Immune evasion mechanisms of mycobacteria and vaccine applications

Abstract

The ability of M. tuberculosis to avoid immune-mediated clearance is likely to reflect a highly evolved and coordinated program of immune evasion strategies, including some that interfere with pathways affecting the innate immune response and others with antigen presentation. Here, I present evidence of six cosmids (M1H4, M2D10, M2H3, M2H6, S1E4 and S1G7) which are able to decrease the MHC class II responses without affecting the total levels of these molecules on the surface of the cells. Moreover, I was able to map the effect of the M2D10 cosmid to the PE_PGRS47 gene which is able to decrease antigen presentation through the inhibition of autophagy. In addition I showed that PE_PGRS47 alters calcium flux at the plasma membrane and at the endoplasmic reticulum (ER). Furthermore, I showed that PE_PGRS47 is detrimental for both the T cell responses and the protection, when the protein is over expressed on the BCG vaccine. Additionally, the deletion of PE_PGRS47 from the virulent increases the T cell responses to MHC class II antigens. As a mechanism that affect innate immune responses, I showed for the first time that M. tuberculosis, M. bovis (BCG) and other mycobacterium species are able to secrete membrane vesicles (MVs). Also, I demonstrate in vivo and in vitro that these MVs have a negative effect for the host, conferring an advantage to the Mtb to grow and disseminate. Besides, we prove that these effects of the MVs, secreted by Mycobacteria, on the host cells are TLR2 mediated. Alternative work done in this thesis, comparing the effects of two growth media, Sauton (Sau-BCG) and Middelbrook-7H9 (7H9-BCG), for the BCG vaccine I showed that the BCG-S can be more persistent and generate higher T cell responses to Mtb epitopes than BCG-M, in vitro and in vivo. This result paradoxically does not translate in a better protection of the host by BCG-S immunized animals but showed that BCG-M protects better. The information provided by this thesis may contribute to future studies of how M. tuberculosis evades and modulates antigen presentation with critical importance for the development of more effective new vaccines to prevent Mycobacterium infections.