Multiple bovine FcγRIIb sub-isoforms generated by alternative splicing

Abstract

Receptors for the Fc portion of immunoglobulin molecules (FcR) provide an important and vital link between circulating antibody and cellular effector functions. These receptors have been well characterized in human and murine species, however few of these receptors have been investigated in livestock. FcγRII (CD32) is an FcR previously shown in mice and humans to exist in multiple isoforms, both activating (FcγRIIa, FcγRIIc) and inhibitory (FcγRIIb), on a wide variety of cells including B cells, T cells, dendritic cells, monocytes, macrophages and platelets. On B cells, FcγRIIb acts to suppress cell activation and immunoglobulin production by means of an intracellular immunoreceptor tyrosine-based inhibitory motif signaling domain. Two sub-isoforms of FcγRIIb, designated b1 and b2, distinguished by the inclusion of an additional cytoplasmic exon in the b1 form, have been demonstrated in humans and mice, whereas only one sequence corresponding to the human and mouse b2 isoform has been identified in cattle. In this study, the expression profile of FcγRIIb in bovine blood mononuclear cells was characterized by collecting blood samples from mature cattle of dairy and beef breeds, and determining their FcγRIIb mRNA expression profile by RT-PCR. Analysis revealed the presence of two uncharacterized bovine FcγRIIb transcripts in addition to the single previously published transcript. Analysis of the first unknown transcript revealed high homology with published human and murine FcγRIIb1 sequences. This transcript was present in all cell types examined, with little variation in primary sequence between individuals or among breeds. The second unknown sequence was found to be homologous to the murine FcγRIIb3 (IgG-binding protein or soluble FcγR in humans) sequence. This transcript appears to have a much more limited expression profile, which may indicate that expression varies with the cellular activation-state of the cell. These results indicate that cattle, like humans and mice, express multiple sub-isoforms of FcγRIIb. These findings add further complexity to the regulation of IgG-mediated immunity and provide new insight into the role Fc receptors play in antigen acquisition and presentation in cattle.