High affinity binding site-mediated prevention of chemical absorption across the gastrointestinal tract.

Preventing mucosal absorption of low-molecular weight compounds such as carcinogens, toxins and drugs could help prevent many diseases. To characterize the effects of dose and timing on high-affinity binding site mediated sequestration of specific chemical ligands in the gastrointestinal tract, avidin was perorally-administered to mice either prior to or mixed with 3H-biotin. Avidin enhanced fecal 3H-biotin excretion in a dose-dependent manner, consistent with the accepted mechanism of egg white-induced biotin deficiency syndrome. Avidin administration up to 4 h before 3H-biotin administration also enhanced fecal 3H-biotin excretion. Activated charcoal (AC) reduced 3H-biotin absorption when mixed with 3H-biotin before ingestion, but was ineffective when ingested prior to 3H-biotin. These studies suggest that ingestion of high-affinity protein binding sites can establish an absorptive barrier at the gastrointestinal mucosa to prevent the uptake of unwanted low molecular-weight chemicals.

Peroral administration of specific antibody enhances carcinogen excretion.

Ingested carcinogens may exert effects directly on the gastrointestinal epithelium or after absorption and transport to other tissues. To determine the effect of anti-carcinogen antibody ingestion on dietary carcinogen excretion, a mixture of specific IgA or IgG and the model carcinogen 125I-N-2-(4-hydroxyphenyl-acetamido) fluorene (125I-pHP-AAF) was perorally administered to mice. These mice excreted more total and antibody-bound radiotracer in feces compared with controls given a similar mixture containing nonspecific antibody. In addition, urinary radiotracer excretion was reduced by 96% in specific-antibody dosed mice, indicating reduced gastrointestinal absorption of 125I-pHP-AAF. Reduced radiotracer absorption was also reflected by a 56% reduction in radiotracer content in tissues from mice receiving specific antibody. Other mice received peroral IgA before i.p. injection of 125I-PH-AAF. Specific antibody treatment consistently increased intraluminal radiotracer sequestration, as indicated by the level of total and antibody-bound radiotracer partitioning to aqueous fecal extracts. Similarly, when a mixture of 125I-pHP-AAF and IgG were injected directly into the small intestine, more radioactivity appeared in the feces of mice given specific antibody. High-performance liquid chromatography analysis of aqueous fecal extracts indicated that the majority of fecal radiotracer from specific-antibody dosed mice was unmetabolized parent compound. Thus, peroral administration of AAF-specific antibodies mixed with 125I-pHP-AAF decreased gastrointestinal absorption and increased fecal excretion of the radiotracer, suggesting a novel mechanism for protection against environmental carcinogens.

Immunoprophylactic intervention in chemical toxicity and carcinogenicity.

Preventing or diminishing the effects of environmental carcinogens and toxicants using immunoprophylactic vaccines has been tested by various investigators for the last 60 y. This approach has not gained widespread interest in scientific or medical communities due to ambiguous results. However, advances in vaccine design and immunization protocols now offer a reliable mechanism for the induction of mucosal immunity which may afford protection from these agents at their site of absorption. Previous studies aimed at immunoprophylactic vaccination against carcinogens and toxicants are reviewed, and future directions for this area of research are discussed.

Selective induction of mucosal immune responses to 2-acetylaminofluorene.

Mucosal vaccination with chemical carcinogens coupled to enterotoxins such as cholera toxin (CT) can elicit carcinogen-specific immunoglobulin secretion into the intestinal lumen. The present study examines the ability of several related bacterial enterotoxins and their subunits to act as adjuvants or carrier proteins in stimulating an intestinal secretory IgA (S-IgA) response to 2-acetylaminofluorene (AAF). Using Thiry-Vella loops in rabbits, CT, cholera toxin B subunit (CTB) and the recombinant B subunit of the heat labile enterotoxin from E. coli (rLTB) were all found to be effective carrier proteins and adjuvants for eliciting S-IgA anti-AAF. However, marked differences in the ratio of mucosal S-IgA to serum IgG production were observed. CT elicited the highest luminal S-IgA anti-AAF titers as well as the highest ratio of intestinal S-IgA/serum IgG when used as an adjuvant. Conversely, rLTB elicited a high serum IgG anti-AAF titer but only a modest intestinal S-IgA response. Dialysis studies using monoclonal IgA versus IgG anti-AAF on opposing sides of a semipermeable membrane demonstrated the potential importance of the intestinal S-IgA/serum IgG ratio. A high "intestinal" IgA/"serum" IgG ratio abolished carcinogen transfer to the "serum" side of the membrane, while a low ratio enhanced transfer. Thus, to generate an active mucosal immune response capable of blocking carcinogen absorption, the carrier protein or adjuvant should be selected to optimize the intestinal S-IgA/serum IgG ratio.

Effects of laminin monoclonal antibodies on the development of cultured rat embryos.

Whole rat embryos (9.5 days of gestation) were exposed to six different monoclonal antibodies to laminin during 48 hr of culture. Four (LAM I, LAM V, 5A2, 9D2) of the six were teratogenic or lethal and two (LAM II, 5D3) were not toxic at comparable levels. Teratogenicity and lethality were not related to antibody level, subclass or affinity for whole laminin. Indirect immunofluorescence studies using mouse embryo sections revealed that the toxic antibodies bound in a diffuse manner, while the nontoxic antibodies showed distinct labeling of tissues. These observations suggest that previous varied responses seen in cultured rat embryos exposed to laminin antibodies obtained from humans, monkeys, and rats were the result of differences in the epitope specificity of those antibodies.