Abstract :
Optimal stimulation of antigen specific T cells requires that peptide antigens bound by class I and class II major histocompatibility complex molecules (MHC) are displayed, in conjunction with a variety of costimulatory ligands, on a large surface approximating that of a cell. We have incorporated these principles in the design of cell-free artificial antigen presenting cell surfaces (solid phase matrices, SPMs) constructed on 5 micron latex beads for use in ex vivo T cell stimulation for adoptive immunotherapy and as platforms for T cell vaccines. We demonstrate that naturally expressed or recombinant peptide-MHC complexes and costimulator molecules can be immobilized and displayed simultaneously on the surface of SPMs. Eight or more distinct costimulatory molecules or ligands can be presented on SPMs at physiological densities and the relative density of each ligand can be varied in a precise and independent manner. This allows for control of the contribution of individual ligands relative to others, with the possibility of altering the type of resulting T cell response. The ligands that can be presented on the SPMs include among others: human or mouse class I and class II MHC peptide complexes (antigen specific T cell receptor ligands), B7-1 and 2 (CD28 ligands), 4-1BBL (TNF family costimulator), ICAM-1 (LFA-1 adhesion ligand), CD1d (NKT cell ligand), MICA and Rae-1 antigens (NKG2D costimulatory ligands). All of the immobilized ligands retain their native conformations, since recognition by conformation dependent antibodies is indicated by FACS analysis of the SPMs. Recently, it has been shown that IL-15 receptor alpha (α) can bind the IL-15 cytokine with high affinity and present IL-15 on antigen presenting cells in trans to T cells for stimulation. We are able to display IL-15Rα bound with IL-15 on the SPMs, in addition to peptide-MHC complexes and costimulator ligands to aid T cell responsiveness. We are presently examining the ligand requir- ments for optimal stimulation of na??????ve and effector/memory T cell subsets. We have found that IL-15Rα bound and presented on the SPM has a rapid and profound effect on the recruitment and binding of T cells to the SPMs. Within 2-4 hours, T cells bind avidly to the SPMs bearing the IL-15Rα-IL-15 complex. The ability of the IL-15R-IL-15 complex to recruit T cells to the SPMs may provide improvement to the utility of SPMs as vaccine vehicles, allowing the attraction of T cells to the SPMs displaying T cell antigens and costimulators, despite the non-motile nature of SPMs.
