Temetomo repelling smallpox demon
Temetomo repelling the demon of smallpox
(Utagawa Yoshikazu, ca. 1847 – 1852)

I talk a lot on this blog about viral immune evasion. I’m most interested in the ways by which, and the evolutionary reasons for, viral evasion of T cell recognition; but there are lots of other branches of the immune response, and viruses have ways of evading most of those branches. I’m even prepared to say that, all in all, evasion of T cell immunity is probably relatively a minor component of pathogenesis for most viruses.

For one thing, most viruses don’t even bother to evade T cell recognition, as far as we know. Although a handful of viruses, like some of the human adenoviruses, apparently use it, only one virus family — the herpesviruses — seem to have evolved T cell immune evasion as a common, highly conserved function. And to the extent that it’s been examined, which isn’t very far, T cell evasion doesn’t make a huge difference to the virulence of the virus, or otherwise have massive effects on the virus’s ability to replicate. That’s not to say there’s nothing out there1 but all in all, it’s not particularly impressive.

On the other hand, as I’ve pointed out before, there are forms of immune evasion that do have a very large effect on the virus’s virulence. The example I mentioned before was influenza virus. Different strains of influenza virus are more or less able to prevent innate immune recognition; swapping efficient immune evasion into a relatively mild influenza virus turns it into a much more severe pathogen.

Testimonial to smallpox vaccination
“Expositions on the Inoculation of the Small Pox and of the Cow Pock”
 By John Coakley Lettsom (1806)

So is this something we can take advantage of? If there’s increased immune recognition of a virus, then perhaps the virus would make a better vaccine; especially if it’s less virulent at the same time. Frank Ennis’s lab2 has recently demonstrated that this might be a useful approach to vaccine development.

In this case, they used vaccinia virus. Vaccinia, of course, is the virus that was used as a vaccine against smallpox virus. With bioterrorism in the spotlight over the past few years, there’s renewed interest in vaccines against smallpox. Although vaccinia worked very well to eliminate smallpox, it’s not particularly safe; the risk of adverse effects is far higher than is tolerated in most vaccines these days. It was worth the risk to eliminate smallpox, but it’s not so clear that the tradeoff is worth it for the purely hypothetical chance of bioterrorism. Accordingly, there’s a lot of interest in developing safer, yet still highly effective, vaccinia-based vaccines. The Ennis lab approached this by knocking out an immune evasion gene in the virus, and has tested their new strain for safety and immunogenicity.

Safer virus, equal immunity

Note that avirulence and immunogenicity, in many cases, have to be balanced against each other. Immune responses tend to be stronger for more dangerous pathogens, because the inflammation associated with cell death and large numbers of viruses stimulates more cytokines, and triggers a more potent immune response. Weakening the virus so that it can’t replicate as well drops the danger, but also drops the stimulation to the immune system. The potential advantage of knocking out immune evasion genes is that any given amount of virus may be more effective at inducing an immune response, because the usual dampening effect of immune evasion is gone.

That’s precisely what Mathew et al found. The knockout virus lacking an immune evasion gene “N1L” is less virulent — it causes less disease, and replicated to much lower levels. This was already known from previous experiments by Geoffrey Smith’s group,3 and, long ago, by Bernie Moss’s lab4 but here they tested the virus in a more biologically meaningful way, by intranasal infection (a better match with the way smallpox naturally infects) and measuring replication in the lungs. The knockout virus only reached 1/10,000 the level of infection as did wild-type vaccinia. (Irritatingly, they don’t specifically compare symptoms, such as weight loss, between the wild type and knockout virus; but at least at one dose the knockout virus didn’t kill mice while the wild-type virus did.)

This was not a particular surprise, but the interesting part was the immune response. Even though there was 10,000 times less virus present, the T cell response to the knockout virus was equivalent to that against the wild-type virus:

Our data indicate that the attenuated vGK5 virus is immunogenic and elicits robust immune responses that are comparable to the wildtype VACV-WR when administered by multiple routes.2

It’s worth repeating that this immune evasion molecule does not specifically affect the T cell response per se. It affects recognition upstream of the T cell response, at a very early point after infection. By reducing this innate immune recognition, the N1L gene reduces overall inflammation. Presumably, the increased inflammation associated with the knockout virus counteracted the reduced numbers of virus, and allowed an equal immune response to a less virulent virus.

A universal solution?

Is this a universal approach that will work with any virus that can evade the innate immune response? It’s certainly worth looking at, but it’s also worth keeping in mind that increasing inflammation is a two-edged sword. Inflammation itself is hazardous, and especially in the lungs inflammation is often the actual killer following viral infection. There’s a well-known study in adenoviruses that concluded that eliminating viral immune evasion increased the virulence of the virus, and this increase in virulence was associated with increased lung inflammation. Although this conclusion is, to my mind, now a very dubious one for reasons I outline here, the principle that inflammation can be dangerous holds. I suspect that using these sorts of immune evasion knockout viruses as vaccine platforms will have to be tested on a case by case basis.

  1. For example: Stevenson PG, May JS, Smith XG, Marques S, Adler H, Koszinowski UH, Simas JP, Efstathiou S (2002) K3-mediated evasion of CD8(+) T cells aids amplification of a latent gamma-herpesvirus. Nat Immunol 3:733–740.[]
  2. Anuja Mathew, Joel O’Bryan, William Marshall, Girish J. Kotwal, Masanori Terajima, Sharone Green, Alan L. Rothman, Francis A. Ennis, Linqi Zhang (2008). Robust Intrapulmonary CD8 T Cell Responses and Protection with an Attenuated N1L Deleted Vaccinia Virus PLoS ONE, 3 (10) DOI: 10.1371/journal.pone.0003323[][]
  3. Bartlett N, Symons JA, Tscharke DC, Smith GL (2002) The vaccinia virus N1L protein is an intracellular homodimer that promotes virulence. J Gen Virol 83: 1965-1976.[]
  4. Kotwal GJ, Hugin AW, Moss B (1989) Mapping and insertional mutagenesis of a vaccinia virus gene encoding a 13,800-Da secreted protein. Virology 171: 579-587.[]