Rationale: We have previously demonstrated that oral treatment with live Lactobacillus reuteri can attenuate major characteristics of the asthmatic response in a mouse model of allergic airway inflammation. However, the mechanisms underlying these effects remain to be determined.

Objectives: We tested the hypothesis that regulatory T cells play a major role in mediating L. reuteri–induced attenuation of the allergic airway response.

Methods: BALB/c mice were treated daily with L. reuteri by gavage. Flourescent-activated cell sorter analysis was used to determine CD4⁺CD25⁺Foxp3⁺T cell populations in spleens following treatment with L. reuteri or vehicle control. Cell proliferation assays were performed on immunomagnetic bead separated CD4⁺CD25⁺ and CD4⁺CD25⁻ T cells. CD4⁺CD25⁺ T cells isolated from, ovalbumin naive, L. reuteri treated mice were transferred into ovalbumin-sensitized mice. Following antigen challenge the airway responsiveness, inflammatory cell influx and cytokine levels in bronchoalveolar lavage fluid of recipient mice were assessed.

Measurements and Main Results: Following 9 days of oral L. reuteri treatment, the percentage and total number of CD4⁺CD25⁺Foxp3⁺T cells in spleens significantly increased. CD4⁺CD25⁺ cells isolated from L. reuteri–fed animals also had greater capacity to suppress T-effector cell proliferation. Adoptive transfer of CD4⁺CD25⁺ T cells from L. reuteri–treated mice to ovalbumin-sensitized animals attenuated airway hyper-responsiveness and inflammation in response to subsequent antigen challenge.

Conclusions: These results strongly support a role for nonantigen-specific CD4⁺CD25⁺Foxp3⁺ regulatory T cells in attenuating the allergic airway response following oral treatment with L. reuteri. This potent immuno-regulatory action may have therapeutic potential in controlling the Th2 bias observed in atopic individuals.