The IgG1 Fc is a dimeric protein that mediates important antibody effector functions by interacting with Fc gamma receptors (Fc gamma Rs) and the neonatal Fc receptor (FcRn). Here, we report the discovery of a monomeric IgG1 Fc (mFc) that bound to Fc gamma RI with very high affinity, but not to Fc gamma RIIIa, in contrast to wild-type (dimeric) Fc. The binding of mFc to FcRn was the same as that of dimeric Fc. To test whether the high-affinity binding to Fc gamma RI can be used for targeting of toxins, a fusion protein of mFc with a 38 kDa Pseudomonas exotoxin A fragment (PE38), was generated. This fusion protein killed Fc gamma RI-positive macrophage-like U937 cells but not Fc gamma RI-negative cells, and mFc or PE38 alone had no killing activity. The lack of binding to Fc gamma RIIIa resulted in the absence of Fc-mediated cytotoxicity of a scFv-mFc fusion protein targeting mesothelin. The pharmacokinetics of mFc in mice was very similar to that of dimeric Fc. The mFc's unique Fc gamma Rs binding pattern and related functionality, combined with its small size, monovalency and the preservation of FcRn binding which results in relatively long half-life in vivo, suggests that mFc has great potential as a component of therapeutics targeting inflammation mediated by activated macrophages overexpressing Fc gamma RI and related diseases, including cancer.
[1]Fudan Univ, Shanghai Med Coll, Minist Educ, Key Lab Med Mol Virol, Shanghai 200433, Peoples R China.
[2]Fudan Univ, Shanghai Med Coll, Minist Hlth, Key Lab Med Mol Virol, Shanghai 200433, Peoples R China.
[3]NCI, Prot Interact Grp, Canc & Inflammat Program, Ctr Canc Res,NIH, Frederick, MD 21701 USA.
[4]Geneva Fdn, Tacoma, WA USA.
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