RESUMEN
BACKGROUND: Primary IgA nephropathy (IgAN) is associated with elevated levels of circulating IgA and is characterized by deposition of primarily IgA1 in the renal mesangium. It has not yet been clarified which mechanisms govern the deposition of IgA1 in the mesangium. One of the factors which may play a role in trapping of IgA in the mesangial area is the interaction of IgA with specific IgA receptors (Fc alphaR, CD89) on the mesangial cells. METHODS: In the present study IgA derived from patients with IgAN and controls was investigated for its interaction with human CD89, expressed on the surface of the murine B cell line IIA1.6. RESULTS: IgA binding to CD89 expressing cells was specific, concentration dependent and binding of dIgA and pIgA occurred in a more efficient fashion than that of mIgA. IgA binding to CD89 directly from serum of patients compared to controls showed no significant difference. However these experiments are affected by differences in IgA concentration and combinations of different sizes of IgA. Using purified fractions of mIgA, dIgA, and pIgA isolated from serum, a significantly reduced binding of mIgA to CD89 from patients compared to controls was observed. Finally, the binding of aIgA2 to CD89 was less inhibited using mIgA from patients with IgAN compared to controls. CONCLUSIONS: The reduced binding of mIgA to CD89 seems to contradict a direct role for CD89 in deposition of IgA. However reduced binding of mIgA to CD89 may affect IgA clearance, leading to higher serum IgA. Furthermore, since it has been demonstrated that mIgA can interfere with binding of di- and pIgA, CD89 could still contribute to pIgA deposition in the mesangial area.
Asunto(s)
Antígenos CD/metabolismo , Glomerulonefritis por IGA/sangre , Inmunoglobulina A/metabolismo , Receptores Fc/metabolismo , Adulto , Animales , Antígenos CD/genética , Linfocitos B/metabolismo , Línea Celular , Fraccionamiento Químico , Femenino , Citometría de Flujo , Humanos , Inmunoglobulina A/sangre , Inmunoglobulina A/química , Masculino , Ratones , Persona de Mediana Edad , Receptores Fc/genética , Valores de Referencia , TransfecciónRESUMEN
The IgA Fc receptor (FcR; CD89) is expressed on several types of cells of the myeloid cell lineage. We investigated whether different sizes of heat-aggregated IgA (aIgA) bind to CD89 and subsequently induce cellular activation. As a model we used the murine B cell line IIA1.6 transfected with CD89 or IIA1.6 cells transfected with CD89 as well as with the FcR gamma chain to study the binding of IgA to CD89. When these cells expressing CD89 were incubated with monomeric IgA, no significant binding of IgA to the cells was detectable by fluorescence-activated cell sorter analysis; however, incubation of the cells with aggregated IgA resulted in 93 +/- 2% positive cells. Incubation of the cells with different sizes of IgA-containing aggregates revealed optimal binding with aggregates containing five to six molecules of IgA per aggregate. No difference was observed between the binding to CD89 of both IgA1- or IgA2-containing aggregates. Furthermore, the binding of aIgA was found to be CD89-specific, since the binding of IgA was completely inhibited by the CD89-specific monoclonal antibody My43 and no detectable binding occurred to the IIA1.6 parent cell line. Activation studies using interleukin-2 (IL-2) production as a marker, showed that the FcR gamma chain is necessary to induce cellular activation. Only cells transfected with both CD89 and the FcR gamma chain (CD89+/gamma +) enhance the IL-2 production 10-12-fold upon stimulation with aggregates of IgA. Furthermore, triggering of CD89 only results in increase of intracellular calcium concentration ([Ca2+]i) in cells co-expressing FcR gamma chain. Mutation of the tyrosine residues in the FcR gamma chain immunoreceptor tyrosine-based activation motif of the FcR gamma chain abolishes this increase in [Ca2+]i, indicating association and involvement of the FcR gamma chain in CD89-mediated signaling.
Asunto(s)
Inmunoglobulina A/química , Receptores Fc/fisiología , Receptores de IgG/fisiología , Sustitución de Aminoácidos , Animales , Sitios de Unión , Calcio/fisiología , Humanos , Inmunoglobulina A/inmunología , Interleucina-2/fisiología , Ratones , Receptores Fc/química , Receptores de IgG/química , Proteínas Recombinantes , Transducción de Señal , Relación Estructura-Actividad , Tirosina/químicaRESUMEN
An alternatively spliced CD89 transcript is present in peripheral blood mononuclear cells (PBMC) and U937 cells. The alternatively spliced CD89 mRNA species lacks the exon 4 sequence, encompassing 288 nucleotides, that encodes the extracellular membrane-proximal immunoglobulin-like domain (EC2).
Asunto(s)
Empalme Alternativo/genética , Antígenos CD/genética , Exones/genética , Receptores Fc/genética , Secuencia de Bases , Mapeo Cromosómico , Datos de Secuencia MolecularRESUMEN
CD32 (Fc gamma RII) is the most abundantly distributed class of IgG Fc receptors in the human body. In this study, we analyzed the effect of transforming growth factor (TGF)-beta 1, a cytokine with strong immunosuppressive function, on the expression and function of CD32 on freshly isolated peripheral blood monocytes and three human monocytic cell lines, U937, THP-1 and Mono mac-6. We found that TGF-beta 1 down-regulates CD32 expression on monocytes and all monocytic cell lines in a dose- and time-dependent fashion. A mean down-regulation of CD32 expression on THP-1 cells of 54 +/- 3.2% after 24 h was found at a concentration of 1 ng/ml TGF-beta 1. At the mRNA level, TGF-beta 1 induced a twofold down-regulation of CD32. Cross-linking of CD32 induced an increase in the concentration of intracellular Ca2+, which was reduced by 50% by TGF-beta 1, suggesting a decreased downstream signaling mediated by the receptor.
Asunto(s)
Regulación hacia Abajo/inmunología , Monocitos/efectos de los fármacos , Monocitos/metabolismo , Receptores de IgG/biosíntesis , Transducción de Señal/inmunología , Factor de Crecimiento Transformador beta/farmacología , Antígenos de Superficie/biosíntesis , Reactivos de Enlaces Cruzados , Regulación hacia Abajo/efectos de los fármacos , Humanos , Monocitos/inmunología , ARN Mensajero/biosíntesis , Receptores de IgG/metabolismo , Transducción de Señal/efectos de los fármacosRESUMEN
Depositions of IgA in the renal glomerular mesangial area are a hallmark of IgA nephropathy, and are thought to be crucial for the onset of inflammation processes in IgA nephropathy. In this report we show that human mesangial cells (MC) in vitro bind IgA and that binding of IgA enhances the production of IL-6 by MC. Furthermore we show that the size of IgA is crucial in its capability to enhance IL-6 production. Monomeric IgA does not affect basic IL-6 production, whereas dimeric and polymeric IgA enhance IL-6 production up to 3- to 9-fold respectively. Additional studies demonstrate that enhanced IL-6 production by MC is not accompanied by increased proliferation of human mesangial cells, a finding which is distinct from that found with rat mesangial cells. Taken together, these fmdings suggest that deposition of dimeric and polymeric IgA in the mesangial area of human kidneys in IgA nephropathy may amplify local inflammation.
RESUMEN
IgA is the predominant immunoglobulin in human secretions and the second most important immunoglobulin in the circulation on a quantitative basis. The clearance of IgA is dependent on the function of at least three types of receptors. One of these receptors recognizes the Fc portion of the IgA molecule, Fc alpha R, which has been cloned recently. Fc alpha R, also designated CD89, is found on a number of cells, including human glomerular mesangial cells, and monocytes. In this study we analysed the effect of TGF-beta 1, a cytokine with strong immunosuppressive function, on the expression of CD89 on freshly isolated monocytes. We found that TGF-beta 1 down-regulates CD89 expression on human peripheral blood monocytes in a dose-dependent fashion. Optimal down-regulation occurred at a concentration of 5 ng/ml. The down-regulation of CD89 by TGF-beta 1 is linear in time, with a mean down-regulation of 34 +/- 13% after 24 h. Also at the mRNA level, CD89 expression was down-regulated by TGF-beta 1, suggesting regulation of CD89 at the transcriptional level. Monocytes pre-treated with TGF-beta 1 displayed a reduced response to IgA, as measured by IL-6 production by monocytes, in contrast to monocytes pre-treated with medium alone. These results suggest an important role for TGF-beta 1 in the regulation of CD89. This down-regulation may have direct consequences for the handling of IgA by human monocytes.
Asunto(s)
Regulación hacia Abajo/inmunología , Inmunoglobulina A/fisiología , Monocitos/metabolismo , Receptores Fc/antagonistas & inhibidores , Factor de Crecimiento Transformador beta/fisiología , Antígenos CD/genética , Humanos , Interleucina-6/biosíntesis , Monocitos/efectos de los fármacos , Receptores Fc/genética , Transcripción Genética/inmunologíaRESUMEN
Extensive chromosome size polymorphism arises in Plasmodium berghei during in vivo mitotic multiplication. Size differences between homologous chromosomes involve rearrangements occurring in the subtelomeric portions while internal chromosomal regions do not contribute significantly to chromosome size polymorphism. Differences in the copy number of a 2.3-kb subtelomeric repeated unit are shown to correlate with size variations, and in at least one case to account completely for the size difference between two variants of the same chromosome.