Mike Clark¹, Kathryn Armour¹, Lorna Williamson^1,2
1Cambridge University, Cambridge, U.K. 
²National Blood Service, Cambridge, U.K.

Designing Antibodies for Effector Recruitment

In the design of therapeutic antibodies there are many strategies and choices for the appropriate selection of antibodies with suitable variable regions, which are specific for the chosen therapeutic targets. However in therapeutic uses the full biological efficacy of these antibodies is often dependent upon both the consequences of the antibody specificity and also the interactions of the antibody with the patients effector systems. These interactions are predominantly determined by the class and subclass of the antibody Fc regions. Human IgG antibodies, and in particular IgG1 subclass antibodies, have been chosen almost exclusively as the most appropriate human immunoglobulin class or subclass for therapeutic applications.

Human IgG antibodies are able to interact with a number of diverse effector systems and cell types. These effector systems include the complement system and also Fc receptors, which are expressed by many different cell types. For IgG there are the classical Fcg receptors, RI, RIIa, RIIb, RIIc, RIIIa and RIIIb, which are expressed on a range of immune system effector cell types, as well as the Brambell, or neonatal receptor FcRB/FcRn, which is involved in IgG placental transport and also IgG plasma half-life. In addition some viruses such as HSV-1 encode Fc receptors, which will bind to human IgG subclasses. The complexity of these systems is further increased by the existence of naturally occurring alleles of the immunoglobulins, complement components, and FcRs, many of which vary in their interactions (specificity and affinity) and in cellular expression.

We have been involved in several studies to produce antibodies for use in different therapeutic situations, including cancer therapy, immunosuppression, and more recently inhibition of allo- and auto-antibody mediated cell destruction. In order to have antibodies in which both the specificity and the effector functions have been appropriately selected for the particular proposed therapeutic application we have been comparing antibodies made with different IgG constant regions. We have looked at wild type human IgG1, IgG2 and IgG4 antibodies as well as mutated variants of these in which homologous amino acid regions from one isotype are exchanged with regions from another isotype. These constructs have then been compared in various assays for their abilities to bind to FcR and also to trigger effector mechanisms in cellular cytotoxicity assays.

From the results of these studies we have identified human IgG constant regions with differential effector abilities, including differences in binding to both activatory and inhibitory Fc receptors. All of the sequences are of human IgG origin so should also hopefully be of low immunogenicity. We are now planning to take selected constructs forward for volunteer studies as well as for potential future therapeutic applications.

Relevant Publications:

1. Armour, K.L., Clark, M.R., Hadley, A.G. and Williamson, L.M. (1999), Eur J Immunol 29: 2613-2624 Recombinant human IgG molecules lacking Fc-gamma receptor I binding and monocyte triggering activities

2. Atherton, A., Armour, K.L., Bell, S., Minson, A.C., and Clark, M.R. (2000), Eur J Immunol 30: 2540-547 The herpes simplex virus type 1 Fc receptor discriminates between TgG1 allotypes

3. Jaakkola, K., Nikula, T., Holopainen, R., Vahasilta, T., Matikainen, M.-T., Laukkanen, M.-L., Huupponen, R., Halkola, L., Nieminen, L., Hiltunen, J., Parviainen, S., Clark, M.R., Knuuti, J., Savunen, T., Kaapa, P., Voipio-Pulkki, L.M., and Jalkanen, S. (2000), American Journal of Pathology 157, 463-471 In vivo detection of vascular adhesion protein-1 in experimental inflammation

Further information is available on my web site:
www-immuno.path.cam.ac.uk/~mrc7/