Susceptibility to nasal and oral tolerance induction to the major birch pollen allergen Bet v 1 is not dependent on the presence of the microflora.

The indigenous microflora plays an integrative role in the maintenance of immunological homeostasis. Several studies reported that immunological tolerance is dependent on microbial colonization of the gut. In the present study, we investigated whether the absence of the microflora influences the sensitization process to an allergen as well as the ability to develop mucosal tolerance in a mouse model of birch pollen allergy. Germ-free or conventional BALB/c mice were intranasally or intragastrically pre-treated with the major birch pollen allergen Bet v 1 prior to sensitization with this allergen. Both germ-free and conventional mice displayed comparable Th2 biased immune responses after allergic sensitization. Oral as well as intranasal tolerization led to suppression of allergen-specific serum antibodies (IgG1, IgE, IgA) as well as cytokine production by splenocytes (IL-5, IFN-gamma) in both germ-free and conventional animals. Peyer's patches of germ-free animals were approximately 20 times smaller than in conventional animals, but the relative distribution of lymphocyte subpopulations was equal. We conclude that the absence of the microflora does not influence the ability to mount Th2 responses nor to establish tolerance towards the aeroallergen Bet v 1. Our findings may challenge the view that the commensal microflora is a key factor for breakdown of physiological tolerance and allergy development.