IgA nephropathy-specific expression of the IgA Fc receptors (CD89) on blood phagocytic cells.

We analysed the biochemical features of receptors for the Fc-region of IgA (Fc alphaR, CD89) on blood monocytes and granulocytes of patients with IgA nephropathy (IgAN). Fc alphaR on monocytes of IgAN were found to have a higher Mr (60-80 kD) than those of control monocytes (50-75 kD) and granulocytes (55-75 kD) in both IgAN and controls as shown by immunoprecipitation analysis. Removal of N-linked carbohydrates from Fc alphaR on monocytes of IgAN revealed a 32-36 kD protein core, the Mr of which was still higher than that of controls (28-32 kD). When Fc alphaR transcripts were analysed by reverse-transcription-PCR, only one prominent band was visualized in PCR products from IgAN monocytes. Since the results thus far show that IgAN monocytes express Fc alphaR protein and mRNA differently from granulocytes and control monocytes, PCR products were then cloned and sequenced. The predominant band in PCR products from IgAN monocytes was identical to that of the Fc alphaR a.1 transcript, and an additional 10 transcripts containing five novel transcripts were obtained from granulocytes and control monocytes. In three transcripts, we found an insertion sequence between the S2 and EC1 domains, suggesting the existence of a new exon. These results suggest a predominant usage of Fc alphaR a.1 among various transcripts of Fc alphaR in IgAN monocytes.