Functionally Altered GPI-Anchor Negative Treg Following Alemtuzumab-Based T-Cell Depletion Are Associated with Acute Gvhd.

The monoclonal anti-CD52antibody Alemtuzumab is frequently used for T-cell depletion (TCD) in the context of allogeneic hematopoietic stem cell transplantation (HSCT) to prevent graft versus host disease (GVHD). We previously demonstrated the long term persistence of functionally impaired glycosylphosphatidylinositol (GPI)-anchor negative effector T-cells in patients receiving high dose (100mg) Alemtuzumab in combination with a dose reduced conditioning regimen (Fludarabin + Melpahlan) (Meyer, Wagner et al. BMT 2010). Despite of Alemtuzumab-mediated TCD, half of our patients developed acute GVHD. Since regulatory T cells (Treg) play a major role for controlling GVHD, we asked whether GPI-anchor negative Treg are present in patients with or without GVHD.

We analyzed peripheral blood samples of 12 patients with acute GVHD (aGVHD), 7 patients with chronic GVHD (cGVHD), and 10 patients who never developed GVHD after Alemtuzumab-mediated TCD. To analyze Treg-subsets, we stained for CD3, CD4, CD25, CD127, FoxP3, CD52 as well as for the activation-markers GARP, HLA-DR and CD45RA. Treg were identified as CD3+CD4+CD25+CD127- or CD3+CD4+CD25+FoxP3+ cells and subdivided according to their CD52-expression. We used FLAER staining to confirm that the loss of CD52 on Treg resulted from the loss of the GPI-anchors themselves. We were able to study Treg subpopulations in the time course of patients who recovered from acute GVHD in comparison to patients with persisting late acute GVHD. In individual patients, we isolated GPI-anchor positive and negative Treg by FACS-Sort, expanded them and performed Treg suppression assays.

GPI-anchor negative Treg were observed in all patients, independent of the development of GVHD. However, the frequency of GPI-anchor negative Treg varied considerably between patients with acute GvHD and those with chronic GVHD or without GvHD. The percentage of GPI-anchor negative Treg was significantly elevated in patients with aGVHD: median 80.35% (range 56,2–96,8%) in comparison to 17,4% (range 0–57,8%) in patients with cGVHD or without GVHD. Activated Treg were almost exclusively detected among GPI-anchor positive Treg-subpopulation. Patients who resolved from aGVHD restored GPI-anchor positive Treg and the amount of activated Treg rose. The percentage of GPI-anchor negative Treg populations remained high in patients with ongoing aGVHD. In addition, these patients had no GARP-positive activated Treg even under long term immunosuppressive treatment. Preliminary experiments with sorted and expanded Treg populations suggest that GPI-anchor negative Treg were unable to suppress T-cell proliferation upon IL-2 stimulation.

We demonstrate for the first time the reconstitution of GPI-anchor negative Treg in patients following Alemtuzumab-mediated TCD. These T cells were functionally altered and were less likely to exhibit an activated phenotype in vivo. Ongoing acute GVHD was associated with high percentages of GPI-negative Treg suggesting that their functional alteration might play a role in aGVHD pathophysiology. This is in line with the finding that only in patients who resolved aGVHD, the frequency of GPI-anchor positive Treg increased significantly. Further functional analyses are ongoing to estimate the cellular consequence of missing GPI-anchored proteins. In addition, correlating the reconstitution of GPI-anchor negative T-cell populations with further clinical events is ongoing.

No relevant conflicts of interest to declare.