In addition, we examined the expression of CD69 (an early activation marker) and the major histocompatibility complex (MHC) class II. reduced bacterial uptake by macrophages. These results were substantiated by increased bacterial virulence when rHcp was added along with the vasHmutant in a septicaemic mouse model of contamination. Analysis of the cytokine profiling in the intraperitoneal lavage as well as activation of host cells after 4 h of contamination with the vasHmutant supplemented with rHcp indicated that this T6SS effector inhibited production of pro-inflammatory cytokines and induced immunosuppressive cytokines, such as interleukin-10 and transforming growth factor-, which could circumvent macrophage activation and maturation. This mechanism of innate immune evasion by Hcp possibly inhibited the recruitment of cellular immune components, which allowed bacterial multiplication and dissemination in animals, thereby leading to their mortality. == INTRODUCTION == Aeromonas hydrophilais among the most common species associated with wound and soft tissue infections, Sorafenib gastroenteritis and septicaemia in the host (Chopra & Houston, 1999;Janda & Abbott, 1998). Our laboratory group as well as others have characterized several virulence factors fromAeromonasspecies, which are secreted via the type 2 and type 3 secretion systems (Carvalho-Castroet al., 2010;Chopraet al., 2000;Shaet al., 2005,2007;Sierraet al., 2007;Tanet al., 2008;Vilcheset al., 2009). Our most recent studies are focused on the type 6 secretion system (T6SS) and its effectors from diarrhoeal isolate SSU ofA. hydrophila(Suarezet al., 2008,2010). Even though T6SS is highly conserved inProteobacteria, the general mechanism as to how this system operates remains poorly comprehended (Das & Chaudhuri, 2003). A. hydrophilaSSU has one chromosomally located T6SS gene cluster which is regulated by the54activator encoded by the virulence-associated gene,vasH(Suarezet al., 2008). We reported that this vasHmutant ofA. hydrophilaSSU was unable to express genes encoding haemolysin-coregulated protein (Hcp) and the valine glycine repeat G (VgrG) family of proteins Sorafenib VgrG2 and VgrG3, which constitute part of the T6SS gene cluster (Suarezet al., 2008). However, this mutant was Sorafenib able to express, but was unable to secrete or translocate, VgrG1 that resides outside of the T6SS gene cluster (Suarezet al., 2010). Further, deletion ofhcporvgrGprevents secretion of the other, thereby demonstrating dual roles of Hcp and VgrG as structural components of the T6SS apparatus and as effector proteins (Cascales, 2008). Importantly, both Hcp and VgrG proteins represent a hallmark of the T6SS secreted proteins in all of the bacteria that possess this system (Bingleet al., 2008;Cascales, 2008;Fillouxet al., 2008;Pukatzkiet al., 2009). Even though role of VgrG1 in bacterial virulence was convincingly exhibited recently by us inA. hydrophilaSSU (Suarezet al., 2010) andV. cholerae(Maet al., 2009;Ma & Mekalanos, 2010), the mechanism of Hcp in modulating the organism’s virulence is poorly understood. Innate immunity is the first line of host defence against the challenged organisms, and pattern acknowledgement receptors sense different microbial ligands (known as pathogen-associated molecular patterns, PAMPs) (Barton & Medzhitov, 2003;Janeway & Medzhitov, 2002;Medzhitov & Janeway, 1999;Tayloret al., 2005), resulting in the triggering Rabbit Polyclonal to APC1 of signalling cascades that determine the host immune response by modulating maturation, activation and recruitment of cellular effectors [e.g. neutrophils, macrophages, dendritic cells (DCs) and natural killer cells] (Henneke & Golenbock, 2004;Hume, 2006;Medzhitov & Janeway, 1999;Plddemannet al., 2006). Phagocytosis is crucial for both innate and adaptive immunity (Coombeset al., 2004;Henneke & Golenbock, 2004;Medzhitov & Janeway, 1999), and macrophages and DCs are professional antigen-presenting cells which act as tissue sentinels and are able to present antigens to nave T-cells (Gordon & Taylor, 2005;Hume, 2006,2008). Bacteria have developed different mechanisms to Sorafenib avoid innate immunity ranging from their ability to avoid acknowledgement by toll-like receptor-4 (Ernstet al., 1999;Hajjaret al., 2002;Kawasakiet al., 2004), altering antigenicity of surface molecules to avoid phagocytosis (Seifert, 1996), interfering with mitogen-activated protein kinase signalling cascades (Parket al., 2002;Sweetet al., 2007;Thiefeset al., 2006), modulating actin polymerization and apoptosis (Abrahams & Hensel, 2006;Pujol & Bliska, 2005;Ruckdeschelet al., 2002;Viboud & Bliska, 2005), manipulating phagosome trafficking and maturation (Dramsi & Cossart, 2002;Sturgill-Koszyckiet al., 1994;Uchiyaet al., 1999), and inducing the production of immunosuppressive cytokines, particularly interleukin (IL)-10. The latter mechanism avoids activation of macrophages, maturation of DCs and recruitment of granulocytes (McGuirket al., 2002;Singet al., 2002;Zuany-Amorimet al., 2002). Previously, we exhibited thatA. hydrophilaSSU vasHand vasKmutants were easily phagocytosed by murine Natural 264.7 macrophages compared with the phagocytosis of wild-type (WT) bacteria (Suarezet al., 2008). Further, we showed.