Inhibitory Activity of Fenoterol on Dermatophagoides-, Parietaria-, Tetanus-Toxoid-, and Candida albicans-Stimulated Blood Mononuclear Cells: Differences in β₂-Adrenoreceptor Stimulation but Not in Cell Apoptosis

Abstract

β₂-adrenoreceptor agonists have the ability to downregulate in vitro the proliferative response of peripheral blood mononuclear cells (BMCs). This activity could be related to a variety of β₂-adrenoreceptor-mediated functions, including induction of cell apoptosis in activated T-cells. To test this hypothesis, BMCs from atopic subjects, sensitized to house dust mites (Dermatophagoides [Der p]) and/or to Parietaria were incubated with fenoterol (10⁻⁸-10⁻⁵M) in the presence of (a) purified allergen extracts (Der p [5 μg/mL] or Parietaria [5 μg/mL]) or (b) antigens (tetanus toxoid [1 μg/mL] or Candida albicans [5 ± 10⁵ bodies/mL]). The BMC proliferation was assessed by [³H] thymidine incorporation and cell apoptosis was assessed by evaluating DNA fragmentation by a fluorescence technique, using propidium iodide. In cultures stimulated with Der p or with Parietaria, fenoterol induced a dose-dependent inhibition of BMC proliferation, significant also at the lowest concentration tested (10⁻⁸M) (p < 0.05, each comparison). In contrast, the inhibitory activity of the drug on tetanus-toxoid-stimulated BMCs was significant only at the highest dose tested (10⁻⁵M) (p < 0.05), whereas no effect was seen when BMCs were stimulated with C. albicans extract (p > 0.05). The different inhibitory efficacy of fenoterol appeared to be related to the degree of activation of pYadrenoreceptors on the different BMC populations that responded to the different stimuli, indeed, in the presence of fenoterol (10⁻⁶ and 10⁻⁵M), a significant increase in cyclic adenosine monophosphate (cAMP) levels was seen in Der p- or Parietaria-stimulated cells (p < 0.05; each comparison), but not in cell cultures stimulated with tetanus toxoid or with C. albicans extracts (p > 0.05; each comparison). Finally, the percentage of cells with fragmented DNA was lower in cultures stimulated with Der p or Parietaria than in those stimulated with tetanus toxoid or C. albicans, and the presence of fenoterol did not modify cell apoptosis (p > 0.05; each comparison). Thus, the different inhibitory activity of fenoterol on BMCs activated by allergens (Der p or Parietaria) or by antigens (tetanus toxoid or C. albicans) seems to be related to differences in β²-adrenoreceptor expression and/or function in the different antigen-specific T-cell subsets, but it is not influenced by changes in cell apoptosis.

Key Words:

• Mononuclear cells
• Fenoterol
• Allergens
• Cyclic AMP
• Apoptosis