Murine monoclonal IgG antibodies: differences in their IgG isotypes can affect the antibody effector activity when...

The current studies were aimed at investigating why certain murine anti-CD3 isotypes (e.g. IgG2b) were poor activators of human T cells despite binding to the same epitope as the IgG2a anti-CD3. Most experiments were conducted with a human T cell line (HuT-78) and U937 a human histiocytic cell line with Fc gamma RI and Fc gamma RII. HuT-78 cells lack Fc gamma R. Anti-CD3-TCR of IgG2a isotypes and some antibodies of the IgG1 isotype, but not IgG2b, activated HuT-78 to secrete IL-2 only in the presence of U937. Epitope binding differences could not entirely explain these observations as OKT3E(IgG2a) but not its switch variant (IgG2b) activated HuT-78 in the presence of U937. Of interest, OKT3D(IgG1) in its intact form, but not its F(ab')2, activated HuT-78 to secrete IL-2 in the absence of U937 suggesting therefore that OKT3D activates T cells in the absence of macrophages and provided its Fc domain is intact. All three anti-CD3 with IgG2b isotypes (but none with the IgG2a isotypes) activated HuT-78 to secrete IL-2 only when these antibodies were immobilized onto plastic (in the absence of U937). Of considerable importance, anti-CD3 activation of human T cells with the IgG2b isotype was efficient in the presence of murine macrophages. These observations would suggest that murine anti-CD3 with IgG2b and certain antibodies of IgG1 isotypes, have inherent human T cell activating potential provided the Fc domain is in some manner activated, as for example, by binding to plastic or to Fc gamma R on murine macrophages. Macrophage cytokines are not essential. Therefore, ineffectual anti-CD3-mediated human T cell activation when using murine mAb with the IgG2b isotypes can best be explained on lack of Fc domain activation by human macrophages. This may indeed be the case as IgG2b and certain antibodies of IgG1 isotype, failed to bind to Fc gamma R on macrophages at 37 degrees C. These studies would indicate that murine mAb with the IgG2b isotype may be potentially useful for human use, especially in situations where receptor blockade (without cell activation) is of prime importance.