028 - Studies on Pneumococcus Immunity III. the Nature of Pneumococcus Antigen

Well, I've already discussed pneumococcal vaccines a decent amount (017, 018, 019, 021, 023, 024, 026), but none of the vaccines in these studies really resemble the ones used against pneumococcus today, which, instead of using whole killed bacterial cells, use only a component (polysaccharide) of the bacterial capsule as an antigen. So the study today is one done in the early 1920s where people began to discover that it was not necessary or helpful to use whole cells in vaccines.

The purpose was to investigate the chemical nature of the antigen that provided the main protection against infection with pneumococcus bacteria. If people could purify and use just the important component, the vaccines' toxicity would be reduced and the effectiveness increased with lower doses.

So the authors studied the effectiveness of vaccines made by different components of pneumococcus cells, testing their ability to protect white mice against live pneumococcus infection. Unvaccinated control mice all died of infection within 3 days, so test mice were considered "survivors" if they lasted at least 5 days.

Whole-cell killed vaccines protected most mice against infection. When the researchers used different methods to isolate proteins (and "nucleoproteins" containing phosphorus, adenine, and guanine; in other words, nucleic acids) and whatever was attached to them, and made vaccines out of those, they worked decently well also. So it must be protein (or nucleic acid) or something attached to it that induced the immunity.

Following the lead of other researchers, they tried digesting the proteins with proteolytic enzymes, and then seeing if the resulting solutions still worked as vaccines. Depending on the method used, these vaccines were decently effective, especially when the protein was degraded more thoroughly.

Finally, they separated the components in a degraded protein solution that could dissolve in alcohol from those that came out of solution. The latter were very bad as vaccines, but the former were just about as good as whole cells. It protected 2 out of 3 mice against a dose of pathogens that was several million times larger than the minimum lethal dose. So they had gotten as far as they could in isolating the important antigen component, and as far as they could tell, it contained very little protein.

They did some further tests, determining that this antigen could survive well in heated acid but not heated base, and that it could last at least several months when refrigerated, even after being heated pretty hot for an hour. And while the initial experiments were all done with pneumococcus type I, the same type of antigen was present in types II and III. And it was not toxic to mice, as far as they could tell.

So that's pretty interesting as a step forward in pneumococcus vaccine development. The numbers of animals were pretty small for each experiment, and it was done in mice, not humans, but it's important as preliminary data.

The papers that cite this study generally agree that the antigen these authors probably isolated was polysaccharide in nature. As Felton et al. say:

"Although not then identified, from the description of their preparation the product undoubtedly contained the capsular polysaccharide."

Citation: Perlzweig, W. A. & Steffen, G. I. Studies on Pneumococcus Immunity III. the Nature of Pneumococcus Antigen. J Exp Med 38, 163–182 (1923).