Characterization of Fc gamma receptors on rat mucosal mast cells using a mutant Fc epsilon RI-deficient rat basophilic leukemia line.

A novel rat basophilic leukemia (RBL) 2H3 subline of rat mucosal mast cells deficient in the expression of the gamma chain (RBL-gamma-) has permitted functional characterization of their low-affinity Fc gamma receptors (Fc gamma R). A rat Fc gamma RII analog of the mouse b2 isoform has been earlier identified and its transcript detected in RBL-2H3 cells. We have noew isolated and sequenced the rat Fc gamma RIIb1 isoform and observed differences between its expression in RBL-2H3 and RBL-gamma-. Furthermore, we demonstrate that rat mucosal mast cells express a second, low-affinity Fc gamma receptor, namely the Fc gamma RIII. Stimulation of either cell line with IgG complexes decreased the expression of transcripts for all Fc gamma R. Hence, ligation of Fc gamma R on rat mucosal mast cells apparently regulate their transcription. Selective stimulation through the Fc gamma RII or Fc gamma RII/III systems, respectively, was accomplished by either using RBL-gamma- line or by saturating the Fc epsilon RI on RBL-2H3 with monomeric IgE. RBL-gamma-cells, which do carry Fc gamma RII (but lack Fc epsilon RI and Fc gamma RIII), do not respond to IgG (and IgE) immune complexes as monitored by specific protein tyrosine phosphorylation, degranulation or cytokine secretion. This finding, together with the restoration of the functional phenotype of parental cells upon gamma chain cDNA transfection into RBL-gamma- cells, unequivocally excludes the possible stimulation of rat mucosal mast cells by clustering of their Fc gamma RII. Fc epsilon RI saturation by IgE on parental RBL-2H3 cells completely blocks their response to IgG immune complexes. Thus the Fc gamma R on these cells do not trigger degranulation and this is not due to the absence of Fc gamma RIII as previously suggested. Therefore, co-clustering of Fc gamma RII and Fc gamma RIII on rat mucosal mast cells does not seem to stimulate them. A possible inhibitory role of Fc gamma RII in this process is suggested and discussed.