Antigen‐specific CD4+CD25+ T cells induced by locally expressed ICOS‐Ig: the role of Foxp3, Perforin,Granzyme B and IL‐10 ‐ an experimental study

We have previously reported that ICOS‐Ig expressed locally by a PIEC xenograft induces a perigraft cellular accumulation of CD4⁺CD25⁺Foxp3⁺ T cells and specific xenograft prolongation. In the present study we isolated and purified CD4⁺CD25⁺ T cells from ICOS‐Ig secreting PIEC grafts to examine their phenotype and mechanism of xenograft survival using knockout and mutant mice. CD4⁺CD25⁺ T cells isolated from xenografts secreting ICOS‐Ig were analysed by flow cytometry and gene expression by real‐time PCR. Regulatory function was examined by suppression of xenogeneic or allogeneic primed CD4 T cells in vivo. Graft prolongation was shown to be dependent on a pre‐existing Foxp3⁺ Treg, IL‐10, perforin and granzyme B. CD4⁺CD25⁺Foxp3⁺ T cells isolated from xenografts secreting ICOS‐Ig demonstrated a phenotype consistent with nTreg but with a higher expression of CD275 (ICOSL), expression of CD278 (ICOS) and MHC II and loss of CD73. Moreover, these cells were functional and specifically suppressed xenogeinic but not allogeneic primed T cells in vivo.