pH sensitive liposomes provide an efficient means of sensitizing target cells to class I restricted CTL recognition of a soluble protein.

Exogenous antigens are normally endocytosed and enter the class II pathway of processing and presentation. It had been shown earlier that soluble antigen could be introduced into the class I pathway of processing and presentation by osmotic loading. In this report, we have demonstrated that OVA containing liposomes that destabilize on exposure to low pH, referred to as pH sensitive liposomes, could sensitize target cells to lysis by class I MHC-restricted OVA-specific CTL. However, OVA-containing pH insensitive liposomes, native OVA, or OVA subjected to the same protocol as was used to make the liposomes, failed to sensitize targets to OVA-specific CTL lysis. The pH sensitive liposomal approach was less toxic and more efficient (about 20-fold) in delivering than the osmotic loading approach. The pH liposome approach may prove valuable to study CTL recognition characteristics of less available proteins such as viral proteins.