Effects of CAMPATH-1 antibodies in vivo in patients with lymphoid malignancies: influence of antibody isotype

The CAMPATH-1 family of antibodies recognize an abundant glycoprotein expressed on virtually all human lymphocytes. All rat IgM and IgG antibodies of this specificity are lytic with human complement, but only IgG2b is active in antibody-dependent cell-mediated cytotoxicity (ADCC). We compared the ability of IgM, IgG2a, and IgG2b to deplete lymphocytes in vivo in two patients with prolymphocytic transformation of B-cell chronic lymphocytic leukemia (CLL). The IgM (CAMPATH-1M) produced transient depletion of blood lymphocytes with consumption of complement but had no effect on solid masses or bone marrow. Similar transient depletion of blood lymphocytes was noted with the IgG2a (YTH34.5). In contrast, the IgG2b (CAMPATH-1G) produced long-lasting depletion of lymphocytes from blood and marrow and improvement in splenomegaly but no detectable changes in complement levels. These differences probably reflect the importance of Fc receptor binding for effective clearance of target cells in vivo. We treated 16 more patients with a variety of lymphoid malignancies and noted consistent effects on blood lymphocytes, marrow infiltration, and splenomegaly. At this dose level, there was comparatively little improvement in affected lymph nodes or extranodal masses. Nevertheless, the in vivo lympholytic ability of CAMPATH-1G is very potent as compared with other monoclonal antibodies (MoAbs) and may have applications in therapy of lymphoid malignancies and as an immunosuppressive agent.