Skin CD4+ Memory T Cells Exhibit Combined Cluster-Mediated Retention and Equilibration with the Circulation

The study demonstrated that memory CD4(+) T cells in mouse skin maintain a balance between residing in the tissue and circulating in the blood. These cells are distributed in the dermis and cluster around hair follicles with antigen-presenting cells. Following infection or exposure to a sensitizing agent, there is a significant and sustained increase in the number of CD4(+) T cells within these clusters. This increase is associated with a rise in CD4(+) T cell recruitment driven by the chemokine CCL5, which is produced by CD11b(+) cells and CD8(+) T cells. Notably, removing CD8(+) T cells leads to a reduction in CD4(+) T-cell numbers. The findings indicate that the presence and dynamics of CD4(+) memory T cells in the skin are influenced by both the tissue's retention mechanisms and its history of infection and inflammation.