Background <p>Immune checkpoint blockade (ICB) has transformed cancer therapy; however, its efficacy remains limited in immunologically “cold” tumors such as breast cancer. These tumors are typically characterized by low T-cell infiltration and an immunosuppressive tumor microenvironment (TME), which restrict the effectiveness of current immunotherapies. Therefore, strategies that enhance antigen presentation and promote coordinated activation of innate and adaptive immunity are needed to improve therapeutic outcomes.</p> Results <p>We developed a lipid nanoparticle platform (IC-LNP) for the co-delivery of interleukin-33 (IL-33) messenger RNA (mRNA) and the stimulator of interferon genes (STING) agonist cyclic di-adenosine monophosphate (c-di-AMP). This formulation enabled sustained intratumoral IL-33 expression and was associated with enhanced dendritic cell maturation and antigen cross-presentation, accompanied by activation of nuclear factor kappa B (NF-κB) signaling. In parallel, c-di-AMP activated the STING pathway, induced type I interferon responses, and enhanced cytotoxic T-cell activity. Consistent with these immunostimulatory effects, IC-LNP increased immune cell infiltration, reduced immunosuppressive cell populations, and shifted the tumor microenvironment toward a more immune-active state. In vivo, IC-LNP improved tumor control and potentiated the therapeutic efficacy of programmed death-ligand 1 (PD-L1) blockade, together with enhanced systemic antitumor immune responses.</p> Conclusions <p>By enabling the coordinated delivery of IL-33 mRNA and a STING agonist, IC-LNP offers a dual-component immunotherapeutic strategy to enhance innate and adaptive antitumor immunity. These findings provide a basis for further development of combination immunotherapy strategies for poorly immunogenic breast cancer, including strategies to improve responses to checkpoint blockade.</p> Graphical Abstract <p></p>