CD66 Expression on Malignant and Normal Plasma Cells: A Potential Target for Therapy.

CD66, a member of the carcinoembryonic antigen family, is known to be expressed on cells of myeloid and monocytic origin and has also been demonstrated on blasts from patients with B-lineage acute lymphoblastic leukaemia. An analysis of CD66 expression has been undertaken in bone marrow samples from patients with plasma cell disorders. Diagnostic bone marrow aspirate samples from 53 patients with multiple myeloma or monoclonal gammopathy were examined by multiparametric flow cytometry in order to demonstrate and quantitate plasma cells. Samples were analysed initially using a primary screening panel of antibodies including a combination of antiCD19 fluoroscein isothiocyanate (FITC)(Pharminogen), antiCD5 phycoerythrin (PE)(in-house), antiCD45 peridin chlorophyll protein (PerCP)(Becton Dickinson) and antiCD38 allophycocyanin (APC)(Phar). Samples shown to contain a CD45 negative, CD19 negative, CD38 positive population, consistent with the presence of plasma cells were then examined with a myeloma panel, including antiCD66. Samples from 41 patients were analysed using antiCD66 naked antibody (TheraPharm, GmBH) double-layered with sheep anti-mouse IgG F(ab’)2 fragment conjugated to FITC, antiCD138 PE, antiCD38 APC and antiCD45 PerCP. The correlation between CD38/CD138 and CD38/CD66 dual expression was 0.997. A further 13 samples were analysed using a commercial antiCD66 antibody conjugated to FITC (Dako). Again the correlation between CD38/CD138 and CD38/CD66 dual expression was 0.990. All patients apart from one, with morphologically detectable plasma cells, co-expressed CD66 with CD38. One patient with highly plasmablastic morphology did not express either CD38/138 or CD38/66 on plasma cells.The bone marrow of a normal individual was also examined and found to contain plasma cells which co-expressed CD19, CD138, CD38 and CD66. Plasma cells express CD66 in almost all patients with plasma cell disorders and expression has also been demonstrated on plasma cells in a normal individual. CD66 is therefore an attractive target for immunotherapy. A clinical trial using anti-CD66 targeted radiotherapy as part of the conditioning regimen for patients undergoing autologous or allogeneic stem cell transplantation, as therapy for multiple myeloma, is currently being conducted at our institution.