FocusCancer Immunotherapy

Targeting TNFR2, an immune checkpoint stimulator and oncoprotein, is a promising treatment for cancer

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Science Signaling  17 Jan 2017:
Vol. 10, Issue 462, eaal2328
DOI: 10.1126/scisignal.aal2328

Abstract

Tumor necrosis factor receptor 2 (TNFR2) is expressed both by some cancer cells and by tumor-infiltrating immunosuppressive CD4+FoxP3+ regulatory T cells (Tregs). TNFR2 stimulates the activation and proliferation of Tregs, a major checkpoint of antitumor immune responses, and promotes cancer cell survival and tumor growth. In this issue of Science Signaling, Torrey et al. found that dominant antagonistic antibodies against human TNFR2 may be a potential therapy for ovarian cancer patients by simultaneously suppressing Treg activity and inducing the death of the cancer cells.

There is now clear evidence that tumor necrosis factor receptor 2 (TNFR2) mediates the stimulatory activity of TNF on CD4+FoxP3+ regulatory T cells (Tregs), resulting in the activation, proliferative expansion, and phenotypic stability of Tregs (1, 2). TNFR2 is preferentially expressed on rodent and human Tregs, and expression of TNFR2 identifies the maximally suppressive subset of Tregs. Tumor-infiltrating Tregs express high levels of TNFR2 and are potent immunosuppressive cells (3, 4). The TNFR2-expressing Tregs present in the tumor environment represent a major cellular mechanism of tumor immune evasion. In addition to Tregs, CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs) also contribute to tumor immune evasion. Recent studies show that the generation, accumulation, and function of MDSCs depend on TNF-TNFR2 signaling (5). Furthermore, the immunosuppressive function of mesenchymal stem cells (MSCs) is also associated with TNFR2 signaling (6). As central regulators of immune responses, Tregs represent a major immune checkpoint, whereas MDSCs and MSCs also contribute to the immune regulation in a collaborative manner with Tregs. Intriguingly, the activation of all immunosuppressive cells in this group is TNFR2-dependent. Thus, TNFR2 behaves as an immune checkpoint stimulator. Consequently, therapeutically targeting TNFR2 provides a strategy to eliminate tumor-associated Treg activity and the function of other types of immunosuppressive cells, which may result in revealing naturally occurring host immune responses against the tumor and thus enhance the efficacy of cancer therapy. An additional scientific rationale for targeting TNFR2 in tumor treatment is that this receptor is also expressed by some malignant cells per se, and the expression of TNFR2 promotes the survival and growth of tumor cells (7). Therefore, blockage of TNFR2 may have the effect of killing two birds with one stone: boosting antitumor immune responses and directly killing tumor cells. In this issue of Science Signaling, Torrey et al. test this idea with antibodies to human TNFR2, which outcompete TNF, the natural ligand of both TNFR1 and TNFR2, and consequently dominantly inhibit the activation of TNFR2 signaling pathways (8).

Torrey et al.’s study results show that antagonistic antibodies to TNFR2 inhibit TNF-induced activation of human Tregs derived from peripheral blood and consequently reduce the number, proportion, and immunosuppressive activity of Tregs. Antagonistic antibodies also inhibit the activation of nuclear factor κB pathways and gene expression associated with TNFR2 signaling in Tregs. The antagonistic antibodies bind to the same region of TNFR2, independent of Fc or cross-linking of antibodies. Using linear and three-dimensional epitope mapping, the authors show that the antagonistic antibodies bind to and stabilize antiparallel dimers, rather than homotrimers, of TNFR2 protein, resulting in (i) blockade of TNF binding to TNFR2, (ii) inhibition of intracellular signaling, and (iii) suppression of cleavage of TNFR2 from a membrane-bound form (mTNFR2) to the soluble form (sTNFR2). They find that Tregs from ovarian cancer tissues, as compared with Tregs derived from healthy donors, are more sensitive to death induced by TNFR2 antagonists, which is likely attributable to the higher TNFR2 expression on tumor-infiltrating Tregs (3, 4). Therefore, the dominant TNFR2 antagonists preferentially suppress tumor-associated Treg activity but have no or only minor inhibitory effects on regular Tregs in the periphery, which play a crucial role in the maintenance of immunological homeostasis. This feature is important because it can mitigate the possibility of developing systemic autoimmune inflammatory responses, which cause severe collateral damage to normal tissues seen after treatment with cancer immunotherapeutic regimens. In addition, TNFR2 antagonists created by Torrey et al. have the capacity to induce the death of OVCAR3, an ovarian cancer cell line with surface expression of TNFR2. This is an ideal scenario, given that the tumor antigen released from dead cells can promote the quiescent antitumor immune responses triggered by attenuation of Treg activity.

Although TNFR2 is constitutively and predominantly expressed by highly suppressive Tregs (3, 9), the expression of TNFR2 can also be induced and up-regulated on CD4+FoxP3 T effector cells (Teffs) upon T cell receptor stimulation (10). TNFR2 expression on Teffs can costimulate the activation of Teffs and empower their ability to resist Treg-mediated suppression (10). Thus, the potential inhibitory effect of TNFR2 antagonists on the activation and expansion of tumor-reactive Teffs in human patients should be considered and carefully evaluated. Nevertheless, tumor-infiltrating Tregs are known to persistently express much higher levels of TNFR2 than Teffs (3, 4, 10). Thus, it is reasonable to predict that in vivo treatment of TNFR2 antagonist will have more profound impact on Tregs than on Teffs. If this is the case, the net outcome of TNFR2 antagonism will favor the activation and expansion of Teffs, resulting in a more effective antitumor immune response. This hypothesis needs to be carefully tested in a suitable mouse tumor model and subsequently studied in human patients.

Membrane-bound TNFR2 can be immunosuppressive or immunostimulatory, depending on the cell type that expresses this receptor. In contrast, the function of sTNFR2 is consistently immunosuppressive. Activated Tregs can release high amounts of sTNFR2 (9), which, at least partially, represents another immunosuppressive mechanism of Tregs. Intriguingly, TNFR2 antagonist shown in Torrey et al.’s study appears to stabilize the surface expression of TNFR2 and to inhibit the release of sTNFR2 from Tregs. This effect should contribute to the immunostimulatory effect of TNFR2 antagonist. Because TNF by itself can up-regulate TNFR2 expression on T cells, interruption of TNF-TNFR2 interaction thus presumably down-regulates TNFR2 surface expression on Tregs.

The study by Torrey et al. clearly shows that TNFR2 antagonists have promise as cancer therapies by simultaneously blocking both an immune checkpoint molecule on T cells and an oncoprotein on tumor cells (Fig. 1, A and B). To achieve an optimal antitumor effect, the TNFR2 antagonist may have to be used in conjunction with other therapeutics. For example, activation and expansion of Tregs are likely side effects of cancer immunotherapy, which, in turn, abolish the antitumor effect of the therapy. TNFR2 antagonists have the capacity to eliminate Treg activity and thus may further enhance the efficacy of current immunotherapeutics. In addition, suppression of tumor cell survival by the TNFR2 antagonist may improve the outcome of chemotherapy or radiotherapy in cancer patients. Further understanding of the fundamental biological processes, such as signaling pathway and molecular mechanism underlying Treg- and tumor-promoting effects of TNFR2, may help design safer and more effective targeted therapeutics. In addition to TNFR2, a number of TNF receptor superfamily members—such as OX40, 4-1BB, and DR3—are also preferentially expressed by Tregs and have similar stimulatory effect as TNFR2 on Tregs; thus, they should also belong to the checkpoint stimulators family (CPSFs). The potential effect of targeting these CPSFs, as well as their respective ligands—namely, TNF/LTα, OX40L, 4-1BBL, and TL1A—for cancer therapy warrants further study.

Fig. 1 TNFR2 antagonism in the treatment of cancer.

(A) In the tumor environment, Tregs are TNFR2-expressing potent immunosuppressive cells. Interaction of TNF-TNFR2 results in the activation and proliferative expansion of Tregs. Consequently, the number and function of Tregs are increased, which, in turn, potently inhibit the function of tumor-reactive Teffs and assist immune evasion by the tumor. Further, TNF can promote the release of sTNFR2, which further suppresses the antitumor activity of Teffs. Moreover, ligation of TNF on TNFR2 expressed by tumor cells provides signaling that promotes the survival, growth, and metastasis of the tumor. Thus, TNF-TNFR2 interaction contributes to tumor immune evasion and development. (B) Blockade of TNF-TNFR2 interaction with neutralizing or blocking monoclonal antibodies or other agents has the capacity to inhibit the activation of Tregs, resulting in a reduction of their function and number. TNFR2 antagonist also intriguingly inhibits release of sTNFR2 from Tregs. Surface abundance of TNFR2 is also down-regulated by the TNFR2 antagonist. Consequently, the number and function of tumor-reactive Teffs are increased, resulting in stronger antitumor immune responses. Furthermore, TNFR2 antagonist can also induce the death of tumor cells and inhibit the growth and metastasis of tumor. Therefore, TNFR2 antagonism is a promising treatment for cancer.

CREDIT: C. BICKEL/SCIENCE SIGNALING

REFERENCES

Funding: Research in the authors’ laboratories is supported in part by the Intramural Research Program of the U.S. NIH, National Cancer Institute, Center for Cancer Research. This project was also funded by University of Macau research grants (SRG2014-00024-ICMS-QRCM and MYRG2016-00023-ICMS-QRCM) and a grant from the Science and Technology Development Fund [Macao S.A.R. (014/2015/A1)]. Competing interests: The authors declare that they have no competing interests.
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