Protective Effect of Immunization of Rats with Holotoxin or B Subunit of Escherichia coli Heat-Labile Enterotoxin

Abstract

The relative immunogenicities of three forms of the Escherichia coli heatlabile enterotoxin (LT), the holotoxin, its B subunit, and the polymyxin-release form (PM LT) were compared by immunizing rats with various dosages of each given exclusively by the parenteral (IP/IP) or peroral (PO/PO) routes or by a combination of the two (IP/PO). The degree of protection was evaluated by challenge in ligated ileal loops, and the serum antitoxin response was determined by an enzyme-linked immunosorbent assay with homologous antigens. When given by the PO/PO route, each LT antigen provided only weak protection against the toxin and virtually none against viable LT-producing strains; serum antitoxin titers were not significantly increased. When the toxins were given after a parental primary immunization by either the IP/IP or the IP/PO routes, each LT antigen provided a dose-related increase in serum antitoxin titers and in the degree of protection against the toxin as well as against viable strains which produce LT alone (LT'/ST-) or in combination with the heat-stable toxin (LT'/ ST'). The degree of protection against viable bacteria, particularly the LT'/ST' strain, was stronger in animals which received booster immunizations by the PO route. When expressed on the basis of molar equivalents, holotoxin provided significant protection (a protection index of >5 against toxin challenge and >50% reduced secretion with bacterial challenge) with 4 to 15 times fewer moles than PM LT and up to 50 times fewer moles than the B subunit. These observations indicate that, on the basis of molar equivalents, the holotoxin (which contains one A plus five or six B subunits) is a more potent immunogen than either PM LT (which contains one A and probably one B subunit) or the B subunit.

INTESTINAL ADAPTATION TO CYCLIC AMP‐MEDIATED HYPERSECRETION INDUCED BY THE HEAT‐LABILE ENTEROTOXIN OF VIBRIO CHOLERAE AND ESCHERICHIA COLI

“…The values represent the mean f SEM of two experiments comprising 2 and 3 animals respectively, mucosal membranes being pooled within each group. numbers of epithelial cells (1 6, 171, or via a direct regulation by nerves and hormones (1,14). The present study suggest that desensitization of adenylate cyclase, mediated by (an) inducable factods), might affect intestinal hypersecretion by similar mechanisms.…”

Molecular Characterization of Bacterial Virulence Factors and the Consequences for Vaccine Design The 1988 Fleming Lecture

“…Toxin genes can also be readily manipulated to direct the expression of toxoids rather than toxins. An early example of this was the construction of a plasmid that directed the expression of the E. coli LT-B toxoid rather than the active LT toxin Klipstein & Engert, 1981). More recent work has been undertaken using pertussis toxin (Black et al, 1987;Gross et al, 1989).…”

Protection in rabbits immunized with a vaccine of Escherichia coli heat-stable toxin cross-linked to the heat-labile toxin B subunit

“…Enterotoxigenic strains of Escherichia coli cause acute diarrhea by means of elaborating heat-labile or heat-stable enterotoxin (LT or ST, respectively), either singly or together. LT and its B subunit are immunogenic (13,16); lowmolecular-weight, haptenic ST becomes immunogenic when coupled to a high-molecularweight carrier (7,9). Immunization of experimental animals with either of these toxins raises a specific antitoxin response that provides protection against viable strains of heterologous serotypes which produce that particular toxin.…”

“…Immunization of experimental animals with either of these toxins raises a specific antitoxin response that provides protection against viable strains of heterologous serotypes which produce that particular toxin. Thus, immunization with either LT or its B subunit provides protection only against LTproducing strains (LT'/ST-) (13,16), immunization with ST yields protection only against STproducing strains (LT-/ST+) (14,18), and antitoxin to each toxin form contributes to protection against strains which produce both toxins together (LT'/ST+) (13, 17a). Conjugation of ST to LT yields an immunogen that provides protection against strains which produce either form of toxin since ST acquires immunogenicity as a result of its coupling to LT and the antigenicities of both toxin components are largely re-tained although their toxicities are greatly reduced (15).We recently described a vaccine for enterotoxigenic E. coli based on cross-linking synthetically produced ST to the nontoxic B subunit of LT (17).…”

“…However, p.o. immunization with either LT or its B subunit is strongly effective in rats only when delivered after primary immunization is given by the parenteral route (13,16,19). It has been uncertain whether this requirement is due to properties of these immunogens or whether it is a function of the rat model itself.…”