Transforming tumor cells into professional antigen-presenting cells using poly(β-amino ester) nanoparticles to deliver CD80-encoding mRNA

Effective T-cell activation requires both antigen recognition and co-stimulatory signaling. However, tumor cells typically lack the necessary co-stimulatory molecules, rendering T-cell receptor engagement with tumor antigens insufficient to induce potent anti-tumor immune responses. To overcome this limitation, we converted tumor cells into professional antigen-presenting cell (APC)-like cells by equipping them with the co-stimulatory molecule CD80. Electroporation of melanoma and lung cancer cell lines with CD80-encoding mRNA significantly increased their ability to stimulate tumor-antigen-specific T cells, as indicated by increased activation markers and cytokine secretion. Most importantly, CD80 expression converted melanoma cells into APC-like cells capable of priming and expanding naïve T cells that recognize endogenous tumor antigens. To enable clinical application, we subsequently used poly-(beta aminoester) (pBAE) polymers, which efficiently encapsulate mRNA into nanometric polyplexes. As previously reported, pBAE nanoparticles (NPs) efficiently penetrate plasma membranes and escape endosomal degradation both in vitro and in vivo. Here, we evaluated pBAE NPs encapsulating GFP-encoding mRNA in human cell lines derived from melanoma, lung cancer, leukemia, adenocarcinoma, uveal melanoma, and Merkel cell carcinoma. The pBAE NPs effectively transfected adherent tumor cells, and tumor spheroids in vitro and in ovo. When delivering CD80 mRNA, NPs achieved expression rates of more than 50% in most cell lines, which lasted for at least 72 h. Here as well, nanoparticle-mediated CD80 expression enabled melanoma cells to prime and expand naïve T cells recognizing endogenous tumor antigens. Importantly, T cells primed by CD80-expressing tumor cells were also capable of killing tumor cells that had not been transfected. In line with these findings, T cells stimulated with untransfected tumor cells showed high levels of checkpoint receptors CTLA-4 and PD-1, whereas the expression was significantly lower when using CD80-transfected tumor cells. This proof-of-concept study indicates that conversion of tumor cells into professional APC-like cells with CD80 mRNA is feasible and results in potent anti-tumor CD8⁺ T-cell responses. With the use of mRNA-loaded pBAEs, the prerequisites for a future application in vivo are given. Therefore, the application of pBAE NPs encapsulating CD80 mRNA to specifically transfect tumor cells presents a new promising strategy for the in vivo treatment of various cancers.