We know that, during infection, treponemes are cleared from lesions following development of a Th1 response and opsonophagocytic killing of the bacteria. A number of studies have demonstrated that passive administration of very large quantities of antiserum from chancre-immune rabbits are able to delay lesion development in response to
infectious challenge, but are not sufficient to prevent it , suggesting that antibodies alone cannot eradicate infection. Adoptive transfer of T cells (in inbred hamster [using T. pallidum subsp. endemicum] and guinea pig studies) yielded only transient and incomplete resistance to infection . Our vaccine studies over the past 15 years have led us to conclude that protection GW786034 nmr from initial infection in rabbits is dependent upon both induction of a Th1 response TGF-beta inhibitor in which T cells infiltrate and produce IFN-γ (appearing as
a delayed-type hypersensitivity response) and development of opsonic antibodies. We therefore used an adjuvant with components most likely to induce a Th1 response and functional antibody: the Ribi adjuvant containing monophosphoryl lipid A, trehalose dicorynomycolate, and cell wall skeleton. Immunization using this adjuvant with a number of recombinant peptides induced significant protection against infection, as measured by reduction in development of lesions with either demonstrable T. pallidum and reduction in proportion of lesions that progress to ulceration , , ,  and . Unfortunately, the adjuvant used in the above studies is no longer being produced, and attempts to substitute available adjuvants have led to reductions in the level of protection achieved, emphasizing the need for adjuvant research. Little is known about the correlates of immunity in humans.
It is well recognized that people who acquire syphilis can be re-infected following treatment, and this cycle can be repeated many times. Human challenge studies have shown that persons with late latent syphilis are resistant to symptomatic reinfection with a heterologous strain of T. pallidum, but that those with earlier stages show evidence of infection following challenge . This correlates with the lengthy immunization period necessary to induce protection in Miller’s successful vaccine. Development of immunity seen in rabbits has components of subspecies- and even strain-specificity , most likely related to antigenic differences among strains. Thus, syphilis vaccine development efforts will need to include evaluation of long immunization schedules, and the selection of immunogens will need to recognize antigenic diversity among strains and accommodate the effects of antigenic variation in immune evasion.