Make friends with the fungi

Regulatory T (T_Reg) cells prevent inappropriate immune responses that can be damaging to host tissues. However, it remains unclear which antigens they respond to, particularly in humans. In this study, Bacher et al. assessed human CD4⁺ T cell responses to two ubiquitous mucosal fungi, Aspergillus fumigatus and Candida albicans. They found that high frequencies of T_Reg cells in adult blood are specific for these fungi; indeed, T_Reg cells specific for A. fumigatus exceeded in number and functionally suppressed memory T cells specific for A. fumigatus. By contrast, human blood contained similar numbers of memory T cells and T_Reg cells specific for C. albicans. The authors propose that the expansion of fungus-specific T_Reg cell populations is important for avoiding pathological immune responses. In support of this, they found that, in patients with severe allergic reactions against A. fumigatus, the antigen-specific T cell response to the fungus is dominated by memory T cells as opposed to T_Reg cells.