This is a 56 year old male patient with relapsed T-cell prolymphocytic leukemia 2.5 years after matched unrelated donor stem cell transplantation.

He was initially diagnosed in 2015 with white cell count of 54,400/ul at the time with extensive bone marrow infiltration and diffuse lymphadenopathy as well as splenomegaly. Flow cytometry at that time demonstrated expression of CD45, CD3 and CD4 without CD8 on the neoplastic T cell population. However when he relapsed with disease after transplant,  there was a change in immunophenotype of neoplastic T cells  in the skin and bone marrow with notable double expression of CD4 and CD8 with loss of CD45.

Since the most recent relapse, he was on started on anti-CD52 therapy alongwith pentostatin and bendamustine but CBC showed progressively rising blood counts with lymphocytosis. The slides and flow plots since relapse (on therapy) are depicted below.

Learning point from case below:

1.T-PLL is a rare, aggresive T cell leukemia characterised by small to medium sized prolymphocytes that show mature/ post thymic immunophenotype. The main affected organs are PB (peripheral blood), spleen, liver, bone marrow and skin.  
2. Three morphological variants have been identified as- classic (75%), small cell variant (25%) and cerebriform or sezary cell like variant (5%).

3.T-PLL often presents as high counts in the blood with atypcial small to medium sized lymphoid cells showing classic punched out nucleoli and cytoplasmic blebs.

4. Immunophenotyping shows the neoplastic T cell population positive for CD3 (weak), CD2, CD5, CD7 and CD52. Mostly CD4+/CD8- T cell population is seen. In certain cases a co-expression of CD4 and CD8 may be seen. TCL1 immunohistochemistry has better sensitivity than FISH for TCL1 rearrangement. 
5. Change in immuophenotype may occur in T-cell propulation, so careful attention must be paid to such changes at relapse.