Extended freeze‐dried Mycobacterium bovis Bacillus Calmette–Guérin induces the release of interleukin‐12 but not tumour necrosis factor‐α by alveolar macrophages, both in vitro and in vivo

Abstract

Airway hyper-responsiveness (AHR), chronic airway inflammation and predominance of the T helper type-2 (Th2; IL-4, IL-5, IL-13) over the Th1 (IL-2, IFN-gamma) immune response are hallmarks of asthma. Alveolar macrophages (AM) are the most numerous cells in the airway lumen, where they represent the first immune cell population encountered by inhaled antigens. AM act as antigen-presenting cells (APC) and they release various soluble mediators and enzymes. AM thus play a prominent role in the modulation of the local immunity in airways. In allergic airways, AM have been implicated in the pathogenesis of inflammation by promoting the Th2 versus the Th1 cytokine patterns. Infections with attenuated bacteria or challenges with bacterial products may involve AM. Such stimuli have been shown to potentially restore the Th1/Th2 balance in asthmatic airways, but they also induce the release of inflammatory mediators. We investigated the response of AM when stimulated by two preparations of non-proliferating Bacillus Calmette-Guérin (BCG). We evaluated the cytokine production by AM from BP2 and C57BL/6 mice when cultured with heat-killed (HK) and extended freeze-dried (EFD) BCG. We then investigated in vivo the release of soluble factors in the airway lumen of mice after instillation of these BCG preparations. Finally, we studied the profile of cytokine transcripts in the lung of mice pre-treated with BCG and then challenged with ovalbumin (OVA). HK BCG induced the production of both TNF-alpha and IL-12, and did not prevent high levels of Th2 cytokine transcripts. In contrast, EFD BCG induced a response dominated by the production of IL-12, with no later over-expression of Th2 cytokine transcripts. Our results show that EFD BCG induce the release of the Th1-promoting cytokine IL-12 by AM, without the deleterious effects of HK BCG. These data suggest that EFD BCG may be considered as a potential novel treatment to restore the Th1/Th2 imbalance in asthma.