Regulatory T cells induce activation rather than suppression of human basophils.

Basophils are a rare granulocyte population that has been associated with allergic and inflammatory responses. It is essential to understand the regulatory mechanisms by which basophils are kept in check, considering the impact of dysregulated basophil function on immune responses under different pathological conditions. Among immunoregulatory cells, CD4CD25FoxP3 regulatory T cells (T) are the key players that maintain immune tolerance. The mechanisms by which T regulate and suppress diverse immune cell subsets have been studied extensively, but the impact of T on basophil functions is not well understood. We report that human basophils are refractory to T-mediated suppression and found that T stimulate resting basophils to induce the expression of activation markers including CD69, CD203c, and CD13 and the release of basophil cytokines including IL-13, IL-8, and IL-4. Mechanistically, T could induce human basophil activation via IL-3 and STAT5 activation, whereas cellular contact was dispensable. Inhibition of either IL-3-IL-3 receptor interactions or STAT5 phosphorylation abrogated T-mediated activation of basophils. These results provide evidence of direct positive effects that human T have on basophil activation and reveal a previously unrecognized feature of this cell subset well known for immunosuppressive functions.