Survival of lethal poxvirus infection in mice depends on TLR9, and therapeutic vaccination provides protection

Abstract

Poxviruses such as the causative agent of smallpox have developed multiple strategies to suppress immune responses, including the suppression of DC activation. Since poxviruses are large DNA viruses, we hypothesized that their detection by DCs may involve the endosomal DNA recognition receptor TLR9. Indeed, we have shown here that DC recognition of ectromelia virus (ECTV), the causative agent of mousepox, completely depended on TLR9. The importance of TLR9 was highlighted by the fact that mice lacking TLR9 showed drastically increased susceptibility to infection with ECTV. In contrast, we found that the strongly attenuated poxvirus modified vaccinia virus Ankara (MVA) activated DCs by both TLR9-dependent and -independent pathways. We therefore tested whether we could use the broader induction of immune responses by MVA to protect mice from a lethal infection with ECTV. Indeed, MVA given at the same time as a lethal dose of ECTV protected mice from death. Importantly, MVA also rescued TLR9-deficient mice if administered 2 full days after an otherwise lethal infection with ECTV. Therefore, these data suggest an essential role for TLR9 in the defense against poxviruses. In addition, postexposure application of MVA may protect against lethal poxvirus infection.

Nonplasmacytoid, High IFN-α–Producing, Bone Marrow Dendritic Cells

“…ECTV has previously been shown by us to activate pDC via TLR9 but to potently inhibit the largely TLR9-independent activation pathways of cDC (21). Both SFV and ECTV did not induce either surface activation of miDC or measurable IFN-a production (Fig.…”

Innate immune activation of NFκB and its antagonism by poxviruses

“…2). Poxvirus infection is detected by TLR7 [47], TLR8 [48] and TLR9 [49,50] in endosomes of conventional and plasmacytoid dendritic cells (cDCs and pDCs). Survival of mice after a lethal ECTV infection was critically reliant on TLR9, while MVA infection protected mice from ECTV in a manner dependent on TLR9 [49].…”

“…Poxvirus infection is detected by TLR7 [47], TLR8 [48] and TLR9 [49,50] in endosomes of conventional and plasmacytoid dendritic cells (cDCs and pDCs). Survival of mice after a lethal ECTV infection was critically reliant on TLR9, while MVA infection protected mice from ECTV in a manner dependent on TLR9 [49]. Recognition of poxvirus by TLR7 was proposed to involve detection of viral RNA transcripts [47], while surprisingly, TLR8-dependent responses were suggested to be mediated by recognition of poly(A)/T-rich poxviral DNA sequences [48].…”

Smallpox vaccines: New formulations and revised strategies for vaccination

“…109,110 This rather long disease free period seems vital to allow for the demonstration of protective post-exposure vaccination with VACV. 109,111 In contrast, respiratory challenge infection with VACV produces a fulminant acute disease which appears to hamper the feasibility of therapeutic immunization. 112 Additional parameters including the increase in hemagglutination inhibiting or neutralizing antibodies were not tested in all vaccinees, yet neutralizing antibodies appeared to better correlate with "vaccine take".…”

“…Recently, the efficacy of vaccination shortly before or post exposure was demonstrated in animal models. 60,109,111 Future research needs to carefully address the evaluation of immune correlates for rapid protection and to investigate the usefulness of new approaches and schedules for emergency immunization in human clinical trials. These activities should be supplemented by the evaluation of other options for therapeutic intervention.…”