Induction of a type 1 immune response to a recombinant antigen from Mycobacterium tuberculosis expressed in Mycobacterium vaccae

Abstract

A 19-kDa lipoprotein from Mycobacterium tuberculosis was expressed as a recombinant antigen in the nonpathogenic mycobacterial host strain M. vaccae. Immunization of mice with the recombinant M. vaccae resulted in induction of a strong type 1 immune response to the 19-kDa antigen, characterized by immunoglobulin G2a (IgG2a) antibodies and gamma interferon (IFN-␥) production by splenocytes. Immunization with the same antigen in incomplete Freund's adjuvant induced a strong IgG1 response with only low levels of IFN-␥. Subsequent intravenous and aerosol challenges of immunized mice with virulent M. tuberculosis demonstrated no evidence of protection associated with the response to the 19-kDa antigen; in fact, the presence of the recombinant 19-kDa antigen abrogated the limited protection conferred by M. vaccae (vector control). The recombinant M. vaccae system is a convenient approach to induction of type 1 responses to M. tuberculosis antigens. However, the unexpected reduction in protective efficacy of M. vaccae expressing the 19-kDa antigen highlights the complexity of testing recombinant subunit vaccines and the need for a better understanding of the immune mechanisms required for effective vaccination against tuberculosis.

The 19-kD antigen and protective immunity in a murine model of tuberculosis

“…Vaccination with an equivalent number of live M. smegmatis or M. vaccae conferred a much lower degree of protection, although a significant increase in MST (P , 0´01) was observed during challenge with the lower dose of M. tuberculosis ( Table 1). The weak protection associated with immunization with saprophytic mycobacteria in this model is consistent with the results of previous experiments involving multiple immunization with heat-killed M. vaccae [7], and our previous observation of rapid clearance of live M. smegmatis following s.c. injection [14]. There was no evidence of a significant difference in the degree of protection conferred by M. vaccae compared with M. smegmatis.…”

“…We have previously reported that immunization with the purified 19-kD antigen in Freund's incomplete adjuvant (FIA) had no effect on MST in the lethal challenge model [7]. However, we also noted that this protocol was less effective than immunization with the M. vaccae recombinant in induction of a type 1 immune response.…”

“…In previous experiments to explore novel tuberculosis vaccines, we compared the effect of immunization of mice with M. vaccae alone with that of M. vaccae expressing the 19-kD antigen [7]. Despite induction of a strong type 1 immune response specific to the 19-kD protein, we observed an unexpected reduction in efficacy of vaccination with the 19-kD recombinant in comparison with control M. vaccae in low-dose aerosol and in high-dose i.v.…”

“…Despite induction of a strong type 1 immune response specific to the 19-kD protein, we observed an unexpected reduction in efficacy of vaccination with the 19-kD recombinant in comparison with control M. vaccae in low-dose aerosol and in high-dose i.v. challenge models [7]. This paradoxical result has, to our knowledge, no analogues amongst a variety of mycobacterial antigen vaccine candidates studied so far.…”

“…Mice receiving DNA vaccines were bled to obtain sera 17 days after the last immunization. Antibody responses were measured in an ELISA format using microtitre plates coated with 19-kD protein purified from recombinant E. coli as described previously [7]. To test the T cell response to DNA vaccination, splenocyte suspensions were prepared for cytokine assay as described below.…”