O

Figure 8-3. Tartrate-resistant acid phosphatase (TRAP) in hairy cell leukemia, demonstrating granular cytoplasmic staining (x 100).

liferative disorders.37 The staining of CD11 is 30-fold higher than that of chronic lymphocytic leukemia, which therefore aids in its differentiation.

Hairy cell leukemia was the first B-cell lymphoproliferative disorder identified that expressed CD25 (an interleukin-2 [IL-2] receptor). Serum levels of soluble IL-2 receptors are elevated in HCL patients and correlate with disease activity following treat-ment.38 CD103 (Bly-7) has the greatest sensitivity and specificity for HCL. This monoclonal antibody is against an antigen associated with the subunit of the B7 integrin, thought to be involved in the process of lymphocyte homing and adhesion.39 CD22 is also expressed more intensely in HCL than in other B-cell chronic lymphoproliferative disorders; CD22 stains 50 times more intensely in HCL than in chronic lymphocytic leukemia, in which there is only weak expression of CD22.35 In 26 percent of cases there is weak expression of CD10 (the CALLA antigen), and in 5 percent of cases there is weak expression of CD5; CD5 is the anomalous T-cell antigen strongly expressed on chronic lymphocytic leukemia cells.35 Peripheral blood flow cytometry revealed that 92 percent of 161 patients with HCL had identifiable circulating hairy cells, in some patients representing less than 1 percent of lymphocytes; this compares with a thorough morphologic evaluation of the peripheral blood, which revealed hairy cells in 80 percent of patients.35

Immunohistochemistry

Immunohistochemistry performed on bone marrow biopsy samples may aid in the diagnosis of HCL and is also useful for evaluating minimal residual disease after effective systemic therapies. Hairy cells stain with CD20 (L26) (Figure 8-5) and DBA.44;40 L26 staining is membranous and accentuates the ruffled abundant cytoplasm, whereas DBA.44, an undefined antigen, stains in both a cytoplasmic granular and a membranous pattern. DBA.44 is again not specific for HCL, because 30 percent of low-grade lymphomas also stain positively with this stain.

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