Abstract
Several lines of evidence have recently suggested that natural killer (NK) cells develop immunological memory against viral infections. However, there is no apparent evidence that NK cells acquire specific memory against Mycobacterium bovis bacillus Calmette—Guérin (BCG), the only currently licensed vaccine for preventing tuberculosis. In the present study, we investigated whether murine splenic NK cells can be activated by BCG in a dendritic cell (DC)-independent or -dependent manner, and furthermore examined whether these NK cells acquire specific memory following BCG vaccination. NK cells isolated from spleens of BCG-immunized mice produced interferon (IFN)γ through direct BCG stimulation in the absence of antigen-presenting cells; however, NK cells from control animals similarly directly responded to BCG, and the response level was not statistically significant between the immunized and the naïve NK cells. When purified NK cells that had been exposed to BCG were cocultured with RAW murine macrophages infected with BCG, the antibacterial activity of the macrophages was strongly enhanced; however, its level was similar to that by naïve NK cells, which had not been exposed to BCG. When splenocytes harvested from BCG-immunized mice were stimulated with purified protein derivative (PPD) derived from Mycobacterium tuberculosis, a specific IFNγ response was clearly observed, mainly attributed to NK cells and memory CD4+ T cells. To investigate whether these NK cells as well as the T cells are activated by cell−cell interaction with DCs presenting mycobacterial antigens, NK cells isolated from BCG-immunized mice were cocultured with splenocytes harvested from naïve mice in the presence of PPD stimulation. However, no IFNγ response was found in the NK cells. These results suggest that murine splenic NK cells do not develop BCG-specific immunological memory in either a DC-independent or -dependent manner.
Highlights
Mycobacterium tuberculosis, the etiological agent of tuberculosis, primarily infects macrophages and dendritic cells (DCs)
Murine splenic natural killer (NK) cells are directly activated by bacillus Calmette— Guérin (BCG) and purified protein derivative (PPD) antigen To assess whether murine splenic NK cells can be directly activated by BCG and PPD antigen in an antigen-presenting cell (APC)-independent manner, we isolated NK cells from spleens of naïve mice, stimulating them either with BCG or PPD for 24 h, and measured the level of IFNγ produced in the culture supernatants as an activation marker
We have demonstrated that: 1) purified murine splenic NK cells produced IFNγ through direct stimulation with BCG or PPD antigen in the absence of the mediation of APCs; 2) the magnitude of the BCG-induced IFNγ response of NK cells in the BCG-immunized mice was similar to that in phosphate-buffered saline (PBS) control animals; and 3) the activity of RAW macrophage cells to eradicate BCG was remarkably enhanced by NK cells; the contribution of the NK cell to the event did not differ between the naïve and BCG-sensitized NK cells
Summary
Mycobacterium tuberculosis, the etiological agent of tuberculosis, primarily infects macrophages and dendritic cells (DCs). IFNγ in particular plays an important role in inducing resistance to M. tuberculosis infection. The IFNγ has been shown to induce CD8+ T cell-mediated protective immunity against the bacteria in mice [5]. While T cells exert the induction of acquired immune responses, NK cells are considered to contribute to evoking early protective immunity against many intracellular pathogens because of their ability to produce IFNγ during innate immune responses [6,7,8,9,10]. The role of NK cells in contributing resistance to intracellular bacterial infections including M. tuberculosis remains poorly understood [11, 12]
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