­­Delineating the role of co-stimulatory signals on human virus-specific memory CD8+ T cell activation and expansion using highly selective T cell receptor-targeted biologics

To delineate the minimum signals required to activate and expand antigen-specific memory CD8+ T cells, we recapitulated the discrete activation signals delivered by antigen-presenting cells using antigen-specific T-cell receptor-specific biologics termed Immuno-STAT (IST), which can specifically deliver a T cell receptor (TCR) signal alone or paired with a defined co-stimulatory signal to antigen-specific CD8+ T cells. Using a reductionist approach that provides defined TCR and co-stimulatory signaling, we demonstrated that different IST could engage TCR alone or in combination with either CD28 or 4-1BB to initiate distinct transcriptional programs in memory CMV-specific CD8+ T cells. As compared to activation by TCR signaling alone, the combination of TCR and CD28 or 4-1BB co-stimulatory signals significantly altered the transcriptome and drove expression of genes related to growth and survival. Nevertheless, TCR signaling in the presence of IL-2 (5 U/ml) was sufficient for robust recall activation and expansion of functional cytomegalovirus CMV(NLV)-specific and HIV(SL9)-specific memory CD8+ T cells. This response contrasts with naïve CD8+ T cells, which required CD28 co-stimulation in addition to TCR stimulation for antigen-specific activation and expansion even in the presence of high levels of IL-2 (100 U/ml). These results have important implications for antigen-specific immunotherapy by indicating that biologics that deliver TCR signals alone in the presence of IL-2 may be sufficient for immunotherapeutic strategies designed to expand memory CD8+ T cells to eliminate cancerous or infected cells. In contrast, strategies that aim to stimulate naïve T cell responses may benefit from the codelivery of TCR and CD28 signals.