Cord Blood CD4+CD25+ Derived T Regulatory Cell Lines Express FoxP3 Protein and Manifest Potent Suppressor Function.

CD4+CD25+ T regulatory (Treg) cells have been shown to critically regulate self and more recently allograft tolerance in mice. Studies of human Treg have been hindered by the presence of CD25-dim conventional T cells that copurify during Treg isolation. We compared adult and cord blood sources for Treg, with the hypothesis that cord blood should lack significant numbers of memory cells or environmentally reactive T cells. We found that cord blood was a superior source for Treg isolation compared to adult blood. CD4+CD25+ cells were readily purified, and generated cell lines that consistently exhibited potent suppressor activity, with >95% suppression of allogeneic MLR (29/30 donors). The cells could markedly suppress an allo-MLR at a 1/16-1/32 suppressor/responder cell ratio. The cultured Treg cells blocked cytokine accumulation in MLR, with a less robust inhibition of chemokine production.

Cultured Treg uniformly expressed CD25, CD62L, CCR7, CD27, and intracellular CTLA4. Upon re-stimulation with anti-CD3/CD28 beads, the cultured Treg produced minimal cytokines (IL2, IFN-gamma, and IL10), and preferentially expressed TGF-beta latency associated protein on the cell surface, while conventional CD4+CD25− derived cell lines did not. Cytokine production however, could be largely restored by stimulation with PMA/ionomycin. Abundant FoxP3 mRNA was detected in fresh, cultured, and anti-CD3/CD28 bead restimulated CD4+CD25+ cells (approximately 2–4 fold less than cyclophillin A). Low amounts of FoxP3 mRNA were present in fresh CD25− cells, and they increased 32 fold on culture. However, FoxP3 protein, as assessed by western blot, was specifically expressed in the CD25+ derived cell lines, and was not detected in the CD25− derived cell lines. Restimulation with anti-CD3/CD28 beads led to increased expression of FoxP3 protein in CD25+ derived cell lines, but not in CD25− derived cell lines.

Cord blood derived cultured suppressor cells were not cytolytic in chromium release assays, and mediated suppression of alloreactivity that was cell contact dependent and predominantly independent of IL10 and TGF-beta. Antibodies to GITR, GITR-L, OX40, OX40-L, CTLA4, and PD1 did not significantly affect suppression of the culture activated Treg cells. These results demonstrate virtually pure populations of potent suppressor cells can be cultured from cord blood, and these cell lines form an ideal model system for the evaluation of suppressor cell biology and mechanisms of action.