Intra-Lineage Phenotypic Switch in T-Cell Prolymphocytic Leukemia.
Authors: Osuji Nnenna , Giudice Ilaria Del , Matutes Estella , Brito-Babapulle Vasantha , Morilla Alison , Owuso-Ankomah Kwasi , Morilla Ricardo , Catovsky Daniel 
Published: October 2019

Abstract
T-cell prolymphocytic leukemia (T-PLL) is a rare T-cell disorder characterized by an aggressive clinical course and short survival. Immunophenotypically, the leukemic cells show heterogeneous CD4 and CD8 expression; most cases are CD4+CD8− but, CD4+CD8+ and CD4−CD8+ cases are recognized. CD4+CD8+ phenotype is conventionally associated with cortical thymocytes, however, double positive T-prolymphocytes, are TdT −, CD1a − and CD3+ suggesting a post-thymic origin. Phenotypic changes in malignant cells while recognized in acute leukemias, are rare in mature T-cell disorders. In T-PLL, there is only 1 report describing 2 cases of phenotypic change (Tuset et al, Leukemia and Lymphoma 2001.42:1379-83). We retrospectively investigate the frequency of this phenomenon amongst 20 patients with T-PLL for whom sequential immunophenotyping of leukemic cells was available. This included the 2 cases previously reported. Diagnosis of T-PLL was based on clinical features, morphology, immunophenotype and karyotype. Membrane markers were sequentially evaluated by flow cytometry on peripheral blood and bone marrow. Sequential chromosome analysis of metaphases derived from T-mitogen stimulated metaphases cultures was available in a proportion of cases. Intra-lineage phenotypic switch occurred at relapse in 5 cases (25%) (table 1). No characteristic pattern of switch was seen. Serial karyoptying was available in 3 of these 5 patients (60%) and showed acquisition of additional cytogenetic abnormalities at the time of relapse. Change in morphology was documented in 1 case where cells displayed a more ‘blastic’ appearance with loss of CD52. Change in phenotype was mirrored in clinical features with increasingly progressive disease and refractoriness to treatment. In all cases, a mature TdT negative phenotype was maintained. Splenectomy was not performed in the interval between initial and documented change in phenotype and LGL’s were not apparent morphologically or by flow cytometry in these 5 patients
Change in phenotype in T-PLL
Subsequent phenotype Initial phenotype CD4+CD8− CD4+CD8+ CD4−CD8+ Total * change in phenotype CD4+CD8− 8 1* 0 9 (45%) CD4+CD8+ 0 2 2* 4 (20%) CD4−CD8+ 1* 1* 5 7 (35%) Total 9 (45%) 4 (20%) 7 (35%) 20
. All patients received alemtuzumab treatment and while it is possible that this agent selectively depleted sub-clones of the disease, the lack of a consistent emergent phenotype, together with acquisition of additional cytogenetic abnormalities as opposed to replacement by a different abnormal karyotype, favors clonal evolution as a likely cause. Such change in phenotype was not observed in the remaining 15 patients despite alemtuzumab treatment in the majority. Our findings indicate that intra-lineage phenotypic switch in T-PLL occurs in up to 25% of cases. This indicates the need to use comprehensive T-cell panels in monitoring for minimal residual disease and/or relapse. Sequential evaluation of phenotype and cytogenetics may provide valuable insights into mechanisms of relapse and resistance and may impact on treatment selection in T-PLL.