One possible candidate for E-cad cleavage during C. burnetii infections could be the human MMP-9, since: (i) MMP-9 levels were reported elevated in the sera of patients with endocarditis (Thuny et al., 2012), and patients with acute Q fever (Krajinović et al., 2012); (ii) its production was induced in PBMCs of healthy persons following in vitro exposure to C. burnetii; (iii) MMP-9 SNP was found more frequently in patients with persistent Q fever (Jansen et al., 2017); and (iv) MMP-9 was recently identified as a key gene in mantle cell lymphoma (Yan et al., 2018). Here, MMP9 is linked to Q fever.