Designed IgM from glycoengineering.

Among the immunoglobulins, the IgGs have received the lion’s share of attention and dominate the monoclonal antibodies (mAbs) that are US Food and Drug Administrationapproved or are in research pipelines leading to the clinic. Whereas IgG is relatively easy to produce and purify and a number of host expression systems have been developed for that isotype, interest in the utility of other isotypes has been increasing in the last few years. In particular, IgM has begun to receive considerable attention as an isotype that may be ideal for specific clinical indications (1). IgM differs from IgG in a number of respects. First and foremost, IgMs are extraordinarily complex molecules consisting of an array of 10 (pentameric IgM) or 12 (hexameric IgM) combining sites compared with the 2 combining sites that comprise IgG. This extensive array consists of heavy chains (μ), light chains (predominantly κ), and may have J chains covalently attached to the pentameric form. As a consequence, IgMs are enormous molecules (∼900 kDa as opposed to ∼150 kDa for IgG). This complexity comes with certain clinically relevant features. In particular, the high level of valency of IgMs can give rise to a very significant degree of avidity for antigen. This is in contrast to IgG, where affinity for antigen is key, and avidity plays a secondary role. Because of the high avidity and relatively low affinity, IgM can be polyreactive. Because a significant proportion of IgM is germ lineencoded and emerges during embryogenesis without apparent antigenic stimuli, they have been referred to as “natural antibodies” (2). Indeed they are the first isotype to be expressed during embryogenesis, as well as the first isotype of an immune response to antigen (3). Due to the abundance of Fc regions, IgM can very efficiently activate complement, leading to potent cytotoxic and cytolytic activity. A single IgM molecule is capable of activating complement. Activation by IgM can therefore be about 1,000 times more efficient than IgG (3, 4). IgM in human serum (∼5% of total Ig) consists predominantly of the pentameric form with only a trace of the hexameric form. Different species have different pentamer:hexamer ratios, but invariably the two forms of IgM are functionally distinct, with the hexameric form being significantly more efficient in complement activation (5). We don’t yet understand the factors that control the pentamer:hexamer ratio or why only ∼50% of pentameric IgM contains a J chain. An additional component of IgM complexity is glycosylation. IgG ordinarily has a single glycosylation site on each constant region, whereas pentameric IgM has 10 sites at five different locations in the constant region. Also, whereas we are beginning to understand how glycosylation can affect