Figure 30.3 Flow immuno-cytometry of a patient with HCL. Hairy cells (red) express B-cell antigens CD19/CD20, X light-chain restriction and are positive for CD11c and CD25. CD10 positivity is noted in the minority of patients with HCL

As hairy cells exhibit distinctive light scatter characteristics and immunophenotype Figure 30.3, they are identified by peripheral blood flow cytometry in most of the patients with HCL, even when the HC represent less than 1% of lymphocytes.43 This observation is useful at the time of diagnosis and after therapy to assess residual disease.44'45

Hairy cells show bright CD45 expression with increased forward and side scatter, resembling that of large lymphocytes or monocytes. They express one or more heavy chains and monotypic light chains. The number of cases expressing either kappa or lambda light chains is approximately equal.4346

Hairy cells strongly express the pan B-cell antigens CD 19, CD20, CD22, and CD 79b.47 They also express high levels of CD11c, CD25, FMC7, and CD103 surface antigens.43 48 The CD11c antigen, the 150-kD a chain of the 150/95 p2-integrin normally expressed on neu-trophils and monocytes, is expressed at levels 30-fold higher than those in chronic lymphocytic leukemia.4950 CD25, the a chain of interleukin-2 (IL-2) receptor, is expressed in HCL, while the p-chain of IL-2 receptor is expressed in variant HCL.51 Serum levels of soluble IL-2 receptor can be monitored in HCL patients and correlates with disease activity following treatment.52

CD103 has the greatest sensitivity and specificity for HCL. CD103 is a aE subunit of the aEp7-integrin, also known as human mucosal lymphocyte 1 (HML-1) antigen due to its primary expression by intraepithelial T lymphocytes. This integrin is believed to be involved in the process of lymphocytes homing and adhesion.53

CD 123, a monoclonal antibody that identifies a chain of human interleukin-3 receptor, is expressed in HCL cells as well as in other acute leukemias and a variety of normal hematopoietic cells. Recently, CD123 was determined to be a useful marker for distinguishing HCL from hairy-cell leukemia variant and marginal zone splenic lymphoma, with 91-96% sensitivity and 97-100% specificity.54

In a small study of nine patients with HCL, CD52 was expressed in most leukemic cells in all the patients. Specifically, greater than 92% of HC stained positive in all nine cases, while 99% positivity was observed in six. The presence of CD52 on HC provides a theoretical basis for use of targeted therapy, such as the monoclonal antibody alemtuzumab, for the treatment of HCL.55 CD10 (CALLA antigen) and CD5 are weakly expressed in 26% and 5% of patients with HCL, respectively.

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