CCR5 (Chemokine-CC Motif-Receptor-5) is a member of the chemokine receptor subclass of the GPCR (G-Protein-Coupled Receptor) superfamily. It regulates leukocyte chemotaxis in inflammation and serves as a coreceptor for M-tropic (Macrophage tropic) HIV (Human Immunodeficiency Virus) entry. It serves as a functional receptor for various inflammatory CC-chemokines, including MIP-1 Alpha (Macrophage Inflammatory Protein-1-Alpha), MIP-1 Beta (Macrophage Inflammatory Protein-1-Beta), and RANTES (Regulated on Activation Normal T-Expressed and Secreted protein) and is the main co-receptor for M-tropic HIV1 strains. CCR5 is expressed in lymphoid organs such as the thymus and spleen, as well as in peripheral blood leukocytes, including macrophages and T-cells. The extracellular loops and amino-terminal domain of CCR5 are critical for binding of ligands, whereas the transmembrane carboxy-terminal helix bundle is involved in receptor activation and activation of G-proteins, along with other signaling molecules.
M-tropic HIV infection of primary human T-cells and macrophages requires optimal cell surface expression of the chemokine receptor CCR5 in addition to CD4 (Ref. 1 & 2). Macrophages are important targets for HIV-1 in vivo. Infected macrophages serve as a reservoir for viral persistence and participate in person-to-person transmission and are central to the pathogenesis of brain, lung, and other end-organ diseases in AIDS (Acquired Immunodeficiency Syndrome). The chemokine receptor CCR5 in macrophages is activated by its peptide ligands and also by the HIV envelope protein GP120 during HIV infection. CCR5 transduces its downstream signals through the G-proteins. Cells expressing CCR5 bind MIP-1 Alpha, MIP-1 Beta and RANTES with high affinity and generate inositol phosphates in response to these chemokines (Ref. 3). Chemokine-induced activation of CCR5 promotes the recruitment of GRKs (G-protein-coupled Receptor Kinases) to the plasma membrane, and the GRKs mediate the phosphorylation of serine residues in the carboxyl-terminal tail of the receptor. Through binding to the chemokine, this receptor plays a crucial role in inflammatory processes. Binding of a ligand (a chemokine or HIV-1) to these receptors induces a characteristic Ca2+ flux and tyrosine phosphorylation. This pathway elevates calcium influx into the cell through CRAC (Ca2+ Release operated Ca2+ Channels) ion channels that are activated by calcium release. Calcium release is facilitated by the activation of PLC-Gamma (Phospholipase-C-Gamma), PIP2 (Phosphatidylinositol 4,5-bisphosphate), and IP3 (Inositol Triphosphate) by G-AlphaQ. Elevated calcium from CRAC is required for activation of Calm (Calmodulin) and the subsequent downstream activation of PYK2 (Proline-Rich Tyrosine Kinase-2), a focal adhesion-associated protein kinase (Ref. 4).
CCR5 is also the principal co-receptor of HIV that, in concert with CD4, mediates the binding of the viral envelope protein to the cell surface, allowing subsequent entry into target cells. When infecting a host cell, the membrane protein complex of HIV-1, GP120, first binds to CD4, an integral membrane protein with a single transmembrane alpha-helix. CD4 binding triggers conformational changes in GP120, which leads to an increased affinity for the co-receptor CCR5 present in the same host cell membrane. Further conformational changes and interaction with CCR5 ultimately results in membrane fusion and viral entry.
CCR5 stimulation by GP120 activates protein kinase pathways in primary macrophages through activation of DAG (Diacylglycerol) and PKC (Protein Kinase-C), and protein phosphorylation is an important molecular mechanism by which extracellular signals produce biologic responses in cells. The pathway activates the focal adhesion–related kinase PYK2, activates ionic signaling responses, and stimulates phosphorylation of p38 and JNK/SAPK (c-Jun amino terminal Kinase/Stress- Activated Protein Kinase). The JNK and p38 MAPK pathways lead to the activation of c-Fos and c-Jun genes, and the consequent induction of chemoattractant pro-inflammatory mediator (Beta-Chemokine) secretion. GP120 stimulation of macrophages mainly induces the production of both MIP-1 Beta and MCP1 (Monocyte Chemotactic Protein-1), potent chemoattractants for monocytes and macrophages. Ionic signaling responses are modulated by PYK2 activation. Intracellular calcium elevation mediated by CRAC channels is a principal link between CCR5 stimulation and PYK2 activation in macrophages. In T-cells, GP120 activation of the chemokine receptors results in elevation of calcium and activation of Lck, ZAP70, PYK2 and FAK (Focal Adhesion Kinase) as well as the MAPK (Mitogen-Activated Protein Kinase) pathway (Ref. 5).
HIV infection is associated with immunosuppression caused by a dramatic reduction in the helper T-cell population. HIV-infected macrophages induce the selective apoptosis of CD4+ T-cells through Fas-FasL (Fas Ligand) interactions. FasL is expressed by macrophages in response to HIV infection and it represents a potential mechanism of peripheral CD4+ T-cell deletion both in physiologic homeostasis but also in disease. Induction of Fas-dependent apoptosis of susceptible CD4+ T-cells by HIV-infected macrophages requires cell-to-cell contact (Ref. 6). CD4 on T-cells is essential for viral cell entry and infection. Crosslinking of CD4 along with TCR (T-Cell Receptor)-CD3 Complex on the T-cell surface is involved in up-regulation of Fas and sensitization of T-cells to apoptosis induced by the Fas-FasL interaction.
CCR5 is thought to be involved in the recruitment of leukocytes in a growing number of inflammatory diseases, such as rheumatoid arthritis, multiple sclerosis and asthma, and it plays a major role in AIDS pathogenesis. Individuals with defective CCR5 alleles, or who lack this receptor, have a reduced susceptibility to infection by HIV. Patients with defective CCR5 co-receptors do not show signs of HIV infection in spite of repeated exposure. The absence of pathological phenotype in individuals lacking functional CCR5, together with the potent HIV-suppressive activity of CCR5 antagonists, makes this receptor an attractive candidate for pharmacological intervention (Ref. 7). Moreover, involvement of CCR5 in a much broader range of human immune diseases, including multiple sclerosis, rheumatoid arthritis and renal allograft rejection, suggests that blocking CCR5 function might be beneficial in these diseases as well (Ref. 1).