Closely Related Mycobacterial Strains Demonstrate Contrasting Levels of Efficacy as Antitumor Vaccines and Are Processed for Major Histocompatibility Complex Class I Presentation by Multiple Routes in Dendritic Cells

Abstract

Mycobacterium bovis BCG, the only vaccine available against tuberculosis (22), has also been used successfully as a vaccine against leprosy (29) and, for over 20 years, as a local immunotherapy against superficial bladder cancer (2). Mycobacterium smegmatis is a related organism which has potential advantages as a recombinant vaccine over M. bovis BCG, as it is nonpathogenic and frequently commensal in humans (36, 42). Indeed, emulsified M. smegmatis and M. bovis BCG have comparable antitumor activity in a tumor immunotherapy model (56). The recent realization that mycobacteria are promising vehicles for a new generation of recombinant vaccines lends urgency to investigating the relative value of different strains in that role.Despite their close relationship, there are several key differences between M. bovis BCG and M. smegmatis. M. bovis BCG is derived from the intracellular pathogen M. bovis and survives in host cells for months (31). In contrast, M. smegmatis is rapidly destroyed within infected cells and is nonpathogenic. Highly pathogenic mycobacteria such as M. tuberculosis can survive and replicate within cells because they inhibit phagosome maturation and hence their own degradation (11). Although the exact mechanism is not yet fully understood (47), it is known that such mycobacteria prevent acidification of the phagosome (13) and subsequent formation of a phagolysosome. M. bovis BCG shares many characteristics with M. tuberculosis and induces biochemical markers of maturation arrest (53, 54). On the other hand, M. smegmatis does not arrest phagosome maturation and is degraded rapidly by phagolysosomal proteases, which probably accounts for its lack of pathogenicity (28,54).Since the development of mycobacterium-Escherichia coli shuttle plasmids (23), there has been much interest in the generation of recombinant mycobacteria as vaccines against infectious disease, such as tuberculosis and human immunodeficiency virus. Furthermore, a recent study has also shown that recombinant M. bovis BCG can also protect against tumor challenge in model systems (15). The authors demonstrated that if they vaccinated mice with 10 2 or 10 4 M. bovis BCG secreting OVA and then challenged at day 30, 120, or 150 with B16-OVA, a significant delay in tumor growth was observed.It is generally accepted that the generation of effective antitumor and anti-infective immune responses requires antigenpresenting cells to prime cytotoxic T lymphocytes and T-helper cells by processing antigen and presenting it on MHC class I and class II molecules, respectively. Mice immunized with recombinant M. bovis BCG expressing human immunodeficiency virus antigens (1) and ␤-galactosidase (49) generate specific cytotoxic T lymphocytes, but the mechanisms underlying anti-* Corresponding author. Present address:

Mycobacterium smegmatisExpressing a Chimeric Protein MPT64-Proteolipid Protein (PLP) 139–151 Reorganizes the PLP-Specific T Cell Repertoire Favoring a CD8-Mediated Response and Induces a Relapsing Experimental Autoimmune Encephalomyelitis

“…This observation supports the idea that intracellular bacteria are effective vaccinia vectors when a strong stimulation of the immune system is needed, such as, for example, in cancer vaccines (23). At the same time, it shows that transient encounters with infectious agents cross-reactive with self-produce profound and lasting effects on the immune system.…”

“…Moreover, its rapid clearance by the host differs from M. tuberculosis or even for the vaccine strain M. bovis bacillus Calmette-Guérin (21). Therefore, M. smegmatis has been used as vaccine vector, because it activates DCs and induces CD8-mediated immune responses (22)(23)(24)(25). Finally, killed M. smegmatis has been shown to provide the same adjuvant activity of M. tuberculosis during induction of EAE (15).…”

Generation of CD8+T-Cell Responses by a Recombinant NonpathogenicMycobacterium smegmatisVaccine Vector Expressing Human Immunodeficiency Virus Type 1 Env

“…Cheadle et al showed that M. smegmatis was better than BCG at promoting DC maturation. However, recombinant M. smegmatis expressing a CTL epitope of the ovalbumin (OVA) antigen did not protect against challenge with a tumor expressing this epitope, whereas BCG expressing the same epitope protected mice against the OVA epitope-expressing tumor (10). This absence of antitumor activity elicited by recombinant M. smegmatis was asso- ciated with poor presentation of peptides by the nonpathogenic mycobacteria to an OVA-specific T-cell line in vitro (10).…”

“…Unlike other mycobacterial strains such as BCG that survive in host cells for months by inhibiting phagosome maturation, M. smegmatis is rapidly destroyed by phagolysosomal proteases in the phagosomes of infected cells (26,32,55,56). Nevertheless, M. smegmatis can induce cytokine production by macrophages better than pathogenic mycobacterial species (8,64) and can activate and induce the maturation of dendritic cells better than BCG by upregulation of major histocompatibility complex (MHC) class I and costimulatory molecules (10). M. smegmatis can also access the MHC class I pathway for presentation of mycobacterial antigens more efficiently than BCG (40).…”

Enhanced Priming of Adaptive Immunity by Mycobacterium smegmatis Mutants with High-Level Protein Secretion

“…Nevertheless, M. smegmatis induces cytokine production by macrophages better than pathogenic mycobacterial species and can activate and induce the maturation of major histocompatibility complex (MHC) class I and costimulatory molecules (6,55). M. smegmatis also facilitates rapid uptake of expressed antigens and cross-presentation of antigens (12,35). Moreover, preexisting immunity to BCG may have only a marginal effect on the immunogenicity of recombinant M. smegmatis (9).…”