Identification of monoclonal CLL phenotype cells in normal individuals.

(A) The gating strategy used to identify B lymphocytes and exclude contamination is shown. (B) The analysis of B-lymphocyte phenotype in 2 representative individuals, 1 with only normal B lymphocytes present (plots i-iv) and 1 with CLL phenotype cells present (plots v-viii). Cells were classified as having a CLL phenotype if a discrete population with stronger CD5, weaker CD20, and weaker CD79b than the other “normal” B lymphocytes was present in all 3 “CLL” regions (R4, R5, and R6, shown in plots i/v, ii/vi, and iii/vii, respectively). This phenotypic profile is unique to CLL, and as such this assay can detect CLL cells when they represent as few as 0.002% of total leukocytes, or 0.5% of Blymphocytes in all samples.20Plot viii demonstrates weak κ expression on the population of CD5⁺ cells. The remaining cells have a normal distribution of κ⁺ and κ⁻ cells similar to that seen in plot iv, which shows the distribution in an individual with no CLL phenotype cells. Although the test is specific for CLL cells, monoclonal populations with a non-CLL phenotype may be detected if there are sufficient neoplastic cells to perturb the normal κ/λ ratio.