Activation and hapten inhibition of mast cells sensitized with monoclonal IgE anti-penicillin antibodies: evidence for two-site recognition of the penicillin derived determinant.

We utilized an in vitro mast cell activation assay and hapten inhibition of mediator release to characterize the fine specificity of two IgE anti-penicillin monoclonal antibodies (mAb). Cultured mouse mast cells were passively sensitized with IgE mAb anti-benzylpenicillin (BP) or anti-amoxicillin (AX) and challenged with a range of penicillin-human serum albumin (HSA) conjugates. Mast cells sensitized with IgE anti-BP degranulated in response to BP-HSA, but not to AX-HSA or ampicillin(AMP)-HSA, whereas mast cells sensitized with IgE anti-AX responded to AX-HSA but not to BP-HSA or AMP-HSA. Because BP, AX and AMP differ chemically only in the structure of their side chain, these results show that this part of the drug molecule is essential for recognition by IgE antibody. Unexpectedly, although IgE-sensitized mast cells responded to only one penicillin in protein-conjugated form, antigen-induced degranulation was inhibited by the monomeric derivative of more than one penicillin. Furthermore, antigen activation of IgE-sensitized cells was inhibited, although less potently, by haptens representative of the specific penicillin side chain or the binuclear portion of the drug molecule. These patterns of recognition and hapten inhibition were also seen in solid-phase enzyme-linked immunosorbent assay (ELISA), although all haptenic inhibitors were approximately 100 times less potent in the ELISA compared to the mast cell assay. To explain these findings we propose a model in which IgE binding to penicillin-protein antigen is dependent on recognition of two distinct epitopes on the drug molecule: the first comprising the side chain, and the second comprising the binuclear portion plus the proximal region of the side chain. This two-site hypothesis provides a generally applicable model of antibody recognition of penicillins and provides a rational basis for understanding the specificity and cross-reactivity of IgE-mediated allergic reactions to penicillins.