Figure 6. VLRB MM3 binding correlates with CD38 enzymatic activity.

(A) VLRB MM3 binding to exogenous (BJAB, top panel) and endogenous (Daudi, bottom panel) CD38. BJAB cells were transiently transfected with CD38-EGFP-GyrB fusion constructs, followed by coumermycin treatment (1 μM) in the presence of the nonhydrolyzable inhibitor araF or NAD controls for 40 minutes prior to addition of Abs. VLRB MM3 binding to transfected BJAB cells was assessed for GFP-positive cells corresponding to gate II of Figure 5. Data represent the median fluorescence intensity (n = 4) ± SD, normalized to values obtained for cells without coumermycin treatment. *P < 0.05, by Student’s t test. (B) Assessment of CD38 enzymatic activity following VLRB MM3 treatment in BJAB (top panels) and Daudi (bottom panels) B cells. BJAB cells were transiently transfected with CD38-EGFP-GyrB fusion constructs, followed by isolation of transfected cells and coumermycin treatment. NAD hydrolase activity was assessed and is shown as the ratio of coumermycin-treated cells relative to untreated cells (top, left panel). Coumermycin-treated cells were preincubated with either VLR4 or VLRB MM3 prior to addition of the NAD hydrolase substrate. NAD activity of VLRB-treated cells is shown relative to that of VLR4-treated controls (top, right panel). Statistical significance was determined using a 1-sample Student’s t test (n = 6). Assessment of endogenous CD38 enzymatic activity in Daudi B cells was performed following treatment with monoclonal VLR Abs (left panel) or conventional mAbs (right panel). NAD activity of the treated cells is shown as the ratio relative to the control Ab–treated cells. Statistical significance was determined using a 1-sample Student’s t test (n = 6). araF, β-ara-2′-deoxy-2′-fluoro NAD; CM, coumermycin; MFI, mean fluorescence intensity; norm., normalized; rel., relative; VLRB, variable lymphocyte receptor B.