A peptide immunomodulator activates MST1 to expand and stabilize murine and human regulatory T cells for immune tolerance

The expansion and potentiation of regulatory T cells (T_reg cells) offer an appealing approach for achieving immune tolerance and controlling overactive immune responses across a range of diseases. Current therapeutic approaches to expanding and improving function of T_reg cells have predominantly relied on protein-based biologics, such as recombinant interleukin-2 (IL-2); in contrast, peptide-based interventions that can expand T_reg cells with improved functional stability, enhanced specificity, and favorable safety profiles remain underexplored. Here, using a deep learning model, we identified a hexapeptide (DLST-6P) that preferentially expands both human and murine T_reg cells while preserving their stability. When tested in multiple autoimmune and inflammatory disorders, including a humanized mouse model of graft-versus-host disease, DLST-6P exhibited therapeutic efficacy both as a monotherapy and in combination with low-dose IL-2. Mechanistically, we found that DLST-6P directly targets and activates mammalian Ste20-like kinase 1 (MST1) by promoting homodimer formation. After activation by DLST-6P treatment, MST1 phosphorylated the master T_reg cell transcription factor forkhead box protein P3 (FOXP3) at serine-390. This promoted the association of FOXP3 with the acetyltransferase tat-interacting protein 60, leading to enhanced FOXP3 acetylation and protein stabilization. Simultaneously, we observed that DLST-6P-mediated activation of MST1 alleviated suppressor of cytokine signaling-mediated inhibition of IL-2 signaling in T_reg cells, thereby sensitizing the cells to IL-2 stimulation. Together, these findings unveil a dual mechanistic program through which pharmacological activation of MST1 can boost T_reg cell expansion and stability and highlight the translational potential of DLST-6P as a peptide-based immunomodulator for treating autoimmune and inflammatory disorders.