TFAM Deletion Enhances Antitumor Immunity

As TFAM deletion resulted in enhanced maturation and activation of the DCs in vitro, we next investigated whether the humoral and cellular immune responses were promoted in Tfam^-/- mice. To better model tumor responses, OVA was used as the model antigen to validate the activation of immune responses in vivo. Tfam^-/- and control mice were immunized with OVA for three times and the sera were collected. The levels of total IgG and its subclasses were measured. The results showed that the antibody titers of IgG, IgG1, and IgG2c were all elevated in immunized Tfam^-/- mice when compared with that of the control group (figure 4A), which suggested a potent activation of humoral immunity in mice Tfam^-/- BMDCs. When it comes to the cellular immune response, we demonstrated that Tfam^-/- deletion simulated more effective OVA-specific CD8⁺ T cell responses as evidenced by the increase in spleen lymphocytes from Tfam^-/- and control mice after the stimulation with OVA_257-264 peptides (figure 4B, C). In accordance, the ELISA results showed the increased production of IFN-γ by CD8⁺ T lymphocytes in Tfam^-/- cells after OVA immunization when compared with the control (figure 4D). Furthermore, the immune-microenvironment and the critical cell populations in the spleen of Tfam^-/- and control mice were characterized by FCM. The increased populations of DCs (CD11c⁺ DCs, figure 4E), CD8⁺ CD69⁺ T cells (figure 4F), CD8⁺ IFN-γ⁺ T cells, CD8⁺ GzmB⁺ T cells (figure 4G), CD4⁺ CD69⁺ T cells (figure 4H) and the decreased percentages of CD4⁺ FOXP3⁺ cells were also detected (figure 4H).
Next, we used the prophylactic model to identify the anti-cancer immunity activation in Tfam^-/- and control group. Mice were challenged with subcutaneous injection of E.G7-OVA cells after three times of immunization. The tumor growth was significantly inhibited in Tfam^-/- mice in the prophylactic tumor model with the OVA immunization when compared with that of the control mice (figure 4I), and this group of mice showed prolonged survival (figure 4J). To investigate whether such tumor inhibition effect was due to the activation of cellular immune response, the adoptive transfer study was carried out by isolating CD8⁺ T lymphocytes from the immunized mouse spleen. CD8⁺ T lymphocytes from immunized Tfam^-/- mice or control mice were injected intravenously in WT mice on day 1 before E.G7-OVA cell inoculation and on day 1 and day 3 after E.G7-OVA cell inoculation. Interestingly, the tumor growth as detected by tumor weight and tumor volume was significantly inhibited in the group that received the adoptive transfer CD8⁺ T cells from Tfam^-/- mice when compared with that received the control CD8⁺ T cells (figure 4K). To further figure out the myeloid cell subset responsible for the activation of antitumor immunity, we adopted control or Tfam^-/- DCs/macrophages to WT LLC tumor-bearing mice. As expected, adoption of Tfam^-/- DCs instead of Tfam^-/- macrophages significantly inhibited tumor metastasis and tumor growth (online supplemental figure 5). In summary, TFAM deficiency not only caused DC activation, but also led to more efficient activation of antitumor humoral and cellular immunity in vivo.