Autochthonous T cells to the rescue: IL-10 directly activates tumor-resident CD8(+) T cells.

Successful cancer immunotherapy is thought to require de novo priming of tumor specific CD8(+) T cells in lymphatic organs. Contrasting these beliefs, cancer therapy based on interleukin-10 (IL-10) results in tumor rejection without a requirement for T-cell trafficking from lymphatic organs. Rather, IL-10 directly activates autochthonous, tumor-resident CD8(+) T cells.

Successful immune responses against a tumors, be it endogenous or therapeutically induced, consist of three distinct phases. First, dendritic cells (DCs) must take up tumor antigens and mature. These activated DCs then induce an effective anti-tumor T-cell response in the lymph node by the priming and expansion of CD4+ T cells first, and then of cytotoxic CD8+ T cells. Lastly, activated T cells eventually migrate into the tumor and kill antigen-presenting malignant cells. Accordingly, a high number of CD8+ T cells in the tumor correlates with improved prognosis for cancer patients. Cancer vaccines and the adoptive transfer of tumor-reactive T cells expanded ex vivo can result in large numbers of effector T cells in the lymph node and blood, but the therapeutic effects of such therapies have not been as consistent or significant as anticipated.

Two major reasons underlying this lack of efficacy may be the poor infiltration of T cells into the tumor and the immunosuppressive tumor microenvironment. Tumor cells often express low levels of MHC molecules as well as of tumor-associated antigens (TAAs), making them poor targets for cytotoxic CD8+ T cells. Moreover, the tumor environment is often characterized by the accumulation of immunosuppressive cells such as regulatory T cells and myeloid-derived suppressor cells. In addition, tumors actively suppress immune response via inhibitory molecules such as PD-L1 or transforming growth factor β (TGFβ).

Another molecule that can be produced by regulatory T cells, myeloid-derived suppressive cells and tumor cells is interleukin-10 (IL-10), which is thought to contribute to the immunosuppressive tumor microenvironment. In vitro and under inflammatory conditions, IL-10 inhibits the expression of MHC Class II molecules, co-stimulatory molecules, and pro-inflammatory cytokines by antigen-presenting cells (APCs). Inhibition of APC function in turn impairs T-cell responses. In addition, IL-10 directly inhibits the in vitro activation and cytokine secretion of CD4+ T cells and macrophages. IL-10 was shown to impair the efficacy of a tumor vaccine when administered during vaccination. However, when given after vaccination, IL-10 enhanced vaccine-mediated antitumor functions.

Contrasting its immunosuppressive function, IL-10 activates CD8+ T cells in vitro and more importantly, the treatment of tumor-bearing mice with IL-10 leads to tumor rejection in multiple tumor models. The antitumor efficacy of IL-10 depend on the presence of CD8+ T cells, and the IL-10 treatment increased the number of CD8+ T cells within the tumor. Similar to human tumors prior to therapy, CD8+ T cells poorly infiltrated the cancer models employed in our studies.

Contrary to expectations, IL-10 treatment induces interferon γ (IFNγ) expression by CD8+ T cells, which in turn increased the levels of MIG and IP10 in the tumor and serum. These chemokines act as chemoattractants for T cells, suggesting a positive feedback loop of IFNγ-producing CD8+ T-cell recruitment into the tumor initiated by IL-10. Indeed, such a feedback loop has been postulated in mice bearing mammary tumors and treated with IL-10. Surprisingly, however, we found that mice deficient for CXCR3, the receptor for both MIG and IP-10, respond normally to IL-10. Furthermore, even a broad inhibition of T-cell migration from lymphoid organs using the S1P inhibitor FTY720 showed no effect on IL-10 efficacy. Therefore, IL-10 induces the accumulation of activated CD8+ T cells in tumors in the absence of de novo migration from lymph nodes.

These data indicate that the expansion and activation of autochthonous tumor-resident CD8+ T cells is sufficient to induce the rejection of well-established tumors. This comes as a surprise, since most approaches of cancer immunotherapy aim at inducing the priming of naïve TAA-reactive CD8+ T cells or the expansion of TAA-specific T-cell reservoirs in lymphatic organs.

The IL-10 receptor (IL-10R) was upregulated on CD8+ T cells upon stimulation of the T cell receptor (TCR). Also, CD8+ TILs showed a higher surface expression of IL-10R than T cells from other locations, suggesting that they are well equipped to directly respond to IL-10. Accordingly, in tumor-infiltrating lymphocytes, IL-10 treatment induced a high degree of phosphorylation not only of STAT3 but also of STAT1. This pattern of STAT activation was unique to tumor-resident CD8+ T cells and was not seen in other T-cell subsets, not even in CD8+ T cells from lymphoid organs. IL-10-activated CD8+ T cells were characterized by increased expression of the cytotoxic molecules granzyme B and perforin as well as of the effector cytokine IFNγ. Analysis of wild type and IL-10R-deficient CD8+ T cells within the same tumor-bearing mouse showed that IL-10 directly and specifically increases the activity of IL-10R-proficient CD8+ T cells without the requirement of other host cells being able to respond to IL-10. Reciprocally, IL-10 treatment did not stimulate IL-10R-deficient CD8+ T cells in an IL-10R-proficient host, confirming the direct nature of IL-10 signaling in controlling the activity of CD8+ T cells. Finally, the administration of IL-10 increased the proliferation of IL-10R-proficient CD8+ T cells, leading to their relative increase within the tumor, while IL-10R-deficient CD8+ T cells were strongly suppressed.

Our data indicate IL-10 directly activates tumor-resident CD8+ T cells increasing their activity, prevalence and proliferation, hence enabling a potent anti-tumor T-cell response leading to tumor rejection. Since it has already been shown that IL-10 treatment increases the production of IFNγ and granzymes in human peripheral blood mononuclear cells,, IL-10 might hold promises for the immunotherapy of cancer patients. (Fig. 1)

Figure 1. Most immune therapies of cancer increase the number of tumor specific CD8+ T cells by ex vivo amplification or stimulate the amplification of tumor specific CD8+ T cells in lymphoid organs through vaccine strategies (blue pathways). In contrast, treatment with pegylated interleukin-10 (IL-10) induces simultaneously the amplification of autochthonous tumor-specific CD8+ T cells and their cytotoxic activation within the tumor, in the absence of trafficking to and from lymphoid organ (red pathway). To achieve tumor rejection, IL-10 signals are required only within CD8+ T cells and not in other cells of the immune system.