Figure 2.

Synovial mast cell (MC): cellular interactions and effector functions. This illustration summarizes the main functions of synovial MCs with relevance in the pathogenesis of RA. Examples of factors capable of triggering MC activation are shown, such as immune complexes [54], cytokines and chemokines [50], and endogenous toll-like receptor (TLR) ligands [48]. The cellular interactions with immune cells are shown on the left: antigen presentation to T cells [55,56]; B cell co-stimulation via CD40–CD40L and soluble mediators (e.g., IL-6), inducing the production of auto-antibodies [31]; modulation of monocyte-macrophages activation [57]. Finally, on the right, the effector functions of MCs are summarized, with an emphasis on the induction of bone erosions, which can be mediated directly by MC-derived proteases [58] or indirectly via the activation of osteoclasts [59,60]. Green arrows indicate the effects of proinflammatory citokines; the “T” bar indicates the effects of antinflammatory citokines.