Using a sensitive immunohistochemical technique and a specific monoclonal antibody (RFD-3), follicular dendritic cells (FDC) have been demonstrated in a significant proportion (6/12) of bone marrow samples from patients with nodular marrow involvement of B-cell chronic lymphocytic leukemia (B-CLL). In all cases with "packed marrow" involvement and advanced stages of disease the FDC were absent. As these accessory cells are normally present only in the lymph nodes, their presence in the bone marrow is in accord with the view that B-CLL might be the malignancy of an immature subpopulation of lymph node lymphocytes that invade the bone marrow.

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We have investigated the relationship between chronic lymphocytic leukemia (CLL), hairy cell leukemia (HCL), and different normal B cell subsets: Mrbc+, T1+ and slgM+ tonsil cells; germinal center; mantle zone; and peripheral blood B lymphocytes. Both malignant and normal cells were incubated in vitro with the phorbol ester 12-O-tetradecanoyl- phorbol-13-acetate (TPA) for 72 hours and the morphology, cytochemical profile, and surface markers were evaluated. The results show that CLL cells TPA-induced become indistinguishable from HCL by four independent criteria: the morphology; the cytoplasmic tartrate resistant acid phosphatase (TRAP) enzyme activity; the membrane positivity with anti- Leu M5 (SHCL3); and anti-Tac monoclonal antibodies which, in the uninduced state, react only with HCL. The features of TRAP and Tac positivity are also expressed (though in variable degree) by different normal B cell populations activated with TPA or pokeweed mitogen (PWM). It is concluded that HCL might represent an aberrantly activated variant of CLL (or of a CLL-related disorder).

This study confirms previous reports that MFHB is a chemosensitive tumour. In view of its rarity collaborative trials are needed to establish the optimum drug treatment including drug selection dose and duration.

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