Pre-existing immunity against Ad vectors: Humoral, cellular, and innate response, what's important?

Fausther-Bovendo, Hugues; Kobinger, Gary P

Ad vectors, like other viral vectors can activate the innate immune response upon injection. Various pattern recognition receptors (PRR) including retinoic acid-inducible gene 1 (RIG-I), nucleotide oligomerization domain (NOD)-like receptors (NLR), factor X, and Toll-like receptors (TLR) 4 and 9 are involved in Ad recognition. In addition, Ad binding to the Coxsackievirus and Adenovirus receptor (CAR), as well as integrins, are also involved in this process. [38] [39] [40] [41] Upon activation, the above PRR molecules trigger signaling pathways such as NF-KB, which can induce the production of numerous pro-inflammatory mediators including cytokines (interferon (IFN) a/ b, interleukin (IL)-1a/ b, tumor necrosis factor (TNF)a, and IL-6) and chemokines (IL-8, macrophage inflammatory protein (MIP)-1a/b, IP-10, RANTES, and monocyte chemoattractant protein (MCP)-1). [38] [39] [40] Among these pro-inflammatory molecules, IL-1 and its receptor IL-1R1, as well as type I IFN, play a central role as they induce positive feed-back loops that in turn, produce additional pro-inflammatory mediators. [42] [43] [44] The role of innate immunity on antigen expression and vector efficacy after Ad immunization was revealed by comparing different HAd of various immunogenicities. It was first observed that when compared with the benchmark HAd5, both in vitro and in vivo, HAd vectors with lower immunogenicity such HAd35, HAd26 and HAd48, 12, 45, 46 induce greater inflammatory responses, as illustrated by higher levels of inflammatory cytokines (IFNa, IFNg, IL-6) and chemokines (IP-10, I-TAC, MIP1a/b). 47, 48 In addition, HAd35 and HAd28 also induced stronger NK cells activation. Stronger IFNa dependent NK cells activation can lead to lower transgene expression and persistence in vitro, due to NK killing of Ad transduced monocytes. 48 The inverse relation between NK cell activation, type I IFN production and transgene expression were confirmed in vivo. For example, in rodents, high type I IFN levels were associated with lower transgene expression. Indeed, compared with HAd5, ChAd68 induces a higher level of type I IFN but lower transgene expression in vivo. 49, 50 Furthermore, higher transgene expression was detected after ChAd68 and HAd5 injection in IFN-a/b-R knockout mice when compared with the same vectors in wild type mice. 42, 50, 51 Similarly, NK depletion prior to HAd5 immunization increases transgene expression level while NK transplantation had the opposite effect. 51 The above studies indicate that Ad vectors inducing higher inflammation and NK activation have lower transgene expression and persistence. Of note, type I IFN and NK cells are also able to control the expressed level of antigen, which influences the T cell response in mice models of mouse cytomegalovirus (MCMV) infection. 52, 53 In individuals or animals with pre-existing Ad immunity, high inflammatory responses will further reduce transgene expression and persistence due to Ad specific CD8 T cell recruitment to the site of inflammation and subsequent killing of Ad transduced antigen presenting cells (APC) (Fig. 2) .