MV-specific IgM stimulates MV-specific B-cell responses in a C3d-dependent manner and overcomes inhibition by MV-specific IgG. Monoclonal IgM antibodies were tested for their ability to stimulate B-cell responses in the presence of inhibitory IgG. All antibodies (IgM 21 and IgM 14) were specific for MV-H and did not neutralize MV. (A) The addition of IgM to bone marrow cells from MV immune cotton rats increased the number of stimulated MV-specific B cells significantly (***P < .001). In the presence of heat-inactivated serum or serum depleted of complement protein 3 (C3), no activation was found. This could be reversed by the addition of C3d. (B) The addition of monoclonal IgG specific for MV-H or polyclonal MV-specific IgG to bone marrow cells from MV immune cotton rats led to low numbers of stimulated MV-specific B cells, whereas addition of IgM led to high numbers. In the presence of a combination of IgM and IgG, numbers were lower than with IgM alone but higher than with IgG alone (***P < .001). (C-D) Cotton rats were inoculated with MV-specific IgG (1 mL of 640 NT, C; or 320 NT, D) intraperitoneally and one day later immunized with 5 × 105 pfu MV (strain Edmonston B, C) or 105 pfu MV (strain Schwarz). At the time of immunization, animals were also inoculated subcutaneously at a different site with different amounts of IgM21 (C) or IgM 14 clone (D). Neutralizing antibodies in serum of cotton rats were determined 7 weeks after immunization; 1 EU of IgM was determined as the amount of IgM with twice the optical density than background.
