Analysis of Peripheral Blood CD4+/CD25^high Treg Cells and FOXP3 mRNA Expression in Patients Treated with Ipilimumab (Monoclonal Human Anti-CTLA4, MDX-010) for Relapse of Malignancy Following Allogeneic Hematopoietic Transplantation.

CTLA-4 is expressed upon activation of T-cells,and serves as an important negative regulator of their effector function. It is also expressed constitutively on CD4+/CD25+ regulatory T cells (Treg), where its function is not clear. Following allogeneic hematopoietic stem cell transplantation (allo-HCT), CTLA-4 function may be involved in suppression of alloreactive T cells that mediate the graft-versus malignancy effect and GVHD. We have studied the administration of a single dose of Ipilimumab (MDX-010), a fully human monoclonal anti-CTLA-4 antibody, in a dose escalation trial in patients with relapse/progression of malignancy following allo-HCT. Here we report effects of ipilimumab on peripheral CD4+/CD25+ cell levels and FOXP3 mRNA expression in these patients. Seventeen patients with a variety of malignancies were enrolled in this study. Ipilimumab was given intravenously at a dose level of 0.1, 0.33, 0.66, 1, and 3mg/kg. The blood samples were obtained prior and after infusion at day 7, 14 and 30. The immunophynotyping of peripheral blood mononuclear cells (PBMC) was analyzed by flow cytometry. CD4+/CD25+ cells from nine patients at day 0, 7, and five normal donors were separated using a Dynal CD4+/CD25+ Treg kit. FOXP3 mRNA expression on CD4+/CD25+ cells were analyzed by a quantitative RT-PCR. Expression level of FOXP3 was normalized to 18S rRNA. Within CD4+ cell population, the percentage of CD4+/CD25^high cells was significantly higher in patients at day 0 (11.6±6.7%, n=17), compared with normal donors (3.8±1.6%, n=12; P<0.001). There was no significant change in CD4+/CD25^high Treg cells in 17 patients after infusion. However, there was a transient 55±24% decrease of CD4+/CD25^high Treg cells in 6 patients at day 7. FOXP3 mRNA expression in CD4+/CD25+ cells was significantly higher in patients at day 0 (2451±1731, n=9) compared with normal donors (918±348, n=5; P<0.001). After ipilimumab infusion, 3/9 patients showed a greater than 50% decrease, 4/9 patients showed no significant change, and 2/9 patients showed a 3–10 fold increase of FOXP3 mRNA expression at day 7. There was no correlation between the dose of ipilimumab and the percentage of CD4+/CD25^high cells, or the FOXP3 mRNA expression. We did not observe a correlation of CD4+/CD25^high Treg cells and FOXP3 mRNA expression in patients who had a clinical response or immune breakthrough adverse events in response to ipilimumab. These data suggest that in-vivo CTLA-4 blockade does not consistently impact the number of CD4+ CD25^high Treg cells and that the clinical effects observed are probably related to the effects of ipilimumab upon activated effector T-cells.