CD123 constitutes the alpha subchain of the human interleukin 3 receptor (IL-3R). Interleukin 3 binds to its receptor with high and low affinities, induces tyrosine phosphorylation and promotes the proliferation and differentiation of haematopoietic stem cells. CD123 expression has been associated with a poor prognosis in AML and has been shown to be a useful marker in hairy cell leukaemia (HCL). However, it has been little studied in CLL, with data in the literature suggesting few cases of positivity. Here we studied CD123 expression by flow cytometry on fresh peripheral blood in 134 unselected, consecutive cases of CLL. We utilised anti CD123 PE (BD Pharminogen) in a dual combination with anti CD19 FITC (Coulter Immunotech) to target B cells. The results were correlated with expression of CD38 and ZAP70, markers of poor prognosis in CLL. CD123 was expressed in = 20% (median 33, range 20–92%) of cells in 32/134 cases (24%). Typical of CLL, the intensity of surface CD123 expression was weak-moderate, unlike HCL cases, where the intensity is moderate-strong. CD38 positivity was seen in 20/32 CD123+ cases (63%). Of the remaining 102 cases where CD123 was negative, a CD38 count was available in 92 cases and 29/92 (32%) were positive. ZAP70 positivity was seen in 18/32 CD123+ cases (56%). ZAP70 data was available in 98/102 CD123− cases and positivity was seen in 26/98 (27%). These findings represent the first account of a significant percentage of CLL cases expressing CD123 positivity. Furthermore we have been able to show that CD123 positivity strongly correlates with two known markers of poor prognosis ZAP70 (p= 0.004) and CD38 (p=0.004) (Chi-Square with Yate’s correction). CD123 may therefore be of prognostic significance in CLL and should be further investigated.

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