The Antigen 85B of mycobacterium tuberculosis: friend or foe in vaccination against tubercolosis?

Mycobacterium tuberculosis (MTB) ability to escape from host immune system is central in tuberculosis (TB) pathology, and raises a major issue to develop effective TB vaccines. We found that anti-TB Th1 response, essential in controlling the acute phase of infection, is highly dynamic and greatly affected by mycobacterial antigen pressure. In our study, we used Ag85B, a recognized immunodominant constituent of the mycobacterial cell wall, as primary DNA vaccine and/or as protein booster or primary stimulant. The Ag85B-encoding DNA (DNA-85B) vaccine was protective in mice but this protection vanished when the protein Ag85B, but not successive DNA- 85B injections, was used as a booster. The loss of protection was associated with the expansion of a population of CD4+ T cells, highly proliferating and secreting a huge amount of IFN-γ in response to the exogenous Ag85B protein but incapable to recognize Ag85B-expression on MTB-infected macrophages. The balance between IFN-γ needed for the control of the pathogen and those sufficient for dampening T cells seems important to determine the fate of MTB infection. CD8+ T cell-dependent regulatory mechanisms as well as IL-10 produced by the same Ag85B responsive CD4+ T cells were simultaneously induced, however, they restrained but could not abolish the potent Th1 response. Although the mechanisms involved in the expansion of this non-protective Ag85B-specific Th1 response are not exactly understood, the high immunogenicity of Ag85B protein and its the ability to induce “cross-presentation” by both CD4+ and CD8+ T cells probably played an important role. Moreover, Ag85B-specific memory CD8+ T cells, induced only by DNA- 85B immunization, were mandatory in the generation of the phenomenon as indicated by the requirement of DNA-85B priming. The peculiar antigenic features and the behaviour in DNA priming-protein boosting protocol vaccination were typical of Ag85B protein and could not be replicated by immunizing with other mycobacterial secretory antigens such as Psts3. In order to modify the microenvironment where the Th1 response is initiated, the Ag85B protein, with or without DNA-85B priming, was administered with the LTK63 adjuvant in other vaccination attempts. Delivering of protein with adjuvant greatly ameliorated protection against MTB infection in both cases although with opposing effects on specific Th1 response: with a reduction in DNA- 85B primed mice and an increase in mice immunized with protein alone. In DNA-85B primed mice, LTK63 reduced the number of antigen-specific memory CD4+ T cells rather than activate CD4+ T cell-independent regulatory mechanisms or increase secretion of CD4+ regulatory cytokine such as IL-10. Of note, a consistent increase in Ag85B-specific antibodies, although without a switch in Th2-like response was observed in LTK63-vaccinated animals. The appearance of Ag85B-specific IgG2a antibodies, besides the huge increase in IgG1 subclass, could be involved in the enhancement of Th1 response in mice not receiving DNA-85B priming. Conversely, the exclusive increase in Ag85B-specific IgG2b antibodies, through TGFβ required for IgG2b class switch and secretion, may have be involved in restrain of IFN-γ response in DNA-85B primed mice. The preferential upregulation of both CD80 and CD86 surface expression, as well as the increase in IL-6 secretion in dendritic cells stimulated with Ag85B protein and LTK63 also supports the role of this adjuvant as enhancer of B cell response. It appears that antibody-mediated immunity, historically thought to be almost irrelevant in host response against MTB, could participate in the regulation of cell-mediated immunity, rightly considered to be the major component of the TB immune response. In conclusion, although the mycobacterial Ag85B antigen induces protective responses in the host, its use as a component of TB vaccine should be carefully assessed, in view of the peculiar protein ability to bias protective antigen-specific Th1 response towards a more potent but non protective one