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Dendritic cells as possible determinants for bone marrow restricted t cells activation and bone loss in Estrogen depleted mice

Final Activity Report Summary - OSTEODEN (Dendritic cells as possible determinants for bone marrow restricted T cells activation and bone loss in estrogen depleted mice)

The project OSTEODEN investigated the role of bone marrow dendritic cells in the development of post-menopausal osteoporosis, aiming to further elucidate the involvement of the immune system in this pathology and to develop new potential therapeutic approaches. Osteoporosis is the most prevalent disease in the world with an estimated 200 million people affected worldwide (Lane NE, American Journal of Obstetrics and Gynecology, 2006). Because of the increase in life expectancy, such an estimate is projected to constantly increase in the next future, causing an enormous burden for the public health system due to the high incidence of fractures among osteoporotic patients.

OSTEODEN stemmed from early observations that the immune system, and particularly T cells, was a key player in post-menopausal bone loss. After estrogen depletion, T cells were activated and increased the production of tumour necrosis factor a (TNFa), the key cytokine that drove increased formation of osteoclasts in bone. However, pathways leading to T cell activation and the role of the bone marrow microenvironment in this process were still unclear. Using ovariectomised mice, the animal model for post-menopausal osteoporosis, the project investigated the role of Dendritic cells (DCs), the most potent antigen presenting cells, and their role in T cell activation after estrogen deficiency. Key to the research project was the observation that antigen presentation by DCs was markedly increased in the bone marrow after ovariectomy (ovx); therefore, aim of this project was to understand what was the cause of DCs activation in the bone marrow after ovx, what pathways were implicated in the downstream activation of T cells and increased TNF production and whether mice findings could be translated to human osteoporotic patients.

Regarding these questions, the project came to two key scientific achievements. Firstly, oxidative stress was a key stimulator of immune cells activation in ovx mice; indeed, we demonstrated that increased production of Reactive oxygen species (ROS) was necessary after ovx for inducing DC activation and increased antigen presentation to T cells. Although the notion that ROS production was implicated in osteoclasts formation was not new, we first made a link between oxidative stress, T cell activation, TNF production and osteoclasts differentiation, thereby contributing to reconcile two different major hypotheses, i.e. that ovx-induced bone loss was either caused by increased oxidative stress or by increased TNF production by T cells. Therefore, we came to the conclusion that estrogen deficiency sent a stress signal in bone through ROS, resulting in DC and T cell activation and ultimately leading to increase TNF production by T cells and osteoclast differentiation.

Secondly, in response to oxidative stress DC activated the CD80 / CD28 costimulatory pathway and caused T cell activation and TNF production. The relevance of this pathway was conclusively demonstrated by a pharmacological approach, where the immunosuppressant CTLA4-Ig, a recently developed drug that inhibited the CD80 of T cell activation, was employed and proved to be effective in preventing ovx-induced T cell activation, T cell TNF production, increased osteoclasts differentiation and bone loss. These findings were completely novel and exciting as they provided for the first time a proof-of-concept that an immunosuppressant could be effectively used to treat osteoporosis, also providing a conclusive demonstration that T cells were key players in the pathogenesis of this disease.

These findings and achievements were collected in a main paper in the journal Proceedings of the National Academy of Science (PNAS), which was anticipated to be available on-line by mid-September 2007. Additional key publications of this time period included:

1. Grassi et al., Endocrinology 2006, a side paper stemmed from the hypothesis that nitric oxide could be a mediator of ovx-induced activation of DCs. Although this hypothesis was not confirmed, novel and interesting new data were published here on the role of the enzyme eNOS in the pathogenesis of osteoporosis;
2. Gao, Grassi et al., Journal of Clinical Investigation, 2006, which was a key paper on the role of IFNgamma in ovx-induced T cell activation and bone loss.