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In the present work we investigated the role of killed Propionibacterium acnes or a soluble polysaccharide extracted from bacterium cell wall in modulated experimental immunization with plasmidial DNA. We used a plasmid, p154/13, containing a gene‐encoding catalytic domain of Trypanosoma cruzi (T. cruzi) trans‐sialidase. As previously described, immunization of BALB/c mice with p154/13 elicited humoral, cell‐mediated and protective immune responses against T. cruzi infection. In this study we describe that both P. acnes and its soluble polysaccharide fraction have the ability to modulate the immune response elicited by p154/13. Treatment with these adjuvants enhanced specific trans‐sialidase Th1 immune response, as revealed by a lower IgG1/IgG2a ratio and stronger in vitro IFN‐γ synthesis by CD4+ T cells. The most important fact was that treatment with P. acnes or its soluble polysaccharide fraction in the presence of p154/13 significantly reduced the peak of parasitemia observed 7 to 8 days after T. cruzi challenge. These data suggest that P. acnes or its soluble polysaccharide fraction may improve the protective potential of a DNA vaccine against experimental T. cruzi infection.

Propionibacterium acnes has been described as a potent adjuvant to immune responses in vitro and in vivo. Presently, we analysed the modulation of peritoneal exudate cells (PEC) by heat‐killed P. acnes or its purified soluble polysaccharide (PS), both injected intraperitoneally in C57Bl/6 mice, aiming at their recruitment and cytotoxicity. Both treatments induced an increase in macrophages, immature dendritic cells, B1a lymphocytes and NK1.1+ CD3+ cells. The bacterium caused a remarkable increase in a NK1.1+ CD3+ CD4− CD8− cells subpopulation, whereas the PS component seemed responsible for the recruitment of mainly macrophage cells. To assess P. acnes and PS adjuvant effect on PEC cytotoxicity we evaluated their in vitro effect on murine B16F10 melanoma cells. The effector cells from the heat‐killed bacteria and PS‐treated groups lysed melanoma cells in co‐cultures with PEC. Mice genetically deficient in IFN‐γ, when stimulated with P. acnes or PS, had reduced PEC cytotoxicity, and the cytotoxic effect was completely abrogated in PEC from iNOS−/− mice. The tumoricidal activity of PEC from P. acnes‐treated mice was mediated by macrophages and NKT cells stimulated with IL‐12. In PS‐treated mice the cytotoxicity was mediated mainly by macrophages. Moreover, both treatments increased IL‐4 and IFN‐γ production by NKT cells. In conclusion, we show that P. acnes act mainly by recruiting and activating NKT double‐negative cells in PEC, which were shown to be tumoricidal in vitro when induced by IL‐12. Macrophages induced by both P. acnes and PS have their antitumour effect dependent on NO production.

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