Therapy of tuberculosis in mice by DNA vaccination.

Mycobacterium tuberculosis continues to kill about 3 million people every year, more than any other single infectious agent. This is attributed primarily to an inadequate immune response towards infecting bacteria, which suffer growth inhibition rather than death and subsequently multiply catastrophically. Although the bacillus Calmette-Guerin (BCG) vaccine is widely used, it has major limitations as a preventative measure. In addition, effective treatment requires that patients take large doses of antibacterial drug combinations for at least 6 months after diagnosis, which is difficult to achieve in many parts of the world and is further restricted by the emergence of multidrug-resistant strains of M. tuberculosis. In these circumstances, immunotherapy to boost the efficiency of the immune system in infected patients could be a valuable adjunct to antibacterial chemotherapy. Here we show in mice that DNA vaccines, initially designed to prevent infection, can also have a pronounced therapeutic action. In heavily infected mice, DNA vaccinations can switch the immune response from one that is relatively inefficient and gives bacterial stasis to one that kills bacteria. Application of such immunotherapy in conjunction with conventional chemotherapeutic antibacterial drugs might result in faster or more certain cure of the disease in humans.

Protection against tuberculosis by a plasmid DNA vaccine.

Past attempts to use fractions of mycobacteria as an alternative to BCG have given disappointing results. The availability of cloned genes and suitable vectors has now opened a new avenue in which individual mycobacterial protein antigens are synthesised within transfected mammalian cells. In an ex vivo transfection approach with a retroviral vector we found that even a single antigen (hsp65) could evoke strong protection when expressed as a transgene and that expression of protection was largely a function of antigen specific cytotoxic T cells. We now find that intramuscular injection of plasmid DNA expressing the antigen from either a viral or a murine promoter can also give protection equivalent to Bacillus Calmette-Guérin (BCG). Plasmids expressing some other mycobacterial antigens, hsp70, 36 kDa and 6 kDa, are also effective, suggesting that this approach may lead to a new vaccine.

Towards a DNA vaccine against tuberculosis.

Expression of the gene for a single mycobacterial antigen (Mycobacterium leprae hsp65) in adult Balb/c mice resulted in substantial cell-mediated protection against challenge with M. tuberculosis. CD4 and CD8 T cells cloned from spleens of such immunized mice passively transferred protection to non-immunized mice, and CD8 cells selectively lysed macrophages infected with M. tuberculosis. Three modes of expressing the gene have been tested: (1) expression from a retroviral vector (pZIPNeoSV) in implanted J774 tumour cells, (2) expression from the same vector via bone marrow cells transfected in vitro and used to reconstitute irradiated mice, and (3) in a preliminary experiment, from CMV immediate-early and hydroxymethylglutaryl Co-A reductase promoters injected as plasmid DNA into muscle.