FcRn mediates the perinatal transfer of IgG. DOI:10.1038/nri2155
The neonatal Fc receptor, or FcRn, plays an important role in both maternal IgG transport and protection of IgG from degradation.
First described in the 1960s, these processes were originally attributed to separate receptors: the neonatal transport receptor (FcRn) and the IgG protection receptor (FcRp). However, in 1996, it was determined that both functions were carried out by the same receptor, now commonly referred to as FcRn. This discovery solidified FcRn’s importance in regulating IgG homeostasis and serum persistence.
Structurally, FcRn is a heterodimer composed of a β2-microglobulin (B2M) light chain and an MHC class I-like heavy chain. Binding occurs between the CH2 and CH3 domains of IgG. This interaction is highly pH-dependent, which is crucial for FcRn’s ability to protect IgG from catabolism. At a pH of 6.0, which is found in the acidic endosomal environment, FcRn tightly binds IgG. This complex then recycles IgG through the cell, preventing its degradation in lysosomes. Once the complex reaches a neutral pH environment, such as the bloodstream, IgG is released back into circulation.
This mechanism ensures IgG has an exceptionally long serum half-life, up to 21 days for most subclasses (IgG1, IgG2, and IgG4). IgG3, due to a single amino acid change at the FcRn binding site, has a reduced half-life of about 7 days. This FcRn-mediated recycling is pivotal for the immune system's ability to maintain adequate levels of circulating antibodies over time. FcRn’s role in antibody longevity has broad implications, particularly in therapeutic antibody development, where maintaining a prolonged serum presence can greatly enhance treatment efficacy.
The ability of FcRn to extend the half-life of IgG molecules is a factor in therapeutic antibody development. By leveraging FcRn-mediated recycling, scientists can enhance the longevity of therapeutic antibodies, thereby reducing the frequency of dosing required in clinical treatments. Modifying Fc regions to optimize FcRn binding is a common strategy used to improve pharmacokinetics in antibody therapeutics.
For example, engineering the Fc region of an antibody to bind more tightly to FcRn at acidic pH but release efficiently at neutral pH can result in a longer duration of action. This has been particularly important for monoclonal antibodies (mAbs) used to treat chronic diseases such as cancer and autoimmune disorders. By extending the half-life of these mAbs, patients can benefit from fewer administrations, reducing healthcare costs and improving patient compliance.
Furthermore, understanding the differential recycling capabilities of various IgG subclasses can influence the design of therapeutic antibodies. While IgG1 is often the preferred subclass for therapeutic use due to its long half-life and strong immune activation capabilities, IgG3’s shorter half-life may be advantageous in certain scenarios where rapid clearance is desired. In some treatments, the ability to quickly eliminate the therapeutic antibody after its function is fulfilled may reduce potential side effects.
Protein A and Protein G: Antibody Purification Tools
In antibody production, particularly for therapeutic applications, the purification of IgG from complex biological mixtures is a critical step. Two bacterial proteins, Protein A and Protein G, have become indispensable tools for this purpose. Both proteins bind to the Fc region of antibodies, but their specific binding properties vary across antibody isotypes and species, making them uniquely suited for different purification tasks.
Protein A, a 56 kDa protein found in Staphylococcus aureus, binds primarily to the CH2-CH3 interface of human IgG1, IgG2, and IgG4 with high affinity, but not to IgG3. In contrast, Protein G, a 65 kDa protein from Streptococcal bacteria, binds more broadly across species and isotypes. Both proteins exhibit pH-dependent binding characteristics, making them ideal for affinity chromatography. In this process, antibodies are bound to a Protein A or G column at neutral pH and then eluted at low pH for collection, providing a highly efficient method for antibody purification.
In therapeutic antibody manufacturing, the scalability and precision of Protein A and Protein G affinity chromatography have made these proteins essential. Their ability to bind selectively to IgG ensures high purity and yield, which is crucial for producing biologics that meet stringent regulatory requirements for clinical use.
Additionally, the understanding of how these proteins interact with different IgG subclasses helps biotechnologists tailor purification processes for specific therapeutic antibodies. For instance, antibodies of different subclasses or from non-human species may require the use of Protein G rather than Protein A, depending on their binding affinities. This fine-tuning in the purification process ensures that the final therapeutic product is both effective and safe for human use.
Impact of FcRn and Antibody Purification in Biotech
The role of FcRn extends beyond the recycling of endogenous IgG. In biotechnology, FcRn is an important consideration when designing and testing new antibody therapies. Its ability to extend the half-life of therapeutic antibodies means that drugs can be engineered to last longer in the bloodstream, improving their overall efficacy. Understanding the intricacies of FcRn binding can also aid in reducing dosing frequencies, which can enhance patient compliance and reduce treatment costs.
Similarly, the use of Protein A and Protein G in antibody purification demonstrates the vital intersection of biology and technology in drug development. Without these tools, producing high-purity antibodies at scale would be far more difficult, and therapeutic antibody development would be significantly hindered.
References:
Roopenian, D. C., & Akilesh, S. (2007). FcRn: The neonatal Fc receptor comes of age. Nature Reviews Immunology, 7(9), 715-725. https://doi.org/10.1038/nri2155
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