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PubMed1987

Immunohistochemical and histochemical markers of primary lung cancer, lung metastases, and pleural mesotheliomas.

Sections of primary lung carcinomas, lung metastases, mesotheliomas, and lung metastases of some rare mesenchymal tumors were incubated with different cytokeratin (CK), vimentin, desmin, and tissue polypeptide antigen (TPA) antibodies and with antibodies reactive with different hormones (ACTH, PTH, alpha-HCG, Calcitonin CT), CEA, carcinoma-associated antigen (CA1), secretory component (SC), neuron-specific enolase (NSE), alpha-1-antitrypsin (alpha-1-AT), lysozyme (lyso), and S-100 protein (S 100). CK antibodies derived from a 49 kD (reactive with simple epithelia [SE]) and a 67 kD CK polypeptide fraction (reaction with complex epithelia [CE] were useful differentiation markers for the four major groups of lung carcinomas. In one half of small cell carcinomas a positive reaction with NSE antibodies was found. S 100 and SC were good markers for papillary and bronchioloalveolar adenocarcinomas, whereas CEA was less important because of its reactivity with different types of lung carcinomas. To discern clear cell carcinomas of lung and renal origin a positive reaction with vimentin antibodies (some renal but not lung types) and with CA1 (no renal but all lung types) seemed to be useful. All hormone antibodies were of no importance as markers for difficult differential diagnosis, because positive reactivities were found in cases from every major carcinoma group. In addition, a Ca2+-activated adenosine triphosphatase (ATPase) was found in mesotheliomas but not in papillary adenocarcinomas.

Adenocarcinoma↗
PubMed1990

Immunohistochemical study of jaw cysts: different existence of keratins in odontogenic and non-odontogenic epithelial linings.

Keratins and secretory component (SC) were immunohistochemically examined in fresh tissue samples from 45 odontogenic and 35 non-odontogenic cysts. Lining epithelia of almost all cases contained keratins which reacted with polyclonal antibodies (Dako, Bio-Science), and no difference could be found between the two groups of lesions. By staining with two monoclonal antibodies against keratins, i.e., RGE53 (Bio-Science) and RKSE60 (Bio-Science), it was revealed that the epithelia of non-odontogenic cysts, which were columnar epithelium in most cases, had fully and positively reacted with RGE53, while none of the cases was positive for RKSE60. In contrast, the squamous linings of odontogenic cysts except for two cases did not react with RGE53, and few cases possessed RKSE60-reactive keratin. SC was also contradictory. All non-odontogenic cysts exhibited SC. Regarding each pair of non-odontogenic and odontogenic cysts covered with RGE53 and SC-positive, and RKSE60-negative squamous epithelium, it seemed reasonable from the staining results to conclude that the squamous linings were metaplastic from the columnar epithelium. Based on the results, concomitant examinations of SC with keratins will be helpful in deciding the epithelial derivation of jaw cysts.

Dentigerous Cyst↗
PubMed2000

Increased immunoglobulin A levels in milk by over-expressing the murine polymeric immunoglobulin receptor gene in the mammary gland epithelial cells of transgenic mice.

The polymeric immunoglobulin receptor (pIgR) transports dimeric immunoglobulin A (dIgA) across the epithelial cell layers into the secretions of various mucosal and glandular surfaces of mammals. At these mucosal sites, such as the gastrointestinal tract, respiratory tract, urogenital tract and the mammary glands, dIgA protects the body against pathogens. The pIgR binds dIgA at the basolateral side and transports it via the complex mechanism of transcytosis to the apical side of the epithelial cells lining the mucosa. Here, the extracellular part of the receptor is cleaved to form the secretory component (SC), which remains associated to dIgA, thereby protecting it from degradation in the secretions. One pIgR molecule transports only one dIgA molecule (1 : 1 ratio) and the pIgR is not recycled after each round of transport. This implies that the amount of available receptor could be a rate-limiting factor determining both the rate and amount of IgA transported per cell and therefore determining the total IgA output into the lumen or, in case of the mammary gland, into the milk. In order to test this hypothesis, we set up an in vivo model system. We generated transgenic mice over-expressing the murine pIgR gene under lactogenic control, by using a milk gene promoter, rather than under immunological control. Mice over-expressing the pIgR protein, in mammary gland epithelial cells, from 60- up to 270-fold above normal pIgR protein levels showed total IgA levels in the milk to be 1.5-2-fold higher, respectively, compared with the IgA levels in the milk of non-transgenic mice. This indicates that the amount of pIgR produced is indeed a limiting factor in the transport of dIgA into the milk under normal non-inflammatory circumstances.

Animals↗
PubMed1996

[Molecular aspects of secretory IgA (S-IgA) in gut-associated lymphoid tissues].

Secretory IgA, which plays an important role in the defense of the exocrine tissue, is composed of a polymeric IgA, joining (J) chain and secretory component (SC). Polymeric IgA and J chain are produced by plasma cells and SC by glandular epithelial cells. We here described the molecular aspects of J chain and SC. Study of the J chain has been confined to vertebrates which produce immunoglobulin (Ig) because the function of J chain is considered to be a polymerization of Ig. Recent molecular studies indicate that the role of J chain has been questioned. The J chain is expressed in invertebrates, as well as, representative species of vertebrates and that J chain is a primitive polypeptide that arose before the evolution of Ig molecules. SC is a 80 kDa glycoprotein functioning as a receptor for J chain-containing polymeric Ig. The expression of SC is regulated by various inflammatory cytokines such as, IL-1, IL-4, IL-6, IL-8, IFN-gamma and TNF-alpha, suggesting SC upregulation in vivo in inflammatory conditions. The human SC cDNA analysis reveals that it consisted of 11 exons with no functional TATA-box or CCAAT-box in the putative promoter region. Further upstream, there are several interesting motifs such as NF-kB and IFN-gamma response element, suggesting possible regulation of SC by cytokines through cellular signal transduction pathways.

Animals↗
PubMed1985

Antibodies recognizing different domains of the polymeric immunoglobulin receptor.

The receptor responsible for the transepithelial transport of IgA dimer antibodies is a transmembrane glycoprotein known as membrane secretory component (SCm). During transport, the membrane anchoring domain is cleaved and the ectoplasmic domain of the receptor (SCs) remains tightly bound to the IgA dimer in exosecretions. We have produced monoclonal antibodies with distinct specificities against both cytoplasmic and ectoplasmic epitopes of rabbit SCm. One antibody (anti-SC303) reacted both with SCm and free SCs but not with SCs bound to IgA dimer (SIgA). Therefore, it recognized an epitope close to the IgA dimer binding site. The other monoclonal antibody (anti-SC166), which was unable to react with SCs, bound to the 15-kDa cytoplasmic extension of the membrane-spanning domain of the receptor. A polyclonal antibody (GaR-SC), raised in a goat against rabbit milk SCs, reacted with a subpopulation of SCs not recognized by the anti-SC303 monoclonal antibody and in addition also reacted with covalently bound sIgA. The three antibodies cross-reacted with rat SCm. We demonstrate the ability of the anti-SC166 monoclonal antibody to immunoadsorb subcellular organelles as a result of the cytoplasmic orientation of its epitope. Our data indicate that there are functional differences between the high- and low-molecular-weight families of SC in terms of IgA dimer binding.

Animals↗
PubMed1984

Immune aspects of intestinal metaplasia of the stomach: an immunohistochemical study.

Immune characteristics of intestinal metaplasia of the stomach were analyzed by the immunoperoxidase technique in frozen and paraffin-embedded specimens. In fetal and minimally inflamed adult gastric mucosa, secretory component (SC) was absent from epithelial cells. Non-intestinalized gastric mucosa with evident inflammatory changes showed weak SC immunoreactivity at the generative cell zone. Enhanced immunoreactivity of SC with evidence of transepithelial transport of IgA and IgM, but not of IgG, was demonstrated in intestinalized glands of either the complete or incomplete type. The number of inflammatory cells and lymphoid follicles was decreased in intestinalized mucosa when compared with that in non-intestinalized gastritic mucosa; J chain-negative IgG plasma cells and T cells, both of which were fairly abundant in the latter mucosa, were remarkably decreased in the former mucosa, whereas the decrease of J chain-positive IgA or IgM plasma cells was slight or equivocal. In either mucosa, IgA was the most popular immunoglobulin class in plasma cells. IgD plasma cells were very rare. In the germinal centers of lymphoid follicles which were preferentially distributed in non-intestinalized gastritic mucosa, IgM or IgG germinocytes predominated over IgA germinocytes, and a few T cells and NK cells also were present. Intraepithelial lymphoid cells with a T-suppressor phenotype were detected in intestinalized glands. The possibility that intestinal metaplasia is an adaptation to long-standing chronic gastritis is discussed.

Adult↗
PubMed1988

Prognostic value of immunohistochemically identified CEA, SC, AFP, and S-100 protein-positive cells in gastric carcinoma.

Gastric tissues from 85 patients with advanced carcinoma were studied immunohistochemically for localization of carcinoembryonic antigen (CEA), secretory component (SC) and alpha-fetoprotein (AFP), and for S-100 protein-positive cells. Our objective was to determine if these factors could serve as prognostic indicators. Forty patients died of recurrence or metastases within 2 years (Group I), and the remaining 45 survived 10 years or longer (Group II) after resection. CEA-positive cells were present in 93% and 89% of Groups I and II, respectively. In case of staining for CEA, positive results were seen not only in the malignant cells but also in the stroma in 35% of Group I but none in Group II. The immunohistochemical status of SC was not essentially different between Groups I and II; the positive staining was recognized in 50% and in 57%, respectively. Gastric carcinomas rarely reacted to AFP. S-100 protein-positive cells, which play an important role in local immunity, were often present in Group II (47%) but rarely in Group I (10%). Thus, in addition to the various prognostic factors available to the surgical pathologist, the presence of CEA and the appearance of S-100 protein-positive cells in tumor tissue will aid in predicting the prognosis of patients with advanced gastric carcinoma.

Biomarkers, Tumor↗
PubMed1988

Immunohistochemical study of palatal salivary glands of denture wearing patients.

The binding pattern of antibodies against different cytokeratin (CK) polypeptides, tissue polypeptide antigen (TPA), carcinoembryonic antigen (CEA), lactoferrin (Lf), lysozyme (Ly) and secretory component (SC) in palatal glands (PSG) of long-term denture wearing patients has been studied to investigate immunohistochemically the localization of these marker proteins in normal PSG and in denture-induced sialadenitis of PSG. The study included palatal gland biopsies from 28 patients (15 f, 13 m; mean age 59 years), 17 of them with normal PSGs, 8 with focal obstructive sialadenitis, and 3 with diffuse sialadenitis. Presence of CK and TPA was found in all intra- and extraglandular salivary ducts, in the basophilic portions of acini, in some mucous acini, and in all atrophic acini. Increased expression of CEA and Lf was observed in inflammed areas of PSG which, on the other site, were devoid of Ly and SC. In the mucous acini of healthy PSG considerable basal Ly immunoreactivity was seen. SC was localized in almost all ductal cells and in some acinar cells. Appearance of Lf in the ductal cells of PSG indicates an early sign of palatal sialadenitis. Some distinctions in the expression pattern of the marker proteins between the mucous acini of major salivary glands and PSG point to differences in the functional activities of either group of salivary glands.

Denture, Complete, Upper↗
PubMed1989

Cell lineage markers in premalignant and malignant colonic mucosa.

Normal colonic epithelial cells consist of several cell types or lineages that are thought to arise from a common stem cell precursor. Neoplastic transformation may occur at different stages in the differentiation of a colonic stem cell to produce tumors that may retain characteristic cell lineage phenotypes. In this study, immunohistochemical techniques were used to identify cell lineage-related markers in fetal, normal, hyperplastic, adenomatous, and cancerous colonic tissue. These markers consisted of secretory component (columnar cells), a purified mucin antigen (mucous or goblet cells), chromogranin A (enteroendocrine cells), lysozyme (Paneth cells), and carcinoembryonic antigen (panepithelial cell marker). Colonic neoplasms, like normal mucosa, predominantly expressed the markers of columnar and goblet cell lineages. Chromogranin A was expressed in a small population of cells in most normal and fetal colonic crypts. Chromogranin A reactive cells were found in 55% of hyperplastic polyps, 31% of adenomatous polyps, and 33% of carcinomas. Lysozyme reactivity was rare in fetal, normal, and hyperplastic specimens, but was present in 86% of adenomas and 40% of carcinomas. Of 42 primary carcinomas, 9% were "pluripotent" and expressed markers of all four cell lineages. In addition to columnar and goblet cell markers, 7% expressed both enteroendocrine and Paneth cell markers, 17% expressed enteroendocrine cell markers, and 24% expressed Paneth cell markers. Two cases (5%) lacked expression of any of the cell lineage markers. The remainder expressed only columnar and goblet cell markers. The markers used in this study appear to identify the major cell lineages of fetal and normal colonic epithelium and can be used to delineate the altered cell lineage phenotypes in premalignant and malignant colonic mucosa.

Biomarkers, Tumor↗
PubMed1988

Intestinal development in the suckling rat: effect of insulin on the maturation of villus and crypt cell functions.

The influence of insulin on the postnatal development of intestinal functions linked to villus cells (sucrase, lactase, maltase and aminopeptidase) and crypt cells (secretory component of immunoglobulins, SC) has been studied in suckling and weanling rats. At 9 days of age, the animals received a daily injection of insulin 12.5 mU g-1 body weight day-1 for 4 days. Compared with saline-treated controls, insulin had no effect on the development of the intestinal mucosal mass parameters determined in the jejunum, ileum and colon. A premature appearance of sucrase was noted in isolated jejunal villus and crypt cells, the level of activity reached by the enzyme being dependent of the amount of insulin injected. By 6 and 12 h after a single injection of the hormone (12.5 mU g-1 body weight), sucrase activity was detected in all the cell fractions along the villus-crypt axis. In villus cells of insulin-treated rats, maltase, lactase and aminopeptidase activities were significantly (P less than 0.001) increased (+201%, +50%, +207%, respectively, vs. controls), whereas the concentration of SC measured by a sensitive immunoradiometric assay was enhanced over the controls by 75% in villus cells, 83% in crypt cells and 172% in the liver. Weanling rats treated from day 10 to day 20 postpartum with 12.5 mU insulin also exhibited a higher intestinal production of SC (+93%, P less than 0.01) than did saline controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminopeptidases↗
PubMed1985

Gall bladder: the predominant source of bile IgA in man?

The sedimentation profiles of IgA and Secretory Component (SC) and the concentrations of IgA, IgG, IgM, SC and albumin were evaluated after an overnight fast in gall bladder bile of six adult subjects without hepatobiliary disease. The sedimentation profiles differed from those previously obtained in hepatic bile in three ways: gall bladder bile contained a greater percentage of free-SC, a greater percentage of polymeric-IgA (p-IgA), and a major peak of 14 to 19 S p-IgA associated to SC. In contrast to hepatic bile in which IgG is the predominant Ig, IgA clearly was the predominant Ig in gall-bladder bile, its concentration averaging 92 micrograms/ml. Relative-to-albumin coefficients of excretion of proteins in gall bladder bile averaged 0.99 for IgG, 8.6 for monomeric IgA, 196 for p-IgA and 31 for IgM, indicating that there was a selective excretion of IgA and IgM into gall bladder bile. As compared to hepatic bile, the enrichment of gall bladder bile with IgA and IgM was respectively 6.5 and 11.5 times greater than with IgG. These results suggest that quite a significant amount of p-IgA could have been added to bile during its storage in the gall bladder which should therefore be regarded as the predominant source of bile IgA in humans.

Bile↗
PubMed1984

Search for specific markers of neoplastic epithelial duct and myoepithelial cell lines established from human salivary gland and characterization of their growth in vitro.

The neoplastic epithelial duct cells human salivary gland (HSG) and myoepithelial cells human pleomorphic adenoma (HPA) established from human salivary gland were examined by the immunoperoxidase method for the presence of specific antigens such as carcinoembryonic antigen (CEA), S-100 protein, secretory component (SC), lactoferrin (LF), and myosin. Isolation of the cells and their morphologic features were reported previously. Consequently, the presence of CEA, SC, and LF in the HSG cells was demonstrated. The HPA cells were identified to express the specific antigens reactive to anti-S-100 protein, anti-myosin and anti-CEA sera in addition to the presence of oxytocin receptor. When the two cell lines were co-cultured in monolayer culture or within the sponge matrix, a large number of ductlike or tubular structures were formed in an optimal ratio of 1:2 in HSG and HPA cells, whereas the cultures of HSG cells only grew with occasional formation of ductlike structure. In addition, in HSG and HPA cells in an area with their contact in the mixed cultures, CEA staining was intensified as compared with the culture of HSG or HPA cells only and further S-100 protein was detected in HSG cells, whereas S-100 protein was not detected in the culture of HSG cells only. These findings strongly suggest that the intercalated duct and myoepithelial cells from human salivary gland propagate with their interaction together in the expression of specific antigens such as CEA and S-100 protein or in the morphogenesis of salivary gland epithelial cells.

Adenoma, Pleomorphic↗
PubMedDate not supplied

The secretory immune system as part of the placental barrier in the second trimester of pregnancy in humans.

The objective of this study was to describe the development of the secretory immune system (SIS) in the placenta of 32 human fetuses who had died from different causes during the second trimester of pregnancy. The distribution of SIS protein elements, including the secretory component (SC), J-chain, IgA, IgM, IgG, as well subsets of lymphocytes and macrophages, were studied by immunohistochemical methods. Both the fetal and maternal parts of the placenta were found to contain these elements. In the fetal part, the immunoglobulins, SC and J chain were located in the syncytio- and cytotrophoblast of the chorion and in the epithelium of the amnion. The villous stroma contained a small amount of different subsets of lymphocytes. Macrophages accounted for up to 45% of the stromal cells of the villi and contained IgG and J-chain. In the maternal part of the placenta, the SIS proteins were in the decidual cells. Relatively few lymphocytes and macrophages were observed in the decidual stroma. Our data suggest that the placenta has two different SIS, one in the fetal part and the other in the maternal part. They differ in their structure and orientation: the maternal SIS protects the mother against paternal antigens from the fetus, while the fetal SIS protects the fetus against macromolecules and infectious agents penetrating from the mother. The placenta represents the largest extracorporal immune system that is functionally active during the whole second trimester of gestation. We suggest that the concept of placental barrier should be expanded to include both fetal and maternal parts of the SIS, fetal membranes and the decidual tissue.

Chorionic Villi↗
PubMed1986

Secretory immune responses in human kidneys.

The secretory immune system has been well studied in the intestinal, bronchial, and biliary systems and breast. Tissue studies of secretory immunoglobulins in the kidney are scanty, mostly related to nephropathies with IgA. Renal tissues from 37 autopsies selected for any history of renal dysfunction were processed for immunohistologic studies on frozen sections with several antisera, including a purified rabbit anti-human secretory component (SC). By immunohistology, gel diffusion, and immunoblotting, the anti-SC antibody reacted appropriately with purified human SC, saliva, intestinal epithelium, and breast milk and did not cross-react with immunoglobulin heavy or light chains, lactoferrin, and other tissue proteins. IgA and SC were seen in tubular casts in 70% of patients, whereas less impressive staining with IgM, IgG, and albumin was seen, respectively, in 24%, 13%, and 22% of the patients. SC was present in the cytoplasm of distal tubule and Henle's loop cells in 78% of specimens. A control group of 10 healthy individuals who died suddenly showed minimal staining of casts and tubules in 2 specimens. Renal pathology in the group with IgA-SC+ casts included acute tubular necrosis (54%), severe chronic renal disease (61%), and mild chronic renal injury (38%). The group with negative IgA-SC casts included acute tubular necrosis (64%), infectious interstitial nephritis (36%), and negligible renal disease (36%). This study suggests that discrete distal segments of the nephron may have the capability of secreting SC, which is probably coupled with serum-derived IgA and incorporated into luminal tubular secretions. The low level of immunosecretions in kidneys which are normal or minimally damaged suggests that this system may need to be turned on by unknown, probably pathogenic stimulating factors.

Adolescent↗
PubMed1987

Ligand-dependent redistribution of the IgA receptor on cultured rat hepatocytes and its disturbance by cytochalasin B.

The topography and dynamics of IgA-secretory component (SC) complexes on the surface of cultured hepatocytes and its disturbance by cytochalasin B were investigated using the colloidal gold technique in conjunction with surface replication. The distribution of IgA-gold conjugates after incubation at 4 degrees C was similar in normal and cytochalasin B-treated hepatocytes and was characterized by diffusely scattered single and clustered particles, the latter often associated with coated pits. After raising the temperature to 37 degrees C, redistribution of particles and their gradual uptake into coated vesicles was observed in control cultures. This ligand-induced redistribution led to a progressive gathering of single and grouped particles in larger clusters (50-200 particles), which appeared to be the site of the most intensive endocytotic activity. In contrast, huge patches of IgA-gold conjugates were formed at the cell periphery of cytochalasin B-treated hepatocytes within 20-60 min at 37 degrees C, while central areas were cleared. Patch formation was triggered by binding of both unlabeled and labeled IgA, but could not be observed with the unoccupied receptor as demonstrated by gold-labeled antibodies against SC. These results show that the topography of SC is markedly changed by binding of its ligand, IgA, and suggest that the dynamics of the IgA-SC complexes in hepatocyte plasma membrane are affected by microfilaments.

Animals↗
PubMed1985

Biogenesis of the polymeric IgA receptor in rat hepatocytes. II. Localization of its intracellular forms by cell fractionation studies.

In the companion paper (Sztul, E. S., K. E. Howell, and G. E. Palade, J. Cell Biol., 100:1248-1254), we have shown that pulse labeling of hepatic proteins with [35S]cysteine can be obtained in vivo in intact rats. Soluble label clears the plasma in approximately 5 min, and incorporated label reaches peak values in the liver approximately 20 min after injection. In the present study, we show that the 105,000-mol-wt protein (105K), kinetically the earliest intracellular form of secretory component (SC), is the predominant form found, between 5 and 20 min postinjection, in homogeneous rough microsomal fractions. The second kinetically defined form, i.e., 116K, is the predominant species present in relatively homogeneous, light Golgi fractions in which it appears at approximately 15 min, and peaks at approximately 25 min, postinjection. The third kinetically defined form, 120K, is found 30 min after injection as the major SC species (albeit still accompanied by its immediate precursor, 116K), in a sinusoidal plasmalemmal fraction isolated by immunoadsorption to anti-SC-coated Sepharose beads. These findings lead to the following conclusions: (a) SC is synthesized on polysomes attached to the rough endoplasmic reticulum (ER) membrane; (b) it is partially translocated across the ER membrane and core glycosylated co-translationally to give a 105K peptide; (c) 105K moves from the ER to the Golgi complex where it is terminally glycosylated to give the 116K form; (d) the latter moves to the sinusoidal plasmalemma where it appears together with the final mature form, 120K. Kinetic evidence indicates that the vesicular carriers involved in the transport of SC from the Golgi complex to the sinusoidal plasmalemma, and from the latter to the biliary front of the hepatocytes, are present in a Golgi heavy fraction and a crude carrier vesicle fraction from which they remain to be isolated, purified, and characterized.

Animals↗
PubMed1998

In vitro comparison of the biologic activities of monoclonal monomeric IgA, polymeric IgA, and secretory IgA.

Secretory IgA (S-IgA), a major humoral mediator of mucosal immunity, is a polymeric Ig containing an unusual extra polypeptide, secretory component (SC), added during transcytosis through epithelial cells. Polymeric S-IgA is more effective than monomeric IgA (mIgA) and IgG in neutralizing viruses. It is not known whether this increased efficacy is due solely to the polymeric structure of the molecule or whether SC itself makes S-IgA more efficient; a quantitative in vitro comparison of the biologic activities of S-IgA and pIgA has not been reported. We prepared purified pIgA and mIgA mAbs directed toward the H1 hemagglutinin of PR8 influenza virus and purified monoclonal S-IgA (made from monoclonal pIgA injected into a Lewis rat and collected as S-IgA from bile) and compared their abilities to carry out hemagglutination inhibition (HI) and neutralization of the infectivity of PR8 influenza virus in vitro. The polymeric Igs (pIgA and S-IgA) were 5 times more effective than mIgA in HI and 7 to 10 times more effective than mIgA in virus neutralization. Addition of SC to pIgA did not modify its ability to mediate HI and had only a minimal effect (S-IgA was 1.4 times more effective) on its ability to neutralize influenza virus in vitro. Trypsin preincubation partially abolished mIgA- or pIgA-mediated, but not S-IgA-mediated, viral neutralization. Thus, although S-IgA is more stable functionally than pIgA, the addition of SC does not influence, positively or negatively, the biologic activity associated with the Fab of S-IgA.

Animals↗
PubMed1993

A conserved binding site on the receptor for polymeric Ig is homologous to CDR1 of Ig V kappa domains.

We have previously shown that the binding of human secretory component (SC) to polymeric IgA (PIgA) is inhibited by a synthetic peptide fragment of SC (SC[15-37]), and also by a mAb (mAb 6G11) which specifically recognizes the unbound form of SC and SC[15-37]. In this report we show that the binding of bovine, rabbit, and rat SC to human PIgA can also be inhibited by SC[15-37], and that the relative affinity of these three species of SC for human PIgA correlated with their relative affinity for mAb 6G11. The binding of mAb 6G11 to SC isolated from various species was especially affected by amino acid substitutions at position 26 of SC. Structural modeling revealed that the side chain of residue 26 is located on the external face of a loop in SC domain I which is homologous in size and position to the first complementarity determining region (CDR1) of Ig V kappa domains. However, the surface topography of the CDR1-like loop of SC is distinct from other known V kappa CDR1 loops due to the unique distribution of charged and bulky polar side chains which locate to the exterior face of the loop. Together, these findings indicated that the binding site on the receptor for PIg is highly conserved among species, and is composed, in part, of residues of domain I of SC which may form a distinct CDR1-like loop on the surface of the molecule.

Amino Acid Sequence↗