Immune responses induced by recombinant BCG strains according to level of production of a foreign antigen: malE.

A variety of viral, bacterial and parasitic antigens have been expressed in BCG and the capacity of these recombinant bacteria to induce immune responses has been well documented. However, little is known about the parameters influencing the induction of immune responses by recombinant BCG (rBCG), such as level of production and localization of the recombinant antigen. In the present study, we have constructed several rBCG strains expressing the malE gene from Escherichia coli which is either secreted or targeted to the cytoplasm or plasma membrane. Expression of malE was quantified by ELISA and localization was analyzed by flow cytometry. Even when using the same promoter, levels of cytoplasmic or membrane MalE production were far less than those from secreting strains using either mycobacterial or E. coli secretion signals. Stronger and more rapid immune responses were induced by rBCG strains with the highest levels of secreted MalE compared to cytoplasmic or membrane constructs, including both good humoral and proliferative responses in BALB/c, C57BL6 and even C3H mice, previously shown to be poor MalE responders. These results suggest that the levels of foreign antigen production play an important role in the induction of immune responses by rBCG strains.