Concerted functions of Streptococcus gordonii surface proteins PadA and Hsa mediate activation of human platelets and interactions with extracellular matrix.

A range of Streptococcus bacteria are able to interact with blood platelets to form a thrombus (clot). Streptococcus gordonii is ubiquitous within the human oral cavity and amongst the common pathogens isolated from subjects with infective endocarditis. Two cell surface proteins, Hsa and Platelet adherence protein A (PadA), in S. gordonii mediate adherence and activation of platelets. In this study, we demonstrate that PadA binds activated platelets and that an NGR (Asparagine-Glycine-Arginine) motif within a 657 amino acid residue N-terminal fragment of PadA is responsible for this, together with two other integrin-like recognition motifs RGT and AGD. PadA also acts in concert with Hsa to mediate binding of S. gordonii to cellular fibronectin and vitronectin, and to promote formation of biofilms. Evidence is presented that PadA and Hsa are each reliant on the other's active presentation on the bacterial cell surface, suggesting cooperativity in functions impacting both colonization and pathogenesis.

Multiple sites on Streptococcus gordonii surface protein PadA bind to platelet GPIIbIIIa.

Infective endocarditis is a life threatening disease caused by a bacterial infection of the endocardial surfaces of the heart. The oral pathogen, Streptococcus gordonii is amongst the most common pathogens isolated from infective endocarditis patients. Previously we identified a novel cell wall protein expressed on S. gordonii called platelet adherence protein A (PadA) that specifically interacts with platelet GPIIb/IIIa. The interaction between PadA and GPIIb/IIIa resulted in firm platelet adhesion, dense granule secretion and platelet spreading on immobilised S. gordonii. This study set out to identify specific motifs on the PadA protein that interacts with platelet GPIIb/IIIa. Proteomic analysis of the PadA protein identified two short amino acid motifs which have been previously shown to be important for fibrinogen binding to GPIIb/IIIa and contributing to the generation of outside-in signalling. Site directed mutagenesis on the PadA protein in which 454AGD was substituted to AAA, and the 383RGT was substituted to AAA suggests the RGT motif has no role in supporting platelet adhesion however plays a role in dense granule secretion and platelet spreading. In contrast to this the AGD motif has no role to play in supporting firm platelet adhesion or dense granule secretion however plays a role in platelet spreading. These results suggest that multiple sites on S. gordonii PadA interact with GPIIb/IIIa to mediate a number of platelet responses that likely contribute to the thrombotic complications of infective endocarditis.

The role of the innate immune system in granulomatous disorders.

The dynamic structure of the granuloma serves to protect the body from microbiological challenge. This organized aggregate of immune cells seeks to contain this challenge and protect against dissemination, giving host immune cells a chance to eradicate the threat. A number of systemic diseases are characterized by this specialized inflammatory process and granulomas have been shown to develop at multiple body sites and in various tissues. Central to this process is the macrophage and the arms of the innate immune response. This review seeks to explore how the innate immune response drives this inflammatory process in a contrast of diseases, particularly those with a component of immunodeficiency. By understanding the genes and inflammatory mechanisms behind this specialized immune response, will guide research in the development of novel therapeutics to combat granulomatous diseases.

Human platelets recognize a novel surface protein, PadA, on Streptococcus gordonii through a unique interaction involving fibrinogen receptor GPIIbIIIa.

The concept of an infectious agent playing a role in cardiovascular disease is slowly gaining attention. Among several pathogens identified, the oral bacterium Streptococcus gordonii has been implicated as a plausible agent. Platelet adhesion and subsequent aggregation are critical events in the pathogenesis and dissemination of the infective process. Here we describe the identification and characterization of a novel cell wall-anchored surface protein, PadA (397 kDa), of S. gordonii DL1 that binds to the platelet fibrinogen receptor GPIIbIIIa. Wild-type S. gordonii cells induced platelet aggregation and supported platelet adhesion in a GPIIbIIIa-dependent manner. Deletion of the padA gene had no effect on platelet aggregation by S. gordonii but significantly reduced (>75%) platelet adhesion to S. gordonii. Purified N-terminal PadA recombinant polypeptide adhered to platelets. The padA mutant was unaffected in production of other platelet-interactive surface proteins (Hsa, SspA, and SspB), and levels of adherence of the mutant to fetuin or platelet receptor GPIb were unaffected. Wild-type S. gordonii, but not the padA mutant, bound to Chinese hamster ovary cells stably transfected with GPIIbIIIa, and this interaction was ablated by addition of GPIIbIIIa inhibitor Abciximab. These results highlight the growing complexity of interactions between S. gordonii and platelets and demonstrate a new mechanism by which the bacterium could contribute to unwanted thrombosis.