Stromal cells' B7-1 is a key stimulatory molecule for interleukin-10 production by HOZOT, a multifunctional regulatory T-cell line

We have previously shown that xenogeneic stromal cell stimulation of naïve T cells resulted in the generation of a new type of regulatory T (Treg) cell termed HOZOT, which has multifunctional properties and a CD4/CD8 double‐positive phenotype. Even after the establishment of HOZOT, stromal cells can function as an antigen‐presenting cell (APC) by inducing these cells to produce interleukin (IL)‐10. When compared with other stimuli, stromal cells showed an IL‐10‐producing ability comparable to anti‐CD3 antibody (Ab) stimulation, and much greater than dendritic cell (DC) stimulation. Distinct from professional APCs, stromal cells express only major histocompatibility complex (MHC) class I and B7‐1 costimulatory molecules, and not MHC class II or other costimulatory molecules, such as ICOSL (CD275), PD‐L1 (CD274), PD‐L2 (CD273), CD40, OX40L (CD252) and 4‐1BBL (CD137L) in the absence of stimulation. Blocking experiments revealed that, in addition to anti‐H‐2Kd Ab and anti‐human CD8 Ab, anti‐mouse B7‐1 Ab could effectively block IL‐10 production, indicating a key role of the B7‐1/CD28 pathway. Using stromal cells expressing different levels of B7‐1, IL‐10 production correlated with the levels of B7‐1 expression. Distinct from ICOSL or PD‐L1 expressed on DCs (which are regarded as IL‐10‐inducing costimulatory molecules), this study showed that B7‐1 on stromal cells is a key molecule regulating IL‐10 production by multifunctional Treg cells, HOZOT.