Class I negative CD8 T cells reveal the confounding role of peptide-transfer onto CD8 T cells stimulated with soluble H2-K^b molecules

Crosslinking of the T cell receptor has been proposed to be a prerequisite for T cell activation. Although the evidence supports this notion for CD4 T cells, the situation for CD8 T cells is less clear. Soluble class I monomers have been used to determine activation requirements in vitro with contradictory results. The possibility of transfer of peptide from soluble class I molecules onto class I molecules present on the surface of CD8 T cells, with ensuing presentation to other CD8 T cells, has been widely ignored. We show that monomeric and tetrameric class I molecules as well as free peptide can stimulate naive CD8 T cells in vitro. We generate and characterize CD8 T cells that express the OT-I T cell receptor (for K^b/SIINFEKL) yet lack K^b and D^b molecules, and show that their activation requirements differ from their class I positive counterparts when stimulated with soluble K^b molecules. By eliminating the confounding effect of peptide transfer, we unmask the true activation requirements for naïve CD8 T cells and show that multivalent engagement of T cell receptors, as well as costimulation, is required for optimal stimulation.