According to Henderson3
, only variola, vaccinia and monkeypox viruses are transmissible to humans, and only the variola viruses cause widespread, disseminated infections. Consequently, in recent times vaccinia has been enlisted as a vector in the creation of vaccines for other microbes, such as HIV. However, to generate vaccines that can be delivered safely to normal individuals, the one characteristic of vaccinia that made it such an effective vaccine for smallpox, i.e.
its ability to replicate in human cells, was crippled via genetic engineering. Thus, gene-deleted modified vaccinia strains unable to replicate have been developed, e.g.
Modified Vaccinia Ankara (MVA) and New York Vaccinia (NYVAC), as well as avian pox (canarypox and fowlpox) strains unable to replicate in mammalian cells are under development for use as vectors to deliver genes from other microbes. In this regard it is noteworthy that Louis Pasteur introduced the concept of attenuating virulent microbes to create live vaccines8,11
. Pasteur hypothesized that the live, replication-competent microbes depleted the host of vital trace nutrients, thereby rendering the host incapable of supporting the viability and reproduction of the virulent pathogenic organisms8,11
. Accordingly, Pasteur was totally naïve of how the immune system functions, by actively recognizing and engaging pathogenic microbes, as well as attenuated vaccines.
Pasteur was correct that live attenuated microbes made for the best vaccines, but he was right for the wrong reasons. Live, replication-competent vaccines are more effective than replication-incompetent vaccines, because these establish a self-limited local infection that efficiently activates all of the immune cells and molecules involved in immunity, both innate as well as adaptive immunity. Thus, a self-limited infection ensures microbial peptide presentation via molecules encoded by both class I and class II major histocompatibility complex (MHC) genes, thereby promoting the activation of both class I and class II restricted T cell receptors (TCRs). Also, a replication competent, attenuated microbe ensures the generation of a high enough concentration of microbial peptides, so that there will be a ‘strong’ TCR activation, enough to overcome negative feedback loops in place that attenuate a meaningful immune response12
. In essence, a successful adaptive immune response is one that leads to the production of antibodies, as well as all of the cytokines that promote maximal cellular immune responses, mediated by both cytolytic and helper T cells. Moreover, a replication-competent attenuated vaccine can be delivered in smaller doses than a replication-incompetent vaccine, an important point when a worldwide vaccination programme is under consideration and billions of people must be vaccinated.
Thus, Jenner's observation was that cowpox virus resulted in an attenuated local infection, and not a systemic infection. But why is this possible? The answer to this question is perhaps the most important in vaccinology today. With regard to smallpox, the hallmark of the disease is the 1-2 wk latent period followed by a generalized type IV delayed type hypersensitivity (DTH) reaction to a disseminated very high viral load. Apparently, the virus goes undetected by the first line of defense, i.e.
the innate immune system6,13
, so that when the adaptive immune system finally reacts the viral load is huge, so that the immunopathology is also huge. From the virus’ viewpoint, this sort of an infectious cycle ensures that enough progeny will be produced and disseminated that ensures sustained viability.
By comparison, vaccinia infection remains localized to the site of inoculation, so that this virus is not disseminated systemically. However, the local infection does result in viral replication, albeit limited to the local tissues. Vaccinia then, represents the prototype of vaccines. One needs to identify how vaccinia differs from variola, and logic dictates that variola has genes encoding molecules that circumvent innate immunity, thereby allowing rapid viral replication and dissemination before the adaptive immune response takes hold. By comparison, probably due to species differences of gene products, vaccinia cannot effectively circumvent innate immune responses, which makes for a locally contained infection, and not widespread dissemination.
The $64 question is whether vaccinia itself can be used as a vector to deliver the gene products of other organisms, instead of genetically modified vaccinia, which circumvents the capacity of the virus to replicate in human cells. Experience with the modified vaccinias as well as the avian poxviruses indicates that although these are safe, these are relatively weak immunogens14
. Because of the recent threat of bioterrorism and the news that the Soviets had a large bioterrorist research and development programme focused on variola, the U.S. government contracted to stockpile >200 million doses of vaccinia smallpox vaccine15,16
. The previously licensed smallpox vaccine in the U.S., Dryvax® (Wyeth Laboratories Inc.) had a questionable safety profile, because it consisted of a pool of vaccinia virus strains with varying degrees of virulence. Thus, six individual clones of viruses were isolated by plaque purification in tissue culture from a pool of 30 vials (3,000 doses) of Dryvax®, NYCBH, and were tested for immugenicity and virulence by comparison with Dryvax®. A test for immunogenicity included the diameter of erythema and lesions on day eight after scarification of rabbit skin. The rabbit scarification model mimics the vaccine “take
” observed following human vaccination with Dryvax® because of local replication of vaccinia, which activates a typical adaptive DTH cellular immune response4,5
. The virulence test measured survival time and viral replication in brain tissue after intracerebral injection of suckling mice. One of the six clones selected for further testing was found to be less virulent than Dryvax® and just as immunogenic. A pilot lot of 750,000 doses of the vaccinia clone propagated on a human lung fibroblast cell line was prepared and tested in 100 volunteers compared with 30 subjects who received Dryvax®. The tissue culture derived clone elicited a 100 per cent take-rate, 100 per cent seroconversion, and a comparable T cell response, monitored via cytotoxic T lymphocyte (CTL), proliferation, and γ-interferon (IFNγ) ELISPOT16
. Of note, further testing of this clone showed that a full-length copy of the IFN-αβ receptor and the TNFα receptor were both absent whereas these ‘virulence factors’ were intact in other isolates16
To enhance preparedness in the event of a possible variola terrorist attack, a 2002 Presidential initiative recommended vaccination of enlisted military personnel and civilan health care workers who might become first responders. By June 2004, 39,566 civilians and by September 2006, > 1 million soldiers were vaccinated16
. The appearance of adverse events (AE) was carefully monitored. Cases of progressive vaccinia, eczema vaccinatum, and foetal vaccinia were completely avoided by careful screening of potential vaccinees. One serious AE noted in Dryvax® vaccinations was a myopericarditis that appeared within 1-2 wk post vaccination. When carefully monitored with EKG and cardiac enzyme tests, the cloned vaccine gave an incidence of 0.5 per cent (seven cases in 1307 subjects), compared with 0.8 per cent of individuals who received Dryvax® (three cases in 363 subjects), a difference not statistically significant16
. However, most of these cases were subclinical or asymptomatic. Moreover, no cases of myopericarditis occurred among 1819 vaccinia-experienced subjects vaccinated with either vaccine16
Accordingly, given these data, which represent an extensive testing of the safety and efficacy of a vaccine in humans, it can be concluded that this strain of plaque-purified vaccinia could serve as a safe and effective vector for many other dangerous microbes, such as HIV. The regulatory authorities, especially of those countries where HIV is endemic at a high prevalence, should be advised to carefully consider the risk/benefit ratio of replication-competent vaccinia as a vector to reduce and perhaps eradicate HIV and other intractable infectious diseases.