A method for predicting or monitoring whether a patient suffering from cancer or an auto immune disease will respond to treatment with a TLR-9 (toll-like recptor 9) agonist by determining the frequency of activated natural killer T (NKT) cells. The agonist may be a DNA construct comprising a sequence motif N1N2CGN3N4, wherin N1N2 and N3N4 is any combination of A, C, G and T. Also claimed is the use of a TLR-9 agonist to treat cancer.