CD32

CD32 (cluster of differentiation 32), also known as FcγRII or FCGR2, is a surface receptor

CD32A is an activating subtype of CD32 that can be found on a variety of immune cells - notably, CD32A is found on platelets, neutrophils, macrophages, and dendritic cells (DCs). On platelets, it is known to aid in the internalization of IgG-opsonized Escherichia coli, and it is more generally implicated in mediating bacterial-activated platelet responses.^[4] CD32A also plays an important role in platelet activation, adhesion, and aggregation in response to injured blood vessels.^[3] When bound to an IgG immune complex, the cytosolic ITAM can promote phagocytic activity and cytokine secretion in neutrophils and macrophages.^[2] CD32A is known to aid in the activation of clathrin coat-mediated endocytosis on various cell types. On DCs, CD32A plays an important role in maturation and the upregulation of costimulatory molecules on the cell surface, strengthening the DC's ability to present antigen to T cells. CD32A activation is necessary and sufficient to produce T cell anti-tumor cellular immunity. CD32A is also linked to autoimmunity; for example, the production of antibodies against platelet factor 4 (PF4) bound to CD32A is linked to the development of heparin-induced thrombocytopenia.^[4]

CD32A is also found on Langerhans cells, mast cells, basophils, eosinophils, monocytes, megakaryocytes, and a subpopulation of activated CD4+ T cells. CD32A is unique to primates.^[3]

CD32B

CD32B is an inhibitory surface receptor that is part of a large population of B cell co-receptors, which act to modulate signaling.^[3] Activated CD32B has the ability to cross-link with B cell receptors (BCRs), which increases the threshold for B cell activation and downregulates antibody production in the presence of IgG.^[5] This feedback loop lowers the production of IgG by B cells when there is a surplus of IgG in the body. CD32B is also found on the surface of follicular dendritic cells (FDCs), which utilize CD32B for the retention and recycling of immune complexes that they later present to B cells.^[3][7] Thus, CD32B plays an important role in both antibody and memory immune responses.^[3]

The balance between CD32B and its activating counterparts is crucial to appropriate cell function. Having too little CD32B has been associated with dysregulated antibody function, as well as increased antibody-dependent inflammatory cell responses.

CD32B is also found on basophils, neutrophils, monocytes, and macrophages.[2]

Non-immune system locations

CD32B can be found on airway smooth muscle cells, as well as liver sinusoidal endothelial cells and salivary gland epithelial cells.^[3][7]

CD32C

CD32C is expressed in ~20% of the human population, and is not well-understood.^[2] It can be found on B cells and natural killer (NK) cells. When expressed, CD32C plays an important role in the activation of antibody-dependent cell cytotoxicity (ADCC).^[3] Animal studies have linked CD32C to augmentation of pathological inflammatory responses.^[3]

See also

• FCGR2A
• FCGR2B
• FCGR2C
• Fc-gamma receptors

References