PLL
Meaning of PLL: Named for the shape of a cell rather than its lineage, so the abbreviation has always covered two separate diseases at once.
Definition of PLL
PLL is an abbreviation for prolymphocytic leukemia, a rare and aggressive blood cancer in which large lymphoid cells with a prominent nucleolus, called prolymphocytes, fill the blood, bone marrow and spleen. It occurs in a B cell form and a T cell form. Both usually produce a very high white cell count and a markedly enlarged spleen, with fatigue and weight loss.
How do the two forms differ?
| B cell form | T cell form | |
|---|---|---|
| Cell of origin | B lymphocytes, the antibody producing line | T lymphocytes |
| Standing today | No longer recognized as a disease in its own right | A defined disease with its own criteria |
| Typical features | Very high white cell count, large spleen, few enlarged nodes | Large spleen and liver, skin involvement, fluid around the lungs |
| Genetic marker | Changes affecting TP53 and MYC | A rearrangement on chromosome 14 switching on the TCL1 gene |
Sharing an abbreviation has caused confusion for a long time, since the two behave differently, respond to different treatment and are diagnosed by different tests. Only the microscope makes them look alike.
Is it a form of chronic lymphocytic leukemia?
No, and this is the point most older descriptions get wrong. B cell prolymphocytic leukemia was once filed as an aggressive relative of chronic lymphocytic leukemia, but current disease classifications have removed it as a separate entity. Cases formerly given that label are now assigned to chronic lymphocytic leukemia with an increased proportion of prolymphocytes, to a splenic B cell lymphoma with prominent nucleoli, or to mantle cell lymphoma in its leukemic phase. The T cell form was never related to chronic lymphocytic leukemia at all. The practical consequence is that a report naming PLL is really naming an appearance, and the underlying diagnosis has to be pinned down before anything follows from it.
How is it identified?
It starts with the blood film, where prolymphocytes are counted as a share of the lymphoid cells present, traditionally with more than half required before the term is used. A blood cell count showing a strikingly high lymphocyte number prompts that examination. Flow cytometry then reads the proteins on the cell surface, which separates a B cell process from a T cell one and rules out lookalikes such as hairy cell leukemia, mantle cell lymphoma and T cell large granular lymphocyte leukemia. Genetic testing follows, and a bone marrow sample shows how far the disease has spread through it. Imaging measures the spleen, which is often enormous and accounts for much of the abdominal discomfort people describe.
Also known as
Used in a sentence
Blood counts and bone marrow findings supported a diagnosis of PLL rather than typical chronic lymphocytic leukemia.
Good to know
PLL is not a subtype of chronic lymphocytic leukemia. The B cell form has been removed as a separate entity in current classifications and its cases reassigned, while the T cell form is a distinct T cell disease, so older descriptions linking the two no longer match how either is defined.