Antibodies are affinity proteins that play a central role in humoral immunity. Their ability to bind to foreign invaders with high affinity and specificity is central to their function. Equally important is their ability to serve as adaptor molecules and recruit immune cells for various effector functions. There are five main classes of antibodies with diverse functions: immunoglobulin (Ig)A IgD, IgE, IgG, and IgM (1). IgGs are the most abundant class of antibodies, as they constitute approximately 75% of the serum immunoglobulin repertoire. There are four subclasses of IgGs, which vary in their abundance and ability to elicit specific effector functions.

The overall architecture of IgGs is conserved across its four subclasses, and consists of two light chains and two heavy chains. The light chains contain variable (VL) and constant (CL) domains, and the heavy chains contain one variable (VH) and three constant (CH1, CH2, and CH3) domains. One notable difference between IgG subclasses is the location of the disulfide bonds between CH1 and CL (which link the heavy and light chains) and the number of disulfide bonds in the hinge region (which link the heavy chains). The multidomain nature of IgGs elegantly divides their bioactivity into different subdomains. The antigen-binding fragment (Fab) contains both variable domains, and mediates antigen recognition via six peptide loops known as the complementarity-determining regions (CDRs). In contrast, the crystallizable fragment (Fc) contains the constant domains (CH2 and CH3) that mediate effector function by binding to immunological receptor molecules such as complement proteins and Fc receptors.

The complexity of optimizing several different antibody attributes (summarized in Figure 2) using traditional immunization and screening methods has led to intense interest in developing antibody-design methods. The most important antibody attributes are binding affinity and specificity, which involve optimizing the variable domains and the CDRs in particular. Colloidal stability (solubility) and conformational (folding) stability are also critical attributes of antibodies because therapeutic mAbs must be soluble for high-concentration delivery and stable for long-term storage. This typically requires optimizing solvent-exposed residues for solubility and solvent-shielded residues for conformational stability. The effector functions of antibodies are also critical to their bioactivity, and can be tailored by manipulating the hinge and Fc regions.

Another increasingly important antibody attribute—which is uncommon in natural antibodies—is bispecificity for either multiple antigens or multiple epitopes on the same antigen. Achieving bispecificity requires methods for combining multiple antibodies into a single one as well as optimizing the key attributes of conventional antibodies. A second nonconventional attribute of antibodies that continues to grow in importance is their bioactivity when attached to small-molecule drugs. Developing antibody–drug conjugates (ADCs) requires optimizing many aspects of the chemistries and linkers used to derivatize antibodies in addition to the other key attributes of conventional antibodies.

This review highlights progress in designing and optimizing each of these key antibody attributes. Given the large size and complexity of antibodies, most design efforts have focused on redesigning or optimizing existing antibodies rather than on de novo design of new antibodies. These design methods vary greatly, and range from knowledge-based methods based on previous mutagenesis results to advanced computational methods based on first principles. A commonality of these diverse methods is that they attempt to guide the design of antibodies in a systematic manner to reduce the need for screening and immunization methods. We discuss these design methods and their application to improve the properties of antibodies that are critical for their activity and stability.