ROR1 is an oncofetal protein expressed in chronic lymphocytic leukemia (CLL). We generated a monoclonal antibody (mAb) specific for ROR1 (4A5) and used this to stain blood or marrow cells from patients with CLL and other adults. We found that 4A5 reacted with the CLL cells of all patients examined and also with some cells that had an immunophenotype of B lymphocyte precursors (BLP) (CD10+, CD19+, CD20-variable, dim CD45+, surface immunoglobulin-negative). 4A5 showed no reactivity with other blood or marrow cells. We speculated that this mAb could be used to detect CLL cells in blood or marrow of patients after therapy, providing a means with which to assess minimal residual disease (MRD). For this purpose, we evaluated a four-color combination of mAbs using CD10-FITC, CD19-PE, CD5-PerCP-Cy5.5, and 4A5-Alexa-647. MRD detection limits were established through reconstitution studies in which we made serial dilutions of CLL cells into normal blood or marrow samples. MRD was measured by whole blood lysis and acquisition of 100,000 events using a FACSCalibur and a gating strategy designed to exclude CD5-negative, CD10-positive, and 4A5-negative B lymphocytes (CD19+). CLL cells stained with 4A5 had a mean fluorescence intensity (MFI) of 31 (N=100) and a MFI ratio (MFIR) relative to that of isotype control stained CLL cells of more than 10:1. On average, 94% of the CLL cells in each sample had higher fluorescence intensity when stained with 4A5-Alexa-647 than when stained with an isotype-control mAb (range 88–98%). Marrow samples (N=9) from adults with diagnoses other than CLL and that had 1–10% BLP had CD5− negative, CD10+, CD19+ cells that reacted with the 4A5-Alexa 647. These cells in such samples had a MFI of 19 and a MFIR relative to that of isotype-control-stained BLP of 6.3. On average, 48% of such cells in each sample had higher fluorescence intensity when stained with 4A5-Alexa-647 than when stained with an isotype-control mAb (range 18–79%). Four color flow cytometric analysis with CD10, CD19, CD5, and 4A5 detected CLL cells present at less than 0.1% in reconstituted blood or marrow samples, including marrow with 3–5% BLP. Background was between 0.01% and 0.1% with whole blood (marrow) lysis when we acquired a total of 100,000 flow-cytometric events. These data indicate that anti-ROR1 mAbs can be used with mAbs specific for CD10, CD19, and CD5 for sensitive detection of MRD in CLL.
Disclosures: No relevant conflicts of interest to declare.