The Proportion of CD52/GPI Negative T Cells Early Following Alemtuzumab Conditioned Haematopoetic Stem Cell Transplantation Is an Independent Risk Factor for Acute GvHD
Authors: Kinsella Francesca , Inman Charlotte F , Murray Duncan , Croft Wayne , Zuo Jianmin , Pearce Hayden , Barbieri Sara , Nunnick Jane , Nagra Sandeep , Craddock Charles , Malladi Ram K , Moss Paul 
Published: November 2019

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+ CD127lowFoxp3+ 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.

Disclosures
No relevant conflicts of interest to declare.