Monoclonal antibodies that inhibit ADP-ribosyltransferase but not NAD-glycohydrolase activity of pertussis toxin

Antibodies recognizing protective pertussis toxin epitopes are preferentially elicited by natural infection versus acellular immunization

Despite more than 50 years of vaccination, disease caused by the bacterium Bordetella pertussis persists, with rates increasing in industrialized countries over the past decade. This rise may be attributed to several factors, including increased surveillance, emergence of vaccine escape variants, waning immunity in adults, and the introduction of acellular subunit vaccines, which include chemically detoxified pertussis toxin (PTd). Two potently protective epitopes on pertussis toxin (PTx) are recognized by the monoclonal antibodies 1B7 and 11E6, which inhibit catalytic and cell-binding activities, respectively. In order to determine whether the PTx exposure route affects antibody responses to these epitopes, we analyzed sera from 30 adults with confirmed pertussis exposure and from 30 recently vaccinated adults for specific anti-PTx antibody responses and in vitro CHO cell neutralization titers. While overall titers against PTx and the genetically detoxified variant, PTg, containing the R9K and E129G substitutions, were similar in the two groups, titers against specific epitopes depended on the exposure route. Natural infection resulted in significantly higher titers of anti-PTx-subunit 1, 1B7-like, and 11E6-like antibodies, while acellular vaccination resulted in significantly higher titers of antibodies recognizing PTd. We also observed a correlation between in vitro protection and the presence of 1B7-like and 11E6-like antibodies. Notably, chemical detoxification, as opposed to genetic inactivation, alters the PTx tertiary and quaternary structure, thereby affecting conformational epitopes and recognition of PTx by 1B7 and 11E6. The lower levels of serum antibodies recognizing clinically relevant epitopes after vaccination with PTd support inclusion of PTg in future vaccines.

Detection of antibodies inhibiting the ADP-ribosyltransferase activity of pertussis toxin in human serum

Bordetella pertussis produces a protein virulence factor termed pertussis toxin. Many candidate pertussis vaccines are based on the rationale that an immune response that neutralizes the virulence activities of this toxin, which are thought to arise from its catalytic ADP-ribosyltransferase activity, would be beneficial. The report describes two methods that quantify the inhibition of this activity by human serum. One, termed a direct assay, involves an initial incubation of toxin with serum, a second incubation that activates the toxin, and a third incubation that measures the ADP-ribosyltransferase activity of the mixture. The other assay, termed a plate assay, involves immobilization of the toxin, exposure of the immobilized toxin to serum and washing of the plate, and then activation and assay of the toxin's ADP-ribosyltransferase activity. The plate assay may be more selective than the direct assay in terms of identifying antibodies that neutralize the toxin in vivo. Sera from controls, selected patients presenting with cough, and vaccinated infants were first analyzed by the direct assay. In contrast to sera from controls, sera from several of the patients and vaccinated infants strongly inhibited activity. Dose-response curves of inhibition were determined for samples from three vaccinated infants by both the direct and plate assays. One of the samples had a dose-response curve of a different shape and thus differed not only in titer but also in functional characteristics. A comparison of inhibition of ADP-ribosyltransferase activity and neutralization in a CHO cell assay indicated that there was incomplete agreement between the two assays. Taken together, these results indicate that measurement of inhibition of ADP-ribosyltransferase activity by human serum is practical and may be useful in the evaluation of responses to pertussis vaccines.

Characterization of monoclonal antibodies directed against domains of pertussis toxin involved in receptor recognition

The exotoxin pertussis toxin (PT) produced by virulent Bordetella pertussis bacteria is regarded as the main virulence factor of the organism and held responsible for most of its pathological effects. Identification of functional sites on PT would greatly facilitate site-specific detoxification and thus also the development of a new vaccine. For the investigation of structure-function aspects of PT we have prepared and characterized eleven monoclonal antibodies (mAbs) (UB-A1, UB-A2, UB-A10, UB-B7, UB-B12, UB-D4, UB-D7, UB-D10, UB-F7, UB-G1, and UB-G12) directed at the native toxin. Only UB-B12 and UB-D10 recognized PT in Western blotting indicating that most of the mAbs were directed against conformational epitopes. The mAbs were assayed for their ability to interfere with the binding of PT in model receptor systems like a solid phase binding assay using fetuin as receptor moiety, hemagglutination of chymotrypsin-sensitized goose erythrocytes, and the PT-mediated induction of the clustered growth pattern (CGP) of Chinese hamster ovary (CHO) cells. Five of the eleven mAbs (UB-A1, UB-A2, UB-B7, UB-B12, and UB-D7) interfered with the binding of PT to fetuin on solid phase and with PT-mediated hemagglutination. UB-A2, UB-B7, and UB-B12 also inhibited the induction of the clustered growth pattern of CHO-cells. This indicates that the determinants recognized by these mAbs are associated with the formation of the carbohydrate recognition sites of PT. Thus, the monoclonal antibodies described in this study will be valuable tools in the further analysis of the structure-function relationship of pertussis toxin with respect to receptor recognition and binding.