V

Figure 8-2. Hairy cell leukemia in the bone marrow, characterized by well-spaced lymphocytes with a "fried egg" appearance due to the distinct round to oval nuclei, which are centrally placed within a pale-staining cytoplasmic domain. Extravasated red cells are frequently seen within the infiltrate, along with a background of reticulin fibrosis (x 60).

which are blood-filled spaces lined by hairy cells that have disrupted the normal sinus architecture,19 are a characteristic but not pathognomonic feature. Cases with splenic but not bone marrow involvement have been described, suggesting a splenic origin for the disease.20

Hepatic infiltration is both sinusoidal and portal.21 Lymph node involvement is marked by both sinusoidal and interstitial involvement.22 Bony infiltration can progress from the medullary cavity to the cortex, resulting in osteolytic lesions.23

Cytochemistry

Hairy cell cytoplasm usually stains strongly for tartrate-resist-ant acid phosphatase (TRAP). Isoenzyme 5 acid phosphatase present in the hairy cell cytoplasm resists decoloration with tartrate.24,25 Hairy cells exhibit a range of staining from weak to strongly positive; typical cells stain with moderate intensity (Figure 8-3). A TRAP stain that involves at least two cells with more than 40 granules or with numerous granules obscuring the nucleus is usually diagnostic. When strongly positive TRAP-staining cells with morphology typical of hairy cells are identified, they are highly specific for HCL. In almost 90 percent of cases, TRAP staining of peripheral blood buffy-coat smears is positive.24 Normal neutrophils and platelets contain acid phos-phatases but are not resistant to tartrate and are therefore TRAP stain-negative. Moderate to weak TRAP staining may occur in other diseases, including prolymphocytic leukemia and the non-Hodgkin's lymphomas.

Electron Microscopy

Electron microscopy may help in the differential diagnosis of HCL from other disorders displaying cells with fine filamentous cytoplasmic projections, appearing as ridges or villi.26 In hairy cells, electron microscopy shows circumferential cytoplasmic projections with fewer and blunter microvilli, as compared with splenic lymphoma with villous lymphocytes in which the projections tend to be polarized at one end of the cell27,28 (Figure 8-4). Under electron microscopy, ribosomal lamellar complexes appear as membranous structures composed of concentric rings of lamellae lined by ribosomes.24 Ribosomal lamellar complexes are seen in 50 percent of HCL cases, but because they may be seen in other lymphoid malignancies, they are not specific for HCL.29

Immunophenotypic Profile

Hairy cells, being mature B cells, express single or multiple immunoglobulin heavy chains with monotypic light chains.8,30-34 Hairy cells express the pan B-cell antigens CD19, CD20, and CD22 but not CD21, an antigen lost in the later stages of B-cell ontogeny. They also express PCA-1, an early plasma cell antigen, on their surface. When a panel of multiple monoclonal antibodies was applied to peripheral blood or bone marrow specimens by two-color flow cytometry, a characteristic profile emerged.35 This pattern was best appreciated by the pan B-cell markers CD11c, CD25, and CD103 (Bly-7).36 CD11c stains very potently, with an intensity much greater than seen in chronic lymphocytic leukemia and other chronic lymphopro-

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