Constraints and tunability of antigen-agnostic memory durability.

Apart from antigen-specific immune memory, infection or vaccination can also induce antigen-agnostic memory including bystander T cell activation memory and trained innate immunity. Determinants of the durability of such memory remain unclear. We developed mathematical models to show that antigen-agnostic memory durability is constrained by immune cell turnover and cytokine dependence. Trained immunity durability is further constrained by the fidelity of epigenetic state transmission during self-renewal and differentiation. Using computer simulations and a machine learning-based parameter-phenotype mapping approach, we find that positive feedback mediated by immune cell-secreted cytokines, a motif seen across immune circuits, can lead to tunable memory involving excitable responses to acute challenges followed by slow relaxation to the pre-challenge baseline. We propose specific experiments to test predictions from our models. Our findings thus reveal a cell extrinsic mechanism that can overcome the constraints imposed by cell turnover and epigenetic state transmission to drive long-lasting, tunable antigen-agnostic memory.