Linda M. Pilarski¹, Eva Baigorri¹, Michael J. Mant², Patrick M. Pilarski³, Penelope Adamson⁴, Heddy Zola⁴ and Andrew R. Belch¹

¹Department of Oncology, University of Alberta and Cross Cancer Institute, 11560 University Avenue, Edmonton AB T6G1Z2, Canada. ²Department of Medicine, University of Alberta, Edmonton AB, T6G2R7, Canada. ³Department of Electrical and Computer Engineering, University of Alberta, Edmonton AB T6G2V4, Canada. ⁴Child Health Research Institute, 72 King William Road, North Adelaide SA 5006, Australia.

Abstract

Potential progenitor B cell compartments in multiple myeloma (MM) are clinically important. MM B cells and some circulating MM plasma cells express CD20, predicting their clearance by treatment with anti-CD20. Here we describe two types of clonotypic CD20+ B cell in peripheral blood of myeloma patients, identified by their expression of CD19 and CD20 epitopes, their expression of CD45RA and their light scatter properties. Thus, the circulating component of the MM clone includes at least two distinct CD19+ CD20+ B cell compartments, as well as CD138+CD20+ plasma cells. To determine whether either or both B cell subsets and the CD20+ plasma cell subset were depleted by anti-CD20 therapy, they were evaluated before, during and after treatment of patients with rituximab (anti-CD20), followed by quantifying B cell subsets over a 5 month period during and after treatment. Overall, all three types of circulating B lineage cells persist despite treatment with rituximab. The inability of rituximab to prolong survival in MM may result from this failure to deplete CD20+ B and plasma cells in MM.