Conceptual basis and hypothesis
Conceptual basis for the rBCG30 vaccine
The rationale for constructing a recombinant BCG vaccine overexpressing the M. tuberculosis 30 kDa major secretory protein was based on the hypothesis (the Extracellular Protein Hypothesis). This hypothesis was first developed in the Horwitz laboratory and is based on the notion that extracellular proteins of intracellular pathogens are potent immunoprotective molecules (7-9); extracellular proteins are proteins that are secreted or otherwise exported by the intracellular pathogens into their milieu inside host cells.
The extracellular protein hypothesis
The Extracellular Protein Hypothesis has three components. First, it holds that extracellular proteins of intracellular pathogens are available within infected host cells for processing and presentation to the immune system as MHC-peptide complexes on the surface of the host cell. The MHC-peptide complexes would serve to alert the immune system to the presence of a pathogen sequestered inside the host cell and enable lymphocytes that recognize the complexes to exert an appropriate antimicrobial response against the host cell. The lymphocytes could activate the host cell so that it can inhibit the growth of the pathogen or lyse the host cell, denying the pathogen its preferred intracellular niche. Second, the hypothesis holds that appropriate immunization of a naive host with extracellular proteins would expand a population of lymphocytes able to recognize an infected host cell at some subsequent time when the host is infected with the pathogen. Finally, the hypothesis holds that the major extracellular proteins, that is those released in greatest abundance, would figure most prominently because they would provide the richest display of MHC peptide complexes on the host cell surface.
Initial evidence in support of the Extracellular Protein Hypothesis was obtained in studies of Legionella pneumophila, the agent of Legionnaires' disease. These studies demonstrated that the major secreted proteins of L. pneumophila were potent immunoprotective molecules that protected guinea pigs from lethal aerosol challenge with virulent L. pneumophila (7-9). Subsequent studies of major extracellular proteins of M. tuberculosis demonstrated that these proteins were also immunoprotective molecules (2).
In guinea pig studies, vaccines comprised of purified M. tuberculosis extracellular proteins in adjuvants were immunoprotective but not as potent as BCG. In view of this, Horwitz et al. used a live replicating vector to deliver extracellular proteins so as to obtain a more vigorous immune response to the proteins (1). A mycobacterial host, BCG, was chosen for this purpose because it was nonpathogenic and capable of expressing and secreting M. tuberculosis extracellular proteins in native form. It also allowed the recombinant vaccine to take advantage of and to build upon the immunoprotective capacity of BCG.