Signature Hypothetical Protein Rv1509 And Tuberculosis
Tuberculosis is still leading the number of deaths caused by infectious disease. The emergence of MDR and TDR strains and co-infection with HIV/AIDS has greatly exacerbated the already severe situation. Surprsingly, M. tb lacks the classical virulence factors used by other pathogens like capsule to evade phagocytosis, pilli for adhesion, and other toxin proteins. Despite M. tb is the most successful pathogen. Therefore, the role of proteins present in M. tb only highlights their special importance. Our results showed that signature hypothetical protein Rv1509 elicits a strong pro-host immune response in immunized mice, induces maturation of dendritic cells, is pro-apoptotic and stimulates strong IgG humoral response in TB patients and immunized mice.
For the host to neutralize the consequence of infection by a pathogen it must equally understand the weaknesses of the pathogen in a similar fashion that the pathogen has exploited the host response to enable productive infection. The key mechanism employed by M. tb for successful invasion and infection of host cells also includes disturbing the cell-mediated and humoral immune response balance. A number of proteins in M. tb have been found to be involved in reducing the pro-inflammatory cytokines and enhancing secretion of anti-inflammatory cytokines. Members of PE-PPE family have been shown to subvert innate immune response and help the pathogen in establishing infection.
Our results shows that hypothetical protein Rv1509 shows properties similar to Dps family proteins (data comminicated). One of the interesting Dps category protein known as HP-NAP from H. pylori was found to be highly immunogenic and involved in activation of inflammatory cells. The protein is also a component of the H. pylori vaccine, currently undergoing clinical trial. Similar homologous proteins from Borrelia burgdoferi and Treponema pallidum were also found to be immunogenic. Cellular immune responses controlled by CD4+/CD8+ T cells form the central element of adaptive immune response against M. tb. CD4+ cells are important during acute phase of infection while as CD8+ has role in clearance of infection during chronic phase. We also evaluated the effect of antigenic protein Rv1509 on the poly-functional IFN-γ and TNF-α secreting CD4+ and CD8+ cells. The number of poly-functional CD4+ and CD8+ cells in mice immunized with rRv1509 increased as compared to mice injected with PBS buffer only. Macrophages and dendritic cells process and present antigen to other cells and modulate host immune response through secretion of cytokines.
Cell-mediated immunity governed by cytokines like IFN-γ and TNF-α are mainly responsible for inhibiting growth and clearance of M. tb. Interestingly, host is also able to induce apoptosis of infected macrophages – yet another pro-host response to clear infected cells. IgG immunoreactivity with human TB patients sera suggest the protein is expressed under in-vivo conditions during infection and is antigenic. The higher response in human TB relapse cases induced by Rv1509 is indicative of the ability of this protein to form memory cells. Evaluation of B-cell response of signature protein in TB patients showed significant difference between TB patients and healthy controls. The host targets virulence factor Rv1509 (Data comminicated) to generate innate and adaptive immune response to counter the pathogen survival, the critical players being T cells and antigen presenting cells (APCs) like macrophages and DCs. DCs are potent APCs that can activate naïve T cells to initiate an immune response against pathogens. DCs express diverse cell surface markers and Rv1509 treated DCs displayed up-regulated CD80 and MHCII denoting mature phenotype that could effectively stimulate T cells. The adaptive mechanism involves the antigen specific Th1 response by CD4+ cells along with CD8+ cytolytic killing of infected host cells. Th1 type of immune response is responsible for containment and control of M. tb replication and involves production of pro-inflammatory cytokines mainly TNF-α or poly-functional (IL-2, IFN-γ and TNF-α producing) CD4+ and CD8+ T lymphocytes. Moreover, a substantial effector memory response among splenocytes also indicated that Rv1509 may act as a recall antigen in a Th1 memory response to counter recurrent infection.
To conclude, we describe a hypothetical signature protein with properties to mount a strong pro-host immune response including an effector memory response as evident from a series of experiments involving in-vitro, mice and human TB patients (PCT Application filed). These findings highlight the host pathogen tussle for their individual existence and the outcome is dependent on a likely complex set of interactions with other microbial/host factors. Studies are underway to create recombinant BCG vaccine carrying Rv1509 for enhancing the memory response, target Rv1509 using drug repurposing approaches and use this signature gene as a DNA probe for specific diagnosis of TB.
