Even though cytotoxic T lymphocytes are called “cytotoxic”, it was no surprise when new techniques in the mid-90s suggested that CTL might have other strings to their bows. (I talked about this the other day.) Some experiments were also pointing the same way. For example, Frank Chisari’s group offered evidence that CTL might be able to control hepatitis B virus without actually being cytotoxic.
Hepatitis B virus was a particularly difficult case for study in general, because it’s pretty much a human-specific virus. (HBV is a pain in tissue culture. There are a couple of animal models, but they’re hardly convenient. I mean, ducks? Woodchucks?) From studies of the natural disease, it seemed pretty clear that natural HBV infection was often handled by CTL; people who control HBV show a strong CTL response, those in whom HBV progresses and becomes chronic usually do not. But testing the function of CTL in HBV infection wasn’t easy without an animal model.
So Chisari made an animal model. He made transgenic mice that express the HBV genome (that is, the mouse genome contained the HBV genome) under a liver-specific promoter.2 There you go, hepatitis B virus in the mouse liver. It can’t spread and infect new cells as would normally happen in humans, but on the other hand you’re expressing the virus in essentially all the liver cells anyway, so you don’t need to infect new cells.
Of course, the “virus” is “self” under those conditions, meaning that the immune system doesn’t react to it. So Chisari’s group immunized other mice to raise HBV-specific CTL, and transferred the CTL into the transgenic mice.3 These HBV-specific CTL did two things: They apparently controlled the virus “infection”, and they damaged the liver.4 The presumption was that the two findings were the same effect, and the CTL were trying to eliminate virus by killing liver cells: “ Our data show that antigen-specific immune effector mechanisms can destroy HBV-positive hepatocytes in vitro and in vivo … “.
This way a few CTL can affect many target cells. There are some 1011 hepatocytes, and hepatitis viruses can infect a lot of them, very fast. There are different estimates for how long it takes for a CTL to deliver a lethal hit when it’s being cytotoxic, but let’s say something like 30 – 60 minutes per cell. That’s a long, long time for CTL to kill off all the infected cells. If all they have to do is release interferon, they can probably pick off many cells at once and move on. This is a faster way to control viruses, and it doesn’t take as many CTL. Guidotti et al transferred 5 x 106 HBV-specific CTL into the transgenic mice; within 24 hours, all of the virus had been shut down (remember. these are transgenic mice expressing “virus” in every liver cell).
Again: There are some 1011 hepatocytes, and hepatitis viruses can infect a lot of them, very fast. If CTL have to kill all the virus-infected cells, what’s left of the liver to do it’s usual liverly duties? If the infection can be shut down without killing the cells, you’ve saved your liver. Those 5 x 106 CTL shut down the virus without killing more than 10% of the liver cells.
So the finding that CTL are capable of shutting down virus replication without having to kill the infected cells, fit very nicely with the new technology showing that CTL commonly have these mechanisms available.
Guidotti, L. G., Ando, K., Hobbs, M. V., Ishikawa, T., Runkel, L., Schreiber, R. D., and Chisari, F. V. (1994). Cytotoxic T lymphocytes inhibit hepatitis B virus gene expression by a noncytolytic mechanism in transgenic mice. Proc Natl Acad Sci U S A 91, 3764-3768.