Orally Administered Mycobacterium vaccae Modulates Expression of Immunoregulatory Molecules in BALB/c Mice with Pulmonary Tuberculosis

The environmental saprophyte Mycobacterium vaccae induces a Th1 response and cytotoxic T cells that recognize M. tuberculosis, and by subcutaneous injection, it is therapeutic for pulmonary tuberculosis (TB) induced by high-dose challenge in BALB/c mice. However, M. vaccae also drives regulatory T cells that inhibit Th2 responses, and this is seen in allergy models, not only following subcutaneous injection but also after oral administration. An oral immunotherapeutic for TB would be clinically useful, so we investigated M. vaccae given orally by gavage at 28-day intervals in the TB model. We used two different protocols: starting the oral M. vaccae either 1 day before or 32 days after infection with M. tuberculosis. Throughout the infection (until 120 days), we monitored outcome (CFU), molecules involved in the development of immunoregulation (Foxp3, hemoxygenase 1, idoleamine 2,3-dioxygenase, and transforming growth factor β [TGF-β]), and indicators of cytokine balance (tumor necrosis factor, inducible nitric oxide synthase, interleukin-4 [IL-4], and IL-4δ2; an inhibitory splice variant of IL-4 associated with improved outcome in human TB). Oral M. vaccae had a significant effect on CFU and led to increased expression of Th1 markers and of IL-4δ2, while suppressing IL-4, Foxp3, and TGF-β. When administered 1 day before infection, oral M. vaccae induced a striking peak of expression of hemoxygenase 1. In conclusion, we show novel information about the expression in TB of murine IL-4δ2 and molecules involved in immunoregulation and show that these can be modulated by oral administration of a saprophytic mycobacterium. A clinical trial of oral M. vaccae in extensively drug-resistant TB might be justified.