"For several decades the Mycobacterium-bovis-derived bacillus Calmette-Guerin (BCG) has been the only widely used vaccie for tuberculosis and accumulating data from clinical trials and subsequent meta-analysis have tended to reveal its general ineffectiveness in adults," say the researchers.

"The mechanism underlying the gradual loss of effectiveness of BCG as the individual reaches 10 to 15 years of age is poorly understood."

The researchers believe that one possible reason for declining effectiveness is that the immune system's memory slowly declines over time. One way to reverse this effect, say the researchers, is to reintroduce the immune system to tuberculosis by exposing it to a harmless protein from the bacterium.

They tested one such protein (Ag85A) on mice who had previously been vaccinated with BCG. Those mice that received the Ag85A booster before their immunity began to decline retained their ability to fight tuberculosis infection better than the mice that did not or that received a mix of tuberculosis proteins instead.

"While there is a desperate need to develop new tuberculosis vaccines to deal with the global emergency in general, in more developed countries tuberculosis continues to be relatively common in elderly people," say the researchers. "While primary tuberculosis sometimes occurs in these individuals, the majority of cases are thought to be due to reactivation of latent disease acquired decades earlier. In view of this, the vaccine strategy espoused here may also have applications to prevention of reactivation tuberculosis in the elderly."

(Reference: J.V. Brooks, A.A. Frank, M.A. Keen, J.T. Bellisle and I.M. Orme. 2001. Boosting vaccine for tuberculosis. Infection and Immunity, 69: 2714-2717.)

17-Apr-2001