γδ T Cells and Allergic Diseases

Gamma-delta (?d) T cells play an essential role in allergic diseases and have emerged as a potential treatment target in recent decades. To clarify the effects of ?d T cells on atopic illnesses, we reviewed the literature on the physical roles and functions of various subsets of ?d T cells, including type 1 T helper (Th1)-like, type 2 T helper- (Th2)-like, and type 17 T helper (Th17)-like ?d T cells. Mouse V?1 T cells increase interleukin (IL)-4 levels and trigger B cell class switching and immunoglobulin E production. Meanwhile, mouse V?4 T cells and human CD8(low)Vd1 T cells secrete interferon-? and exert an anti-allergy effect similar to that of Th1 cells. Moreover, mouse V?6 T cells produce IL-17A, while Th17-like ?d T cells enhance neutrophil and eosinophil infiltration in the acute phase of inflammation, but exert anti-inflammatory effects in the chronic phase. Human V?9d2 T cells may exhibit Th1- or Th2-like characteristics in response to certain types of stimulation. In addition, the microbiota can modulate epithelial ?d T cell survival through aryl hydrocarbon receptors; these ?d T cells play crucial roles in the repair of epithelial damage, antibacterial protection, antigen tolerance, and effects of dysbiosis on allergic diseases.