Yeah, this is the only way it will work i.e. the mRNA being endocytosed by the APCs, mainly dendritic cells. Some of the spike proteins after synthesis can be diverted to proteosomes where they are broken down into short peptides. These are subsequently transported to the endoplasmic reticulum where they are loaded onto class 1 MHC molecules for display on the surface of the dendritic cells and presentation to the T cells in the lymph nodes.
My assumption all this while is that the muscle cells are the main target of the mRNA vaccines