Protection against tumor challenge required both membrane extract and IL-2 in the same MPL vesicle. (A) Mice (10 per group) were inoculated once i.p. with the indicated preparations and challenged 14 days later with 2 × 10³ 38C13 lymphoma cells. Survival was assessed 50 days after challenge. Vaccines tested included the standard MPL formulation with membrane extract (1.25 μg Id/dose) and 4 × 10⁵ IU/dose IL-2 (MPL); an MPL formulation containing membrane extract but no IL-2; an MPL formulation without IL-2 that was mixed with multilamellar liposomes containing 4 × 10⁵ IU/dose IL-2; multilamellar liposomes containing 4 × 10⁵ IU/dose IL-2 (L[IL-2]); and secreted hybridoma-derived Id conjugated to KLH (Id-KLH; 50 μg Id/dose). All p values shown are with respect to the standard MPL vaccine (closed circle). (B) Vaccine formulations protected against tumor challenge at very low antigen doses. MPL and PPL vaccine formulations were prepared with a constant amount of IL-2 (4 × 10⁵ IU/dose) and different concentrations of 38C13 cell membrane Id. Mice (10 per group) received a single immunization intraperitoneally and were challenged with 2 × 10³ 38C13 lymphoma cells 14 days later. Survival was assessed at 50 days following tumor challenge. No mice survived in the saline control group (data not shown).