CTLA-4 Blockade following Relapse of Malignancy after Allogeneic Stem Cell Transplantation Is Associated with T Cell Activation But Not with Increased Levels of T Regulatory Cells

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is a key negative regulator of T cell activation and proliferation. Ipilimumab is a human monoclonal antibody that specifically blocks the binding of CTLA-4 to its ligand. To test the hypothesis that blockade of CTLA-4 by ipilimumab could augment graft-versus-malignancy (GVM) effects without a significant impact on graft-versus-host disease (GVHD), we conducted a phase I clinical trial of ipilimumab infusion in patients with relapsed malignancy following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Here, we report the analysis of peripheral blood T lymphocyte reconstitution, T regulatory cell (Treg) expression, and T cell activation markers after a single dose of ipilimumab in 29 patients. Peripheral blood samples were collected from all patients before and after ipilimumab infusion. Lymphocyte immunophenotyes, including levels of CD4⁺CD25^high cells and T cell activation markers, were analyzed in all cases. Levels of CD4⁺CD25^highFoxp3⁺ cells and intracellular CTLA-4 in CD4⁺ T cells also were evaluated in the last 11 cases. We found lower baseline levels of CD4⁺ and CD45RO⁺ T cells in patients compared with normal controls. More than 50% of the patients had abnormally low lymphocyte counts (CD4 or/and CD8 T cells), and some had no circulating B lymphocytes. The percentages of both CD4⁺CD25^high and CD4⁺CD25^highFoxp3⁺ T cells were significantly higher in patients before ipilimumab infusion than in healthy donors. Twenty of 29 patients exhibited an elevated level of CD4⁺CD25^low activated T cells at baseline, compared with only 3 of 26 healthy donors. Both CD4⁺ and CD8⁺ T lymphocyte counts were significantly increased after ipilimumab infusion. There was no consistent change in absolute lymphocyte count or in the number of T cells expressing the activation marker CD69. However, increases in CD4⁺CD25^low T cells were seen in 20 of 29 patients and increases in CD4⁺HLA-DR⁺ T cells were seen in the last 10 patients in the first 60 days after ipilimumab infusion. Although the percentages of both CD4⁺CD25^high and CD4⁺CD25^highFoxp3⁺ T cells decreased significantly during the observation period, the absolute cell counts did not change. Intracellular CTLA-4 expression in CD4⁺CD25^lo/- T cells increased significantly after ipilimumab infusion. We conclude that CTLA-4 blockade by a single infusion of ipilimumab increased CD4⁺ and CD4⁺HLA-DR⁺ T lymphocyte counts and intracellular CTLA-4 expression at the highest dose level. There was no significant change in Treg cell numbers after ipilimumab infusion. These data demonstrate that significant changes in T cell populations occur on exposure to a single dose of ipilimumab. Further studies with multiple doses are needed to explore this phenomenon further and to correlate changes in lymphocyte subpopulations with clinical events.