Addition of a mu-tailpiece to IgG results in polymeric antibodies with enhanced effector functions including complement-mediated cytolysis by IgG4.

Abstract

The 18-amino acid carboxyl-terminal tailpiece from IgM (mutp) has now been added to the carboxyl-termini of IgG1, IgG2, IgG3, and IgG4 constant regions to produce recombinant IgM-like IgGs. Polymeric IgGs obtained by this approach possess up to six Fcs and 12 antigen-combining sites, greatly increasing the avidity of their interactions with other molecules. Not surprisingly, the C activity of normally active IgG1 and IgG3 and somewhat less active IgG2 Abs is shown to be dramatically enhanced upon polymerization. The multiple Fcs present in a single molecule apparently allow for more efficient interactions with the multiple C1q heads present in C1, the first component of the classical C cascade. An unexpected result however, is that IgG4, normally devoid of C activity, when polymerized in the same fashion directs C-mediated lysis of target cells almost as effectively as the other polymers. Interestingly though, IgG4mutp does not deplete C activity in a standard consumption assay using soluble Ag. The other gamma mutp isotypes are capable of depleting 100% of the serum lytic ability even in the absence of Ag, whereas IgG4mutp shows no evidence of activity in this assay under any of the conditions tested. Additionally, we show that, in contrast to monomeric IgG, polymeric IgGs bind with very high affinity to Fc gamma receptor II (Fc gamma RII), a low affinity receptor for wild-type antibodies; however, binding to Fc gamma Rl, the high affinity receptor, appears to be unaltered. Finally, the in vivo t1/2 of the gamma mutp proteins is decreased relative to wild-type IgG, apparently because of rapid clearance of the polymeric fraction.

The unique properties of IgG4 and its roles in health and disease

“…Nevertheless, some studies suggest that IgG4 can activate complement in specific contexts. For example, artificially enforcing the hexamerization of IgG4 — a process that normally would take place ‘spontaneously’ as part of complex formation of antibody with C1 — results in complement activation by the classical route 33 , 34 . This shows that IgG4 has a reduced ability to activate complement but is not completely ‘silent’ in this respect.…”

Nasal spray of an IgM‐like ACE2 fusion protein HH‐120 accelerates SARS‐CoV‐2 clearance: A single‐center propensity score‐matched cohort study

“…We recently reported our development of an IgM‐like oligomerized ACE2 fusion protein (named HH‐120) 26 . Each HH‐120 subunit (monomer) consists of the inactive zinc metalloenzyme extracellular domain of human ACE2 (residues 19–615) at the N‐terminus, an Fc fragment from human IgG1, and an IgM tailpiece at the C‐terminus (which enables the pentamerization and hexamerization of the fusion protein) 27 . HH‐120 has demonstrated potent and broad neutralizing activity against all tested SARS‐CoV‐2 variants in vitro, and demonstrated therapeutic effect against the ancestral strain and Delta variant in golden Syrian hamster infection models via aerosol inhalation 26 .…”

Factors affecting IgG4-mediated complement activation

“…Regardless, IgG4 is not completely unable to bind C1q. A fusion of the IgM tail piece to IgG4, which causes it to multimerize, greatly improved the ability of IgG4 to induce lysis ( 68 ). Similarly, this was also shown for the IgG hexamer-enhancing mutation E430G ( 18 , 61 ).…”