IgA binding factors and Fc receptors for IgA: comparative studies between IgA and IgE Fc receptor systems

Channel catfish soluble FcmuR binds conserved linear epitopes present on Cmu3 and Cmu4

A linear epitope on catfish IgM has been identified as the docking site for the catfish soluble FcmuR (IpFcRI). Western blot analyses and latex bead binding assays identified the consensus octapeptide motif FxCxVxHE located at the second cysteine that forms the intrachain disulfide bond of the catfish Cmu3 and Cmu4 immunolglobulin (Ig) domains as the IpFcRI binding sites. Furthermore, molecular modeling of catfish Cmu3 and Cmu4 confirmed that the octapeptide in both of these domains is accessible for IpFcRI interactions. In addition, since this octapeptide motif is also found in other vertebrate Ig domains, IpFcRI binding to Ig heavy (H) and light (L) chains from rainbow trout, chicken, mouse, rabbit, and goat were examined by Western blot analyses and latex bead binding assays. IpFcRI readily bound reduced rainbow trout (Igmu), chicken (Ignu), mouse (Igmu, Iggamma1, Iggamma2a, Iggamma2b, and Igalpha), rabbit (Igmu and Iggamma) and goat (Iggamma) IgH chains, and mouse Igkappa and Iglambda, and chicken Iglambda IgL chains. IpFcRI also bound mouse IgM, IgA and IgG subclasses when examined under native conditions.

Immunoproliferative small intestinal disease associated with Campylobacter jejuni

BACKGROUND

Immunoproliferative small intestinal disease (also known as alpha chain disease) is a form of lymphoma that arises in small intestinal mucosa-associated lymphoid tissue (MALT) and is associated with the expression of a monotypic truncated immunoglobulin alpha heavy chain without an associated light chain. Early-stage disease responds to antibiotics, suggesting a bacterial origin. We attempted to identify a causative agent.

METHODS

We performed polymerase chain reaction (PCR), DNA sequencing, fluorescence in situ hybridization, and immunohistochemical studies on intestinal-biopsy specimens from a series of patients with immunoproliferative small intestinal disease.

RESULTS

Analysis of frozen intestinal tissue obtained from an index patient with immunoproliferative small intestinal disease who had a dramatic response to antibiotics revealed the presence of Campylobacter jejuni. A follow-up retrospective analysis of archival intestinal-biopsy specimens disclosed campylobacter species in four of six additional patients with immunoproliferative small intestinal disease.

CONCLUSIONS

These results indicate that campylobacter and immunoproliferative small intestinal disease are associated and that C. jejuni should be added to the growing list of human pathogens responsible for immunoproliferative states.

Fc alpha RI/CD89 circulates in human serum covalently linked to IgA in a polymeric state

The FcR for IgA CD89/FcalphaRI, is a type I receptor glycoprotein, expressed on myeloid cells, with important immune effector functions. In vitro CD89 can be released from CD89-expressing cells upon activation. Little information is available on the existence of this soluble molecule in vivo. Using specific and sensitive ELISA techniques (detection limit 50 pg/ml), we were not able to detect circulating CD89 in human sera. However, using Western blotting, a 30-kDa soluble CD89 molecule was demonstrated in both serum and plasma. Moreover, using a specific semiquantitative dot-blot system, we found CD89 in all human sera tested (mean concentration 1900 ng/ml). Size fractionation of human serum using gel filtration chromatography showed that the CD89 molecule was predominantly present in larger molecular mass fractions. Direct complexes between IgA and CD89 were demonstrated by anti-IgA affinity purification, and when analyzed under nonreducing conditions appeared to be covalently linked. Size fractionation of affinity-purified IgA showed the presence of soluble CD89 only in the high molecular mass fractions of IgA, but not in monomeric IgA. High molecular mass complexes of CD89-IgA could be distinguished from J chain containing dimeric IgA. These data show that CD89 circulates in complex with IgA, and suggest that CD89 might contribute to the formation of polymeric serum IgA.

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.

Sensitivity of receptors for IgA on T560, a murine B lymphoma, to phorbol myristate acetate and to phosphatidylinositol-specific phospholipase C

A GALT-derived B lymphoma, T560, that bears IgAR is described. T560 is IgG2a kappa +, Ia+, B220+, J11d+, Thy-1-, CD3-, CD4-, CD5-, Mac 1-, Mac 2-, nonspecific esterase negative and binds bromelain-treated mouse RBC but not SRBC or ORBC. It presents antigen, secretes IL-1, IL-4 and IL-6 but not IL-2, IL-5 or TGF beta and appears to be related to the Lyt 1+(CD5) lineage of B cells though it lacks Lyt 1. T560 bears IgAR that, on the cell surface, are completely cross-inhibited by low concentrations of IgM and by high concentrations of IgG2a and IgG2b. They do not appear to represent a cell-surface form of galactosyl transferase. They are inducible by high concentrations of IgA, sensitive to trypsin and insensitive to neuraminidase. They are down-regulated by activation of PKC with PMA, but their recovery is not inhibited by cycloheximide, indicating that they are not degraded or shed. They may either lose their affinity for IgA or be internalized without degradation. Seventy percent of IgA receptor activity is lost when T560 is treated with PI-PLC; part of this loss of activity is due to activation of PKC and is inhibited by staurosporine, but approximately 30% of it is not protected by staurosporine indicating that some, or all, of the IgA receptor of T560 is connected to the cell membrane via a GPI linker. The T560 IgA receptor could be related to the poly-Ig or M cell receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of IgD-binding factor by T cells under the influence of interleukin 2, interleukin 4, or cross-linked IgD

Helper T cells with receptors specific for IgD have immunoaugmenting properties. We have now detected soluble IgD-binding factor in cell supernatants immobilized on nitrocellulose paper by their ability to bind 125I-labeled IgD. IgD-binding factor is released by normal splenic T cells stimulated with recombinant interleukin 2, recombinant interleukin 4, or crosslinked IgD in amounts paralleling the induction of IgD receptors on the cells. IgD receptors are constitutively produced by antigen-specific helper T-cell hybridomas 2H10 and A3.4C6. Incubation of these hybridoma cells with recombinant interleukin 2 increases release of IgD-binding factor while reducing expression of IgD receptors. Specificity of the binding factor for IgD is established by (i) competitive inhibition; (ii) the ability of the binding factor to bind radiolabeled IgD and not monoclonal IgE, IgG2a, or polyclonal IgG; and (iii) the removal of the binding factor on passage through an IgD-Sepharose column and recovery in a subsequent acid eluate.

Cellular immunoregulatory aspects of IgA nephropathy

Polyclonal activation of IgA-producing B cells has been observed in the majority of patients with IgA nephropathy (IgAN) and some of their family members. The precise mechanism of such activation is presently unknown. However, abnormalities of T and B cell interactions have recently been observed in this disease. These abnormalities include the decrease of IgA-specific suppressor T cell activity and the increase of IgA-specific helper T cell activity. A T cell subpopulation with receptors for the Fc portion of IgA and with CD4 antigens (ie, T alpha 4 cells) has been proposed as a candidate of IgA-specific switch T cells that convert IgM-producing B cells to IgA-producing B cells. Further analysis of the immunogenetic mechanism and the clinical correlates of the cellular immunoregulatory aspects of IgAN is necessary to elucidate the pathogenesis of this disease.