HAS1-expressing MM B cells synthesize a pericellular HA matrix. The HA pericellular matrix around the cell was visualized by the addition of fixed erythrocytes to sorted CD19⁺ B cells from MM (n = 15) and healthy donors (n = 3) and from MM CD38^hiCD45^lo PCs (n = 3) in short-term culture. Fixed erythrocytes were physically excluded from the areas surrounding a cell that had synthesized an HA pericellular matrix. HA matrix was detected only around MM CD19⁺ B cells. MM BM CD38^hiCD45^lo PCs and CD19⁺ B cells obtained from healthy donors did not exhibit an HA matrix around their plasma membranes (results not shown). Furthermore, no HA matrix was detected around the MM CD19⁺ B cells at 4 hours and 12 hours of culturing; however, 24 hours later, a small amount of HA matrix was detected around some MM CD19⁺ B cells, while other B cells in the culture did not exhibit an HA matrix. The size of an HA matrix significantly increased around some MM CD19⁺ B cells 48 hours after culture (A, arrows; B), while other CD19⁺ B cells did not develop an HA matrix (A, stars). In the culture some cells were characterized by a prominent coat of HA at one edge of the cell, while the opposite edge of the same cell exhibited lesser amounts of HA matrix (B). This type of distribution of HA matrix around the cells results from cell motility. Motile cells exhibit a prominent HA halo at their trailing edge and a lesser pronounced halo at their leading edge. After HAase treatment no HA matrix was detected around MM CD19⁺ B cells (C). Scale bar = 10 μm. Magnification 40×, numerical aperture 1.3. Images were processed using LSM 510 software.