Cancer chemotherapy agents target intratumoral dendritic cells to potentiate antitumor immunity

Abstract

Cytotoxic drugs capable of killing cancer cells in conjunction with targeted conversion of tumor resident, tolerogenic dendritic cells (DCs) into efficient antigen presenting cells (APCs) are highly complementary therapeutic routes to boost antitumor immunity. Our data suggest that the microtubule-depolymerizing compounds Dolastatin 10 and Ansamitocin P3 may serve as prototypes for a class of agents that display this binary mode of action.

The field of cancer immunotherapy is moving forward at an accelerated pace and was honorably selected as Breakthrough of the Year in 2013 by the editors of Science.¹ Following many disappointments and decades long struggles to successfully implement cancer immunotherapy from the bench to the bedside the concerted efforts of basic and clinical scientists have begun to pay off in the form of efficacious clinical treatment regimens. Ipilimumab, a fully humanized antibody targeting cytotoxic T lymphocyte associated protein 4 (CTLA-4) as well as anti-PD-1 and -PD-L1 (the ligand of PD-1) blocking antibodies targeting the programmed cell death 1 (PD-1) immune checkpoint, have stimulated objective and durable anticancer responses in patients with treatment-refractory solid tumors, including melanoma and cancers of the lung, and kidney.^2-4 These clinical successes highlight the potential of immune-based therapies in oncology. Nevertheless, only a fraction of patients respond and most of the responders eventually relapse. Possible explanations include immune effector cell paucity at the tumor site, as well as the molecular and cellular composition of the tumor microenvironment, both of which may pose a major hurdle for contriving antitumor immunity.

Dendritic cells (DCs)serve as potent antigen-presenting cells (APCs) and as such are crucial for the initiation of antitumor T-cell responses. However the tumor stroma may interfere with DC differentiation, activation and antigen-processing capacity.⁵ As a consequence, immature and/or dysfunctional DCs tend to accumulate at the lesion, where they add to the immunosuppressive armament of the tumor microenvironment. Overcoming these suppressive barriers and converting immature, and therefore, rather tolerogenic, DCs into professional APCs is crucial to the clinical success of immunotherapy-based treatment approaches. An additional, frequently overlooked mechanism by which antitumor immune responses may be potentiated is the physical reduction of the primary tumor burden, an effect that may be induced by ionizing radiation, targeted therapies or chemotherapy. For example, persistent presentation of a tumor antigen may cause tolerance of resident T cells and consequently tumor outgrowth.⁶ It is therefore highly desirable to identify cytotoxic drugs that are capable of inducing tumor cell death, thus shrinking tumor masses, while concurrently facilitating antigen up-take, and subsequent activation, of tumor resident DCs. Furthermore, this scenario may allow de novo priming of tumor antigen-specific T cells in tumor-draining lymph nodes. Treatment approaches that remove immunosuppression and foster antitumor T-cell responses are currently being examined in the clinic, studies that may provide a rational for further immunotherapy combinations, such as in conjunction with immune checkpoint blocking antibodies.

Recently, we were able to demonstrate that 2 families of microtubule destabilizing agents, are not only cytotoxic drugs (i.e., inducing tumor cell death) but also potently provoke phenotypic and functional DC maturation and activation.^7,8 These include dolastatins, namely Dolastatin 10 and the synthetic analog monomethyl auristatin E (MMAE), as well as the maytansinoid, Ansamitocin P3. Interestingly, MMAE has been used as a cytotoxic payload in Brentuximab Vedotin, an antibody-drug-conjugate that has recently been approved for the treatment of patients with CD30⁺ lymphomas. Using various murine and human in vitro and in vivo tumor models, we demonstrated that these microtubule destabilizing agents induce upregulation of maturation markers, facilitate antigen uptake at the tumor site, and foster the migration of antigen loaded DCs to the tumor-draining lymph nodes.^7,8 Taken together, these activities thereby promote T-cell priming and expansion (Fig. 1). Importantly, these agents depend on DCs and the adaptive immune system for their full therapeutic efficacy. Combinations with immune checkpoint inhibition, immunotherapeutic avenues that hold great promise for the clinic, further augmented antitumor immunity and tumor rejection. Mechanistically, the combination therapy reduced Treg numbers and elevated effector function of tumor resident T cells. On a translational level, we demonstrated peripheral immune cell activation and brisk T-cell infiltration into tumors in patients previously treated with Brentuximab Vedotin. The clinical relevance of these findings are further substantiated by our previous observation that long-lasting tumor specific T cells are induced in patients responding to Brentuximab Vedotin (with or without donor lymphocyte infusions) for lymphoma relapse post allogeneic hematopoietic stem cell transplantation.⁹

Figure 1.

Antitumor immunity induced by the microtubule-depolymerizing agents Dolastatin 10 and Ansamitocin P3. The cytotoxic compounds Dolastatin 10 and Ansamitocin P3 do not only induce tumor cell death and subsequent antigen release from dying cancer cells but are capable of directly promoting dendritic cell (DC) differentiation and maturation. Tumor-derived antigens are taken up by immature DCs at the tumor site that, upon maturation, up-regulate co-stimulatory molecules such as the B7 family members CD80 and CD86, as well as CD40 and MHC molecules. Upon migration to the tumor-draining lymph nodes (LNs) these antigen-loaded antigen presenting cells (APCs) prime antigen-specific CD4⁺ and CD8⁺ T cells. Expanded and activated T cells infiltrate the lesion where they recognize and attack antigen-expressing cancer cells.

Little is known about the molecular mechanisms underlying the immune-boosting responses to these particular agents. It appears to be a class-dependent effect as a broad range of cytotoxic drugs, including colchicine, Vinblastine, Vindesine, Vincristine, Combretatstain-A4, Dolastatin10, Dolastatin15 and MMAE as well as AnsamitocinP3, all of which are microtubule-destabilizing agents, also induce DC activation. In contrast, our own data demonstrate that microtubule-stabilizing agents, such as taxanes only display mild DC activating properties in murine DCs and none in human DCs. We recently were able to exclude a myeloid differentiation primary response 88 (MyD88)-dependent mechanism by employing MyD88-deficient mice but cannot currently rule out that activation occurs via toll-like receptor (TLR) 3 or 4, NOD-like receptors (NLRs), or other pattern recognition receptors. On the other hand, microtubule destabilization alone could contribute to DC activation. Further experiments are currently ongoing in our laboratory, including the analysis of key signaling molecules to dissect these mechanisms. It also remains to be determined if dolastatins and maytansinoids, besides their direct effects on DCs, enhance the immunogenic characteristics of cancer cells, similar to the previously described mechanisms of immunogenic cell death induced by agents, such as anthracyclines, mafosfamide and oxaliplatin.¹⁰

From a clinical perspective, induction of antitumor immunity in response to antibody-drug conjugates containing MMAE (e.g., Brentuximab Vedotin) is a highly attractive treatment route, primarily due to the potential to combine this immune potentiating approach with immunological strategies encompassing checkpoint-directed immunotherapeutics, such as anti-CTLA-4 or anti-PD-1/PD-L1 blockade.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.