Sand rat, Psammomys obesus
Sand rat, Psammomys obesus

Although a lot of viruses have ways of blocking recognition by T cells and NK cells, there’s not much known about the importance of these mechanisms in actual infections. That’s because the best-studied viruses in this class tend to be highly species-specific. So, for example, we don’t have good animal models for the human herpesviruses human cytomegalovirus, Epstein-Barr virus, varicella-zoster virus, or Kaposi’s sarcoma herpesvirus. Herpes simplex virus does infect mice, but its immune evasion molecule ICP47 doesn’t work well in mice, so we’re no further ahead.1

Immune evasion by all these viruses has been studied pretty extensively in cultured cells, but because they essentially only infect humans we only have circumstantial evidence for a role in vivo. Similarly, porcine, bovine, and equine herpesviruses encode immune evasion molecules, but pigs, cattle, and horses are not very convenient models for basic research either.Another major group of viruses, besides the herpesviruses, that are noted for immune evasion are the adenoviruses. However, only the human (and primate) adenoviruses contain the classical E3gp19k immune evasion molecule. There’s an animal model for human adenoviruses (the cotton rat) but as I pointed out the other day, there’s little evidence for an important function of CTL immune evasion in this model.

Virus Host Family Genome
Mouse CMV Mouse β NC_004065
Rat CMV Rat β NC_002512
Mouse thymic HV Mouse    
MHV68 Mouse γ NC_001826
Field mouse HV Microtus
Sand rat nuclear
inclusion agent
Sand rat    

So what we need are small animal, and preferably lab mouse, models for infection with adenoviruses or herpesviruses that include immune evasion molecules. As far as we know, mouse adenoviruses don’t have T cell or NK cell immune evasion properties. That leaves us with mouse herpesviruses. Of the hundreds of known herpesviruses, six are known to be murid-specific (see the table at right), and three of those infect lab mice. One of those is totally obscure (there’s very little known about mouse thymic herpesvirus), leaving us with mouse cytomegalovirus and mouse herpesvirus 68 (MHV68). I’ve already commented on immune evasion by mouse CMV. The bottom line is that removing all known T cell evasion molecules from MCMV makes almost no difference to infection or latency; the one difference is that the virus persists longer and at higher levels in salivary glands. That may be important in virus transmission, but lacks a little oomph.

MHV–68 exiting an infected cell on actin-dependent plasma membrane protrusions. EGFP–tagged ORF58 is green, gp150 red, co–localization yellow and nuclei blue (Mike Gill).That leaves MHV68, and I’m pleased to say that there is actually some evidence that T cell immune evasion is important for this guy. (I’ve mentioned this in passing earlier, but it deserves its own post.) MHV68 uses a gene “mK3” to attack MHC class I (MHC class I is recognized by cytotoxic T lymphocytes). 2 In 2002, Philip Stevenson and Stacey Efstathiou made a mutant of MHV68 lacking mK3,3 and tested its ability to infect mice:

Stevenson, P., May, J., Smith, X., Marques, S., Adler, H., Koszinowski, U., Simas, J., Efstathiou, S. (2002). K3-mediated evasion of CD8+ T cells aids amplification of a latent γ-herpesvirus. Nature Immunology DOI: 10.1038/ni818

MHV68 latency +/- mK3 (Stevenson et al, 2002)In cultured cells, where there’s no immune system, the mutant virus grows exactly as well as wild-type MHV68. As well (more surprisingly) there was no difference in the initial virus clearance; the mutant virus and the parent were both cleared from the lungs of infected mice at the same rate, and were undetectable after about 13 days. However (finally!) there was a big difference in the amount of latency. The figure to the right shows latent wild-type (left panel) and mutant (right panel) MHV68, the black dots, in spleens of mice infected 13 days previously. What’s more, during the latent phase there was a better immune response to mutant MHV68; mice infected with the mutant virus had about twice as many CTL specific for MHV68.

The association of higher virus-specific CTL frequencies with lower viral loads suggested that CTLs were responsible for the elimination of DeltaK3 viruses during latency amplification.

Eliminating CTL from the mice removed the difference; in the absense of CTL, the mK3 knockout virus established latency just as well as the wild-type. This shows that effects on CTL, and probably not some other unknown function of mK3, are responsible for the difference.

So with the two and a half authentic models of infection (I count the herpes simplex virus one as a half because it’s more contrived than a natural infection) we have immune evasion molecules helping to establish latency (MHV68), helping to reactivate from latency (herpes simplex) and helping with persistence (MCMV) . In no case is there much effect on acute infection.

  1. However, swapping in a different immune evasion molecule, that does work in mice, helps HSV reactivate from latency; see my previous post here. []
  2. mK3 is so named because it’s highly similar to a Kaposi’s sarcoma herpesvirus gene called “K3”; K3 is one of two KSHV genes that target MHC class I, but we don’t know much about KSHV infection. MHV68 probably isn’t a great model for KSHV, in spite of using a similar protein in immune evasion.[]
  3. This virus was still able to down-regulate MHC class I to some extent, though, so there may be still other immune evasion genes in MHV68[]