Potent costimulation of human CD8 T cells by anti-4-1BB and anti-CD28 on synthetic artificial antigen presenting cells

The in vitro generation of cytotoxic T lymphocytes (CTLs) for anticancer immunotherapy is a promising approach to take patient-specific therapy from the bench to the bedside. Two criteria must be met by protocols for the expansion of CTLs: high yield of functional cells and suitability for good manufacturing practice (GMP). The antigen presenting cells (APCs) used to expand the CTLs are the key to achieving both targets but they pose a challenge: Unspecific stimulation is not feasible because only memory T cells are expanded and not rare naïve CTL precursors; in addition, antigen-specific stimulation by cell-based APCs is cumbersome and problematic in a clinical setting. However, synthetic artificial APCs which can be loaded reproducibly with MHC-peptide monomers and antibodies specific for costimulatory molecules could resolve these problems. The purpose of this study was to investigate the potential of complex synthetic artificial APCs in triggering the costimulatory molecules CD28 and 4-1BB on the T cell. Anti-4-1BB antibodies were added to an established system of microbeads coated with MHC-peptide monomers and anti-CD28. Triggering via CD28 and 4-1BB resulted in strong costimulatory synergy. The quantitative ratio between these signals determined the outcome of the stimulation with optimal results when anti-4-1BB and anti-CD28 were applied in a 3:1 ratio. Functional CTLs of an effector memory subtype (CD45RA⁻ CCR7⁻) were generated in high numbers. We present a highly defined APC platform using off-the-shelf reagents for the convenient generation of large numbers of antigen-specific CTLs.