Mycobacterium indicus pranii as a booster vaccine enhances BCG induced immunity and confers higher protection in animal models of tuberculosis

Abstract

BCG, the only approved vaccine protects against severe form of childhood tuberculosis but its protective efficacy wanes in adolescence. BCG has reduced the incidence of infant TB considerably in endemic areas; therefore prime-boost strategy is the most realistic measure for control of tuberculosis in near future. Mycobacterium indicus pranii (MIP) shares significant antigenic repertoire with Mtb and BCG and has been shown to impart significant protection in animal models of tuberculosis. In this study, MIP was given as a booster to BCG vaccine which enhanced the BCG mediated immune response, resulting in higher protection. MIP booster via aerosol route was found to be more effective in protection than subcutaneous route of booster immunization. Pro-inflammatory cytokines like IFN-γ, IL-12 and IL-17 were induced at higher level in infected lungs of ‘BCG-MIP’ group both at mRNA expression level and in secretory form when compared with ‘only BCG’ group. BCG-MIP groups had increased frequency of multifunctional T cells with high MFI for IFN-γ and TNF-α in Mtb infected mice. Our data demonstrate for the first time, potential application of MIP as a booster to BCG vaccine for efficient protection against tuberculosis. This could be very cost effective strategy for efficient control of tuberculosis.

Introduction

Tuberculosis is a serious and substantial threat to health worldwide resulting in death of about 1.5 million people per annum. Chemotherapy is the only way to cure the disease but it also has limitations in MDR and XDR form of tuberculosis. Further its high cost and long treatment duration restricts the success of chemotherapy. Effective vaccination could be the most convenient and cost effective measure to reduce the tuberculosis associated morbidity and mortality. Current vaccine BCG has saved the lives of many children by protecting against disseminated tuberculosis but its efficacy wanes in adolescence, resulting in increasing number of TB cases in young adults.

Since BCG gives protection against severe form of childhood tuberculosis [1], it is more logical to give a booster vaccine for protection against adult pulmonary tuberculosis till the time a radically effective vaccine is available to replace the BCG.

As BCG mediated protection is not improved by a subsequent injection of BCG later in life, enhancing the BCG mediated immunity by sub-unit vaccine or by any other vaccine comprising of non-pathogenic mycobacterium sharing antigens with BCG and Mtb, could be a good approach towards the development of an efficacious vaccine regimen. Till now several vaccines have been developed either to replace BCG vaccine with more superior vaccine or subunit vaccines that could boost BCG mediated protection; few of these are under clinical trials or preclinical development [2]. Majority of these vaccines are recombinant subunit vaccines which have the potential to boost BCG generated immune response but large scale production and high cost of these vaccines is a limiting factor [3], [4], [5], [6].

Mycobacterium indicus pranii (MIP), a saprophytic, non-pathogenic mycobacterium, has been shown to have strong immunomodulatory properties and modulates the immune response towards Th1 type [7], [8], [9]. MIP shares antigenic repertoire with both M. tuberculosis and M. leprae [10] and it was first evaluated as a therapeutic vaccine against leprosy [11]. Analysis of homologous protein coding genes revealed that ∼82% of Mtb proteins have homology with MIP proteome [12]. Similar comparison between MIP and BCG at 70% cut-off, showed that 55% BCG proteins have homology with MIP proteome (unpublished report). MIP also shares larger repertoire of highly putative antigenic PE/PPE proteins of M. tuberculosis as compared to other vaccine candidate like M. vaccae [13], [14]. MIP modulates immune response towards Th1 type which is crucial for protection against intracellular pathogens. Such unique attributes of MIP have gained attention for evaluation of its protective efficacy against tuberculosis besides leprosy.

In a double blind clinical trial (involving about 29,000 household contacts of leprosy patients) where MIP was used as prophylactic vaccine against leprosy, retrospective analysis after 13 years revealed that MIP gave significant protection against TB also, in highly endemic area. Another important observation was that incidence of tuberculosis was much less in the sub group which received BCG during childhood [15]. Later on protective efficacy of MIP was evaluated in susceptible guinea pig model where MIP was given by parenteral/aerosol route and aerosol immunization of MIP provided higher protection as compared to s.c. route [16], [17]. Immunization through respiratory route induces efficient local lung immune response. Later, when Mtb enters the airways, quick homing of antigen specific cells as well as resident memory cells in the alveolar lumen and lung can effectively control the establishment of infection [18], [19], [20].

Based on the above facts, we hypothesized that booster with MIP aerosol may enhance the memory immune response against antigens which are shared between BCG and MIP, besides inducing immune response against its own unique antigens which possibly would result in higher protection against Mtb.

In this report, we have studied the protective efficacy of MIP, given as a booster to BCG via aerosol or s.c. route. These studies were carried out in susceptible guinea-pig model. We have also evaluated protective immune correlates of protection and qualitative T cell response including induction of multi-functional T cells in mice model of tuberculosis.