How not to be seenI was getting a little concerned and distressed by the lack of evidence for any function of viral MHC class I immune evasion. It’s kind of a relief to see articles demonstrating function coming out.

MHC class I is the target for cytotoxic T lymphocytes (CTL), which are generally believed to be pretty important in controlling viral infection. So when some viruses were shown to block MHC class I in cultured cells, it seemed pretty obvious that this would be a big benefit for the virus. You’d expect these viruses to be exceptionally resistant to CTL, for example.

But when people actually looked in animals (as opposed to in tissue culture), the ability to block MHC class I didn’t seem to do all that much. I’ve summarized some of those experiments here and here. For example, the MHC class I immune evasion genes in adenoviruses and in mouse cytomegalovirus (MCMV) didn’t show much effect on the actual infection at all.1 Mouse herpesvirus 68 (MHV68) had shown an effect, but not at the time point that you might expect — not early after infection, when CTL are kicking in and clearing virus, but rather later on, during the latent phase.2

We all believed there must be a function, because viruses don’t hang on to genes for millions of years unless those genes are important,3 but I was starting to wonder if perhaps we were looking in the wrong places — whether any immune effects might be spillover from some other function, say. But, as I say, we’re starting to get confirmation that these things really are doing more or less what we’d expected all along.

A little while ago, Klaus Fruh and Louise Pickert showed a significant effect of MHC class I immune evasion in rhesus cytomegalovirus: without that ability new viruses couldn’t superinfect hosts that already carry the virus. 4 (I talked about it here.) It’s quite possible — though of course not certain until it’s actually tested — that this is also true for human cytomegaloviruses (which are very closely related to the rhesus version) and for mouse CMV (which are less closely related but in the same family). So now we have functional data for MHC class I immune evasion for representatives of two broad groups of viruses, the betaherpesviruses (the cytomegaloviruses) and the gammaherpesviruses (the MHV68 story).

Now there’s another paper5 showing a function for the MHC class I immune evasion ability of HIV (actually for SIV, but again it’s probably true for the closely-related HIV).

HIV has a gene, nef, that can block MHC class I expression. This has been shown in cultured cells, but understanding its relevance in actual infections has been difficult:

Although these data suggest that Nef-mediated immune evasion could play an important role in AIDS pathogenesis, there has been little direct evidence linking disease progression with MHC-I downregulation in vivo. 5

Obviously you can’t make a nef-less HIV and just throw it into people to see what happens. Even doing the experiment in monkeys with SIV is complicated by the fact that nef is very polyfunctional — as well as downregulating MHC class I, it also targets a number of other molecules.

But you can take advantage of natural variation, both in the virus and the host.  Nef isn’t equally effective on all MHC class I types, for one thing. As well, nef can develop mutations within the host.  It turns out that rapid disease progression correlates with the extent of MHC class I downregulation, whereas effects on other genes affected by nef (CD3 and CD4) didn’t correlate:

The extent of MHC-I downregulation on SIV-infected cells varied among animals …  the level of MHC-I downregulation on SIV-infected cells was significantly greater in the rapid progressor animals than in normal progressors.  … high levels of MHC-I downregulation on SIV-infected cells are associated with uncontrolled virus replication and a lack of strong SIV-specific immune responses.5

This is strictly a correlation study, so we can’t confidently say that MHC downregulation causes disease progression. Still, it’s an interesting finding, and perhaps one that can be followed up in human studies.

  1. Gold MC, Munks MW, Wagner M, McMahon CW, Kelly A, Kavanagh DG, Slifka MK, Koszinowski UH, Raulet DH, & Hill AB (2004). Murine cytomegalovirus interference with antigen presentation has little effect on the size or the effector memory phenotype of the CD8 T cell response. Journal of immunology (Baltimore, Md. : 1950), 172 (11), 6944-53 PMID: 15153514
    Only slightly qualified by
    Lu, X., Pinto, A., Kelly, A., Cho, K., & Hill, A. (2006). Murine Cytomegalovirus Interference with Antigen Presentation Contributes to the Inability of CD8 T Cells To Control Virus in the Salivary Gland Journal of Virology, 80 (8), 4200-4202 DOI: 10.1128/JVI.80.8.4200-4202.2006[]
  2. 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 ?-herpesvirus Nature Immunology DOI: 10.1038/ni818[]
  3. I will admit there’s a certain circular quality to this argument.  “The gene must be important, because viruses don’t carry unimportant genes.  We know that, because this gene that they’ve hung on to must be important.”[]
  4. Hansen, S., Powers, C., Richards, R., Ventura, A., Ford, J., Siess, D., Axthelm, M., Nelson, J., Jarvis, M., Picker, L., & Fruh, K. (2010). Evasion of CD8+ T Cells Is Critical for Superinfection by Cytomegalovirus Science, 328 (5974), 102-106 DOI: 10.1126/science.1185350[]
  5. Friedrich, T., Piaskowski, S., Leon, E., Furlott, J., Maness, N., Weisgrau, K., Mac Nair, C., Weiler, A., Loffredo, J., Reynolds, M., Williams, K., Klimentidis, Y., Wilson, N., Allison, D., & Rakasz, E. (2010). High Viremia Is Associated with High Levels of In Vivo Major Histocompatibility Complex Class I Downregulation in Rhesus Macaques Infected with Simian Immunodeficiency Virus SIVmac239 Journal of Virology, 84 (10), 5443-5447 DOI: 10.1128/JVI.02452-09[][][]