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Departments of
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Pathology,
Pediatrics, and
Medicine, Case Western Reserve University School of Medicine, Cleveland, OH 44106
| Abstract |
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| Introduction |
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Earlier we demonstrated that soluble immune complexes containing
oligomeric IgA Abs were readily transported intact into the apical
medium across polarized epithelial cell monolayers that expressed the
pIgR on their basolateral surface (8, 9). In other words,
the Fc
-dependent mechanism used to transport
IgA across epithelial cells functioned the same for IgA that was
complexed to Ag by its Fab binding sites as it did for free IgA. Based
on these experiments in vitro we suggested that an IgA-mediated
excretory immune system could function in vivo. The present work was
undertaken to explore whether such an excretory immune system indeed
functions in vivo. The experimental design was to immunize mice
intragastrically to stimulate an intestinal IgA Ab response, after
which Ag was injected i.v. to generate soluble IgA-containing immune
complexes in the intestinal lamina propria. Subsequent detection of Ag
in small intestinal crypt cells was taken as evidence of mucosal
excretion by specific IgA Ab.
| Materials and Methods |
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Transgenic BALB/c mice of both sexes expressing the 
TCR
from a T cell hybridoma, DO11.10, that recognizes a chicken OVA peptide
(323339) restricted by MHC class II
I-Ad (10) were obtained from Dr.
Kenneth Murphy (Washington University). They were maintained and bred
in our animal facility with food and water ad libitum. TCR transgenic
progeny were identified by PCR assays on DNA isolated from tail clips
(11). Animal studies were approved by the Case Western
Reserve University Institutional Animal Care and Use Committee and were
performed in compliance with institutional guidelines.
Immunization
Chicken OVA (Sigma, St. Louis, MO) was coupled with biotin (Pierce Chemical, Rockford, IL) according to the suppliers instructions at a weight ratio of 15% biotin. An average of seven biotins were coupled to each molecule of protein. Mice were immunized five to six times at weekly intervals with 10 mg biotinylated OVA or the control protein BSA (Sigma), by stomach tube in the presence of 10 µg of the mucosal adjuvant cholera toxin (12) in 0.5 ml of 0.2 M NaHCO3. IgA Ab titers in serum, measured by ELISA, reached a maximum at this time. In selected mice, it was confirmed that specific IgA Ab was also secreted into the intestinal lumen. For this, a segment of small intestine was rinsed with 5 ml of a mixture of protease inhibitors (Complete Protease Inhibitor Cocktail; Roche, Gipf-Oberfrick, Switzerland) in PBS (pH 7.2). Intestinal secretions collected with polywicks (Polyfiltronics Group, Rockland, MA) (13) were assayed for IgA Ab content by ELISA.
Measurement of Ab response
IgA Ab responses to biotinylated OVA were measured by ELISA. Nunc Immuno Plates (Naperville, IL) were coated by overnight incubation at 4°C with 10 µg/ml Ag in carbonate buffer (pH 9.5). Nonspecific binding sites were blocked with PBS containing either 1% BSA or 40% soy milk. Serial half-log10 dilutions of antisera (50 µl), made in either 1% BSA or 40% soy milk, were added and the plates were incubated at ambient temperature for 90 min. Plates were then washed with PBS and incubated with affinity purified, alkaline phosphatase conjugated goat anti-mouse IgA-specific Ab (Southern Biotechnology Associates, Birmingham, AL) at ambient temperature for 90 min. The plates were washed and developed with disodium p-nitrophenyl phosphate (Sigma) in glycine buffer (pH 9.6). After 60 min, ODs were read at 405 nm with a Molecular Devices Vmax plate reader (Menlo Park, CA). For each serum sample, the least-squares regression of the OD as a function of the log of the serum dilution was used to calculate the titer. The titer was defined as the log of the dilution that generated an OD equal to two SDs above the mean background OD developed with nonimmune syngeneic control serum.
Ag challenge and tissue preparation
Five to seven days after the final intragastric immunization, the mice were injected in the tail vein with 50 mg biotinylated OVA in PBS and sacrificed 30 min later. The small intestine was removed and the lumen was rinsed with 20 ml of cold PBS containing 0.9 mM Ca2+ and 0.49 mM Mg2+. The intestine was filled with OCT Compound (VWR, Bridgeport, NJ), coiled, snap frozen in 2-methyl butane, and stored at -70° before sectioning.
Microscopy
Cryostat sections (78 µm) of coiled small intestine were
air-dried, fixed for 1 min in acetone, air-dried, and stored at
-20°. For detection of biotinylated OVA Ag, sections were hydrated
in PBS, exposed to avidin-biotinylated HRP complex (Vectastain Elite
ABC; Vector Laboratories, Burlingame, CA) for 30 min, rinsed in PBS,
exposed to diaminobenzidine, and rinsed in PBS. Sections were
counterstained with hematoxylin, cleared, and sealed with a coverslip.
For detection of mouse IgA, hydrated frozen sections were exposed to
peroxidase-conjugated goat Ab (Southern Biotechnology Associates),
rinsed, developed with diaminobenzidine, counterstained, cleared, and
sealed with a coverslip. Slides were numbered randomly by one
investigator and examined and scored in blinded manner by another on a
scale of 0 (no staining) to 3 (prominent, dense small intestinal crypt
cell staining easily observed at low power) by direct visual
observation with a Leica DMLB microscope (Deerfield, IL). The
semiquantitative peroxidase staining intensity scores for the amount of
biotinylated OVA in the small intestinal crypt epithelium of the
individual mice were subjected to one-way ANOVA, stratified by mouse
group. Post hoc comparisons among the groups used Scheffes test and
Fishers protected t test. In addition, the distribution of
individual mice scored "positive" (intensity score >0) or
"negative" (intensity score = 0) in each group was assessed by
2 contingency analysis in a two (level) by
three (group) design, with Fishers correction for small numbers
applied. Photographs were made with a Diagnostic Instruments digital
camera.
| Results and Discussion |
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The current experiments were designed to create soluble immune complexes in mucosal lamina propria to test the hypothesis of a local excretory function for mucosal IgA Abs in vivo. To stimulate a potent mucosal Ab response, mice were immunized via the gastrointestinal tract. After the specific IgA Ab response, as reflected in the serum, reached a peak, the mice were challenged i.v. with a large dose of Ag to ensure that the entire extracellular fluid volume would be in Ag excess and to provide sufficient soluble IgA-containing immune complexes in the lamina propria for their excretion through mucosal epithelium to be detectable morphologically.
The requisite immune complexes in the intestinal lamina propria could have arisen by two mechanisms. One is diffusion of excess free Ag from the bloodstream into the lamina propria, where it combined with IgA Abs secreted by local plasma cells. The other possibility is formation of IgA immune complexes in the circulation and their subsequent diffusion into the lamina propria. Both mechanisms would have been facilitated by the large excess of injected Ag, designed to favor the formation of small immune complexes, with a limiting complex of one molecule of Ag and one molecule of Ab, with some Ag remaining free. Regardless, once in the lamina propria, the IgA immune complexes would be in a position to compete for available epithelial pIgR with background levels of free nonspecific oligomeric IgA and IgA immune complexes containing naturally prevalent Ags.
For several reasons we believe the biotinylated OVA that was detected in the small intestinal crypt cells was Ag being transported, i.e., excreted, by specific IgA Ab across the epithelium from basal to apical. First, biotinylated OVA was readily detected in the crypt cells of TCR transgenic mice that had been both mucosally immunized and subsequently challenged i.v. with biotinylated OVA. It was also detectable, but less prominently, in the crypt cells of wild-type BALB/c mice that had been treated similarly, but that, lacking the transgene, made a less vigorous IgA Ab response (results not shown). Specific Ag was not detected in the intestinal epithelium of mice that had been immunized mucosally with an irrelevant Ag (BSA) before i.v. injection of biotinylated OVA. Thus, in the absence of specific IgA Ab, biotinylated OVA in the lamina propria was not taken up by the intestinal epithelium. Second, mice that had been immunized intragastrically with biotinylated OVA but not subsequently challenged i.v. did not evidence biotinylated OVA in small intestinal crypt cells, showing that under the experimental conditions, luminal Ag from the mucosal immunization was not absorbed in detectable quantities into the crypt cells, either nonspecifically or bound to previously secreted IgA Ab. Third, the architecture of the small intestine provides an internal control, namely, a gradient of decreasing transit of IgA across the lining epithelium along the crypt-villus axis. This parallels the expression of epithelial pIgR, the transporter of IgA, which is also most prominent in the crypt cells, diminishing toward the tips of the villi (17, 18, 19). Consistent with this gradient in IgA Ab transport, biotinylated OVA Ag was detected much more prominently in crypt than in villus epithelial cells. This marked decrease in amount of Ag in the epithelium proceeding toward the tips of the villi, in turn, argues against epithelial uptake due to nonspecific events and staining artifacts.
Specific IgM Ab, also present after immunization (data not shown) and also capable of binding to epithelial pIgR, could contribute to uptake of Ag into intestinal epithelial cells. Quantitatively, however, mucosal IgA production greatly exceeds that of mucosal IgM; moreover, the transport of IgA appears to be favored under in vivo conditions (20).
Mucosal surfaces, especially in the gastrointestinal tract, are constantly exposed to foreign substances, and secreted IgA Ab in the lumen has long been known to provide an immunological barrier to limit the penetration of Ags into mucous membranes. From previous experiments in vitro (8, 9) and the present experiments in vivo, we propose that mucosal IgA Abs in the lamina propria additionally provide a backup, internal barrier beneath the epithelium that can trap Ags missed by the initial IgA barrier in the lumen. For example, Ags could by-pass luminal IgA Ab either during the early stages of a mucosal Ab response when there was insufficient Ab to prevent the absorption of all the Ag present or even in the face of an established response if there was sudden exposure to a particularly large quantity of luminal Ag. Moreover, Ags will also be released into the lamina propria from microbial pathogens during infections of the mucosae. Therefore, we believe that to some extent foreign Ags are a regular presence in mucosal lamina propria. Regardless of the origins of particular Ags at those sites, given the ongoing production of IgA by the numerous local plasma cells, IgA Abs are in a position to bind and efficiently transport Ags out of the body proper and into the lumen. Potentially IgA Abs could even excrete particles as large as intact viruses (21).
Removal of a variety of Ags from the lamina propria via the same mechanism that is used for transporting free IgA across mucosal epithelium would serve to limit the amounts of Ag reaching the circulation, where Ags would also be more likely to be bound by Abs of the more abundant and more phlogistic IgG class. In this way, the excretory function of IgA could help to prevent diseases that result from circulating immune complexes. A particularly relevant example is IgA nephropathy, the most common form of glomerulonephritis, which is thought to result from abnormal regulation of the immune response to mucosal infections (22).
In addition to extracellular locales, IgA Abs are also capable of binding to Ags inside epithelial cells during pIgR-mediated transport of free IgA. IgA Abs acting intracellularly have been shown to neutralize viruses (23, 24, 25, 26) and to block their apical to basal transcytosis (27). Thus, overall, IgA Abs appear to be capable of mediating an integrated, multilayered mucosal defense system (28). The first layer, in the lumen, is exclusion of Ag by secreted free IgA. The second layer, within the lining epithelial cells, allows for inhibition of intracellular pathogens like viruses. The third layer, as demonstrated in the current work, is the lamina propria from which IgA Abs can directly excrete Ags into the lumen.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Michael E. Lamm, Institute of Pathology, Case Western Reserve University, 2085 Adelbert Road, Cleveland, OH 44106-4907. ![]()
3 Abbreviation used in this paper: pIgR, polymeric Ig receptor. ![]()
Received for publication October 18, 2000. Accepted for publication January 4, 2001.
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