Decreased Infectivity despite Unaltered C3 Binding by a ΔhbhA Mutant of Mycobacterium tuberculosis

HbhA of Mycobacterium tuberculosis is a multifunctional binding protein, binding to both sulfated sugars such as heparin and to human complement component C3. HbhA may therefore interact with host molecules and/or host cells during M. tuberculosis infection and play a role in the pathogenesis of this bacterium. The purpose of this study was to use allelic exchange to create an M. tuberculosis strain deficient in expression of HbhA to determine whether this protein's C3-binding activity plays a role in the pathogenesis of M. tuberculosis. An in-frame, 576-bp unmarked deletion in the hbhA gene was created using sacB as a counterselectable marker. Southern blotting and PCR analyses confirmed deletion of hbhA in the ΔhbhA mutant. The ΔhbhA mutant strain grew at a rate similar to that of the parent in broth culture and in J774.A1 murine macrophage-like cells but was deficient in growth compared to the parent strain in the lungs, liver, and spleen of infected mice. In addition, the ΔhbhA mutation did not reduce binding of M. tuberculosis to human C3 or to J774.A1 cells in the presence or absence of serum, suggesting that in the absence of HbhA, other molecules serve as C3-binding molecules on the M. tuberculosis surface. Taken together, these data indicate that HbhA is important in the infectivity of M. tuberculosis, but its ability to bind C3 is not required for mycobacterial adherence to macrophage-like cells. Using the ΔhbhA mutant strain, a second M. tuberculosis C3-binding protein similar in size to HbhA was identified as HupB, but the role of HupB as a C3-binding protein in intact organisms remains to be determined

A Mutant of Mycobacterium tuberculosis H37Rv That Lacks Expression of Antigen 85A Is Attenuated in Mice but Retains Vaccinogenic Potential

The fbpA and fbpB genes encoding the 85A and 85B proteins of Mycobacterium tuberculosis H37Rv, respectively, were disrupted, the mutants were examined for their ability to survive, and the strain lacking 85A (ΔfbpA) was tested for its ability to immunize mice. The ΔfbpA mutant was attenuated in mice after intravenous or aerosol infection, while replication of the ΔfbpB mutant was similar to that of the wild type. Complementation of the fbpA gene in ΔfbpA restored its ability to grow in the lungs of mice. The ΔfbpA mutant induced a stronger expression of pulmonary mRNA messages in mice for tumor necrosis factor alpha, interleukin-1 beta (IL-1β), gamma interferon, IL-6, IL-2, and inducible nitric oxide (NO) synthase, which led to its decline, while H37Rv persisted despite strong immune responses. H37Rv and ΔfbpA both induced NO in macrophages and were equally susceptible to NO donors, although ΔfbpA was more susceptible in vitro to peroxynitrite and its growth was enhanced by NO inhibitors in mice and macrophages. Aerosol-infected mice, which cleared a low-dose ΔfbpA infection, resisted a challenge with virulent M. tuberculosis. Mice subcutaneously immunized with ΔfbpA or Mycobacterium bovis BCG and challenged with M. tuberculosis also showed similar levels of protection, marked by a reduction in the growth of challenged M. tuberculosis. The ΔfbpA mutant was thus attenuated, unlike ΔfbpB, but was also vaccinogenic against tuberculosis. Attenuation was incomplete, however, since ΔfbpA revived in normal mice after 370 days, suggesting that revival was due to immunosenescence but not compensation by the fbpB or fbpC gene. Antigen 85A thus affects susceptibility to peroxynitrite in M. tuberculosis and appears to be necessary for its optimal growth in mice