A panel of human cell-based artificial APC enables the expansion of long-lived antigen-specific CD4⁺ T cells restricted by prevalent HLA-DR alleles

Many preclinical experiments have attested to the critical role of CD4⁺ T cell help in CD8⁺ cytotoxic T lymphocyte (CTL)-mediated immunity. Recent clinical trials have demonstrated that reinfusion of CD4⁺ T cells can induce responses in infectious diseases and cancer. However, few standardized and versatile systems exist to expand antigen-specific CD4⁺ T_h for clinical use. K562 is a human erythroleukemic cell line, which lacks expression of HLA class I and class II, invariant chain and HLA-DM but expresses adhesion molecules such as intercellular adhesion molecule-1 and leukocyte function-associated antigen-3. With this unique immunologic phenotype, K562 has been tested in clinical trials of cancer immunotherapy. Previously, we created a K562-based artificial antigen-presenting cell (aAPC) that generates ex vivo long-lived HLA-A2-restricted CD8⁺ CTL with a central/effector memory phenotype armed with potent effector function. We successfully generated a clinical version of this aAPC and conducted a clinical trial where large numbers of anti-tumor CTL are reinfused to cancer patients. In this article, we shifted focus to CD4⁺ T cells and developed a panel of novel K562-derived aAPC, where each expresses a different single HLA-DR allele, invariant chain, HLA-DM, CD80, CD83 and CD64; takes up soluble protein by endocytosis and processes and presents CD4⁺ T-cell peptides. Using this aAPC, we were able to determine novel DR-restricted CD4⁺ T-cell epitopes and expand long-lived CD4⁺ T-cells specific for multiple antigens without growing bystander Foxp3⁺ regulatory T cells. Our results suggest that K562-based aAPC may serve as a translatable platform to generate both antigen-specific CD8⁺ CTL and CD4⁺ T_h.