Abstract B36: mRNA and dendritic cell based immunotherapy
Authors: Thielemans Kris 
Published: October 2015

Abstract
Modification of dendritic cells (DC) with mRNA allows the loading of these cells with tumor antigens and the functional modification of the cellular vaccine. To reprogram immature DC towards potent and immunostimulatory antigen presenting cells, we provide three different molecular adjuvants to DCs with mRNA coding for caTLR4, mimicking TLR-activation, CD40L, mimicking the ‘licensing’ of DCs by Th cells, and CD70 to provide an extra stimulus for the priming of CD8+ T cells. The mixture of these three mRNA's (caTLR4, CD40L and CD70) is referred to as ‘TriMix’
The programming of DC with mRNA can be performed either in vitro, by electroporation, or even in vivo after injection of mRNA into lymph nodes or at the tumor site.
At our institution, clinical trials in advanced melanoma patients are being performed using ex vivo modified DCs. These patients are treated with TriMixDC-MEL, a mixture of TriMix-DC co-electroporated with mRNA encoding a fusion of DC.LAMP and 1 of 4 melanoma associated antigens (TAA) (gp100, tyrosinase, MAGE-C2 or MAGE-A3).
In a first pilot clinical trial, 24.106 TriMixDC-MEL cells were administrated solely by the intradermal (ID) route. Subsequently, a phase IB was conducted to investigate the safety of administrating TriMixDC-MEL by the intravenous (IV) and ID-route. ID administration of TriMixDC-MEL was found to be feasible, safe, effectively stimulating CD8+ T-cell responses, but did not result in objective tumor responses. In contrast, the combined ID/IV administration is associated with distinct but manageable side-effects and has seemingly superior clinical activity as compared to DC administered solely ID in patients with pretreated advanced melanoma. We also investigated the safety and activity of TriMixDC-MEL combined with check-point blockade (ipilimumab). Thirty nine AJCC stage IIIc/IV melanoma pts initiated study treatment). Following DC-administration, gr2 skin injection site reactions were observed in all pts, post-infusion chills (&lt; gr2) in 15 (38%), and transient flu-like symptoms (&lt; gr2) in 33 pts (85%). Most frequent grade 3/4 adverse events of special interest were: dermatitis, diarrhea/colitis, hypophysitis, hepatitis, and pneumonitis. Systemic corticotherapy was used to treat irAE in 18 pts (46%). Best overall tumor response by irRC: 8 CR, and 7 PR (BORR 38%), 6 SD and 18 PD. All CR, and 3 PR are ongoing after a median duration of 19 mths (range 3-29 mths). The 6-mths DCR by irRC is 50% (95% CI 34-66). Median PFS and OS are respectively 6.2 (95% CI 3-9), and 14.4 mths (95%CI 10-18). These encouraging clinical responses indicate that further clinical development of TriMixDC-MEL in combination with immune checkpoint modulators is warranted.
Furthermore, more recent pre-clinical studies indicate that this cellular therapy with ex vivo modified DCs could evolve towards a less complex approach via the direct in vivo modification of DCs. We evaluated whether intranodal (IN) and intratumoral (IT) delivery of TriMix mRNA together or not with TAA mRNA, results in the in situ modification and maturation of DCs hence priming of TAA-specific T-cells. Upon IN and IT delivery of mRNA, we could demonstrate the selective uptake and translation of mRNA by lymph node- and tumor-resident CD11c+ cells, respectively. This process was hampered by co-delivery of classical maturation stimuli but not by TriMix mRNA. Importantly, injection of TriMix mRNA induced a T-cell attracting and stimulatory environment. Enhanced induction of antigen-specific CD4+ and CD8+ T-cells was demonstrated using TriMix mRNA and mRNA's several antigens upon IN immunization. Intranodal mRNA immunization was shown to be as efficient in induction of CTLs and in therapy as vaccination with ex vivo mRNA electroporated DCs in several mouse tumor models. Upon IT injection of TriMix mRNA, the tumor-resident DCs become activated, migrate to lymph nodes and activate CD8+ cytotoxic T lymphocyte responses against several cancer-antigens, including a cancer-testis antigen and a neoepitope. Since no TAA encoding mRNA was added, these results indicate that the tumor-resident DCs are loaded with tumor antigens, can be activated in situ and can be tuned into an auto-vaccine. Intratumoral injection of TriMix mRNA only showed curative potential against disseminated tumors. In conclusion, IN and IT administration of TAA mRNA together with mRNA encoding DC modulating molecules is a promising vaccination strategy that could evolve towards an off-the-shelf anti-cancer vaccine.
Citation Format: Kris Thielemans. mRNA and dendritic cell based immunotherapy. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr B36.