BCR-mediated uptake of Ag-CpG conjugates is regulated by the avidity of the Ag-BCR interaction in vitro. (A-C) CFSE-labeled MD4 B cells were stimulated with 1 μL particulates coated with CpG together with HEL^RD either (left panels), HEL^KD (middle panels), or HEL^RKD (right panels) at high (top panels), intermediate (middle panels), or low density (bottom panels). B-cell proliferation and differentiation were measured 72 hours after stimulation. (A) CFSE dilution in stimulated (black line) or unstimulated (filled gray) MD4 B cells was measured by flow cytometry. (B) IL-6 and (C) IgM^a secretion were measured by ELISA. (D) MD4 B cells were stimulated with 1 μL fluorescent particulates left either uncoated (filled gray) or coated with HEL, HEL^K, or HEL^RKD in the absence (top panels) or presence (bottom panels) of CpG. Flow cytometry was used to assess binding of particulates: gates shown indicate the percentage of live cells binding intermediate levels (left gate) and high levels (right gate) of particulates. Values represent the mean (± SD) from triplicate samples.