Cell-based drug carriers are mostly prepared in vitro, which may negatively affect the physiological functions of cells, and induce possible immune rejections when applied to different individuals. In addition, the immunosuppressive tumor microenvironment limits immune cell–mediated delivery. Here, we report an in vivo strategy to construct cell-based nanomedicine carriers, where bacteria-mimetic gold nanoparticles (GNPs) are intravenously injected, selectively phagocytosed by phagocytic immune cells, and subsequently self-assemble into sizable intracellular aggregates via host-guest interactions. The intracellular aggregates minimize exocytosis of GNPs from immune cells and activate the photothermal property via plasmonic coupling effects. Phagocytic immune cells carry the intracellular GNP aggregates to melanoma tissue via inflammatory tropism. Moreover, an initial photothermal treatment (PTT) of the tumor induces tumor damage that subsequently provides positive feedback to recruit more immune cell–based carriers for enhanced targeting efficiency. The optimized secondary PTT notably improves antitumor immunotherapy, further strengthened by immune checkpoint blockade.