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The Journal of Immunology, 1999, 162: 6046-6052.
Copyright © 1999 by The American Association of Immunologists

Fine Specificity of Ligand-Binding Domain 1 in the Polymeric Ig Receptor: Importance of the CDR2-Containing Region for IgM Interaction1

Målfrid Røe, Inger N. Norderhaug, Per Brandtzaeg and Finn-Eirik Johansen2

Laboratory for Immunohistochemistry and Immunopathology (LIIPAT), Institute of Pathology, University of Oslo, The National Hospital, Rikshospitalet, Oslo, Norway


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The human polymeric Ig receptor (pIgR), also called transmembrane secretory component, is expressed basolaterally on exocrine epithelia, and mediates specific external transport of dimeric IgA and pentameric IgM. The extracellular part of pIgR consists of five Ig-like domains (D1-D5), and a highly conserved D1 region appears to mediate the initial noncovalent ligand interaction. While the human pIgR binds both dimeric IgA and pentameric IgM with high affinity, the rabbit counterpart has virtually no binding capacity for pentameric IgM. This remarkable disparity constitutes evidence that the binding site of the two ligands differs with regard to essential receptor contact elements. Therefore, we expressed human/rabbit chimeric pIgRs in Madin-Darby canine kidney cells and found that human pIgR D1 is crucial for the interaction with pentameric IgM when placed in the context of a full-length receptor regardless of its backbone species. D1 contains three complementarity-determining region-like loops (CDR1–3), and to further map human D1 regions involved in pentameric IgM binding, we transfected Madin-Darby canine kidney cells with human/rabbit chimeric receptors in which the regions containing the CDR-like loops had been interchanged. Our results showed that the region containing the CDR2-like loop is the most essential for pentameric IgM binding. The region containing the CDR1-like loop also contributed substantially to this interaction, whereas only little contribution was provided by the region containing the CDR3-like loop, although it appeared to be necessary for maximal pentameric IgM binding.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Secretory Ig (SIgA3 and SIgM) Abs play a major role in adaptive defense at mucosal surfaces, the largest body area exposed to the external environment (1, 2, 3, 4). Dimers and larger polymers of IgA (collectively called pIgA) and pentameric IgM synthesized by subepithelial plasma cells become specifically bound by the human pIg receptor (pIgR), also known as the transmembrane secretory component (SC). This receptor is expressed on the basolateral surface of secretory epithelial cells (5, 6). The pIgs are next internalized and transported across these cells to their apical domain, where the extracellular ligand-binding portion of the receptor is proteolytically cleaved and released to the lumen, either complexed to the ligand as bound SC or unoccupied as free SC (4, 7).

It remains an enigma that two so structurally different polymers as pIgA and pentameric IgM can bind specifically to the same receptor, although their shared J chain has been shown to be essential for this interaction (8, 9, 10, 11, 12, 13, 14, 15). However, by itself this polypeptide shows only marginal affinity for free SC (10). Therefore, although the J chain in a crucial way contributes to the binding site for pIgR in both pIgA and pentameric IgM, it is unknown how the polymers themselves are involved in this binding site. The fact that pentameric IgM has been shown to interact with free SC with an affinity that is 8–30 times that determined for pIgA (16, 17) suggests that the pIgR binding site of the two polymers is structurally somewhat different. This idea is supported by remarkable species differences shown by pIgR with regard to pentameric IgM interaction (see below).

The pIgR is a glycoprotein of 100–120 kDa (depending on the species) with five Ig-like extracellular domains (D1-D5) that are structurally most similar to the IgV regions (18). Binding of pIgA to pIgR appears to be a sequential process in which an initial noncovalent ligand interaction with D1 progresses to other domains (Ref. 19 and Norderhaug et al.4) and is followed (in most species) by disulfide binding between one of the IgA heavy chains and D5 (reviewed in 20). Several lines of evidence suggest that D1 carries the primary site of interaction with pIgA. First, both proteolytic and recombinant fragments of the receptor that contain D1 have been shown to retain the capacity to bind pIgA, although this initial interaction with certain synthetic receptor peptides appears to be "promiscuous" with regard to Ig class (19, 21, 22, 23) (see below). Second, mAbs that recognize an epitope within D1 have been shown to compete with pIgA for binding to the receptor (21). Furthermore, we have shown recently that binding of pentameric IgM to the human pIgR depends preferentially on a strong interaction with D1, while binding of pIgA in addition depends on determinants within D2 and/or D3 to support the initial noncovalent interaction with D1.4

Located in D1 are three loops corresponding to the complementarity-determining regions (CDR1-CDR3) of IgV regions (24), the sequence that determines their Ag-binding specificity. The CDR1-like loop in pIgR D1 is highly conserved among different species, 82–100% when conservative amino acid changes are not taken into account (25). The CDR2- and CDR3-like loops in D1 show less interspecies homology, but retain some invariant residues that may play important roles in ligand binding. Thus, a study by Coyne et al. (25), based on a mutational approach with modeling of the rabbit pIgR D1 sequence on known Ig variable structures, suggested that all three loops participate in the specific noncovalent binding of human pIgA. The order of importance was not examined, but based on the studies mentioned above, the CDR1-like loop is probably the most important one for the initial interaction. In humans, pIgR binding of pIgA also depends on structural elements outside D1 (Ref. 19 and Norderhaug et al.4), and the importance of interactions between pIgA and elements in CDR2 and CDR3 may therefore be less significant than for rabbit D1.

Human pIgR does not show stable binding of monomeric IgA or other monomeric Ig isotypes, but interacts with both pIgA and pentameric IgM with high affinity, as mentioned above (16, 26). In mice and rabbits, on the other hand, the pIgR binds primarily pIgA (27). To characterize the interaction of human pentameric IgM with pIgR, we exploited this species difference and generated several chimeric receptors to characterize the pentameric IgM binding site of the receptor. We found that D1 of human origin could transfer its pentameric IgM-binding affinity to the rabbit pIgR, thus substantiating its crucial role for both pIgA and pentameric IgM interaction. Furthermore, we demonstrated that the D1 regions containing CDR1- and CDR2-like loops of human origin could transfer substantial pentameric IgM-binding capacity to the rabbit pIgR. However, all three human pIgR CDR-like regions were required for maximal pentameric IgM-binding capacity.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Immunoglobulins

Polyclonal human pIgA (28), mainly IgA1, crude monomeric IgA, and monoclonal pentameric IgM, were isolated and characterized, as previously described (29).

Plasmid constructions

All constructs were cloned in the eukaryotic expression vector pCDNA3neo (Invitrogen, San Diego, CA). The subcloning of the full-length cDNA encoding the human pIgR (30) has been described previously (29). Subcloning of the murine and rabbit pIgR was performed by PCR with pIgR cDNA of mouse (gift from C. Kaetzel, University of Kentucky) (31) and rabbit (gift from K. Mostov, University of California at San Francisco) (32) origin as template, respectively. The human/rabbit chimeras were constructed by PCR with primers that introduced silent mutations in the overlaps to create restriction enzyme cloning sites. Chimeric expression constructs were made with standard molecular biology techniques and encoded the amino acid sequences outlined in Fig. 1Go (details of construction will be provided upon request).



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  		FIGURE 1. A, Comparison of the deduced amino acid sequences of the experimentally interchanged regions A, B, and C of human (H) and rabbit (R) pIgR domain 1 (upper and lower lines, respectively). The CDR-like loops are indicated by boxes. When used in the text, capital letters for region A, B, or C refer to the amino acid sequence of human origin, and corresponding lower case letters rabbit origin; and the backbone species is denoted by -h and -r for human and rabbit, respectively. Crossovers were chosen from stretches of highly conserved amino acid sequence to minimize chances of steric incompatibility and incorrect folding of the chimeric proteins. B, Point mutations generated to create restriction enzyme cloning sites for swapping of regions A, B, or C. Recognition sequences are underlined, and restriction enzymes used are written below. The nature of the mutations is shown in parentheses. For human region C to rabbit D2, SacI was used, and for rabbit region C to human D2, ApaI was used.

 
Cell culture

MDCK (strain II) cells were grown in DMEM (Bio Whittaker, Walkersville, MD) supplemented with 5% FCS, 50 µg/ml of gentamicin, and 1 mM L-glutamine (Life Technologies, Paisley, U.K.).

Transfection and clonal selection of pIgR constructs

MDCK cells were stably transfected by the DNA-calcium phosphate procedure (33) or by electroporation, in which 107 cells were exposed to 10 µg DNA in 0.4 ml PBS at 250 µF, 675 V/cm. Clones expressing the neomycin resistance marker were selected in the presence of 0.5 mg/ml G418 (Geneticin; Sigma, St. Louis, MO). Stable cell lines were established by isolating resistant colonies with cloning cylinders. Clones expressing the pIgRs were identified by immunofluorescence staining, in which cells were first incubated with pIgA (19 µg/ml) for 1 h at room temperature, and then with a rabbit anti-human IgA FITC conjugate (F 0204; Dako, Glostrup, Denmark) diluted 1/50 (1 h, room temperature). At least two clones for each construct that showed uniform, strong staining were randomly selected for further analysis.

Binding of radioiodinated pIgA and pentameric IgM

Preparations of pIgs were 125I labeled with Chloramine-T-catalyzed iodination and purified by gel-filtration chromatography on a PD-10 column (Sephadex G-25 M) (Pharmacia Biotech, Uppsala, Sweden). Stably transfected MDCK cells grown to confluence in microtiter plates (number 3590; Costar, Cambridge, MA) were incubated with 125I-labeled pIgA or pentameric IgM, and with various concentrations of unlabeled corresponding pIg in DMEM/5% FCS/10 mM HEPES at 4°C for 2 h. The medium was then harvested and the cells were washed four times with ice-cold PBS and lysed in 2 M NaOH. The radioactivity was determined in an automatic gamma counter (1470 Wizard; Wallac, Turku, Finland). Nonspecific binding of 125I-labeled pIg was determined by incubation with nontransfected MDCK cells.

Epithelial cell translocation of pIgA and pentameric IgM

Transfected and nontransfected MDCK cells were cultured on 3-µm Transwell-COL filters (Costar) for 6 days, and transepithelial resistance for each filter was measured to be >=180 {Omega} before the start of transcytosis experiments. Polarized cells were incubated with 50 nM pIgA or pentameric IgM together with 50 nM IgG in the basal medium at 37°C for 20 h. The apical medium was harvested and analyzed by ELISA to determine the concentrations of IgA, IgM, and IgG, as previously described (29).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Rabbit pIgR binds pentameric IgM poorly compared with human pIgR

Previous studies on binding of pIgA and pentameric IgM to pIgR from different species have demonstrated that mouse and rabbit pIgR binds pentameric IgM (from any species) poorly but pIgA from all species quite well, whereas human pIgR binds both pentameric IgM and pIgA with high affinity (see above). We used this disparity to localize regions of the human receptor essential for the binding of pentameric IgM. Preliminary experiments were first performed to show that we were able to reproduce the previously observed species difference. MDCK cells were stably transfected with cDNA encoding the human, murine, or rabbit pIgR. Binding of pIgA and pentameric IgM to the transfected receptors was determined with 125I-labeled ligands. The rabbit pIgR bound pentameric IgM poorly compared with pIgA, while the human pIgR bound both pentameric IgM and pIgA with high affinity, pentameric IgM in fact showing the better binding (Fig. 2GoA). The relative binding of pentameric IgM and pIgA to murine pIgR demonstrated binding characteristics in this species intermediate to the binding characteristics for human and rabbit receptors (Fig. 2GoB). Based on these results, we chose to use human-rabbit chimeras to identify the pentameric IgM binding site of the human pIgR, and to use ligand concentrations of 5, 10, and 20 nM to determine relative binding of pentameric IgM and pIgA to these chimeras.



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  		FIGURE 2. A, Saturation-binding experiments with MDCK cells expressing the human, rabbit, and D1 swapped chimeric pIgRs depicted in B. B, Schematic diagram of the human, mouse, and rabbit constructs, and the relative binding of 125I-labeled pentameric IgM (pIgM) and pIgA. The first extracellular domain (D1) and the region-spanning domain two to five (D2–5) are indicated. White and grey boxes denote human and rabbit sequences, respectively. TM, transmembrane region; C, cytoplasmic tail. Transfected MDCK cells were incubated with 125I-labeled pIgM or pIgA for 2 h on ice and washed four times, and the cell-bound radioactivity was determined in a gamma counter. Values for nonspecific binding, determined by binding to untransfected MDCK cells, were subtracted. Binding values represent the average of the ratio of fmol pIgM-to-fmol pIgA bound at three different unsaturated ligand concentrations (5, 10, and 20 nM) expressed as mean (+1 SD) of at least two separate experiments. Binding values that differ from the wild-type human pIgR (p < 0.05; two-tailed Student’s t test) are indicated by a star; and binding values that differ from the wild-type rabbit pIgR (p < 0.05; two-tailed Student’s t test) are indicated by a closed circle.

 
D1 of human pIgR is essential for binding of pentameric IgM

The initial noncovalent interaction of human pIgR with pIgA occurs in D1 (21, 22, 23, 24), whereas the receptor elements required for binding of pentameric IgM are less well characterized. To study whether human pIgR D1 is essential for binding of pentameric IgM as well, we constructed chimeric receptors, namely a human receptor with D1 from rabbit and vice versa. Binding of 125I-labeled pIgA or pentameric IgM to MDCK cells stably transfected with these constructs showed that the human chimeric receptor containing rabbit D1 (rD1-h) exhibited reduced binding affinity for pentameric IgM compared with pIgA, whereas the rabbit chimeric receptor containing human D1 (hD1-r) bound pentameric IgM with relatively high affinity (Fig. 2Go, A and B). Thus, the human pIgR D1 is the primary determining element for the noncovalent initial receptor interaction with pentameric IgM, in the same fashion as with pIgA.

Translocation of pIgA and pentameric IgM by pIgR-transfected MDCK cells

To study the functional capacity of the different wild-type and chimeric receptors, transfected MDCK cells were grown on filters to confluent monolayers, and the translocation of pIgA and pentameric IgM from the basal surface to the apical medium was characterized. Cells were incubated at 37°C with 50 nM pIgA or pentameric IgM in the basal medium, together with 50 nM IgG as an internal control. After 20 h, the apical medium was harvested, and the concentrations of IgA, IgM, and IgG in each fraction were analyzed by ELISA. The capacity for pentameric IgM transport by the human pIgR was found to be quite similar to that for pIgA (Fig. 3Go). By contrast, the rabbit pIgR showed a relatively high capacity for pIgA transport, while it translocated only trace amounts of pentameric IgM. Like the wild-type human pIgR, hD1-r translocated both pIgA and pentameric IgM (although favoring the former ligand), whereas rD1-h, like the wild-type rabbit pIgR, translocated mainly pIgA, but also some pentameric IgM. These results harmonized with the binding results and showed that the chimeric receptors were functional. The level of IgG sampled from the apical medium was virtually the same in cells expressing the different pIgR constructs and in nontransfected MDCK cells (Fig. 3Go). This IgG most likely reflected passive paracellular diffusion, a possibility supported by the fact that even smaller amounts of the larger pIgA and pentameric IgM molecules were translocated across nontransfected cells (Fig. 3Go).



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  		FIGURE 3. Transport of pIgA and pentameric IgM (pIgM) by MDCK cells transfected with human pIgR, rabbit pIgR, and D1 interchange constructs (see Fig. 2Go). The transport was measured over a time period of 20 h after basolateral addition of 50 nM pIgA or pentameric IgM with 50 nM IgG as an internal control. The apical medium was harvested and analyzed by ELISA. The apical levels of IgG when coincubated with pIgA or pentameric IgM are shown as separate columns. Apical levels of IgA (columns 1–4) and IgM (columns 6–9) were significantly higher than background (columns 5 and 10, respectively) for all pIgR transfectants (p < 0.05; two-tailed Student’s t test). IgG transfer (columns 11–20) did not differ significantly for any of the cell lines. Data are from one of three similar experiments, expressed as mean (+1 SD) of three filters.

 
Several human D1 elements participate in pIgR binding of pentameric IgM

To define more precisely the sites for pentameric IgM interaction within D1 of the human pIgR, we constructed a series of chimeras designed to assess the relative role of the various regions containing the different CDR-like loops (25). D1 was divided into three regions (A, B, and C) that contained the CDR-like loop 1, 2, or 3, respectively (Fig. 1Go). These human regions (denoted by the capital letters A, B, and C) were replaced alone, or in combination, with the same region(s) from rabbit D1 (denoted by the corresponding lower case letters), resulting in six different mutants (Fig. 4Go). The use of these chimeras allowed us to define the subdomain interchanges between human and rabbit D1 that decreased or abolished the binding of pentameric IgM to the human pIgR. For unknown reasons, chimera aBC-h was extremely difficult to express stably in the MDCK cells. However, the clones we were able to isolate showed a surface expression of the mutant receptor at similar levels to the clones expressing the human wild-type pIgR based on similar values in a cell-based ELISA with a mAb against human pIgR-D3 (data not shown). The aBC-h clones, however, did not show significant pIgA or pentameric IgM binding at low ligand concentration; therefore, the relative binding of the two ligands for this construct was calculated at ligand concentrations of 20, 40, and 80 nM (Fig. 4Go).



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  		FIGURE 4. Schematic diagram of various chimeric domain 1 (D1) constructs with a human pIgR backbone (D2–5), and their relative binding of 125I-labeled pentameric IgM (pIgM) and pIgA compared with the relative binding of the two ligands to wild-type human or rabbit pIgR (at the bottom). White and grey boxes (on the left) denote the employed human- or rabbit-derived sequences, respectively. D1 is divided into three regions labeled A, B, and C, containing CDR-1, CDR-2, and CDR-3, respectively. Binding values (on the right) represent the average of the ratio of fmol pIgM-to-fmol pIgA bound at three different unsaturated ligand concentrations (5, 10, and 20 nM) expressed as mean (+1 SD) of at least two separate experiments. Binding values that differ from the wild-type human pIgR (p < 0.05; two-tailed Student’s t test) are indicated by a star, and binding values that differ from the wild-type rabbit pIgR (p < 0.05; two-tailed Student’s t test) are indicated by a closed circle. For details of chimeras and experimental setup, see Figs. 1Go and 2Go, respectively.

 
Binding of 125I-labeled pIgs revealed that all chimeras bound pIgA. The chimeras containing only one region of rabbit origin (aBC-h, AbC-h, and ABc-h) all showed some pentameric IgM binding, but this was reduced compared with that of the wild-type human receptor. Replacing the human CDR1-containing or CDR3-containing region with the equivalent rabbit region (Fig. 4Go; aBC-h, ABc-h) resulted in only slightly reduced pentameric IgM binding. However, AbC-h showed a reduction in pentameric IgM binding of about 83% compared with the human pIgR, suggesting that the human CDR2-containing region was most important for pentameric IgM binding. The human chimeras that had two CDR-like loops replaced with the comparable rabbit regions (Fig. 4Go; abC-h, aBc-h, and Abc-h) all bound pentameric IgM poorly; aBc-h (which retained the most pentameric IgM-binding activity) showed a reduction of about 75%, and Abc-h about 90%, compared with the human pIgR, whereas abC-h did not bind pentameric IgM at levels higher than the wild-type rabbit receptor. Taken together, these results demonstrated that elements within the CDR1- and CDR2-containing regions of the human pIgR are particularly important, and that their combined interchange for equivalent rabbit regions (abC-h) was sufficient to abolish pentameric IgM binding to the human pIgR. Exchanging the CDR3-containing region of the human receptor alone (ABc-h) or in combination with region A or B (aBc-h and Abc-h) resulted in only a small reduction in pentameric IgM binding compared with the appropriate parental pIgR constructs (human wild type, aBC-h, and AbC-h, respectively).

A human CDR2-like loop confers pentameric IgM-binding capacity to rabbit pIgR D1

A comparable series of D1 chimeras was constructed on a rabbit backbone (D1 regions of human origin denoted by capital letters, and rabbit origin by lower case letters, as above) to define the minimal interchange between human and rabbit pIgR that could confer efficient pentameric IgM-binding capacity to the rabbit pIgR (Fig. 5Go). All expressed chimeras showed binding properties similar to their human backbone analogues. Of those containing only one human D1 region, aBc-r demonstrated substantial pentameric IgM binding, while Abc-r and abC-r behaved more like the wild-type rabbit pIgR (Fig. 5Go). Of the chimeras containing two CDR-like loops of human origin, ABc-r bound pentameric IgM similar to human wild-type pIgR, whereas AbC-r showed only moderate pentameric IgM binding (Fig. 5Go). For unknown reasons, we were unable to express the aBC-r construct stably in our MDCK cells, comparable with the difficulties expressing aBC-h (see above).



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  		FIGURE 5. Schematic diagram of various chimeric domain 1 (D1) constructs with a rabbit pIgR backbone (D2–5), and their relative binding of 125I-labeled pentameric IgM (pIgM) or pIgA compared with the relative binding of the two ligands to wild-type human and rabbit pIgR (at the bottom). White and grey boxes (on the left) denote human and rabbit-derived sequences, respectively. Binding values (on the right) represent the average of the ratio of fmol pIgM-to-fmol pIgA bound at three different unsaturated ligand concentrations (5, 10, and 20 nM) expressed as mean (+1 SD) of at least two separate experiments. Binding values that differ from the wild-type human pIgR (p < 0.05; two-tailed Student’s t test) are indicated by a star, and binding values that differ from the wild-type rabbit pIgR (p < 0.05; two-tailed Student’s t test) are indicated by a closed circle. For details of chimeras and experimental setup, see Figs. 1Go and 2Go, respectively.

 
Together our results showed that the human pIgR D1 region containing the CDR2-like loop is sufficient to endow the rabbit pIgR with pentameric IgM-binding capacity. Elements in the human CDR1-containing region further enhanced substantially the ability of the rabbit pIgR to bind pentameric IgM. The fact that AbC-r also bound pentameric IgM suggests that the CDR1- and CDR3-like regions contain elements that contribute to pentameric IgM binding, and that their combination suffices to endow the rabbit pIgR with some pentameric IgM-binding capacity. This supports the notion that all three regions of human pIgR D1 contribute to its pentameric IgM-binding activity.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Epithelial transport mediated by pIgR is specific for pIgA and pentameric IgM, but both affinity studies (16, 26, 34) and species differences (27, 35, 36, 37, 38) have suggested that the initial noncovalent ligand-receptor interaction to some extent involves different structural elements. In this study, we describe for the first time domain regions of the human pIgR that are uniquely essential and sufficient to mediate the interaction with pentameric IgM. We constructed chimeric receptors interchanging parts of human pIgR (which binds both pIgA and pentameric IgM) and rabbit pIgR (which in essence binds only pIgA), and found that D1 of human pIgR was sufficient in a full-length context to mediate pentameric IgM binding. Furthermore, we determined that the CDR2-containing and, to a smaller extent, the CDR1-containing region of human D1 were crucial to allow pentameric IgM binding to the pIgR, whereas the region containing a CDR3-like loop at best provided a minor contribution to this end.

The characterization of the interaction between pentameric IgM and pIgR is of both basic and clinical interest. Phylogenetically, SIgM appears to be the first secretory Ab class that evolved (39), so the receptor configuration specific for pentameric IgM may be more ancient than that for pIgA. Selective IgA deficiency is the most common primary immunodeficiency in humans, with a prevalence of one case in 500–700 subjects in our part of the world (40). IgA deficiency predisposes particularly for upper respiratory tract infections and is sometimes associated with an immunoregulatory disorder (e.g., allergy, autoimmunity, or celiac disease) (41). However, most subjects (about two-thirds) remain healthy, which may be partially explained by compensatory SIgM Abs. Thus, when IgA-producing immunocytes are reduced or completely lacking in the gut, IgG- and especially IgM-producing cells are substantially increased (42, 43). Therefore, IgA-deficient subjects generally have increased intestinal and salivary IgM levels (44, 45), but such mucosal IgM compensation does not always take place in the upper respiratory tract of patients with infectious problems there (46).

In agreement with previous studies (17, 27), we confirmed in preliminary experiments that the rabbit pIgR virtually did not bind pentameric IgM, whereas the murine pIgR showed intermediate affinity for this polymer (Fig. 2Go). Similar results have been obtained for rat pIgR (27). In contrast to humans (5), these species express the pIgR on their hepatocytes, and therefore perform efficient removal of pIgA from the circulation (5). Clearly, binding of pentameric IgM to the pIgR on hepatocytes would compromise the protective role of pentameric IgM in the systemic circulation by removing such circulating Abs. In contrast, the high avidity binding of pentameric IgM by the human pIgR would not be disadvantageous, because in humans this receptor is absent from hepatocytes, although they express other binding sites for IgA (3, 47).

The extracellular ligand-binding part of pIgR consists of five Ig-like domains, and it has been suggested that the noncovalent interaction between pIgA and pIgR is mediated by elements in D1 (see later). By constructing different chimeric receptors, we found that as for pIgA binding, the D1 of the human pIgR is responsible for the initial noncovalent pentameric IgM interaction; transfer of this domain to the nonbinding rabbit pIgR conferred a significant increase in pentameric IgM-binding properties, while the reciprocal transfer nearly abolished pentameric IgM binding. Thus, D1 of the human pIgR is necessary and sufficient to bind both pIgA and pentameric IgM, at least when this binding is supported by interactions between the ligand and D2-D5 of either human or rabbit pIgR.

In vivo, pIgR is constitutively expressed by secretory epithelial cells at all exocrine tissue sites (5). We used MDCK cells transfected with different pIgR cDNAs to determine whether their ability to transport pIgA and pentameric IgM was directly related to their polymer-binding capacities. We found that the interspecies exchange did not affect the processing and sorting of the different receptors in MDCK cells, and they were able to translocate the bound ligand corresponding to its binding affinity regardless of the species backbone. It has been shown previously that although the rabbit pIgR, in contrast to the human receptor, does not stabilize the binding of human pIgA by a disulfide-exchange reaction, its transport efficiency was nevertheless as great or greater than that of the human pIgR (48). This observation is consistent with the view that disulfide bonding to the receptor does not enhance ligand translocation, but may rather play an important physiologic role in stabilizing SIgA Abs and protect them against degradation in external body fluids (5, 49, 50, 51, 52, 53). Furthermore, signals for intracellular trafficking have been mapped to the highly conserved cytoplasmic tail of the pIgR (54).

Bakos et al. (19) also studied the binding of human pIgs to different pIgR-derived peptides, and found that only pIgR fragments containing D1 bound to pIgA and pentameric IgM. Interestingly, a synthetic peptide from D1 (SC (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37)) bound not only pIgA and pentameric IgM, but also to monomeric IgA and IgG equally well. Thus, these findings suggest the presence of a common promiscuous Ig binding site in D1. This domain contains three loops corresponding to the CDRs of Ig variable domains (CDR1-CDR3), the sequence that determines their Ag-binding specificity and affinity. The synthetic peptide (SC (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37)) shown to exhibit promiscuous Ig-binding capacity contains the amino acids comprising the CDR1-like loop (19). A mAb that recognizes this pIgR peptide specifically blocked the binding of pIgA to free SC (19, 21), suggesting that this area is an essential initial binding site. Furthermore, the human SC (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37) peptide has also been shown to compete with human, bovine, rabbit, and rat SC for binding of human pIgA (24), indicating that this conserved structure plays an Ig-binding role in all species.

The CDR2- and CDR3-like loops in D1 are less homologous among species than the CDR1-like loop, but retain some invariant residues that might be important in ligand binding. As a matter of fact, a study by Coyne et al. (25) suggested that all three loops of the rabbit pIgR D1 participate in the noncovalent pIgA binding. Replacing regions corresponding to the CDR2- and CDR3-like loops with their counterparts from D2 resulted in complete abrogation of binding activity (25). Our results suggested that several human pIgR D1 regions likewise contribute to pentameric IgM binding. We divided D1 into three regions, each containing one CDR-like loop, and exchanged them between human and rabbit pIgR to study each region in the context of a full-length receptor. Although all three regions were necessary for maximal pentameric IgM binding, we found that the region containing the CDR2-like loop was most important in the pentameric IgM-binding process. Replacing this region of the human receptor with the rabbit counterpart (AbC-h) significantly reduced the pentameric IgM-binding capacity. In support of this observation, we also found that a similar human replacement in a rabbit receptor (aBc-r) sufficed to confer substantial pentameric IgM-binding capacity. Combining the two human D1 regions that contained the CDR1- and CDR2-like loops on the backbone from either species (ABc-h or ABc-r) reconstituted pentameric IgM binding to a level approaching that of the human wild-type receptor. The third region, including the CDR3-like loop, did not appear to be necessary for maximal pentameric IgM interaction, because the chimeras with this region of rabbit origin (ABc-h or ABc-r) showed binding levels similar to that of the human wild-type pIgR. However, a positive effect of the CDR3-containing region was demonstrated by the chimeras, in which it was combined with one of the two first human regions (AbC-r, AbC-h, or aBC-h); this exchange increased the pentameric IgM-binding efficiency compared with the chimeras that contained only one of the first human regions (Abc-r, Abc-h, or aBc-h). Taken together, these data suggested that the CDR1-like loop contains a structural element essential for pIg binding, but that the specificity to discriminate between pIgA and pentameric IgM mainly resides in the CDR2-like loop.

Importantly, we found no IgG binding to our chimeric receptors. On the other hand, all combinations of human/rabbit chimeric D1 constructs retained relatively high pIgA-binding capacity. Thus, rabbit pIgR elements in the CDR2-like region, for example, substituted efficiently for the same human elements without jeopardizing the pIgA binding, whereas the binding of pentameric IgM was diminished. Therefore, although pIgA and pentameric IgM interacting sites in the pIgR are overlapping, the exact amino acids responsible for the initial ligand contact appear to differ. There is a high degree of amino acid sequence identity in D1 between the two species studied, but certain differences must account for the observed differences with regard to pentameric IgM binding. Finer mutational analysis and more detailed structural information of pIgR D1 will be necessary to accurately determine the amino acids involved in binding of the two ligands. Recently, an amino acid motif in C{alpha}3 was identified as essential for dimeric IgA binding to the human pIgR (55). However, considerable structural work is still required to characterize the intact SC or pIgR binding site in the polymers. The only crucial common element identified up until now is the J chain (5). Several studies have suggested that this polypeptide is directly involved in the binding site (11, 14, 15), as well as in correct assembly of IgA and IgM polymers (56, 57). Our finding that the binding of pIgA and pentameric IgM to pIgR D1 involves unique elements, in addition to overlapping structures, does suggest that both the J chain and the respective heavy chains contribute to the pIgR binding site.


   Acknowledgments
 
We thank the technical staff at Laboratory for Immunohistochemistry and Immunopathology for invaluable assistance.


   Footnotes
 
1 This study was supported by the University of Oslo, the Research Council of Norway, the Norwegian Cancer Society, and Anders Jahre’s Foundation for Promotion of Science. M.R. has been a Research Fellow of the University of Oslo. Back

2 Address correspondence and reprint requests to Dr. Finn-Eirik Johansen, LIIPAT, Institute of Pathology, Rikshospitalet, N-0027 Oslo, Norway. E-mail address: Back

3 Abbreviations used in this paper: SIg, secretory Ig; CDR, complementarity-determining region; MDCK, Madin-Darby canine kidney; pIgA, polymeric IgA; pIgR, polymeric IgR, SC, secretory component. Back

4 I. N. Norderhaug, F.-E. Johansen, P. Krajci, and P. Brandtzaeg. Domain deletions in the human polymeric Ig receptor disclose differences between its dimeric IgA and pentameric IgM interaction. Submitted for publication. Back

Received for publication November 16, 1998. Accepted for publication March 3, 1999.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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