Research Article: The Glycosylated Rv1860 Protein of Mycobacterium tuberculosis Inhibits Dendritic Cell Mediated TH1 and TH17 Polarization of T Cells and Abrogates Protective Immunity Conferred by BCG

Date Published: June 12, 2014

Publisher: Public Library of Science

Author(s): Vijaya Satchidanandam, Naveen Kumar, Rajiv S. Jumani, Vijay Challu, Shobha Elangovan, Naseem A. Khan, David M. Lewinsohn.


We previously reported interferon gamma secretion by human CD4+ and CD8+ T cells in response to recombinant E. coli-expressed Rv1860 protein of Mycobacterium tuberculosis (MTB) as well as protection of guinea pigs against a challenge with virulent MTB following prime-boost immunization with DNA vaccine and poxvirus expressing Rv1860. In contrast, a Statens Serum Institute Mycobacterium bovis BCG (BCG-SSI) recombinant expressing MTB Rv1860 (BCG-TB1860) showed loss of protective ability compared to the parent BCG strain expressing the control GFP protein (BCG-GFP). Since Rv1860 is a secreted mannosylated protein of MTB and BCG, we investigated the effect of BCG-TB1860 on innate immunity. Relative to BCG-GFP, BCG-TB1860 effected a significant near total reduction both in secretion of cytokines IL-2, IL-12p40, IL-12p70, TNF-α, IL-6 and IL-10, and up regulation of co-stimulatory molecules MHC-II, CD40, CD54, CD80 and CD86 by infected bone marrow derived dendritic cells (BMDC), while leaving secreted levels of TGF-β unchanged. These effects were mimicked by BCG-TB1860His which carried a 6-Histidine tag at the C-terminus of Rv1860, killed sonicated preparations of BCG-TB1860 and purified H37Rv-derived Rv1860 glycoprotein added to BCG-GFP, but not by E. coli-expressed recombinant Rv1860. Most importantly, BMDC exposed to BCG-TB1860 failed to polarize allogeneic as well as syngeneic T cells to secrete IFN-γ and IL-17 relative to BCG-GFP. Splenocytes from mice infected with BCG-SSI showed significantly less proliferation and secretion of IL-2, IFN-γ and IL-17, but secreted higher levels of IL-10 in response to in vitro restimulation with BCG-TB1860 compared to BCG-GFP. Spleens from mice infected with BCG-TB1860 also harboured significantly fewer DC expressing MHC-II, IL-12, IL-2 and TNF-α compared to mice infected with BCG-GFP. Glycoproteins of MTB, through their deleterious effects on DC may thus contribute to suppress the generation of a TH1- and TH17-dominated adaptive immune response that is vital for protection against tuberculosis.

Partial Text

The scourge of tuberculosis which claimed close to a million non-HIV infected victims in 2011 worldwide [1] aided by multiple (MDR) and extremely drug resistant (XDR) strains [2] of the causative organism Mycobacterium tuberculosis (MTB), has entrenched itself in the human population in its latent form and is undisputedly one of the most dreaded human bacterial diseases. MTB employs multiple mechanisms to interfere with both the innate and adaptive arms of the vertebrate immune system. These include inhibition of (i) phagolysozome fusion within antigen presenting cells [3], (ii) maturation of human monocytes into DC [4], (iii) dendritic cell migration to secondary lymphoid organs [5] as well as antigen processing and presentation to T cells [6], [7]. In addition, MTB-infected macrophages, but not DC, prevented the development of a TH1-polarized T cell response [8].

Immune protection against MTB is an extremely complex phenomenon where the multiple cells of the immune system along with their mediators work both in concert and at cross purposes, often succeeding and occasionally (in 5 to 10% of individuals) failing to achieve just the right balance required to prevent disease {reviewed in O’Garra, 2013 #183}. Subversion of cells comprising the innate immune system by MTB has been extensively documented. Macrophages, dendritic cells and neutrophils are all infected by MTB in the mouse model of aerosol infection [5] with resultant subversion of their functions. In this model, DC were demonstrated to be one of the earliest infected population, second only to neutrophils. Following intranasal inoculation of mice with as little as one million BCG, 5% of lung cells were infected by 24 hrs, of which 15% were DC [49], corroborated by another study that demonstrated phagocytosis of BCG by almost 40% of lung DC by 24 hrs post intranasal infection [50]. This latter study also showed the superior production of IL-12 and TH1 polarization of CD4+ T cells by recruited DC over alveolar macrophages. Both these studies reproducibly implicated both alveolar macrophages and DC as the predominantly infected cells early in lung infection. Both in infected mouse lung and spleens, the subset of DC infected early were lymphoid [47], [50], replaced by myeloid DC post 2 weeks following infection [5], [47]. DC, despite constituting a mere 6.8% of total lung tissue, were found to represent 50% of BCG-infected cells in lungs by 2 to 3 weeks post low dose aerosol infection [5]. All these studies point to the high propensity of tissue DC to be infected regardless of route of infection by the slow growing mycobacteria. Keeping in mind that immature DC are highly phagocytic, it is not surprising that both in vivo and in vitro, DC are efficiently infected, not to mention the numerous pathogen recognition receptors that they boast of, many of which such as DC-SIGN and mannose receptor, have already been shown to play a role in internalization of mycobacteria [5], [51], [52].




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