Background
Currently only few adjuvants are approved, most of which confer a T helper (TH)2 shift. Among new adjuvants conferring a TH1-shift, RNAdjuvant® consisting of non-coding RNA complexed with a cationic carrier is a promising candidate.
Here, we studied the immunoenhancing effects of RNAdjuvant® on vaccine-induced antibody responses. To understand the underlying mechanisms, we elucidated the signaling pathways involved in the sensing of RNAdjuvant® and investigated the induced cytokine responses.
Methods
Bone marrow-derived dendritic cells (DC) were stimulated with RNAdjuvant® in vitro to study the direct effects of RNAdjuvant® on DC. In preclinical mouse studies, the immunoenhancing effects of RNAdjuvant® were investigated with two different vaccination models. Experiments with mice deficient in TLR and RIGI-like helicase (RLH) signaling were conducted to elucidate the sensing of RNAdjuvant®.
Results
Upon RNAdjuvant® treatment, bone marrow-derived DC showed TLR7-dependent IFN‑I induction and upregulation of activation marker. This result was verified by the ex vivo analysis of splenic DC. Immunization of mice with influenza vaccines plus RNAdjuvant® significantly enhanced influenza-specific IgG responses. In particular, IgG2b/c antibodies were elevated, indicating a TH1-biased immune response. RNAdjuvant®‑mediated enhancement of antibody responses was conferred by concomitant MyD88- and Cardif-dependent signaling. Thus, RNAdjuvant® induced two different signaling platforms, which explains the strong immunoenhancing effect. Future studies will show, which specific receptors are responsible for the sensing. Upon intramuscular injection of RNAdjuvant®, IFNβ induction and CD69 upregulation on DC were only detectable in draining lymph nodes. Furthermore, transient lymphopenia as a sensitive readout of serum cytokine responses was only detected upon intravenous, but not upon intramuscular, injection indicating that intramuscular administration of RNAdjuvant® induced primarily local effects.
Conclusions
RNAdjuvant® is a novel efficacious immune enhancer that augments TH1 responses in a MyD88- and Cardif-dependent manner by triggering strictly local effects.