The CD32a
immunoglobulin G (
IgG) receptor (Fcγ receptor IIa) is a potential therapeutic target for diseases in which
IgG immune complexes (ICs) mediate
inflammation, such as
heparin-induced
thrombocytopenia,
rheumatoid arthritis, and
systemic lupus erythematosus.
Monoclonal antibodies (mAbs) are a promising strategy for treating such diseases. However, IV.3, perhaps the best characterized CD32a-blocking mAb, was recently shown to induce
anaphylaxis in immunocompromised "3KO" mice. This
anaphylactic reaction required a human CD32a transgene because mice lack an equivalent of this gene. The finding that IV.3 induces
anaphylaxis in CD32a-transgenic mice was surprising because IV.3 had long been thought to lack the intrinsic capacity to trigger cellular activation via CD32a. Such an
anaphylactic reaction would also limit potential therapeutic applications of IV.3. In the present study, we examine the molecular mechanisms by which IV.3 induces
anaphylaxis. We now report that IV.3 induces
anaphylaxis in immunocompetent CD32a-transgenic "FCGR2A" mice, along with the novel finding that IV.3 and 2 other well-characterized CD32a-blocking mAbs,
AT-10 and MDE-8, also induce severe
thrombocytopenia in FCGR2A mice. Using recombinant variants of these same mAbs, we show that
IgG "Fc" effector function is necessary for the induction of
anaphylaxis and
thrombocytopenia in FCGR2A mice. Variants of these mAbs lacking the capacity to activate mouse
IgG receptors not only failed to induce
anaphylaxis or
thrombocytopenia, but also very potently protected FCGR2A mice from near lethal doses of
IgG ICs. Our findings show that effector-deficient IV.3,
AT-10, and MDE-8 are promising candidates for developing therapeutic mAbs to treat CD32a-mediated diseases.