Date Published: December 21, 2016
Publisher: Public Library of Science
Author(s): Neil Berry, Maria Manoussaka, Claire Ham, Deborah Ferguson, Hannah Tudor, Giada Mattiuzzo, Bep Klaver, Mark Page, Richard Stebbings, Atze T. Das, Ben Berkhout, Neil Almond, Martin P. Cranage, Ronald C. Desrosiers.
In order to evaluate the role of persisting virus replication during occult phase immunisation in the live attenuated SIV vaccine model, a novel SIVmac239Δnef variant (SIVrtTA) genetically engineered to replicate in the presence of doxycycline was evaluated for its ability to protect against wild-type SIVmac239. Indian rhesus macaques were vaccinated either with SIVrtTA or with SIVmac239Δnef. Doxycycline was withdrawn from 4 of 8 SIVrtTA vaccinates before challenge with wild-type virus. Unvaccinated challenge controls exhibited ~107 peak plasma viral RNA copies/ml persisting beyond the acute phase. Six vaccinates, four SIVmac239Δnef and two SIVrtTA vaccinates exhibited complete protection, defined by lack of wild-type viraemia post-challenge and virus-specific PCR analysis of tissues recovered post-mortem, whereas six SIVrtTA vaccinates were protected from high levels of viraemia. Critically, the complete protection in two SIVrtTA vaccinates was associated with enhanced SIVrtTA replication in the immediate post-acute vaccination period but was independent of doxycycline status at the time of challenge. Mutations were identified in the LTR promoter region and rtTA gene that do not affect doxycycline-control but were associated with enhanced post-acute phase replication in protected vaccinates. High frequencies of total circulating CD8+T effector memory cells and a higher total frequency of SIV-specific CD8+ mono and polyfunctional T cells on the day of wild-type challenge were associated with complete protection but these parameters were not predictive of outcome when assessed 130 days after challenge. Moreover, challenge virus-specific Nef CD8+ polyfunctional T cell responses and antigen were detected in tissues post mortem in completely-protected macaques indicating post-challenge control of infection. Within the parameters of the study design, on-going occult-phase replication may not be absolutely required for protective immunity.
Live attenuated SIV has proven to be a highly effective vaccination strategy in non-human primate (NHP) models of HIV/AIDS [1,2], in many cases protecting macaques from detectable superinfection following re-challenge with both homologous and heterologous wild-type SIV administered systemically and mucosally [3–23]. Although safety concerns such as reversion to virulence and recombination with wild-type strains preclude direct application of this vaccine approach in humans, a clearer understanding of mechanisms of pathogenesis and protection may inform the development of more clinically acceptable HIV vaccines. Studies have been performed using vaccine viruses attenuated by genetic disruption of key regulatory genes including nef, vpx, vpr and vif; although the moderately attenuated prototypic vaccine strain SIVmac239Δnef has been used for the majority of studies.
The reported breadth and duration of protection conferred in macaques following vaccination with live attenuated SIV has many of the features required of an effective vaccine against HIV/AIDS. Understanding the mechanisms of protection may allow the informed design of intrinsically safe vaccines. Earlier attempts to improve the safety profile of live attenuated SIV by introducing multiple attenuating mutations revealed that the degree of protection was inversely proportional to the degree of attenuation . Hence, it was perhaps not unexpected that SIV clones molecularly engineered to be limited to a single round of replication conferred only limited protection compared with more vigorously replicating attenuated vaccine strains [23, 33]. The development of SIVrtTA with potential to be temporally modulated for replication in vivo provides a novel tool to further dissect the processes of protection elicited by live attenuated SIV. Previously, we reported this novel virus to replicate in vivo and being fully infectious in rhesus macaques, with the ability to disseminate to lymphoid tissues and elicit a range of immunological responses including reversible changes in the frequency of memory T cell subsets dependent upon the withdrawal of dox .