The CD8 and tetramer signals inside the tumour in pErNP–OVA–CpG B immunized mice were much stronger than those in the spleen or liver (Extended Data Fig. 1a), suggesting extravasation from the blood circulation and specific targeting of our ErNP–CD8 and QD–tetramer probes to antigen-specific CTLs in the tumour microenvironment. Here, CD8A is linked to neoplasm.