Viral vectors, in innate and adaptive immunity

Abstract

Recombinant viral vectors and DNA vectors induce strong immune responses in animal models. However, in clinical trials, the generation of immune responses is less robust, suggesting that further optimization and a deeper understanding of nucleic acid-based vaccines are required. Improvements, such as combining the vaccine vectors in heterologous prime boost regimens and/or using vectors that do not induce strong immune responses against the vector itself may enhance immune responses against the antigen-of-interest. In Paper I, we performed head-to-head comparisons of adenovirus, ALVAC and Semliki Forest virus (SFV) vectors, in homologous and heterologous prime boost regimens. The recombinant viral vectors were evaluated for their potency to generate T cell responses and to protect against a tumor challenge. We show that the memory T cell response induced by the different immunization regimens were distinctly different and that protection against a tumor challenge was more dependent on the quality of the response than the magnitude.

The potency of the nucleic acid-based vaccines depends on the activation of innate signaling pathways. In Paper II-IV, we investigated innate signaling pathways activated by different viral vectors and the role of these pathways for induction of T cell responses. The CD8alpha+ DCs play a major role in cross-priming of anti-viral T cells. This dendritic cell subtype phagocytoses apoptotic bodies, expresses high levels of toll-like receptor (TLR) 3 and has a unique ability to cross-present exogenously derived cell-associated material. In Paper II, we investigated the role of TLR3 expression in the CD8alpha+ DCs and its relevance for cross-priming of T cells. We show that dsRNA activates CD8alpha+ DCs to cross-prime T cells via TLR3.

In addition to TLRs, the cytoplasmic RNA receptors RIG-I and MDA5, expressed by most cell types, are detectors of viral infection. It was initially suggested that both RIG-I and MDA5 recognize double-stranded RNA (dsRNA) intermediates generated in the cytoplasm during viral infection in the host cell. However, negative-sense RNA viruses do not generate detectable levels of dsRNA in infected cells, thus these viruses may be recognized via alternative non-self signatures. In Paper III, we show that RIG-I is a receptor for single-stranded RNA molecules bearing 5 -phophates, illustrating one of the differences between RIG-I and MDA5 virus recognition.

The alphavirus replicon-based DNA (DREP) vectors induce superior immune responses in comparison to conventional DNA (convDNA) vectors in animal models. We hypothesized that DREP vectors induce potent innate signaling pathways that account for the immunogenic properties of these vectors. In Paper IV, we investigated T cell responses in mice deficient in innate signaling pathways, including TLR3, TLR9, MyD88, IRF3 and the interferon alpha/beta receptor (IFN-AR1), after SFV viral and DNA based vector immunization. We show that IFN-AR1 and IRF3, but not detectably the other molecules, influence the T cell response induced by these vectors.