Although murine antibodies are of value in therapy of human diseases, their effectiveness is limited because rodent monoclonal antibodies have a short survival time in humans and induce an immune response that neutralizes their therapeutic effect. To circumvent these difficulties, genetically engineered antibody variants were produced that combine the rodent variable or hypervariable regions with the human constant and/or variable framework regions (21-25). The ability to genetically engineer antibodies represents a quantum leap in immune intervention that is comparable to the immunological revolution initiated by the introduction of monoclonal antibodies.
After the demonstration that lymphoid cells can express cloned transfected immunoglobulin genes, mouse-human chimeric monoclonal antibodies to tumors were generated with specificities directed toward antigens expressed by malignancies (21,22). Jones et al. (23) proposed that because V domains represent a framework of beta sheets topped with antigen-binding loops and because beta-framework structures of most crystallized antibodies are nearly invariant, the specificity of the antibody combining site might be independent of the framework region. Thus, to further reduce the immunogenicity of rodent elements, Winter and colleagues generated humanized antibodies that retained only the antigen-binding complementarity-determining regions (CDRs) from the parent rodent monoclonal antibody in association with human framework regions (23,24). Unfortunately, in some cases humanized antibodies produced by this approach have reduced binding affinity for antigen when compared to the original rodent antibody. Queen and co-workers (25) addressed this problem in two ways. First, the human framework was chosen to b as homologous as possible to the original mouse antibody in order to reduce deformation of the transplanted mouse CDRs. Second, computer modeling was used to identify several amino acids in the mouse antibody framework that, although outside the CDRs, were likely to interact with the CDRs or antigen. These specific amino acids were retained in the humanized antibody.
Effector functions can be improved by introduction of human constant regions that impart biological activity to a murine antibody that lacks effector function but has the desired binding specificity. The human IgG subclasses differ in their antitumor activity and in their capacity to induce complement or antibody-dependent cell-mediated cytotoxicity. The IgG1 subclass appears to be superior to the other subclasses in most functions (33). The human IgG1 versions of the CAMPATH-1 antibody and the L6 monoclonal antibody to a carcinoma-associated antigen were more effective in antibody-dependent call-mediated cytotoxicity (ADCC) than the parent rodent antibodies (8,24). Furthermore, the humanized version of anti-Tac induces ADCC with mononuclear cells, a function absent from the original mouse monoclonal antibody (34).
