Comparative efficiencies of bispecific F(ab′γ)2 and chimeric mouse/human IgG antibodies in recruiting cellular effectors for cytotoxicity via Fcγ receptors

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Abstract
The three forms of Fcγ receptor carried by monocytes (FcγRI, II) and natural killer (NK) cells (FcγRIII) are all capable of mediating cell lysis. Here we compare the use of F(ab′γ)2 bispecific antibodies, specifically targetting individual FcγR, and chimeric IgG mouse/human antibodies which are capable of targetting all FcγR, for their ability to mediate target cell destruction. The derivatives are prepared by linking hinge sulphydryl residues via tandem thioether bonds, using a bismaleimide crosslinker: Fab′ from an anti-FcγR mAb linked to Fab′ from a common anti-target mAb (BsAb), or Fab′ from the common anti-target mouse antibody linked to human Fcγ (FabFc or bisFabFc). All the derivatives targetting chick red blood cells gave efficient lysis, although different effector cell donors yielded differences in both the lytic levels achieved and the comparative efficiencies of derivatives. In contrast, significant lysis of the guinea pig lymphoblastic leukaemia, L2C, regularly resulted only via the anti-FcγRIII BsAb and the chimeric derivatives. These results suggest that the chimeric, Fc-containing derivatives mediate tumour cell lysis principally through FcγRIII on NK cells. This is in contrast to the situation with the chick red blood cells where the chimeric derivatives appear capable of lysing erythrocytes by utilizing either monocytes or NK cells, because significant (≈50%) lysis occurred with effector cell populations magnetically depleted through either FcγRII or FcγRIII. A major difference between these two types of antibody derivative was their ability to function in the presence of high concentrations of normal human Fcγ. The lysis mediated by BsAb reactive with FcγRI or II was unaffected by the presence of human Fcγ at 2.5 mg/ml (a concentration comparable with that yielded by IgG in plasma) whereas the BsAb recognizing FcγRIII and all the Fc-containing derivatives were completely inhibited.