Selection of CMV-specific CD8⁺ and CD4⁺ T cells by mini-EBV-transformed B cell lines

Efficient protocols to generate cytomegalovirus (CMV)-specific T cells are required for adoptive immunotherapy. Recombinant Epstein-Barr virus (EBV) vectors called mini-EBV can be used to establish permanent B cell lines in a single step, which present the CMV antigen pp65 in a constitutive manner. These B cell lines, coined pp65 mini-LCL, were successfully used to reactivate and expand CMV-specific cytotoxic T cells. Here we evaluate this pp65 mini-EBV system in closer detail, focusing on (1) the quantification of T cells with specific effector function and (2) the identification of CMV-specific CD4⁺ helper T cells. The co-expansion of various functional CMV epitope specificities was demonstrated by IFN-γ enzyme-linked immunospot assay (ELISPOT) assays and HLA-peptide tetramer staining. Single-cell cloning resulted in both CD4⁺ and CD8⁺ T cell clones, the majority of which was CMV specific. Thus, mini-LCL present the pp65 antigen on HLA class I and II, mobilizing both arms of the T cell response. Using a peptide library covering the pp65 sequence for further analysis of T cell clones, we identified new pp65 CD8⁺ and CD4⁺ T cell epitopes.