Efficient recognition by rat T cell clones of an epitope of mycobacterial hsp 65 inserted in Escherichia coli outer membrane protein PhoE

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Abstract
PhoE is a pore-forming protein, abundantly expressed in the Escherichia coli outer membrane. Previous investigations have shown the possibility of inserting antigenic determinants in cell surface-exposed regions of PhoE by recombinant DNA techniques without disturbing the biogenesis and the functioning of the protein. This method proved to be successful for foot-and-mouth disease virus B cell determinants. We have now shown for the first time that PhoE can also be used as a carrier molecule for Tcell epitopes. A well-characterized Tcell epitope (180–188) of the 65-kDa heat-shock protein (hsp 65) of Mycobacterium tuberculosis was expressed in PhoE and tested for recognition by specific Tcell clones. Specific and efficient T cell proliferation was found after stimulation with this protein construct in vitro. Interestingly, paraformaldehyde fixation of antigen-presenting cells did not abrogate Tcell recogition. Thus, in contrast to hsp 65 itself, recognition of epitope 180–188 in the context of PhoE appeared to be independent of antigen-processing events. At the level of polyclonal Tcell responses the epitope in the context of PhoE is recognized more efficiently than 180–188 as synthetic peptide or in the context of the hsp 65 molecule itself. These findings indicate that PhoE may serve as attractive vaccine carrier not only for B, but also for Tcell epitopes. Furthermore, the possibility for expression of PhoE constructs in attenuated Salmonella typhimurium strains offers the exciting prospect of new types of live oral vaccines expressing selected combinations of B and Tcell epitopes.