The matched sets of antibodies to CAMPATH-1 (CD52) antigen fall into two subgroups, a subgroup of chimaerics with the original rat V H region and and the rat kappa-1b light chain and a second set which uses a reshaped fully-humanised V H region and a reshaped fully-humanised V L region on a human kappa light chain [18],[31].
The chimeric antibodies were essentially the human correlates of the original rat IgG2b antibody CAMPATH-1G [32] and were used to decide which human isotype seemed the most suitable for therapy. This could be assessed by comparing the performance of the chimaerics in in-vitro assays with that of CAMPATH-1G, the specificity and affinity of the antibodies being identical in all cases [18]. For in-vivo use it was necessary to minimise the possibilities of an antiglobulin response to the rodent V-region, therefore the complimentarity regions were grafted onto human frameworks (see chapter 2 this volume). The antibodies with the fully reshaped V-regions have a slightly lower affinity than the original chimeric antibodies [18] and so this needs to be considered with the proper controls when interpreting the experimental data.
The results from the comparison of the chimeric antibodies are given in Figure 2 . Targets for these experiments were human peripheral blood mononuclear cells. The ADCC effector cells were these same mononuclear cells but after activation with a mitogenic CD3 antibody and expansion in 40 units per ml of recombinant human Interleukin-2. These expanded cells contain a low percentage of CD16 positive cells and are extremely potent as effectors [15],[18],[31].
The IgG1 antibody was found to be the most effective antibody in both complement-mediated lysis and ADCC. The IgG3 antibody was also active in both assays whilst the IgG2 antibody had a lower titre in complement-mediated lysis and gave very little ADCC. The IgG4 antibody did not work in either system. Comparing these results with the NIP chimaerics reveals a large degree of similarity except that the IgG2 antibody to CAMPATH-1 is more active in complement lysis. Following the studies of others using the NIP system [29],[30] this observation may be due to different antigen densities, or clustering of antigen, the CAMPATH-1 antigen being equivalent to the NIP antigen at high concentrations.
The human IgG1 isotype appeared to be the most similar to rat IgG2b and the reshaped version of that isotype, CAMPATH-1H [18],[32], was chosen for therapeutic evaluation and comparison to the previously used CAMPATH-1G [24],[25]. The antibody has now been used in a number of clinical settings and has proved to be very effective at depleting cells [22],[26],[27]. The antibody is currently being evaluated in therapeutic trials in the USA and Europe by the Wellcome Foundation.
More recent experiments have made the interpretation of the ADCC experiments more complex [31]. Repeating the experiments with a larger panel of donors of effectors and targets demonstrated that there were differences in the pattern of ADCC seen with different individuals. Thus some people (3 out of 8) showed the originally described pattern where only the IgG1 gave good levels of ADCC. For other individuals there were significant levels of ADCC seen with all four isotypes of IgG, including IgG4. This variation has been found to be consistent with the source of effectors and is not a difference in target susceptibility [31]. This represents an intriguing polymorphism and also predicts that unexpected results might be obtained with some human isotypes such as IgG2, and particularly IgG4, when used therapeutically in some individuals. It has often been assumed that IgG4 binds only poorly to Fc gamma RI and does not mediate ADCC and, therefore, that it would be a good choice for non-depleting antibodies used, for example, in imaging with radioisotopes (Reviewed by Adair [33]).
Since this article was originally written the development of the CAMPATH-1 antibodies has continued but no longer as a product from Glaxo-Wellcome. For more information see the web pages for the Therapeutic Antibody Centre in Oxford.