Bordetella pertussis and Mycobacterium tuberculosis, routinely used to promote the development of autoimmune disease, were recently reported to also be effective in inducing protection against an autoimmune disease. Thus, we previously demonstrated that SJL/J and (SJL/J x BALB/c)F₁ mice that are genetically susceptible to experimental autoimmune encephalomyelitis (EAE) become highly refractory to the induction of the disease following their exposure to B. pertussis and M. tuberculosis. In the present study, the pertussis toxin (PT) from B. pertussis and the purified protein derivative (PPD) of M. tuberculosis, were found to be sufficient to fully protect against EAE and thus may be the major bacterial components responsible for conferring protection. The 65‐kDa heat‐shock protein played only a marginal role in the protection against EAE induced by these bacteria. Both PT and PPD were protective when given before, but not after, the encephalitogenic challenge, and minute amounts (5–50 ng) emulsified in oil were sufficient to confer long‐lasting resistance to EAE. The effect of PT or PPD on EAE differed from that of mitogens or bacterial superantigens, suggesting that their protection ability was not attributable merely to mitogenic or superantigenic properties.

The mechanism of protection is not yet clear. Preliminary studies revealed a complex mechanism of protection whereby PPD and PT may operate differently. Thus, only PPD‐induced, but not PT‐induced, protection was transferrable by CD4⁺ T lymphocytes bearing an αγ Tcell antigen receptor. Neither PT nor PPD had a protective effect on EAE mediated by preformed pathogenic T lymphocytes and it is most likely that they exert their protection by affecting the development of such T lymphocytes. How bacteria such as B. pertussis and M. tuberculosis can either enhance the development of an autoimmune disease or protect against the disease is not yet clear. However, identifying PT and PPD as the bacterial components active in protection may allow a better understanding of the modulatory effects of bacteria and point to the potential use of such bacterial products in immunomodulation of autoimmune diseases.