T-cell prolymphocytic leukaemia (T-PLL) is a clinicobiological entity that results from the clonal proliferation of post-thymic T cells . It has no relationship to B-cell prolymphocytic leukaemia (B-PLL) other than some degree of similarity of cytological features [2,3]. It is mainly a disease of the elderly. Ataxia telangiectasia patients have an increased risk of developing T-PLL . There is no relationship to human lymphotropic virus 1 or 2 .
Typical clinical features are lymphadenopathy, hepatomegaly and splenomegaly. Skin infiltration is present in about one-fifth of patients . Serous effusions can occur. In most patients this is an aggressive disorder, although in a minority of patients the course is more indolent (smouldering T-PLL). The disease in these patients often progresses in terms of months or a few years .
Haematological and pathological features
The white cell count is usually high and anaemia and thrombocytopenia are common. Serum lactate dehydrogenase can be raised. Serum immunoglobulins are normal. Neoplastic cells can resemble those of B-PLL, being large with a round to oval nucleus, a moderate amount of cytoplasm and a prominent nucleolus; in some cases the nuclei are quite irregular (Figures 21.1, 21.2, 21.3). In the small cell variant of T-PLL the cells are not much larger than those of chronic lymphocytic leukaemia but differ in that the cytoplasm is more basophilic and there are cytoplasmic blebs; nuclei are irregular and a nucleolus is apparent. In the small cell variant the nucleolus is smaller and much less prominent than in cases with larger cells. The two cytological variants represent the same disease. The condition initially described as ‘Sézary cell leukaemia’, in which the cells are medium sized with a highly convoluted or cerebriform nucleus, is now considered to be a cerebriform variant of T-PLL  (Figure 21.4).
Peripheral blood film in T prolymphocytic leukaemia (T-PLL) showing small and medium sized cells with irregular nuclei and nucleoli. Romanowsky stain, ×60 objective.
Peripheral blood film in T-PLL showing medium sized cells with basophilic cytoplasm and irregular nuclei with nucleoli. Romanowsky stain, ×100 objective.
Ultrastructure of a T-PLL cell showing an irregular nucleus with a prominent nucleolus. The arrow shows electron-dense granules, a feature of T-PLL and not B-PLL. Lead nitrate and uranyl acetate stain.
Peripheral blood film in a case of a cerebriform variant of T-PLL showing a hyperchromatic, cerebriform nucleus. Romanowsky stain, ×100 objective.