Abstract
The properties of the human immunodeficiency virus (HIV) pose serious difficulties for the development of an effective prophylactic vaccine. Here we describe the construction and characterization of recombinant (r), replication-competent forms of rhesus monkey rhadinovirus (RRV), a gamma-2 herpesvirus, containing a near-full-length (nfl) genome of the simian immunodeficiency virus (SIV). A 306-nucleotide deletion in the pol gene rendered this nfl genome replication-incompetent as a consequence of deletion of the active site of the essential reverse transcriptase enzyme. Three variations were constructed to drive expression of the SIV proteins: one with SIV’s own promoter region, one with a cytomegalovirus (cmv) immediate-early promoter/enhancer region, and one with an RRV dual promoter (p26 plus PAN). Following infection of rhesus fibroblasts in culture with these rRRV vectors, synthesis of the early protein Nef and the late structural proteins Gag and Env could be demonstrated. Expression levels of the SIV proteins were highest with the rRRV-SIVcmv-nfl construct. Electron microscopic examination of rhesus fibroblasts infected with rRRV-SIVcmv-nfl revealed numerous budding and mature SIV particles and these infected cells released impressive levels of p27 Gag protein (>150 ng/ml) into the cell-free supernatant. The released SIV particles were shown to be incompetent for replication. Monkeys inoculated with rRRV-SIVcmv-nfl became persistently infected, made readily-detectable antibodies against SIV, and developed T-cell responses against all nine SIV gene products. Thus, rRRV expressing a near-full-length SIV genome mimics live-attenuated strains of SIV in several important respects: the infection is persistent; >95% of the SIV proteome is naturally expressed; SIV particles are formed; and CD8+ T-cell responses are maintained indefinitely in an effector-differentiated state. Although the magnitude of anti-SIV immune responses in monkeys infected with rRRV-SIVcmv-nfl falls short of what is seen with live-attenuated SIV infection, further experimentation seems warranted.
Highlights
There are good reasons for believing that development of an effective preventive vaccine against human immunodeficiency virus (HIV)-1 is going to be a very difficult task [1,2,3]
Given the magnitude and impact of the HIV/AIDS pandemic, development of a safe, effective vaccine against HIV remains a top priority for biomedical research
The HIV envelope glycoprotein is shielded with a large amount of carbohydrate and the trimer spike as it exists of the surface of virions is difficult for antibodies to access and difficult for antibodies to block infectivity
Summary
There are good reasons for believing that development of an effective preventive vaccine against HIV-1 is going to be a very difficult task [1,2,3]. HIV-1 is highly variable from one individual to another and even within a single individual evolves to evade ongoing immune responses. The virus encodes a number of gene products that function at least in part to evade intrinsic, innate and adaptive immune responses. During the course of an infection, HIV-1 gradually destroys CD4+ T lymphocytes, a key orchestrator of adaptive immune responses. The inability of infection by one HIV-1 strain to routinely provide protection against superinfection by a different HIV-1 strain supports this perception of great difficulty in development of a protective vaccine [4]. Investigation of a variety of creative, non-standard approaches to a vaccine seem justified given this expected difficulty
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