LAUSR

OBJECTIVES CD161 (NKRP1) is a lectin-like receptor present on NK cells and rare T-cell subsets. We have observed CD161 expression in some cases of T-cell prolymphocytic leukemia (T-PLL) and found it to be useful in follow-up and detection of disease after treatment. METHODS Retrospective review of T-PLL cases with complete flow cytometry data including CD161. RESULTS We identified 10 cases of T-PLL with flow cytometric evaluation of CD161 available. Six of these cases were positive for CD161 expression. All CD161-positive cases were positive for CD8 with variable CD4 expression, whereas all CD161-negative cases were negative for CD8. In a case with two neoplastic subsets positive and negative for CD8, only the former expressed CD161. CONCLUSIONS These novel results suggest that CD161 is often aberrantly expressed in a defined subset of T-PLL positive for CD8. We are showing the utility of this immunophenotype in diagnosis and follow-up.