The Proportion of CD52/GPI Negative T Cells Early Following Alemtuzumab Conditioned Haematopoetic Stem Cell Transplantation Is an Independent Risk Factor for Acute GvHD

T cell depletion with in vivo alemtuzumab is used to ameliorate the graft versus host response and permit donor engraftment in allogeneic haematopoietic stem cell transplantation (allo-HSCT). Previous reports have shown that T cells lacking CD52 are often detectable during the period of early immune reconstitution with this protocol, but the clinical relevance of this cellular population is not understood.

In a cohort of 67 consecutive patients undergoing allo-HSCT between 2013-2016 we investigated the phenotype and function of the CD52-negative T cell fraction and related their presence to clinical outcome. 47 patients (70%) had a myeloid disease (AML or MDS) while 20 patients had lymphoid disease. All patients received in vivo alemtuzumab (10mg/day from day -5 for 5 days). 63 (94%) received reduced intensity conditioning chemotherapy, while 4 (6%) received a myeloablative regimen. All patients received post-transplant ciclosporin A for GvHD prophylaxis. 6 (9%) also received methotrexate, while 2 (3%) patients also received mycophenolate mofetil. Overall survival at 2 years was 68% and relapse free survival was 48%. 29% patients experienced acute GvHD (grade 2 or above) and 15% developed chronic GvHD.

CD52-negative T cells demonstrated low binding of FLAER, indicating downregulation of the glycophosphatidylinositol (GPI) anchor, although we did not detect any mutations in the PIG-A gene as is typically seen in patients with PNH. CD52-negative T cells were almost exclusively CD4⁺ and exhibited a dominant memory phenotype with only small numbers of CD25⁺ CD127^lowFoxp3⁺ regulatory T cells. They exhibited enhanced cytotoxic responses to T cell receptor stimulation in vitro. Early after allo-HSCT, the presence of a significant population of CD52 negative T cells (comprising >51% of the T cell fraction) was found to be an independent risk factor for acute GvHD. This was confirmed in a validation cohort of 28 patients obtained between 2017-2018.

These data suggest that CD52/GPI-negative T cells arise from the selective pressure of alemtuzumab in the setting of lymphopenia and homeostatic proliferation. To our knowledge this is the first study to show that CD52 negative T cells have a reduced regulatory T cell fraction, and have an association with acute GvHD. This suggests that the CD52-negative T cell fraction may represent a 'footprint' of the early alloreactive response focused within the CD4⁺ population, or contribute to an immune environment with reduced T cell tolerance. These data help to delineate the nature of T cell escape from alemtuzumab surveillance providing new insights into the early alloreactive immune response. Furthermore, this study informs the use of alemtuzumab in conditioning regimens for upcoming cellular immunotherapies.