TLR3 ectodomainA week after I talked about toll-like receptors (TLRs) and their role in signaling “danger”, here’s another beautiful real-world example.

TLRs, remember, are pattern-recognition molecules that identify “pathogen-associated molecular patterns” (PAMP). There are a dozen or so of them in humans and mice, and they recognize things like bacterial cell walls, viral DNA, and double-stranded RNA (which is associated with a lot of viral infections). The downstream effects that TLRs trigger after they recognize a PAMP are various, but typically do things like activating dendritic cells and inducing interferon release.

Herpes simplex viruses, like most herpesviruses, are ancient and immensely successful viruses. They’re very common infections (HSV type 1 infects something like 60-80% of North Americans; HSV-2, around 25%), rarely cause any diseases at all, and most of the diseases they are associated with, are fairly mild. But occasionally, HSVs do cause serious disease, most dramatically herpes simplex encephalitis (HSE). HSE isn’t common, but it has a 70% mortality rate, and the survivors often have long-term neurologic problems. Why do some two out of a million people have such major problems, while the vast majority breeze along without even knowing they’re infected?

I think it’s fair to say that we don’t know much about immunity to HSV. We know that cytotoxic T lymphocytes are abundant and may help prevent recurrence, but don’t eliminate the infection. (HSVs do have an immune evasion molecule, ICP47,1 that purportedly provides resistance to CTL, but it’s not clear how important that is in actual infection — as is true of most viral anti-CTL immune evasion molecules.) Natural killer cells are probably involved in resistance, but we don’t have a detailed understanding of that either. For that matter, only a handful of humans lacking NK function have been found, and while a couple of them have been susceptible to herpesvirus infections2 it’s been varicella, rather than herpes simplex, that was the major problem.3

One reason we don’t fully understand immunity to HSV is that, like most herpesviruses, HSVs are very host-specific. Although they will infect mice, the course of infection there is probably quite different than in humans, and we don’t see many humans lining up for the chance to be infected with mutant herpes simplex viruses. On the other hand, without wanting to be too vulture-ish about it, one advantage of a human-specific virus is that humans are a huge population that is naturally riddled with mutations, and those mutations that are associated with disease tend to get examined carefully. An example of one such mutation is in the 14 September issue of Science.4

Herpes simplex virus-infected cellIn this paper, they looked at two children who had herpes simplex encephalitis. Both children, though they are unrelated, turned out to have the same point mutation in their TLR3. TLR3 is the TLR that recognizes double-stranded RNA. Herpes simplex virus produces dsRNA during infection, and although other PAMP receptors also recognize dsRNA, apparently only TLR3 recognizes the form that HSV produces, because it seems that TLR3 is essential for protection against encephalitis caused by herpes simplex.5 It’s not a universal effect: TLR3 does not seem to be important for recognizing several other viruses, including some with double-stranded RNA genomes,6 but is important in protecting against another herpesvirus, murine cytomegalovirus.7

As a side note, TLR3 was the first (and I believe only) TLR that’s been crystallized,8 so we know a bit about its structure. (I made the figure at top, using YASARA, from the Choe et al structure.) The mutation is in a region proposed to be associated with dsRNA binding and with receptor dimerization. The mutation here turns out to be dominant-negative (heterozygotes are unresponsive to dsRNA through TLR3), and this (at least to me) hints that perhaps dimerization is intact but binding is screwed up.

This paper builds on a previous study from the same group that found that HSE was associated with a mutation in UNC-93B,9 a molecule involved in signalling from TLR7, TLR8, TLR9, and, yes, TLR3. However, not everyone with these mutations gets HSE:

Interestingly, five of the seven TLR3-deficient individuals and one of the three UNC-93B-deficient individuals did not develop HSE after HSV-1 infection. The incomplete clinical penetrance of TLR3 and UNC-93B deficiency is consistent with the typically sporadic, as opposed to familial, occurrence of HSE. Multiple factors may affect clinical penetrance, including age at infection with HSV-1, the viral inoculum, and human modifier genes.

It’s not spelled out in the paper if the HSE patients here were selected for further study for some specific reason, or if the authors had methodically tested every case of HSE in France, or if in fact this mutation is a common cause of HSE, so that most patients with HSE would have something wrong with TLR3 or its signalling pathway. If I’m reading that quote right, though, the authors are at least implying that these mutations are the major underlying cause of herpes simplex encephalitis. If that’s the case (and even if it’s not), I think it’s a remarkable link between innate immunity and a real-world disease.

  1. York, I. A., Roop, C., Andrews, D. W., Riddell, S. R., Graham, F. L., and Johnson, D. C. (1994). A cytosolic herpes simplex virus protein inhibits antigen presentation to CD8+ T lymphocytes. Cell 77, 525-535.[]
  2. Biron, C. A., Byron, K. S., and Sullivan, J. L. (1989). Severe herpesvirus infections in an adolescent without natural killer cells. N. Eng. J. Med. 320, 1731-1735. and A. Etzioni, C. Eidenschenk, R. Katz, R. Beck, J.L. Casanova and S. Pollack. (2005) Fatal varicella associated with selective natural killer cell deficiency, J Pediatr 146, 423-425.[]
  3. And Dr Biron has told me that she knows of at least one NK-deficient person who hasn’t been written up because she, or he, is living perfectly normal, herpes-free life.[]
  4. Zhang, S.-Y., Jouanguy, E., Ugolini, S., Smahi, A., Elain, G., Romero, P., Segal, D., Sancho-Shimizu, V., Lorenzo, L., Puel, A., Picard, C., Chapgier, A., Plancoulaine, S., Titeux, M., Cognet, C., von, B., Horst, Ku, C.-L., Casrouge, A., Zhang, X.-X., Barreiro, L., Leonard, J., Hamilton, C., Lebon, P., Heron, B., Vallee, L., Quintana-Murci, L., Hovnanian, A., Rozenberg, F., Vivier, E., Geissmann, F., Tardieu, M., Abel, L., and Casanova, J.-L. (2007). TLR3 Deficiency in Patients with Herpes Simplex Encephalitis. Science 317, 1522-1527. []
  5. Though, perhaps, not for other aspects of HSV infection?[]
  6. This paper, and also Edelmann, K. H. (2004). Does Toll-like receptor 3 play a biological role in virus infections? Virology 322, 231-238. []
  7. Tabeta, K., Georgel, P., Janssen, E., Du, X., Hoebe, K., Crozat, K., Mudd, S., Shamel, L., Sovath, S., Goode, J., Alexopoulou, L., Flavell, R. A., and Beutler, B. (2004). Toll-like receptors 9 and 3 as essential components of innate immune defense against mouse cytomegalovirus infection. Proc Natl Acad Sci U S A 101, 3516-3521.[]
  8. Choe, J., Kelker, M. S., and Wilson, I. A. (2005). Crystal Structure of Human Toll-Like Receptor 3 (TLR3) Ectodomain. Science 309, 581-585. and Bell, J. K., Askins, J., Hall, P. R., Davies, D. R., and Segal, D. M. (2006). The dsRNA binding site of human Toll-like receptor 3. Proc Natl Acad Sci U S A 103, 8792-8797. []
  9. Casrouge, A., Zhang, S.-Y., Eidenschenk, C., Jouanguy, E., Puel, A., Yang, K., Alcais, A., Picard, C., Mahfoufi, N., Nicolas, N., Lorenzo, L., Plancoulaine, S., Senechal, B., Geissmann, F., Tabeta, K., Hoebe, K., Du, X., Miller, R. L., Heron, B., Mignot, C., de, V., Thierry Billette, Lebon, P., Dulac, O., Rozenberg, F., Beutler, B., Tardieu, M., Abel, L., and Casanova, J.-L. (2006). Herpes Simplex Virus Encephalitis in Human UNC-93B Deficiency. Science 314, 308-312. []