M. Bloch

Immune complex-induced enteropathy in the rat

Adult male Sprague-Dawley rats injected with preformed rat anti-bovine serum albumin antibody-bovine serum albumin complexes prepared in fivefold antigen excess, developed intestinal lesions consisting of annular bands of serosal hyperemia alternating with nonhyperemic bands, causing a striped appearance. Histologically, vascular congestion and mucosal edema were observed; more severe lesions were accompanied by hemorrhage, epithelial necrosis and sloughing, and modest polymorphonuclear leukocyte infiltration. The lesions developed rapidly and were accompanied by hemoconcentration. A correlation between the dose of immune complexes injected and the intensity and extent of intestinal lesions was noted. The pathogenetic mechanism of the lesions was not determined. The similarity of the lesions to those observed in systemic anaphylaxis in the rat and experimental and clinical shock was cited. The implications of immune complex-induced enteropathy for studies of immune complex clearance by the mononuclear phagocyte system were considered.

Effect of fish-fat or beef-fat supplemented diet on immune complex-induced entheropathy in the rat

In contrast to animals on a beef fat-supplemented diet (BFD), animals maintained on a fish fat-supplemented diet (FFD) incorporate increased amounts of eicosapentaenoic acid (EPA) into membrane phospholipids. Generation of lipid mediators from such tissues favors the formation of compounds with less pro-inflammatory activity than are derived from tissues poor in EPA. Nevertheless, the FFD has not had a uniformly beneficial effect on animal models of inflammatory diseases. We previously showed that intravenous injection of rat anti-BSA-BSA complexes (IC) prepared in 5x antigen excess rapidly induced a striate pattern of serosal (to mucosal) hemorrhage and vascular congestion throughout the small intestine. In this study, we tested the effect of a BFD and FFD on immune complex-induced enteropathy. After six (Expt. 1) or eight weeks (Expt. 2) on the diet, rats were injected with IC and the severity of serosal hyperemia in the small intestine was scored. In some FFD, no lesions were seen under conditions which elicited moderate to severe lesions in BFD rats. In Expt. 1 involving 22 rats and in Expt. 2 involving 28 rats, those on the FFD had a significantly lower composite lesional score compared to those on the BFD, p less than 0.005 and p less than 0.005, respectively. These results indicate that the FFD had a beneficial effect on IC-induced enteropathy. It is suggested that this effect of the FFD may be mediated primarily by a reduction in availability of platelet-activating factor.

Feeding of antigen reduces antigen-binding activity and blunts the secondary response of actively immunized rats☆

Enteric administration of soluble or particulate antigens to unprimed animals has been repeatedly demonstrated to blunt or abrogate the cellular and humoral immune response to parenteral immunization. We tested the effect of administering bovine serum albumin by gavage or in the drinking water on the serum antigen-binding activity of rats primed by previous immunization with bovine serum albumin and alum. In some animals, this treatment served initially to boost the systemic humoral response; prolonged treatment led to a reduction in antigen-binding activity at a faster rate than was observed in sham-fed controls. In all rats treated with enteric antigen, the booster response to a second parenteral immunization was reduced or absent. Example of the use of enteric antigen in treatment of patients are cited. On the basis of our findings in rats and those of other investigators who studied mice, as well as the results obtained in patients, it is suggested that further exploration of the enteric approach to treating actively immunized subjects appears warranted.

In vivo intestinal uptake of immunoreactive bovine albumin in piglet enteritis

We examined the uptake of bovine serum albumin (BSA) from the intestine into the circulation of 3-week-old piglets infected with transmissible gastroenteritis virus. Transfer of immunoreactive bovine serum albumin (iBSA) from the intestinal lumen into the circulation was enhanced during both the early invasive phase of this viral enteritis (12-h postinoculation) and the diarrheal phase (84-h postinoculation). In some animals, enhanced uptake persisted into the recovery phase, 324 h after inoculation. Gel filtration studies suggested that iBSA had the molecular size characteristic of native BSA; no immunoreactive fragments of BSA were detected. Based on studies of two animal she half-life of iBSA approximated that of porcine albumin. Further study is required to determine the immunological consequences of the enhanced uptake of protein occurring during viral infection of the intestine.

Inhibition of immune complex-induced enteropathy by three different platelet-activating factor receptor antagonists

We previously showed that intravenous injection of rat anti-BSA-BSA complexes (IC) prepared in 5x antigen excess rapidly induced a striate pattern of serosal (to mucosal) hemorrhage and vascular congestion throughout the small intestine of the Sprague-Dawley rat. In this study, we tested the effect of three different platelet-activating factor (PAF) receptor antagonists on the development of lesions. L-652,731, a synthetic derivative of kadsurenone (at doses of 1.3-2.7 mg/kg), SRI 63-675, a substituted quinolinium compound (6.7-15 mg/kg), and WEB 2086, a thienotriazolodiazepine (5-25 mg/kg) were each capable of completely or partially inhibiting IC-induced enteropathy in the majority of animals tested. Pretreatment with WEB 2086 prevented IC-induced hemoconcentration but not neutropenia. The antagonists did not lower the level of blood complement nor interfere with the fall in complement induced by administration of IC. The ability of PAF receptor antagonists to completely or partially inhibit IC-induced small intestinal lesions suggests that endogenous PAF is a major mediator of IC-induced enteropathy.

Blood levels of PAF are elevated during induction of immune complex mediated enteropathy in the rat.

Intravenous injection into rats of immune complexes (IC) prepared in 5 × antigen excess rapidly induces annular bands of vascular congestion and transmural haemorrhage producing a striped appearance of the small intestine. Indirect evidence suggested a major role for PAF in the induction of lesions. In the present study, we showed that blood and leukocyte levels of PAF were elevated in most rats injected 10 min earlier with sufficient IC to induce lesions of 3+ to 4+ intensity. There was no significant difference in the number of rats with elevated plasma levels of PAF. The possibility that changes in blood PAF levels might be mirrored at sites closer to the lesions was considered. The overall effect of PAF on the small intestine of the rats is to induce stasis of flow; the precise target of PAF in mediating this effect is unknown.

Interaction of circulating radiolabelled IgG antibody and fed protein antigen in the rat

We tested the effect of fed protein antigen on circulating IgG antibody in rats passively supplied with a limited dose of antibody. Rats were given 1.0 g BSA or buffer by gavage and were injected i.v. with trace amounts of 125I-rat anti-BSA antibodies plus 35 ug ‘cold’ anti-BSA antibodies. Rats were exsanguinated under anesthesia and perfused with buffer 1 or 18–22 h after feeding. Radioactivity in serum and liver was measured. At 1 h, there was less radioactivity in serum of BSA-fed compared to control animals (p less than 0.05); this difference was greater at 18–22 h (p less than 0.001). At 1 h, the content of radioactivity in the liver was greater in BSA-fed than control rats (p less than 0.005); there was no significant difference at 18–22 h. On A5m gel permeation of serum obtained at 1 h from BSA-fed rats, radioactivity was detected in fractions expected to contain high and intermediate molecular size immune complexes; serum obtained from BSA-fed rats at 18–22 h contained no such fractions. Based on a comparison with the decrease in concentration of labeled antibody resulting from i.v. injection of BSA, BSA-fed rats appeared to have taken up 16.7 to 23.1 ug BSA of the 1.0 gram dose administered. These experiments confirm that immunoreactive protein is taken up from the gut into the circulation of fed animals, that considerable uptake occurs during the first hour but persists thereafter, and that IgG antibody-fed antigen complexes forming early after ingestion are suitable for clearance by the liver.