Th1 immune response is essential in the protection against mycobacterial intracellular pathogens. Lipoproteins trigger both humoral and cellular immune responses and may be candidate protective antigens. We studied in BALB/c mice the immunogenicity and the protection offered by the recombinant 27-kDa Mycobacterium tuberculosis lipoprotein and the corresponding DNA vaccine. Immunization with the 27-kDa antigen resulted in high titers of immunoglobulin G1 (IgG1) and IgG2a with a typical Th1 profile and a strong delayed hypersensitivity response. A strong proliferation response was observed in splenocytes, and significant nitric oxide production and gamma interferon secretion but not interleukin 10 secretion were measured. Based on these criteria, the 27-kDa antigen induced a typical Th1-type immune response thought to be necessary for protection. Surprisingly, in 27-kDa-vaccinated mice (protein or DNA vaccines) challenged by M. tuberculosis H37Rv or BCG strains, there was a significant increase in the numbers of CFU in the spleen compared to that for control groups. Furthermore, the protection provided by BCG or other mycobacterial antigens was completely abolished once the 27-kDa antigen was added to the vaccine preparations. This study indicates that the 27-kDa antigen has an adverse effect on the protection afforded by recognized vaccines. We are currently studying how the 27-kDa antigen modulates the mouse immune response.

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We reported previously that even though immunization with the recombinant mycobacterial 27-kDa lipoprotein (r27) induced a Th1-type response in mice, the vaccinated mice became more susceptible to challenge with Mycobacterium tuberculosis. In this study we show that r27 stimulates naive splenocytes to proliferate. Acylation of r27 was crucial for this effect, since a nonacylated mutant of r27, termed r27⌬SP, failed to stimulate splenocytes either in vitro or in vivo. Depletion experiments indicated that only B cells were proliferating in a T-cell-independent manner. We also found that r27 is recognized by TLR2, which is involved in mitogenic stimulation. Interestingly, r27 but not r27⌬SP induced high gamma interferon levels in splenocyte supernatants, whereas no significant interleukin-2 levels were detected. Since B-cell polyclonal activation might aggravate pathogen infection, we asked whether the antiprotective effect of the r27 lipoprotein is associated with its mitogenicity. We showed that, as in the case of r27, immunization of mice with the nonmitogenic r27⌬SP lipoprotein resulted in increased M. tuberculosis multiplication. We conclude that the antiprotective effect of the r27 lipoprotein must be linked to properties of the polypeptide portion of the lipoprotein rather than to its lipid moiety and its mitogenicity.The 27-kDa antigen, the product of the Mycobacterium tuberculosis lprG gene, is a putative lipoprotein that is present exclusively in the membrane of the M. tuberculosis species complex (5). Previously, it was reported that mice immunized with r27 were more susceptible to M. tuberculosis challenge than nonimmunized mice (15). Splenic CFU counts in mice immunized with the recombinant mycobacterial 27-kDa lipoprotein (r27) were significantly higher, by about 0.5 to 0.8 log unit, than those in nonimmunized mice. Furthermore, the protection afforded following immunization with the Mycobacterium bovis BCG vaccine was completely abolished when r27 was added to the BCG vaccine (15). These results were unexpected, because r27 induces a typical Th1-type immune response thought to be crucial for protection against intracellular pathogens such as M. tuberculosis. It was found recently that M. tuberculosis with a knockout of the 27-kDa (lprG) gene has an attenuated virulence phenotype in mice (A. Cataldi, personal communication). This finding supports our previous results and indicates that the native 27-kDa lipoprotein might also have a role in enhancing M. tuberculosis infection.Lipoproteins have been reported to be powerful antigens that induce strong antibody-and cell-mediated immune responses (7, 10). The lipid moiety on mature acylated lipoproteins or synthetic acylation of peptides increased their immunogenicity and induced their adjuvant-like properties (4). Bacterial lipoproteins are recognized by the innate immune system through toll-like receptor 2 (TLR2) together with TLR1 or TLR6, all of which induce antibacterial activity in macrophages (27-29). These findings indicate that lipoproteins migh...