FcγR expression on hemopoietic cells
The tissue distribution of the human leukocyte FcγR is well documented and reviewed comprehensively elsewhere.7, 11, 17 In the context of effector functions harnessed by therapeutic mAbs, several aspects of the cellular distribution (Table 2) should be emphasized.
FcγR expression profiles differ between cell lineages but almost all mature human leukocytes, and platelets, express at least one FcγR (Table 2). It should also be appreciated that the cellular expression levels and receptor diversity as will be described later is also influenced by the activation state of the cells, anatomical location and the cytokine environment which modulates FcγR expression, particularly for FcγRI and FcγRIIb.18 For example, resting monocyte subpopulations may express only FcγRIIa but activated macrophages express FcγRI, FcγRIIa and FcγRIIIa and/or FcγRIIb.7
Thus, specific characteristics of leukocyte FcγR expression are summarized as follows:
FcγRI is not usually expressed until induced by cytokines such as interferon‐γ on monocytes, neutrophils, macrophages, microglial cells in the brain, dendritic cells and mast cells. The sensitivity of FcγRI to interferon‐γ suggests that its in vivo activity is closely tied to immune activation events, and mouse studies have suggested that it has a critical role early in immune responses.19, 20 Its role in the MOA of antibodies may vary with anatomical location.21
FcγRIIa is expressed only in primates and shows the broadest expression of all FcγRs, being present on all innate leukocytes. It is also present on platelets but its role in effector functions is not established but it is important in certain immune thrombocytopenias. A polymorphic form of this receptor is the only human receptor for human IgG2. This, together with its limited species expression and unique ITAM‐containing cytoplasmic tail (reviewed by Anania et al. 11), suggests a unique function in human leukocytes. Interestingly, polymorphism of the receptor is associated with systemic lupus erythematosus and resistance to Gram‐negative organisms.11 A rare, hyper‐responsive form is a risk factor for neutrophil‐driven anaphylaxis in Ig replacement therapy.22
FcγRIIc is an activating FcγR whose expression is regulated single nucleotide polymorphism that permits expression in approximately 20% of humans and in whom it is present at low levels on natural killer (NK) and B cells.11 It has arisen by gene duplication/recombination resulting in an extracellular region  derived from  FcγRIIb, which binds IgG4, and with an ITAM‐containing cytoplasmic tail related to the activating receptor FcγRIIa. Thus FcγRIIc provides IgG4 with an activation receptor pathway and confers a new biology of IgG4 in these individuals. Its low frequency in the population may also confound in vivo mAb clinical testing or use, but as yet there is no evidence for this.
FcγRIII forms are two highly related gene products, FcγRIIIa and FcγRIIIb. The FcγRIIIa is expressed on NK cells and professional phagocytes, particularly macrophages. It is only recently apparent that FcγRIIIa is expressed on neutrophils, albeit at low levels, but plays a role in their function.23 FcγRIIIb is unique to humans and unlike other FcγRs it is attached to cell membrane via a glycophosphatidyl anchor. It is expressed, predominantly and abundantly, on human neutrophils.7 Its effector function depends in part on its coexpression with FcγRIIa. The lack of FcγRIIIb on macaque neutrophils appears to be compensated for by an increase in FcγRIIa expression.15
FcγRIIbs are the inhibitory‐type FcγR and arise from a single gene. They lack intrinsic proinflammatory signaling and are instead immune checkpoints. They provide feedback regulation by antibodies, in the form of immune complexes, to inhibit B‐cell activation by specific antigen. They also control activating‐type FcγR function on innate cells. Two major splice variant forms of FcγRIIb exist with differential tissue expression profiles. FcγRIIb1 preferentially expressed on B lymphocytes contains a 20‐amino acid cytoplasmic insertion necessary for membrane retention and cocapping with the B‐cell antigen receptor (BCR). FcγRIIb2 is the predominant inhibitory receptor found on basophils and neutrophils, as well as on subpopulations of mast cells, dendritic cells and some monocytes/macrophages. FcγRIIb2 lacks the cytoplasmic insertion of FcγRIIb1 and consequently can internalize rapidly including with the activating FcR when they are co‐cross‐linked.11 It is not clear which form is present on human T cells.
One additional comment on tissue distribution is that FcγR expression on T cells has been difficult to establish unequivocally. However, there is increasing evidence that T‐lymphocyte populations express FcγR. Some γδ T cells express FcγRIIIa and αβ T cells reportedly express FcγRIIa, FcγRIIb or FcγRIIIa but the significance with respect to effector function mediated by antibody is presently unclear.24, 25, 26, 27, 28