HIV- and cytomegalovirus-specific human memory CD8^þ T cells are activated and expanded independent of co-stimulatory signaling by T-cell receptor–specific immunotherapeutics

To delineate the minimum signals required to activate and expand antigen-specific memory CD8^þ T cells, we used immunotherapeutics termed Immuno-STAT (IST) that we designed to selectively engage and activate antigen-specific T-cell receptors alone or combined with a defined co-stimulatory signal to recapitulate the discrete activation signals delivered by antigen-presenting cells to human CD8^þ T cells. Transcriptome analysis of highly purified CD8^þ cytomegalovirus (CMV)–specific memory T cells after delivering defined antigen-specific TCR signals alone or with co-stimulatory signals revealed that the combination of TCR signaling and CD28 or 4-1BB co-stimulatory signals significantly altered the transcriptome compared to activation by TCR signaling alone. Nevertheless, IST-delivered CMV-specific or HIV-specific TCR signaling alone in the presence of IL-2 was sufficient to induce robust recall activation and expansion of CMV (NLV)–specific or HIV (SL9)–specific memory CD8^þ T cells, respectively. This response contrasts with naïve antigen-specific CD8^þ T cells and TCR-engineered T cells, which required CD28 co-stimulation in addition to TCR stimulation for robust antigen-specific activation and expansion. These results have important implications by indicating that immunotherapeutics that deliver antigen-specific TCR signals alone in the presence of adequate environmental cytokine support should be sufficient for immune-based strategies designed to expand antigen-specific memory CD8^þ T cells to eliminate cancerous or infected cells. In contrast, strategies that aim to optimally stimulate and expand virus or cancer-specific naïve or TCR-engineered T-cell responses would benefit from the codelivery of TCR and CD28 signals.