Expression of VEGF Receptors on Plasma Cells: Evidence of Heterogeneity.

Background: In previous studies quantitating VEGF receptors we have found no significant differences in expression between plasma cells from normal controls, multiple myeloma (MM), monoclonal gammopathy of undetermined significance (MGUS), or smoldering myeloma (SMM). (Kumar S, Blood, May 2004; 10.1182/blood-2003-11-3811 ePub). These studies were done using immunohistochemistry or Western Blotting (on CD138+ plasma cells) and may have been limited by low levels of receptor expression and by heterogeneity of expression. We measured expression of VEGF receptors (VEGFR1, VEGFR2 and VEGFR3) on the surface of plasma cells and in plasma cell subsets using direct and indirect flow cytometric assays to determine if significant differences in VEGF receptor expression existed between MGUS, SMM and MM.

Methods: In the indirect flow cytometric assay, 32 bone marrow samples (3 amyloid, 9 MGUS, 12 MM, 8 SMM) were ACK lysed and tested using the FLUOROKINE ^TM rhVEGF biotin kit (R and D Systems, Minneapolis, MN) according to manufacturer’s instructions. In brief, in one tube (tube 1) cells were incubated with VEGF biotin. In tube 2, VEGF biotin preincubated with a blocking antibody was added to the cells (specificity control), while in a third tube a non-specific biotinylated protein (negative control) was added. After incubation, FITC-avidin, CD38 APC and CD45 Percp was added to each tube. Gates were drawn around the cells of interest and fitc staining was evaluated for the % positive cells. The % of signal blocked was calculated by comparing the fitc intensity (channel number) of the blocked VEGF peak (tube 2) to FITC intensity of tube 1. This system does not determine the identity of the receptor, but indicates the presence of VEGF receptors. In the direct flow cytometric assay, bone marrow from 25 individuals (2 amyloid, 5 MGUS, 7 MM, 7 SMM, and 4 normals) were lysed and blocked with mouse Ig and stained with CD38/CD45. In individual tubes, PE labeled VEGF R1, R2, R3 antibodies (R and D Systems, Minneapolis, MN) or isotype control were added. Plasma cells were identified, divided according to CD45 expression, and analyzed for % and intensity of receptor staining.

Results: In the indirect assay, plasma cells in all groups bound VEGF ( 96% positive) at high intensity. There was also no difference in VEGF binding between CD45+ and CD45- plasma cell fractions (93 and 98% respectively).The specificity of VEGF binding (to one of the VEGF specific receptors) was confirmed by a significant drop in peak channel numbers of FITC intensity in the presence of blocking antibody. Specific VEGF binding at a similar intensity was seen in monocytes (95%) and at a lower intensity in lymphocytes (66%) and granulocytes (28%). Staining for VEGFR1, 2, and 3 in plasma cells using the direct assay revealed that except for 2 patients (1 amyloid and 1 SMM) none had >20% cells staining for any of the 3 receptors. The same results were seen in the CD45− fraction as well. In contrast, the CD45+ plasma cell fraction was highly positive for all 3 of the receptors in nearly all cases, with no significant differences between MGUS, SMM, amyloid, or MM.

Conclusions: Plasma cells in MM and related disorders have specific VEGF receptors on the cell surface. The expression of VEGFR1, 2, and 3 seems to be primarily restricted to the CD 45+ subset of plasma cells. The finding of specific VEGF binding in CD45− plasma cells seen in the indirect assay may reflect the higher sensitivity of this assay due to the inbuilt amplification process or the presence of additional VEGF receptors such as neuropilin 1.