Dendritic cell (DC)-derived exosomes (Dexo) are promising vaccine candidates since they carry key molecules, normally found in DCs, that are essential for the activation of adaptive immune responses. Moreover, Dexo have a longer in vivo half-life than DCs, are not subject to the effects of immunomodulators, and are devoid of the risks of cell-based vaccines. As such, clinical trials were conducted to test the feasibility, safety and efficacy of vaccines composed of Dexo carrying tumor antigens for the immunotherapy of melanoma and small-cell lung carcinoma patients. Even though these trials proved the feasibility and safety of Dexo vaccination for the treatment of human cancer patients, the clinical benefits associated with the treatment were limited.

We reasoned that the limited efficacy of Dexo vaccines could be ascribed to the low immunogenicity of the Dexo formulations tested in the clinical trials. Indeed, such formulations were produced from DCs loaded with tumor antigens in the absence of any pro-inflammatory stimuli that could provide the danger signals necessary to elicit a proper anti-tumor immune response in vivo.

The objective of our work was thus to identify a Dexo formulation with stronger immune stimulatory properties as compared to the Dexo formulations previously tested in pre-clinical and clinical settings.

As adaptive immune responses are elicited by antigen-presenting cells activated in the presence of antigens and pro-inflammatory signals, we produced Dexo from DCs loaded with specific antigens (either Ovalbumin, OVA, or tumor-derived antigens) and matured with the Toll-like receptor (TLR)-3 ligand poly(I:C) as danger signal.

As a preliminary assessment, we tested Dexo produced from DCs loaded with OVA and stimulated with poly(I:C) (Dexo(OVA+pIC) in in vivo models of vaccination, proving that Dexo(OVA+pIC) could induce efficient activation of transferred or endogenous OVA-specific CD4+ and CD8+ T cells, leading to pro-inflammatory Th1 immune responses.

We then moved to a model of therapeutic vaccination in melanoma tumor-bearing mice, taking advantage of the B16F10 melanoma model. To this purpose, we produced Dexo from DCs matured with poly(I:C) and cultured in the presence of oxidized B16F10 cell lysates (Dexo(B16+pIC)), in order to provide additional pro-inflammatory stimuli as well as all the B16F10 antigens to the exosome-producing DCs.

Mice bearing subcutaneous B16F10 tumors were vaccinated intradermally in order to target tumor-draining lymph nodes (tdLN). This protocol of vaccination resulted in reduced tumor growth and limited metastasis burden and, most importantly, in prolonged survival of the mice vaccinated with Dexo(B16+pIC). Such benefits were associated with increased frequencies of effector CD8+ T cells in the tdLNs, spleen and tumor masses, reduced frequencies of tumor-infiltrating exhausted PD-1+ CD8+ T cells and increased frequencies of tumor-infiltrating NK and NK-T cells obtained upon vaccination with Dexo(B16+pIC) as compared to vaccination with Dexo(B16).

Our results demonstrate that poly(I:C) is a promising candidate for the production of Dexo vaccines with improved immune stimulatory properties suitable for the immunotherapy of cancer.

Citation Format: Martina Damo, David Scott Wilson, Jeffrey Alan Hubbell. Improving the immunogenicity of dendritic cell-derived exosome vaccines for the immunotherapy of melanoma. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr A09.