Induction of Unconventional T Cells by a Mutant Mycobacterium bovis BCG Strain Formulated in Cationic Liposomes Correlates with Protection against Mycobacterium tuberculosis Infections of Immunocompromised Mice

Abstract

ABSTRACT Earlier studies aimed at defining protective immunity induced by Mycobacterium bovis BCG immunization have largely focused on the induction of antituberculosis CD4 + and CD8 + T cell responses. Here we describe a vaccine consisting of a BCGΔ mmaA4 deletion mutant formulated in dimethyl dioctadecyl-ammonium bromide (DDA) with d -(+)-trehalose 6,6′-dibehenate (TDB) (DDA/TDB) adjuvant (A4/Adj) that protected TCRδ −/− mice depleted of CD4 + , CD8 + , and NK1.1 + T cells against an aerosol challenge with M. tuberculosis . These mice were significantly protected relative to mice immunized with a nonadjuvanted BCGΔ mmaA4 (BCG-A4) mutant and nonvaccinated controls at 2 months and 9 months postvaccination. In the absence of all T cells following treatment with anti-Thy1.2 antibody, the immunized mice lost the ability to control the infection. These results indicate that an unconventional T cell population was mediating protection in the absence of CD4 + , CD8 + , NK1.1 + , and TCRγδ T cells and could exhibit memory. Focusing on CD4 − CD8 − double-negative (DN) T cells, we found that these cells accumulated in the lungs postchallenge significantly more in A4/Adj-immunized mice and induced significantly greater frequencies of pulmonary gamma interferon (IFN-γ)-producing cells than were seen in the nonvaccinated or nonadjuvanted BCG control groups. Moreover, pulmonary DN T cells from the A4/Adj group exhibited significantly higher IFN-γ integrated median fluorescence intensity (iMFI) values than were seen in the control groups. We also showed that enriched DN T cells from mice immunized with A4/Adj could control mycobacterial growth in vitro significantly better than naive whole-spleen cells. These results suggest that formulating BCG in DDA/TDB adjuvant confers superior protection in immunocompromised mice and likely involves the induction of long-lived memory DN T cells.