Macrophage reprogramming by mycolic acid promotes a tolerogenic response in experimental asthma.

Rationale: Mycolic acid (MA) constitutes a major and distinguishing cell wall biolipid from Mycobacterium tuberculosis. MA interferes with the lipid homeostasis of alveolar macrophages, inducing differentiation into foamy macrophages exhibiting increased proinflammatory function.
Objectives: We verified the interference of this altered macrophage function with inhaled antigen-triggered allergic airway inflammation and underlying Th2 lymphocyte reactivity.
Methods: Using ovalbumin (OVA) as model allergen, C57BL/6 or BALB/C mice were sensitized by OVA-alum immunization. Experimental asthma, triggered subsequently by repetitive nebulized OVA inhalation, was assessed, using as readout parameters eosinophilia, peribronchial inflammation, and Th2 cytokine function.
Measurements and Main Results: A single intratracheal treatment of sensitized mice with MA, inserted into liposomes as carriers, prevented the onset of OVA-triggered allergic airway inflammation and promoted unresponsiveness to a secondary set of allergen exposures. The development of this tolerant condition required an 8-d lapse after MA instillation, coinciding with the appearance of foamy alveolar macrophages. MA-conditioned CD11b(+)F4/80(+) macrophages, transferred to the airways, mimicked the tolerogenic function of instilled MA; however, without the 8-d lapse requirement. Indicative of a macrophage-mediated tolerogenic antigen-presenting function, major histocompatibility complex (MHC)-mismatched donor macrophages failed to promote tolerance. Furthermore, Treg markers were strongly increased and established tolerance was lost after in situ depletion of CD25(+) Treg cells. Contrary to the interleukin-10 dependence of tolerogenic dendritic cells, IFN-gamma deficiency but not interleukin-10 deficiency abrogated the tolerogenic capacity of MA-conditioned macrophages.
Conclusions: These results document an innate-driven Mycobacterium tuberculosis MA-triggered immune regulatory mechanism in control of pulmonary allergic responses by converting macrophages into IFN-gamma-dependent tolerogenic antigen-presenting cells.