Vaccine strategies based on mycobacterial heat shock protein 65

Abstract

Mycobacteria can infect macrophages and dendritic cells (DC), where they reside inside the endosomal compartment. The importance of CD4+ T cells in protective immunity against mycobacteria and other intracellular pathogens is well established, and for a long time these cells were thought to be the major defense in controlling an infection. Lately, several reports have shown that also cytotoxic CD8+ T cells contribute to protective immunity.

Heat shock proteins (lisp) are highly conserved molecules that have been shown to contain important antigens both for B and T cell mediated immune responses against several infections. Studies have indicated that in vivo expression of the hsp65 gene from M. leprae could induce protection against live bacterial challenge with M. tuberculosis. CD8+ T cells could confer the observed hsp65 induced protection.

We studied an immunization approach where plasmids encoding mycobacterial hsp65 (Mhsp65) were injected intramuscularly in mice to evaluate the resultant immune response following in vivo transcription of the immunogen. This mode of immunization is associated with efficient CD8+ T cell priming and the generation of a T helper I (Th1) type of immune response. Genetic vaccination further offers possibilities to manipulate the quality of the induced immune responses. We showed that the immune response could be efficiently enhanced after co-administration of either a Schiff base forming drug, tucaresol, or by plasmids encoding the cytokines interferon-gamma (IFN-gamma) or granulocyte-macrophage colony stimulating factor (GM-CSF). Among these, tucaresol had the most prominent effect, resulting in a general enhancement of the immune responses in our studies. This makes tucaresol a potent and interesting adjuvant candidate for plasmid DNA immunization.

Human antigenic epitopes can be evaluated in mice expressing transgenes for human HLA complexes, since antigen processing and presentation is similar in mouse and man. We concluded that Mhsp65 plasmid injection results in human HLA class I and II restricted antigen presentation, and a subsequent priming of CD8+ and CD4+ T cells in HLA transgenic mice.

We did also investigate CD8+ T cells with specificity for Mhsp65. This was studied by the identification of human I-ILA class I restricted cytotoxic T lymphocyte (CTL) epitopes encoded within Mhsp65. One of the identified epitopes, Mhsp659-369, is conserved within hsp65 from several human pathogens. Our results revealed that Mhsp659-369 could be processed and presented in the context of HLA-A*0201 during both Salmonella and mycobacterial infections. Several additional pathogens have this conserved epitope. These results suggested that T cells primed during infections may recognize conserved pathogen-associated epitopes, thereby conferring protection to unrelated pathogens.

In summary, DNA vaccines have the potential to induce protection against intracellular pathogens like M. tuberculosis, where Mhsp65 fulfills the criteria to be an efficient antigen. The use of defined epitopes can ftirther restrict the immune responses.