Chemokine Classification

See affects of chemokines on Th1/Th2 differentiation

Chemokines are a superfamily consisting of 8 to 11 kDa proteins subdivided into four families (C, CC, CXC, CX₃C) according to the position and separation of their first two amino-terminal cystein (C) residues of a four-cysteine motif in the N-terminal regions of the molecules. The CC and CXC chemokines are promiscous, with most binding to two or more receptors (e.g., CCL5 binds to CCR1, CCR3 and CCR5), although the C and CX3C chemokines bind to only one receptor.

Although the structural subclassificaiton of chemokines based on the cystein motif was initally adequeate with relatively few chemokines, the superfamily comprises now over 40 members. Moreover, this classification based on structure does not provide information of the functional role of the chemokines. Thus, chemokines are often also classified based on their functional expression as constitutive or inflammatory/inducible.

Classification based on the cysteine motif:

C: The C family consists of only two members: X chemokine ligand (CL) 1 (lymphotactin-alpha) and XCL2 (lymphotactin-beta) which are homologus to the CC family at their carboxyl end, but lack the first and third cysteine residues found in the other 3 families and recruit T lymphoctyes only.

CC: This family of structurally related secreted proteins involved in inflammation, wound healing, hematopoiesis, and tumorigenesis. The genes encoding this supergene family reside on human chromosome 17. The CC chemokine family evokes monocyte. lymphocyte and DC migration.

MIP-1κ is an active chemokine in the maturation and differentiation of myeloid progenitor cells. It interacts with the CCR5, which is a member of the heptahelical transmembrane spanning G protein coupled receptors. CCR5 has been extensively identified as a co-factor in HIV binding and entry into target cells.

Constitutive Chemokines

CCL17 (TARC; thymus- and activation regulated cytokine): is known to be upregulated by DCs upon maturation. TARC is produced by cytokine stimulated monoctyes expecially under Th2 dominant conditions as well as by monocyte derived DCs. CIn humans TARC binds to the chemokine receptors CCR4 and CCR8. TARC has been reported to be enhanced by IL-4 and suppressed by IFNy and IL-10.

CCL19 (Epstein-Barr virus-induced molecule (EBI1) or (ELC)  (MIP-3B) is reportedly expressed by T cell area dendritic cells. It localizes mature CCR7 expressing DCs (and naive T cells) to T-cells areas of secondary lymphoid tissues.It is related to CCL2 which are made constitutively within lymphod T zones and function in bringing APCs and T cells together in the lymphoid tissue.

CCL19 (MIP-3beta) has reportedly augmented IL-10 expression in activated PBMC, suggesting a role in dampening inflammatory TH1 type resposnes within lymphoid tissues. Interestingly, in vitro studies indicate that expression fo CCL19 can be down regulated by IL-10 which suggests a negative regulatory loop between CCL19 and IL-10.

CCL21 (secondary lymphoid chemokine; SLC) is expressed by high endothelial venules (HEVs) and lymphatic vessels. It is a ligand for CCR7. SLC is believed to mediate the homing of naive lymphocytes to secondary lymphoid tissues.

CCL22 (MDC): Macrophage-derived chemokine. is known to be upregulated in DCs upon maturation. MDC is expressed on about 20% of adult periopheral blood effector/memory T cells. MDC has been identified as an important suppresser factor for HIV-1 infection. 

CXCL10 (IP-10)

CXCL12

Inflammatory Chemokines

Inflammatory chemokines are produced by several cell types such as endothelial, epithelial, and stromal cells as well as leukocytes. They are induced to high level of expression by inflammatory stimuli such as LPS, IL-1 and TNFα.

CCL2(MCP-1): Monocyte chemattactic protein 1 is a CC chemokine which is responsible for recruiting monocytes into sites of inflammation and cancer. MCP1 is abundantly produced in a variety of inflammatory diseases. Consistent with its role in inflammation, MCP1 is known to be a chemotatic for monocytes, T lymphoctes, basophiles and NK cells. Based on its chemotactic effect on monocytes, MCP-1 has been observed to have an anti-tumor effect in certain mouse/tumor experimental designs. However, inhibition of MCP-1 has also been reported to inhibit tumor growth and prolongs survival. The NIH has transgenic mice for which the gene for MCP1 is deleted which is available for further development through collaborative research with inventors.

MCP-1 initiates signal transduction through binding to C-C chemokine receptor 2 (CCR2). In humans there are two receptors for MCP-1, CCR2A and CCR2B, which are splice variants of a single gene. In the mouse, there is a single MCP-1 receptor.

Animals that lack CCL2 show diminished T cell responses with stronger effects on TH2 type responces. This includes decreased production of IL-4 by draining lymph node cells after in vitro restimulation and increased resistance to Leishmania infection, which is indicative of a shift from a TH2 to TH1 response. IgG antibody responses also show shifts in isotype secretion towards subtypes that are typical of TH1 responses. Mechanistic insight into how CCL2 influences T cell differentiation came from the finding that CCL2 neutralization led to increased IL-12 production by ganuloma associated macrophages. CCL2 appeared to be acting directly on the activated macrophages as addition of CCL2 to macrophages in vitro caused a decrease in IL-12 expression. This has been extended with the discovery that MCPs CCL2, CCL7, CCL8 and CCL13 are all able to reduce IL-12 production by activated human monocytes in a pertussis toxin senstive manner. Thus, MCP molecules appear to signal through CCR2 or another Gi coupled receptor to antagonize expression of IL12 in activated moncotyes.

MCP has also been reported as a regulatory factor of oral tolerance. Increase in MCP-1 expression resulted in downregulation of mucosal IL-12 expression with concomitant increase in mucosal IL-4 expression. Fuctionally, MCP-1 upregulation was shown to regulate oral tolerance induction by the ability of antibodies to MCP-1 to inhibit tolerance induction after oral administration of a proteolipid protein peptide.

CCL3/CCL4 (MIP-1 proteins): The genes for CCl3 and CCl4 are inducible in most mature hematopoietic cells. Cells that are directly involved in eliciting immune responses, (i.e., macropahges, T and B lymphocytes, neutrophils, DCs, mast cells and NK cells can produce large amounts of MIP1 (up to several nonogram/106 cells). Generally the synthesis of CCl3/4 requires cell activation and are induced by various proinflammatory agents/cytokines including LPS, TNFα, IFNy, IL-1alpha/beta and others. Treatment with IL-4, IL-10 or other anti-inflammatory signals is known to downregulate MIP1 expression.

MIP-1 proteins mediate their biological effects by binding to cell surface CC chemokine receptors which belong to the G-protein coupled receptor superfamily. Receptor binding involves high affinity interactions and a subsequent cascade of intracellular events that rapidly leads to a wide range of target cell functions including chemotaoxis, and phagocytosis. Signal transduction events are initiated by the G-protein complex leading to its dissociation into Gα and Gβy subunits. Gα indcues phosphoinositide 3-kinase (PI3K) pathway activation, adn Gβy subunits activate phospholipase C and induce Ca2+ influx resulting in protein kinase C isoform activation. It has also been shown that MAP kinases as well as the JAK/STAT signaling cascade are involved.

MIP1 family members orchestrate acute and chronic inflammatory host responses at sites of injury or infection mainly by recruting proinflammatory cells. They are crucial for T cell chemotaxis form the circulation to inflamed tissue and also play an important role in the regulation of transendothelial migration of monocytes, DCs and NK cells. CCR1, CCR3 and CCR5 activation by MIP1 also leads to Ca2+ release, upregulation of activation markers and release of proinflammatory mediators such as LTC4, arachidonic acid or histamine. In addition, MIP1 proteins can regulate immune responses by modulating Th-differentation.

MIP1 proteins can also promote health by inducing inflammatory responses against infectious pathogens such as viruses (e.g., influenza or parasites). For example, in Toxoplasma gondii infection CCl3 and CCL4 (and CCL5/RANTES) increase IL12 release from DCs by binding to CCR5, which results in enhanced Th1 immunity and clearance of the parasite. On the other hand, the MIP-1 receptors CCR3 and CCR5 promote HIV-1 infection as they are important co-receptors for M-tropic HIV 1 viruses on cD4+ target cells. A role for CCL3 in cell mediated immunity was also suggested by the finding that CCL3 is highly expressed in EAE leasion and that neutralization of CCL3 reduces the severity fo EAE.  A mechanisms by which CCL3 and CCL5 may favor cell mediated immune reponses is through preferential recruitment of TH1 cells.

--CCL3 (MIP-1α): macrophage inflammatory protein 1α. CCL3 and its receptor CCR5 promote Th1 skewing. (e.g., CCR5 deficient mice display Th2 skewed cytokine profiles, whereas Th2 cells mainly express CCR2 and CCR4.) One study reported that addition of CCL3 to antigen or anti-TCR-activated T cells promoted development of IFNy producing cells.

--CCL4 (MIP-1β):

CCL5 (RANTES): is regulated on activation, normal T cell expressed and secreted. RANTES displays high affinity binding and signaling through multiple independent chemokine receptors including CCR1, CCR3, and CCR5.

Rantes is a proinflammatory CC-chemokine that has an important role in multiple chronic inflammatory conditions. It is a potent chemoattractant for both T lymphocytes and monocytes/macrophages that are implicated in the pathogenesis of MS lesions.

RANTES is the most potent natural inhibitor of M-trophic HIV-1 infection.

CXCL1 (KC) (GRO-α)

CXCL2 (MIP-2)

CXCL8 (IL-8): is a member of the CXC family of chemokines and is the principle neutrophil chemoattractant and activator in humans. IL-8 is a potent neutrophil recruitment factor that binds to the chemokine receptors CSCR1 and CXCR2 on neutrophils.

CXCL10 (interferon-inducible protein 10; IP10): is known to be upregulated in DCs upon maturation. IP10 induces chemotaxis of Th1 cells. IP-10 has also been reported to selectively activate and enhance antigen and mitogen driven IFNγ (but not IL-4) cytokines gene expression in human PBMCs suggesting a possible role in the maintenance of the default Th1 dominated resonses to environmental antigens usually observed in vivo. See Th1/Th2 defining factors

Constitutive/Inflammatory

CCL20

CCL18: attracts naive B and T cells as well as immature DCs.

Not known

CX₃CL1 (fractalkine): is a T cell attractant/chemokine expressed by mature DCs and is a unique membrane bound CX₃C chemokine, with the chemokine domain perched atop a long mucin-like stalk at the cell surface. Fractalkine can be cleaved by ADA10 ( a disintegrin and metalloproteinase) or tumor necrosis factor alpha-converting enzyme to produce a soluble glycoprotein. The structure of fractalkine allows not only the shed soluble form to recruit T cells and monoctyes expressing its specific receptor, CX₃CR1, in local chemoattractant gradients but also the membrane anchored form to act upon CX₃CR1 positive cells directly and promote cell-cell adhesion.

Endogenous fractalkine expression of DCs has been round to be subtly increased after contact with heat-killed L. pneumophila in BM DCs. Moreover, exogenous overexpression of fractalkine in Legionella capturing DCs resulted in a significant enhancement of T cell mediated protective immunity to lethal Legionella infection.