PPBP and systemic lupus erythematosus: While we identify IL6 as a WNT-responsive contributor to paracrine senescence, several other secreted mediators for paracrine senescence in MSCs have been described: IGFBP4 and IGFBP7 in adult bone marrow MSCs, CCL2 in umbilical cord MSCs, and Pro-platelet basic protein (PPBP, also known as Neutrophil-Activating Peptide 2) and the hormone leptin in MSCs from lupus patients [15, 100, 101].