CCL8 and intervertebral disk degenerative disorder: Some of the 20 factors have been previously associated with IVD degeneration [12,15,51–54], however 10 of the 20 factors (GCSF, GM-CSF, IL-3, IL-5, IL-13, IL-15, CCL8, TNF-β, GRO and CXCL1) have not previously been described in the IVD and two (CCL7 and CXCL9) [54] have not been described in degenerative disc disease.