Microtubule-depolymerizing agents potentiate anti-tumor immunity by stimulation of dendritic cells

Dendritic cells (DC) are unique players in the initiation and regulation of anti-tumor immune responses. Yet, the immunosuppressive tumor microenvironment may hamper the maturation stage and antigen-processing capacity of tumor-residing DCs. Thus, in an optimal setting, anti-cancer drugs have the power to reduce tumor size and, at the same time, modulate DC function towards efficient priming of antigen-specific effector T cells. With this in mind, we screened a small library of classical chemotherapeutics with distinct pharmacological mechanisms for their DC-stimulatory potential. As a result we discovered a previously unrecognized immunostimulatory profile of microtubule-destabilizing agents (MDAs), including dolastatins and maytansines, which so far have been described exclusively for their tumor cell cytotoxicity. Intriguingly, distinct compounds of this class potently provoked phenotypic and functional maturation of murine as well as human dendritic cells, resulting in an enhanced capacity to prime naïve T cells. Local administration of MDAs triggered in situ maturation of skin Langerhans cells and efficiently promoted antigen uptake and migration of tumor-resident DCs to tumor-draining lymph nodes in murine tumor models.
Underlining the requirement of an intact host immune system for the full therapeutic benefit of specific MDAs, the antitumor effect was far less pronounced in immunocompromised mice. Moreover, substantial therapeutic synergies were observed when combining MDAs with immunotherapy such as tumor-antigen-specific vaccination or blockade of the cytotoxic T-lymphocyte antigen 4 (CTLA-4) or programmed cell death ligand 1 (PD-L1) co-inhibitory pathways. Of note, combined T cell checkpoint inhibition and MDA treatment resulted in an increased intratumoral effector T cell to regulatory T cell ratio that is associated with beneficial prognosis in multiple tumor entities. Importantly, synthetic analogues of dolastatins and maytansines are currently used as cytotoxic payloads of the two recently approved antibody-drug conjugates (ADC) brentuximab vedotin and trastuzumab-emtansine (T-DM1), respectively. Treatment with ADCs coupled to microtubule-destabilizing agents induced DC homing in murine models and activated cellular antitumor immune responses in patients, thereby demonstrating the immune-modulating potential of these ADCs.
Ultimately, these data shed light on the MDA-triggered molecular pathways that, when activated in DCs, result in inflammatory responses. We here propose that MDA-mediated microtubule disassembly triggers the release of the microtubule-associated nucleotide exchange factor GEF-H1 from its cytoskeletal anchor. Subsequent induction the small GTPase RhoA results in activation of mitogen-activated protein kinases (MAPKs) including the c-Jun N terminal kinases (JNK). Phenotypic and functional DC maturation is then mediated by JNK-dependent phosphorylation of the transcription factor c-Jun, leading to activator protein -1 (AP-1) target gene expression.
Hence, by providing the molecular basis that links microtubule disruption with triggering of innate immune responses that translate into adaptive anti-tumor immunity, we reveal a novel mechanism of action for MDAs and provide a strong rationale for clinical treatment regimens combining MDA-based therapies with immune-based therapies.