Mycobacterium tuberculosis Surface-binding Antibodies Influence Early Infection Events
Mycobacterium tuberculosis, the etiologic agent of tuberculosis (TB), is among the leading causes of death from infectious disease world-wide. An intracellular pathogen, M. tuberculosis infects phagocytic cells, and subverts the host immune response, preventing eradication once infection has been established. Even after successful chemotherapy, exogenous re-infection occurs, indicating that sterilizing immune responses are not generated during natural infection. While a TB vaccine exists, it does not alter M. tuberculosis infection rate, rather it prevents the progression from latent TB infection to active TB disease. Vaccines against Haemophilus influenzae and Streptococcus pneumonia protect from bacterial colonization and infection through the induction of antibodies to capsular surface components. This dissertation explores if antibodies to the surface of M. tuberculosis can alter the initial interaction between a bacterium and host cell, leading to a reduction in infection rate.
When pre-mixed with M. tuberculosis prior to in vitro infection of macrophages, or retropharyngeal instillation of mice, monoclonal surface-binding, but not non-surface-binding antibodies, decrease bacterial burden and the number of infected cells within the first twenty-four hour of infection. If administered retropharyngeally prior to aerosol exposure, surface-binding antibodies decreased pulmonary bacterial burden at twenty-four hours post infection in an FcγR independent manner. Despite decreasing early bacterial burden, pre-administration of surface-binding antibodies prior to ultra-low dose aerosol infection did not alter infection rate compared to mice instilled with PBS (Chapters 4 and 5).
Infected humans do not produce high-titer, high-avidity surface-binding antibodies. Plasma from uninfected controls, individuals with latent TB infection, and active TB disease was assayed by ELISA to determine the titer, avidity and IgG/IgM ratio for antibodies to the surface and additional bacterial fraction. In contrast to antibodies to bacterial fractions, individuals with active TB disease had decreased avidity, and no augmentation of the IgG/IgM ratio for antibodies to the live M. tuberculosis surface, as compared to uninfected controls (Chapter 3).
Overall these findings demonstrate that surface-binding monoclonal antibodies alter early infection events, both in vivo and in vitro, though the magnitude of protection was not sufficient to decrease M. tuberculosis infection rate. Additionally, the failure of humans to generate high-titer, high-avidity surface-binding antibodies after infection indicates and that induction of surface-binding antibodies may be an appropriate target for future vaccines.
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