Recombinant Mouse Fc gamma RIIB/CD32b Protein, CF

Receptors for the Fc region of IgG (Fc gamma Rs) are members of the Ig superfamily that function in the activation or inhibition of immune responses such as degranulation, phagocytosis, ADCC (antibody-dependent cellular toxicity), cytokine release, and B cell proliferation (1 - 3). The Fc gamma Rs have been divided into three classes based on close relationships in their extracellular domains; these groups are designated Fc gamma RI (also known as CD64), Fc gamma RII (CD32), and Fc gamma RIII (CD16). Each group may be encoded by multiple genes and exist in different isoforms depending on species and cell type. The CD64 proteins are high affinity receptors
(~10^-8 - 10^-9 M) capable of binding monomeric IgG, whereas the CD16 and CD32 proteins bind IgG with lower affinities (~10^-6 - 10^-7 M) only recognizing IgG aggregates surrounding multivalent antigens (1, 4). Fc gamma Rs that deliver an activating signal either have an intrinsic immunoreceptor tyrosine-based activation motif (ITAM) within their cytoplasmic domains or associate with one of the ITAM-bearing adapter subunits, Fc R gamma or zeta (3, 5). The only inhibitory member in human and mouse, Fc gamma RIIB, has an intrinsic cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM). The coordinated functioning of activating and inhibitory receptors is necessary for successful initiation, amplification, and termination of immune responses (5).

Three distinct genes encode the human CD32 group. These proteins share 94 - 99% amino acid identity in their extracellular domains but have divergent cytoplasmic domains and signaling capacities (1, 3). In contrast, a single gene encodes mouse CD32 (4). The protein product delivers an inhibitory signal upon ligand binding and is functionally equivalent to human Fc gamma RIIB (5). Mouse CD32 is the only Fc gamma R expressed on B cells and is also expressed on macrophages, neutrophils and mast cells. Mouse CD16 is closely related to mouse CD32 throughout its extracellular domain (95% amino acid identity), but has a divergent cytoplasmic domain and functions as an activating receptor. Together these proteins constitute an activating/inhibiting receptor pair to regulate immune responses (5).