Initial experiments confirmed that old mice (>68 weeks old) succumb to tumour growth faster than young mice (<40 weeks old). Moreover, whereas immune checkpoint blockade therapy (anti-PD1 antibody alone or together with anti-CTLA4 antibody) increased the survival of tumour-bearing young mice, this therapy was not effective in older mice.

The authors tested whether vaccination could stimulate protective immunity in old mice. Young and old mice were immunized with whole tumour lysates together with various DC stimulatory adjuvants: lipopolysaccharide (LPS) alone, LPS plus alum, or LPS plus 1-palmitoyl-2-glutaryl phosphatidylcholine (PGPC), a combination that was previously shown to induce a state of hyperactivation in DCs. Immunization with PGPC as an adjuvant induced tumour rejection in around 80% of mice regardless of age, whereas immunization with LPS alone or LPS plus alum as adjuvants was ineffective in old mice.