Gene expression of disc cells after 3h and 24h of co-culture with a 10-fold and 100-fold excess of P. acnes relative to disc cells alone. Lipopolysaccaride (LPS) was used as positive control. a1, b1, c1, d1: relative gene expression of IL1, IL6, IL8, and CCL2 of responders, respectively; a2, b2, c2, d2: relative gene expression of non-responders. Significant differences to disc cells alone are indicated: * p<0.05, ** p <0.01, *** p<0.001.