The tetraspan MS4A4A modulates macrophage activation

Membrane-spanning 4A (MS4A) is a protein family whose different members show preferential expression in distinct cell types and share with tetraspanins a general structure as well as the ability to form lateral associations with each other and with other proteins, generating a dynamic web in lipid rafts. The biological function of most of these proteins is poorly characterized, but some members of the family have been shown to regulate vesicular trafficking and membrane receptors’ signaling in immune cells. We have focused our attention on a subset of MS4A proteins restricted to myeloid cells (MS4A4A, MS4A6, MS4A7). Using a split-ubiquitin two-hybrid analysis and a FLIM-FRET approach, we demonstrated that these ‘myeloid’ MS4A proteins interact with themselves most likely in lipid rafts. MS4A4A, which is preferentially expressed in macrophages and is overexpressed in “alternatively activated” and tumor-associated macrophages, interacts and regulates β-glucan-dependent Dectin-1 signaling (Syk phosphorylation) and its consequent functional properties (cytokines and reactive oxygen species production). In the two-hybrid screening we also identified the γ signaling chain of Fcγ receptors as a new possible partner of MS4A4A. In mast cells the γ chain mediates FcɛRI activation in response to IgE immunocomplexes, and MS4A4A has been shown to modulate this event acting on the recruitment of KIT to FcɛRI. As the γ chain also mediates FcγR activation in macrophages, our working hypothesis is that MS4A4A in macrophages modulates the FcγR signalling pathway through the engagement of the γ signaling chain. We are currently working on the validation of the interaction between MS4A4A and γ chain and studying the role of MS4A4A in the regulation of the FcγR-dependent pathway in MS4A4A-deficient macrophages.