Structural requirements of immunoglobulin G for binding to the FC(γ) receptors of the human tumor cell lines U937, HL-60, ML-1, and K562

Fc receptors (FcR) on U937, HL-60, ML-1, and K562 cells were characterized by using competitive binding assays and EA rosette inhibition studies. Like human monocytes, U937, HL-60, and ML-1 cells bound monomeric human IgG Fc with high affinity and mildly reduced and alkylated human IgG and Fc with a somewhat diminished affinity. In contrast, K562 cells had a much lower affinity for monomeric human IgG or Fc. Concentrations of these proteins as high as 10^-5 M were needed to completely block EA rosette formation. There was no binding of reduced and alkylated IgG and Fc. We assessed the influence of various segments of IgG on FcR interactions by using human pFc', rabbit Facb, mouse IgG2b-IgG2a hybrid Ig, and also studied the effect of reduction of the interchain disulfide bonds. The FcR on all four cell types bound rabbit IgG but not rabbit Facb or human pFc', suggesting that rabbit C(γ)3 domains are required for FcR interaction and that isolated human C(γ)3 domains do not have a human FcR binding site. Murine IgG2a, but not IgG2b, was cytophilic for U937, HL-60, and ML-1 cells. Binding studies with the use of several mouse myeloma variant Ig molecules having hybrid (γ2b-(γ)2a heavy chains showed that variants with a complete (γ)2a Fc region bound to these FcR-like IgG2a, whereas those having (γ)2a sequences only in the C(γ)3 region and in a short adjacent segment of the C(γ)2 region behaved like IgG2b and did not bind. These results suggested that additional murine, (γ)2a sequences are required for FcR binding. Interestingly, the Fc fragments from murine proteins with a complete (γ)2a Fc region bound only to a limited extent. These fragments are shorter than the human IgG1 Fc fragments, and they lack that segment of the hinge region that includes the interchain disulfide bonds. Cleavage of the interchain disulfide bonds of murine Ig having a complete (γ)2a Fc region diminished binding to a similar extent as that observed with human IgG. Together, these findings provide additional insight into the roles of the hinge, C(γ)2, and C(γ)3 regions of human, rabbit, and mouse IgG in their interaction with the FcR of human tumor cells.