Altered expression of lymphocyte Fc alpha receptor in selective IgA deficiency and IgA nephropathy.

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Abstract
To study the expression of FcR specific for IgA (Fc alpha R) on human peripheral lymphocytes (PBL), PBL from normal donors were incubated with 300 to 500 micrograms/ml MOPC 315 IgA having anti-trinitrophenyl (TNP) antibody activity at 4 degrees C or 37 degrees C for 60 min. Under this condition, less than 2% of total cells could form rosettes with TNP-coated ox red blood cells (TNP-ORBC). When cultured with MOPC 315 IgA at 37 degrees C for 18 hr, however, there was a dose-dependent increase of the rosette-forming cells (RFC) binding TNP-ORBC. Because 15 to 20% of the total cells bound TNP-ORBC but not unsensitized ORBC, the rosette formation appeared to be due to the cytophilic binding of IgA to the cells. The binding of MOPC 315 IgA was competed by TEPC 15 IgA and human myeloma IgA, but not by murine myeloma proteins of other classes, indicating that the receptor is specific for IgA. Fc alpha R was induced on 15 to 20% of fractionated T and B cells, as well as on 15 to 18% of concanavalin A-(Con A) activated lymphocytes when cultured with IgA. The induction of the receptor was dependent on protein and RNA synthesis, but not on DNA synthesis as suggested by the sensitivity to metabolic inhibitors, such as mitomycin C, actinomycin D, puromycin, and cycloheximide. In five patients with selective IgA deficiency (serum IgA, 0 to 4 mg/dl), only 5.1% +/- 1.7 of PBL formed rosettes with TNP-ORBC after culture with MOPC 315 IgA, whereas 12.5% +/- 2.5 of PBL from normal donors (serum IgA, 90 to 330 mg/dl) formed rosettes. Fc alpha R was induced on more than 15% of the cells from these patients, however, when cultured with IgA in the presence of a conditioned medium obtained from mixed lymphocyte culture from two normal donors. The results suggested that the abnormality in the patients' PBL might be in the induction mechanism rather than in the number of precursor cells that could express Fc alpha R in the presence of IgA. On the other hand, Fc alpha R was induced on 10.4% +/- 1.5 of PBL from the patients with IgA nephropathy (serum IgA, 382 +/- 11 mg/dl) when they were incubated with IgA for 1 hr at 37 degrees C. Because Fc alpha R on normal PBL was not induced by 1 hr of incubation with IgA, it appeared that the receptor was already expressed in vivo on the cells of these patients.