Defining the Critical Valency of Peptide-MHC on Nanoscale Artificial Antigen Presenting Cells for Expanding Endogenous Tumor-Specific T-cells <i>In Vivo</i>

Expanding tumor antigen-specific T-cells <i>in vivo</i> remains a major challenge in developing effective cancer immunotherapeutics. Artificial antigen-presenting cells (aAPCs) have the potential to overcome this challenge, but formulating aAPCs without correlating the <i>in vivo</i> dynamics of T-cell expansion often leads to suboptimal T-cell responses. Here, we have defined the critical effect of the peptide-major histocompatibility complex (p-MHC) valency displayed along with co-stimulatory molecules on iron-oxide nanoparticle-based aAPCs <i>in vivo</i> and demonstrated that the functionality of these aAPCs is governed by the p-MHC valency. Nanoscale aAPCs engage cognate CD8+ T-cells directly to induce the formation of T-cell receptor (TCR) microclusters, leading to nuclear translocation of nuclear factor of activated T-cells (NFAT) in a p-MHC valency-dependent manner. Nanoscale aAPCs, presenting p-MHCs above the threshold valencies, promoted the expansion and tumor infiltration of adoptively transferred and endogenous tumor-specific CD8+ T-cells in tumor situations. The treatment effectively inhibited tumor growth, highlighting the importance of defining the threshold valency of p-MHC in developing aAPCs for effective immunotherapies.