Novel Strategies to Prevent and/or Treat Tuberculosis

Abstract

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), continues to be a leading infectious cause of death worldwide with approximately 10 million new cases and 2 million deaths attributed annually. There has been resurgence in active TB infections since 1980s and the World Health organization (WHO) has declared TB to be a “Global emergency”. The lack of an effective vaccine, the lengthy treatment regimens with multiple chemotherapeutic agents that have serious side effects, the prevalence of co-infection with HIV and the increasing number of cases of multi-, extensively-, and totally-drug resistant tuberculosis (MDR, XDR, TDR-TB), necessitate the development of novel vaccine and therapeutic regimens to treat and control the global spread of tuberculosis. The immune correlates of protection from infection and control of an ongoing infection are not clear yet, but intensive studies suggest the important role of innate immunity and multi-specific, polyfunctional adaptive cellular immunity. In this study, I have designed a novel subunit vaccine by conjugating a palmitoyl-lysine residue to peptides of the ESAT-6 antigen of Mtb, which corresponds to dominant human T cell epitopes. I sought to evaluate their ability to induce protective cellular immunity against Mtb (H37Ra) by subcutaneous (s.c.) and intranasal (i.n.) immunizations in a mouse model of Mtb infection. I have also evaluated how an adjuvant [Poly I:C, MPL, Gardiquimod (GDQ) and heat-killed Caulobacter crescentus (HKCC)] contributes to enhancing the induced responses and resulting protective efficacy of lipopeptides. My results demonstrated that single C-terminal palmitoyl-lysine modified lipopeptides of ESAT-6 elicited significant antigen specific CD4+ and CD8+ T cell responses upon subcutaneous immunizations. Intriguingly, a combination of immunogenic lipopeptides of ESAT-6 antigen exhibited local (pulmonary) and systemic immune responses along with efficient protective efficacy when administered intranasally or subcutaneously. Surprisingly, immunization with ESAT-6 derived lipopeptides with MPL and HKCC enhanced protection, whereas PolyI:C and GDQ led to reduce protection associated with specific local and systemic immune modulation. My studies demonstrate the potential of ESAT-6 derived lipopeptides as a promising vaccine candidate against Mtb, and emphasize that selection of the adjuvant is critical for the success of vaccines. In addition, I have investigated the ability of the newly discovered immunomodulator HKCC to induce immune responses capable of controlling mycobacterial growth. I examined the effect of HKCC in vitro on various human immune cells present in human peripheral blood mononuclear cells (PBMCs). I also evaluated the host-mediated anti-mycobacterial effects of HKCC in human macrophages infected with Mtb (H37Ra) and M. avium, and in a mouse model of Mtb infection. My results demonstrated that HKCC stimulate innate immune cells such as antigen-presenting cells (APCs), NK and NKT cells, and also lead to induction of multiple cytokines upon 24 h and 96 h culture with human PBMCs. Intriguingly, treatment with HKCC stimulated PBMCs supernatant led to significant reduction of Mtb and M. avium replication within human macrophages. Further, immunotherapy with HKCC alone and combination with isoniazid therapy significantly controlled mycobacterial growth in lungs, liver and spleen of Mtb infected mice with the induction of local and systemic protective immune responses. These findings reveal the promise of HKCC as a novel immunotherapy to treat mycobacterial infections. Overall, a promising vaccine candidate and a novel immunotherapeutic approach have emerged from my studies, which may be developed to prevent and/or treat TB infections.