serena Schippa a , Carlo Zagaglia a , Milena Grossi e , Mauro Nicoletti b,∗∗ a Dipartimento di Scienze di Sanità Pubblica e Malattie Infettive, “Sapienza” Università di Roma, 00185 Rome, Italy b Dipartimento di Scienze Sperimentali e Cliniche, Università “G. D’Annunzio”, 66100 Chieti, Italy c San Raffaele, Istituto di Ricovero e Cura a Carattere Scientifico IRCCS, Università Telematica San Raffaele Rome, Italy d Laboratorio di Microbiologia Clinica, II Facoltà di Medicina e Chirurgia, Ospedale “Sant’Andrea”, 00189 Rome, Italy e Dipartimento di Biologia e Biotecnologie Charles Darwin, “Sapienza” Università di Roma, 00185 Rome, Italy a r t i c l e i n f o Article history: Received 15 July 2014 Received in revised form 30 October 2014 Accepted 4 November 2014 Keywords: ospB Shigella flexneri MAP kinases PMN migration IL-8 a b s t r a c t Through the action of the type three secretion system (T3SS) Shigella flexneri delivers several effectors into host cells to promote cellular invasion, multiplication and to exploit host-cell signaling pathways to modulate the host innate immune response. Although much progress has been made in the understanding of many type III effectors, the molecular and cellular mechanism of the OspB effector is still poorly characterized. In this study we present new evidence that better elucidates the role of OspB as pro- inflammatory factor at very early stages of infection. Indeed, we demonstrate that, during the first hour of infection, OspB is required for full activation of ERK1/2 and p38 MAPKs and the cytosolic phospholipase A 2 (cPLA 2 ). Activation of cPLA 2 ultimately leads to the production and secretion of PMN chemoattractant metabolite(s) uncoupled with release of IL-8. Moreover, we also present evidence that OspB is required for the development of the full and promptly inflammatory reaction characteristic of S. flexneri wild- type infection in vivo. Based on OspB and OspF similarity (both effectors share similar transcription regulation, temporal secretion into host cells and nuclear localization) we hypothesized that OspB and OspF effectors may form a pair aimed at modulating the host cell response throughout the infection process, with opposite effects. A model is presented to illustrate how OspB activity would promote S. flexneri invasion and bacterial dissemination at early critical phases of infection. © 2014 Elsevier GmbH. All rights reserved. Introduction The human enteropathogenic bacterium Shigella flexneri invades the colonic mucosa and induces an intense inflammatory reaction that leads to tissue destruction and the dysenteric syndrome. Effi- cient invasion of epithelial cells of the colonic mucosa by S. flexneri occurs mainly at the basolateral membrane, through access to the submucosa. This access is accomplished via three distinct mech- anisms: transcytosis through M cells, tight junction disruption or induction of polymorphonuclear (PMN) leucocyte transepithe- lial migration (Ashida et al., 2011). The latter two mechanisms maximize the available surface for bacterial entry into host cells. ∗ Corresponding author. Tel.: +39 6 49914622. ∗∗ Corresponding author. Tel.: +39 6 49914657. E-mail addresses: cecilia.ambrosi@uniroma1.it (C. Ambrosi), mauro.nicoletti@uniroma1.it (M. Nicoletti). 1 These authors contributed equally to this work. Following invasion, bacteria break free in the cell cytosol, replicate intra-cellularly and, using actin-based motility, disseminate from cell to cell without extracellular steps (Ashida et al., 2011). Critical to its virulence, S. flexneri expresses a type III secretion system (T3SS) to inject type III effectors into the surrounding space and directly into the host cell cytoplasm (Parsot, 2009; Sasakawa, 2010; Ashida et al., 2011). Secreted effectors enable the bacteria to be internalized, to survive intra-cellularly, to spread the infec- tion to adjacent cells and to modulate the host inflammatory and immune responses (Sasakawa, 2010). Effectors are classified into three categories depending on whether their expression is not, par- tially or fully controlled by the T3SS activity (Le Gall et al., 2005; Parsot, 2009). Early effectors are produced and presumably stored in the cytoplasm prior to activation of the T3SS and, therefore, are readily secreted into host cells to induce bacterial internalization (Le Gall et al., 2005; Ashida et al., 2011). Late effectors are pro- duced only after activation of the T3SS apparatus and are likely to be involved at a later stage. In addition, middle effectors are pro- duced both before and after T3SS activation and could be involved http://dx.doi.org/10.1016/j.ijmm.2014.11.004 1438-4221/© 2014 Elsevier GmbH. All rights reserved.