Chemokine fusion vaccination elicits protective antitumor responses in C57BL/6 mice. (A) Ten mice per group were gene-gun immunized 3 times over a 2-week interval with pMIP3α-gp100, pMIP3α-D-gp100, or PBS. Two weeks after the last immunization, mice were challenged subcutaneously with a lethal dose of B16 tumor cells. Tumor growth suppression was subsequently assessed and mice with a tumor larger than 400 mm² were killed. The data shown are representative of 2 independent experiments that yielded similar results. P = .02. Error bars represent the SD of the mean in 10 mice per group. (B) Mechanism of chemokine-mediated antigen presentation in APCs. Chemokine (MIP3α) is usually internalized upon binding to its chemokine receptor (CCR6) (1-2), and the internalized receptor then can be recycled back to the surface (5). Tumor antigens (TAA), if physically linked with MIP3α (MIP3α-TAA), are preferentially taken up via chemokine receptor–mediated and clathrin-dependent process (1) and internalized into endosomal and lysosomal compartments (2-3), where they can be degraded and presented to MHC class II molecules (3) and subsequently delivered to the cell surface to be presented and activate CD4 T⁺ cells (4). However, some portion of MIP3α-TAA escapes from (3) to the cytosol (6) where it is degraded by proteasomes (7). The degraded peptides are then transported by TAP-1 to the ER (8) to be loaded to the MHC class I molecules (9) and exposed on the cell surface, thus, inducing CD8⁺ T cells (11).