Cathepsins and non-apoptotic caspases involvment during immunosuppressive macrophages generation

Macrophages are immune cells resulting from embryogenesis or from the differentiation of monocytes. Schematically, three populations of macrophages co-exist: naive macrophages (M0), pro-inflammatory macrophages (M1) and anti-inflammatory macrophages (M2 or immunosuppressive). Naive macrophages have phagocytic functions and recognize pathogens but rapidly undergo polarization into pro- or anti-inflammatory macrophages to acquire all their functions. Pro-inflammatory macrophages are partly responsible for the inflammatory response, during which they exert anti-microbial and anti-tumor functions. Conversely, anti-inflammatory macrophages are involved in the resolution of inflammation, phagocytosis of cellular fragments and tissue repair. However, macrophages are also involved in the development of some pathologies such as atherosclerosis (M1), fibrosis or cancers (M2). Thus, a better understanding of the generation mechanism of these three kinds of macrophages is essential to develop new therapeutic strategies.During my thesis, I was interested in the role played by cathepsins and caspases during the differentiation of monocytes into naive macrophages and then during their polarization into anti-inflammatory macrophages. Indeed, it has been previously demonstrated in my team that the activation of caspases -8, -3 and -7 are needful for the differentiation of monocytes into macrophages in response to the cytokine M-CSF (CSF-1). The activation of caspases in the context of macrophage differentiation is original since the cleavage fragments obtained (respectively 34 ; 26 and 30 kDa for caspases -8, -3 and -7) are different in size from those found during apoptosis and, moreover, do not lead to cell death. The more precise characterization of caspase activation during the differentiation of monocytes into macrophages has demonstrated the crucial role of cathepsin B in the cleavage and activation of caspase-8. The activation of caspases leads to the cleavage of some substrate proteins, such as p47 Phox, NPM or LYN on non-consensus peptide sequences, highlighting an original activation mode of caspases during the differentiation of monocytes into macrophages. From these original sequences, we generated new tetrapeptides allowing to selectively measure and inhibit the activities of non-apoptotic caspases found during the differentiation of monocytes into macrophages.These results, obtained during the differentiation of monocytes into macrophages, were then transposed to the polarization of naive macrophages. Interestingly, activation of cathepsin B as well as caspases is required for the generation of anti-inflammatory macrophages in response to IL-4/IL-13. Conversely, these activations are completely inhibited during pro-inflammatory polarization of macrophages in response to IFN-γ/LPS, pointing to a strict specificity of cathepsin B and caspase activation in anti-inflammatory macrophages. Moreover, pharmacological, or genetic inhibition of cathepsin B and caspases leads not only to a blockage of the polarization of anti-inflammatory macrophages but also to their repolarization towards a pro-inflammatory phenotype. The important role played by cathepsin B and non-apoptotic caspases during the polarization of anti-inflammatory macrophages makes them very promising targets to limit the functionality of these immunosuppressive macrophages, especially in pathologies where they are deleterious, such as fibrosis or cancers.