In vitro priming of cytotoxic T lymphocytes against poorly immunogenic epitopes by engineered antigen-presenting cells

Cytotoxic T lymphocytes (CTL) recognize antigenic peptides presented by major histocompatibility complex class I (MHC-I) molecules on the surface of target cells. Optimal induction of CD8⁺ CTL depends on the amount of relevant peptide/MHC-I complexes and the presence of co-stimulatory molecules on antigen-presenting cells (APC). The antigen-processing defective mutant cell line RMA-S, when cultured at low temperature. expresses high amounts of MHC-I molecules that do not contain endogenously derived peptides. These 'empty' MHC-I molecules can be stabilized by addition of MHC-binding peptides. RMA-S cultured at low temperatures with selected peptides have been used for in vitro CTL induction with conflicting results. RMA-S cells do not express detectable amounts of B7 co-stimulatory molecule. This could explain their unpredictable efficiency as APC. We have evaluated whether RMA-S cells, stably transfected with cDNA encoding for the human B7.1 molecule could provide effective co-stimulation for CD8⁺ T lymphocytes. RMA-S/B7 cells, loaded with different synthetic peptides, demonstrated a high and sometimes unique efficiency for in vith primary CTL induction, even when 'sub-optimal' antigen peptides were used. Most importantly, RMA-S/B7 cells pulsed with naturally processed peptides extracted from the poorly immunogenic B16 melanoma cells were able to prime CD8⁺ cells against B16 melanoma. We conclude that the use of RMA-S/B7 cells as APC represents an ideal strategy for in vitro CTL immunization without prior in vivo priming. This system may also help to address the issue of the different contributions of co-stimulation and relative occupancy of MHC-I by single peptide epitopes in CTL priming.