My first foray into research, when I started grad school, was to work on a vaccine against bovine adenovirus type 3.1 From a technical viewpoint, it was a great introduction to research; I got to do classical virology, animal work, tissue culture, protein purification, all kinds of immunology, and so on. Still, I ended up unenthusiastic about vaccinology (though not about vaccines, which I regard as one of the most important benefits to civilization in history). The problem was that so much of vaccine research seemed purely empirical, with no solid theory underlying it. The way to design a new vaccine was just to try a whole bunch of different things (sometimes applying rules of thumb) and see what worked.
To some extent that’s still true — there’s a great deal we still don’t understand about the immune system, and predicting how to drive a safe, protective response still has a lot of guesswork to it. 2 At the time, though, the most frustrating aspect of the field for me was adjuvants.
Adjuvants are factors that you include with your antigen, in order to drive a potent immune response. In general, the more pure an antigen is, the worse the immune response to it. Adjuvants allow you to provide a clean, defined antigen, without losing the immunogenicity of the filthy natural antigen.
The problem was that no one knew how adjuvants worked. They just … worked. There were a myriad of choices (for animals; in the US and Canada there’s only one adjuvant, alum, that’s licensed for humans), and they all mostly worked, and sometimes one worked better and sometimes another worked better, or differently; but there was no understanding of how, or why. Sometimes toe of newt was the best choice, and sometimes you were better off with eye of toad, and it depended on the phase of the moon and on which malign vapours were influencing your system.
Had I but known, of course, just about the time I switched out of vaccines and into other aspects of viral immunity, Charlie Janeway was offering up a grand unified theory of adjuvants3 which has for the most part proven triumphantly true. (I talked about it here.) He suggested that the immune system normally initiates a response when it recognizes conserved features of pathogens, and that adjuvants work because they mimic these conserved pathogen-associated molecular patterns. (Polly Matzinger also proposed a related model, in which immune responses start because cells are damaged — the danger hypothesis.) Since then, many of the pathogen-associated patterns have been identified, and many of the pattern receptors have been identified; adjuvants are no longer magic, they’re science.
Well, all except for one: Alum, the most important one of all (because it’s the main adjuvant used for human vaccines). We had no real idea how that works, because it looks nothing like any plausible pathogen pattern.
A paper in J Exp Med4 now argues that alum’s adjuvant activity comes from uric acid. As it happens, this is less related to Janeway’s hypothesis, and is closer to Matzinger’s. Uric acid is released by dying or damaged cells, and is a powerful natural adjuvant5 — it’s an indicator to the immune system that cells are being damaged in the vicinity, meaning that there is “danger” nearby.
I’m not entirely convinced that this is the whole, or even the main, story. The implication is that alum acts by damaging cells. The authors say that “alum has been shown to induce a considerable degree of necrosis”. That may be true with the intraperitoneal injection model they used, but alum-adjuvanted vaccines in people are more often given intramuscularly, and I don’t know that alum is all that nasty in that context.6 After all, the reason alum is approved for use in humans is that it is so innocuous. And simple experience says that while vaccines sting, you don’t expect any kind of large-scale necrosis in your injected arm afterward — no more than you’d get from a modest bruise, which isn’t enough to trigger the kind of adjuvant effects we see with alum. Perhaps there is a small release of uric acid effect, and alum somehow amplifies the effect (perhaps by facilitating uric acid crystallization, which is essential for its adjuvant activity). Or perhaps uricase is important in intraperitoneal inject, but is less so in more clinically-relevant injections. I don’t know.
Still, it’s nice to see that adjuvant activities, nowadays, can actually be tested within well-defined theoretical contexts. That’s just a huge advance since the days when I had to play with them.
Update: Since I wrote this article there have been several more papers on alum’s mechanism of action. As I guessed here, alum can in fact work in other ways, and though the mechanism described here may be part of its effect it seems very likely that alum mainly acts in different ways. For updates see these posts:
Alum, take 2: A better answer
Silicosis parallels alum
A general rule for (some) adjuvants
- York, I., and Thorsen, J. (1992). Evaluation of a subunit vaccine for bovine adenovirus type 3. American journal of veterinary research 53, 180-183.[↩]
- Partly, of course, because most of the easy targets for vaccines, where we could predict protective responses, are already out there, and now we’re working on the hard cases like malaria and HIV.[↩]
- Approaching the asymptote? Evolution and revolution in immunology. Janeway, C.A.Jr.. Cold Spring Harb. Symp. Quant. Biol. 54, 1-13 (1989) [↩]
- Kool, M., Soullie, T., van Nimwegen, M., Willart, M.A., Muskens, F., Jung, S., Hoogsteden, H.C., Hammad, H., Lambrecht, B.N. (2008). Alum adjuvant boosts adaptive immunity by inducing uric acid and activating inflammatory dendritic cells. Journal of Experimental Medicine DOI: 10.1084/jem.20071087[↩]
- Shi, Y., Evans, J. E., and Rock, K. L. (2003). Molecular identification of a danger signal that alerts the immune system to dying cells. Nature 425, 516-521.[↩]
- A quick and superficial scan through PubMed doesn’t turn up much support for the statement either , for what that’s worth.[↩]