Mast cells in chronic inflammatory diseases : new insights in mast cell function and phenotype

Abstract

Mast cells are multifunctional cells that play important roles in health and disease. They are long lived cells with the capacity to be activated several times. Upon activation mast cells release a huge array of mediators. The nature of the mediators released is dependent on the stimuli. The microenvironment in which the mast cell resides determines its phenotype and modulates its function. The aims of this thesis were to study the phenotype and role of mast cells in different chronic inflammatory diseases, particularly the importance of activation via CD30L/CD153 and the effects of hypoxia on mast cells.

It has previously been shown that cells expressing CD30L can be activated by CD30 and that CD30 is associated with different inflammatory diseases, such as Hodgkin s lymphoma. Here we show that mast cells can be activated via CD30L with a CD30-Fc fusion protein, secreting chemokines in a degranulation-independent mechanism. We also provide evidence for an involvement of the MAPK and PI3K pathways in the signalling downstream of CD30L. Furthermore, we show that mast cells are the predominant CD30L expressing cells in atopic dermatitis and psoriasis and that IL-8 expressing mast cells are upregulated in lesional skin from both diseases. IL-8 secretion by mast cells was induced in CD30-treated healthy skin cultures.

We next extended these findings to another chronic inflammatory disease, rheumatoid arthritis. In some studies mast cells have been implicated in the pathogenesis of the disease. The soluble form of CD30 is increased in the serum and synovial fluid of rheumatic patients, however there are no reports regarding the soluble form of CD30L in the serum or synovial fluid or the expression of CD30L in the synovium. We found a strong correlation between elevated levels of soluble CD30L in the serum and synovial fluid of patients suggesting a common origin for the molecule. A CD30L+ cell population was found in biopsies of the rheumatic synovium, but in contrast to what we observed in the first study, mast cells were not the CD30L predominant expressing cells.

Hypoxic areas are found in different inflammatory conditions. In situations of low oxygen tension some cells die while others change their metabolism and adapt to the hypoxic condition. In the previous studies we discussed the involvement of mast cells in different chronic inflammatory diseases. In this study we show that mast cells regulate their survival to hypoxia in an autocrine way by secreting IL-6. In response to hypoxia, mast cells also secrete the proinflammatory cytokine TNF-alpha. Furthermore, we show that hypoxia does not affect degranulation and that the response to different activators is still sustained, although attenuated, under hypoxia when compared to normoxic conditions. These results suggest that mast cells are capable of resisting hypoxia and still respond to different stimuli even under hypoxic conditions, which can have important consequences in health and disease.

In the last study presented in this thesis we validated the method used in previous studies and in the projects included in this thesis to identify human mast cells in vivo. Tryptase is the most abundant protein in mast cell granules and is frequently used as a mast cell marker in vivo. Here we show that different Hodgkin lymphoma cell lines also express tryptase in vitro. In vivo the tryptase expression is almost exclusively restricted to mast cells and the rare tryptase positive Hodgkin and Reed Stenberg cells are easily distinguished from mast cells by their characteristic morphology.

In conclusion, this thesis elucidates some aspects of the function of mast cells in different chronic inflammatory diseases.