Functional and structural characterisation of human colostrum free secretory component

Secretory component (SC) in association with polymeric IgA (pIgA) forms secretory IgA (SIgA), the major antibody active at mucosal surfaces. SC also exists in a free form in secretions, with innate neutralizing properties against important pathogens. IgA-bound SC and free secretory component (FSC) are both produced by proteolytic cleavage of the polymeric Ig receptor whose function is to transport IgA and IgM across mucosal epithelia. Although the proteases have not been characterised and the site(s) of cleavage of the polymeric Ig receptor has been debated, it has been assumed that bound and free SC are produced by cleavage at the same site. Here we show by SDS-PAGE analyses that FSC is slightly smaller than SIgA1- or SIgA2-bound SC when purified simultaneously. The FSC preparation was functionally active, shown by binding to dimeric and polymeric IgA, and by its ability to trigger a respiratory burst by binding to 'SC receptors' on eosinophils. We also show that FSC from different human secretions have different molecular sizes. The solution structure of FSC from colostrum was studied by analytical ultracentrifugation and X-ray scattering. The sedimentation coefficient of 4.25S is close to that for recombinant FSC. The X-ray scattering curve showed that FSC adopts a compact structure in solution which corresponds well to the J-shaped domain arrangement determined previously for recombinant FSC which terminates at residue Arg585. The smaller sizes of the FSC forms are attributable to variable cleavages of the C-terminal linker region, and may result from the absence of dimeric IgA. The FSC modelling accounts for the lack of effect of the C-terminal linker on the known functions of FSC.

The J chain is essential for polymeric Ig receptor-mediated epithelial transport of IgA

Local production of secretory (S)IgA provides adaptive immunologic protection of mucosal surfaces, but SIgA is also protective when administered passively, such as in breast milk. Therefore, SIgA is a potential candidate for therapeutic administration, but its complex structure with four different polypeptide chains produced by two distinct cell types complicates recombinant production. The J chain is critical in the structure of SIgA because it is required for efficient polymerization of IgA and for the affinity of such polymers to the secretory component (SC)/polymeric (p)IgR. To better understand the role of the J chain in SIgA production, we have generated various mutant forms of the human J chain and analyzed the function of these mutants when coexpressed with IgA. We found that the C terminus of the J chain was not required for the formation of IgA polymers, but was essential for the binding of pIgA to SC. Likewise, we found that two of the intrachain disulfide bridges (Cys(13):Cys(101) and Cys(109):Cys(134)) were also required for the binding of pIgA to SC but, interestingly, not for IgA polymerization. Conversely, the last intrachain disulfide bridge (Cys(72):Cys(92)) was not essential for either of these two J chain functions. Finally, we demonstrated that the presence of only Cys(15) or Cys(69) was sufficient to support polymerization of IgA, but that these polymers were mostly noncovalently stabilized. Nevertheless, these polymers bound free SC with nearly the same affinity as pIgA containing wild-type J chain, but were transcytosed by pIgR-expressing polarized epithelial cells at a reduced efficiency.

The IgA/IgM receptor expressed on a murine B cell lymphoma is poly-Ig receptor

T560, a mouse B lymphoma that originated in gut-associated lymphoid tissue, expresses receptors that bind dimeric IgA and IgM in a mutually inhibitory manner but have little affinity for monomeric IgA. Evidence presented in this paper indicates that the receptor is poly-Ig receptor (pIgR) known in humans and domestic cattle to bind both IgA and IgM. The evidence includes the demonstration that binding of IgM is J chain dependent, and that pIg-precipitated receptor has an appropriate Mr of 116-120 kDa and can be detected on immunoblots with specific rabbit anti-mouse pIgR. Overlapping RT-PCR performed using template mRNA from T560 cells and oligonucleotide primer pairs designed from the published sequence of mouse liver pIgR indicate that T560 cells express mRNA virtually identical with that of the epithelial cell pIgR throughout its external, transmembrane, and intracytoplasmic coding regions. Studies using mutant IgAs suggest that the Calpha2 domain of dimeric IgA is not involved in high-affinity binding to the T560 pIgR. Inasmuch as this mouse B cell pIgR binds IgM better than IgA, it is similar to human pIgR and differs from rat, mouse, and rabbit epithelial cell pIgRs that bind IgA but not IgM. Possible explanations for this difference are discussed. All clones of T560 contain some cells that spontaneously secrete both IgG2a and IgA, but all of the IgA recoverable from the medium and from cell lysates is monomeric; it cannot be converted to secretory IgA by T560 cells.

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

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.

Ontogeny of J chain expression in pig lymphocytes

The ontogeny of lymphocytes expressing J chain in the cytoplasm (J+) was studied in pig foetuses by the immunofluorescent technique. Peripheral blood lymphocytes were the first J+ cells in prenatal life. The spleen and lymph nodes contained J+ cells in the last days of gestation. J+ cells were found in the lamina propria of the gut and some glands of conventional but not of germ-free piglets. J chain was not detected on or in cell membranes at any developmental stage.

Immunohistochemical characterization of intracellular J-chain and binding site for secretory component (SC) in human immunoglobulin (Ig)-producing cells

J-chain staining of IgA- and IgM-producing immunocytes was significantly enhanced when tissue sections were pretreated with acid urea, apparently because molecular unfolding exposed concealed J-chains. This indicated substantial completion of the Ig polymers at the cytoplasmic level, which was verified by diffuse binding of SC in vitro to the cytoplasm of most J-chain-positive IgA and IgM cells. This process involved specific non-covalent forces which showed the same interrelation as that noted for isolated dimeric IgA and 19S IgM--the latter as well as IgM cells exhibiting stronger binding of SC than the IgA counterparts. Conversely, J-chain staining of IgD and IgG immunocytes was not enhanced by acid urea and these cells did not generally express affinity for SC; rare exceptions could apparently be ascribed to artifacts or dual isotype production including IgA or IgM polymers. Parallel demonstration of J-chain and SC binding seems to be the best available method for studies of polymer-producing immunocyte populations and offers the advantage of in situ evaluation of cell distribution in relation to morphology. The reliability of this approach was attested to by the fact that IgA immunocytes in all secretory tissues investigated (salivary, mammary and lacrimal glands; nasal and intestinal mucosae) expressed J-chain (87-97%) and SC affinity (84-87%) in comparable proportions, indicating that almost 90% of the cells were engaged mainly in dimer production. The observation that most IgD and 50-70% of the IgG immunocytes in secretory tissues expressed J-chain, has implications for the differentiation of B-cell clones homing to such sites. Conversely, IgG cells in extra-glandular tissues showed strikingly reduced J-chain production and such sites contained IgA immunocytes with heterogeneous expression of J-chain and SC affinity. Thus, in the extra-follicular area of palatine tonsils 70-80% of the IgA cells seemed to be pure monomer producers and the remainders apparently generated a mixed product. Most immunocytes in extra-glandular tissues may therefore belong to mature clones with completely or partially repressed J-chain synthesis.

Identification of Hodgkin and Sternberg-reed cells as a unique cell type derived from a newly-detected small-cell population

In this study the antigenic profile of Hodgkin (H) and Sternberg-Reed (SR) cells from cases of Hodgkin's disease was analysed using a large panel of monoclonal and polyclonal antibodies reactive with cells of lymphoid and haemotopoietic origin. The aim of this investigation was, firstly, to throw light on the origin of H and SR cells and, secondly, to determine whether there is any evidence to support recent suggestions that H and SR cells differ antigenically between different histological categories of Hodgkin's disease. Frozen sections (from 24 cases) and paraffin sections (83 cases) were stained by immunoenzymatic methods and the results compared with those obtained from staining a wide variety of reactive and neoplastic tissue samples (including examples of tuberculosis, sarcoidosis, malignant histiocytosis, histiocytosis X, osteomyelosclerosis and non-Hodgkin's lymphoma). The results revealed that H and SR cells of all types of Hodgkin's disease consistently lack markers found on null cells, B cells, T cells, cells of monocyte/macrophage series, interdigitating reticulum cells, dendritic reticulum cells and erythropoietic and thrombopoietic cells. However, H and SR cells constantly expressed an antigen detectable with the recently produced monoclonal antibody Ki-I. The vast majority of typical and lacunar type H and SR cells contained the granulocyte-related antigens detected by monoclonal antibodies TU5, TU6, TU9 and 3C4, whereas other more or less specific granulopoietic cell markers (such as peroxidase, chloroacetate esterade, lysozyme, cationic leukocyte antigen and OKMI) were consistently absent. H and SR cells in cases of nodular paragranuloma (nodular type of Hodgkin's disease with lymphocyte predominance) were not monotypic in light chain type (as has been previously reported), but rather contained chi and lambda chains within the same cells, as do typical and lacunar type H and SR cells. Immunostaining of normal and hyperplastic lymphoid tissue with the Ki-I antibody led to the detection of a new, as yet unidentified, small-cell population of unknown origin and function, which is present between, around, and within cortical follicles. It is concluded from these findings that H and SR cells constitute a unique cell type that differs in many properties from all other known cell types. Furthermore, H and SR cells of the various histological types of Hodgkin's disease are more closely related than previously believed. It is suggested that the hitherto unknown cell population detected with the monoclonal antibody Ki-I in normal lymphoid tissue is the normal equivalent of H and SR cells.

Concentration of immunoglobulins in human whole saliva: effect of physiological stimulation

The concentrations of total protein, albumin, IgA, IgG and IgM in human whole salivas were measured with a solid phase radioimmunoassay before and after physiological stimulation of salivary secretion. The geometric mean concentrations (mg/l) before stimulation were: total protein 1600, albumin 60, IgA 140, IgG 16 and IgM 4.1. Physiological stimulation of salivary secretion caused an increase of the total protein concentration to 2400 mg/l, had little effect on the concentrations of albumin and IgG, but lowered the concentrations of IgA and IgM to 56 mg/l and 1.7 mg/l, respectively. These findings indicate that a considerable portion of salivary IgA and IgM are produced locally depending on selective transport and that the release from local storage sites is not increased during stimulation as much as the total volume of the saliva.

Immunohistochemical evaluation of J-chain expression by intra- and extra-follicular immunoglobulin-producing human tonsillar cells

Cytoplasmic J-chain expression by Ig-producing cells was characterized immunohistochemically in normal specimens of palatine and nasopharyngeal tonsils (adenoids). Altogether follicular immunocytes, which were mainly of ther IgG and IgM isotypes, showed a much higher percentage of J-chain positivity than the extrafollicular ones, in agreement with the idea that this polypeptide is principally a marker of differentiating or newly differentiated Ig-producing cells. Thus, in concurrence with the predominating IgG isotype, J-chain expression was almost 50% in the germinal centres of lymphoid follicles but only about 2% in the extrafollicular compartment. By contrast, a substantial number of J-chain-positive IgA immunocytes were found in the latter compartment. Since it is believed that extrafollicular Ig-producing cells are mainly derived from follicular centre cells, this result indicated that significant isotype switching is involved in tonsillar B-cell differentiation. Most tonsillar IgD immunocytes were J-chain-positive, in accordance with the notion that IgD expression is a feature of relatively early clonal development. It is postulated that J-chain-positive B-cell blasts may leave the tonsils and, through isotype switching, contribute to the dimer-producing IgA-cell populations normally found in the exocrine glands of the upper aero-digestive tract.

The local immunological defence system of the human endometrium

The distribution of immunoglobulins and secretory component (SC) in the human endometrium has been studied by an immunoperoxidase technique. SC is present in a proportion of the epithelial cells and the amount of this substance is increased during the secretory phase of the cycle. A variety of immunoglobulins are present, in low concentrations, in the stromal interstitium during the secretory phase of the cycle and these are thought to diffuse passively from the plasma as a non-specific accompaniment of stomal oedema. Only IgA is found in the epithelial cells and this appears solely in those cells containing SC. No immunoglobuln-containing lymphoid cells are present in the endometrium. It is suggested that the endometrium lacks a true local secretory immune system but is able, because of its content of SC, to compensate for this by extracting polymeric IgA from the plasma. This system presumably helps to protect the endometrium against infection but the biological significance of its apparent control by progesterone is uncertain.

Naturally occurring polymers of IgA lacking J chain

Two of twenty IgA myeloma proteins studied were found to lack J chain. Both IgA proteins contained dimers and higher polymers (trimers, tetramers, pentamers) in proportions similar to those found in most classical 'J-positive' proteins. Both the 'J-negative' proteins contained bound albumin and alpha-1 antitrypsin (alpha1AT),and reduction with mercaptoethylamine caused a release of albumin and alpha1AT concomitant with depolymerization of the higher polymers of IgA. These proteins formed complexes with secretory component (SC) in vitro, indicating that the presence of J chain is not a requirement for SC binding.

Complex formation between secretory component and human immunoglobulins related to their content of J chain

The J-chain content of 3 IgM and 24 IgA preparations was quantitated by an immunochemical technique after reduction with 20mM dithiothreitol. The amounts released ranged from undetectable (less than 0.1 mg) to 7.3 mg per 100 mg of Ig. Most of the J-chain-deficient proteins were monomeric, but four polymeric IgA preparations were found to contain only 0.2-0.8 mg of J chain per 100 mg. The SC-binding capacity of these polymers, expressed as percentage of the amount added (2.5 mu g SC/100 mug Ig), was 6%-12% compared with 69%-82% for IgA and IgM polymers that contained more than 4.0 mg of J chain per 100 mg. Some monomeric IgA preparations showed a slight SC-binding capacity, which was explained by the presence of contaminating J-chain-positive polymers. Bound J chain therefore seems to be a structural prerequisite for a specific noncovalent complexing of Ig polymers with SC.

Immunochemical studies on free and bound J chain of human IgA and IgM

J chains purified from colostral IgA, polymeric (mainly dimeric) polyclonal IgA from serum, and monoclonal IgM appeared to be immunochemically identical in tests with antisera produced against the two latter antigens. None of the antisera precipitated with the native immunoglobulins, but J-chain antibodies became bound to all three types of Ig polymer. On a molar basis polymeric IgA was almost twice as efficient as IgM and 17 times more efficient than colostral IgA in antibody inhibition tests. The J-chain antibodies were able to link two or more IgA dimers and two or more IgM pentamers. After incubation in large antibody excess, the sedimentation properties of most IgM molecules were altered, whereas about 40% of the IgA dimers were unaffected by the treatment. THus, although the J chain on the whole was slightly more exposed in polymeric IgA than in IgM, it was more homogeneously available in the latter polymer. After denaturation in acid urea, the J chain became markedly more exposed in all three types of Ig polymer, and they were all precipitated by the most potent antiserum. Immunochemical quantitation of J chain released after reduction with 20mM dithiothreitol indicated that on the average there are about 2 mol of J chain per mol of polymeric IgA, and 3 to 4 mol per mol of IgM.

J chain in malignant human IgG immunocytes

J chain was demonstrated directly by immunofluorescence in the cytoplasm of malignant IgG-producing human tumor cells. This was in contrast to IgG immunocytes accumulating in foci of chronic inflammation, which generally were devoid of J chain.

Purification of J chain after mild reduction of human immunoglobulins

Two methods are described for the purification of J chain from polymeric IgA after mild reduction without the use of alkylating or dissociating reagents. The released peptide was separated from other protein components by immunoadsorption combined with gel filtration or anionic-exchange chromatography, or both. J chain was thus obtained in a yield of about 30% of the total release. Most of it consisted of dimers (molecular weight, approximately 25,000 to 30,000) or larger polymers, but re-reduction and alkylation produced a quite homogeneous fraction that sedimented slightly more slowly than egg-white lysozyme. The purity was high enough for successful immunization. When J chain coupled to bovine serum albumin was used as an antigen, all of five rabbits showed a good immune response. Although the same principle could be used for the purification of J chain from IgM and colostral IgA, high purity was more difficult to achieve and the yield was much lower. These preparations contained an unidentified slow-moving component, and the J chain was more prone to become rapidly degraded to smaller fragments.