Efficient recruitment into inflamed skin requires exceptional adhesive interactions between mononuclear cells and the inflamed endothelial cell lining of draining blood vessels. In addition to the expression of E-selectin, CCL17, CXCL16, CX3CL1, and increased numbers of integrin ligands [33–35], we show here for the first time a very early production of CS-1 fibronectin by activated endothelial cells in patients with ACD. The expression of this molecule may contribute to the enhanced stickiness of monocytes and T cells expressing α4β1 integrin.
Within the extracellular matrix components fibronectin play an important role in lymphocyte trafficking by integrins-ligands interaction. Activated lymphocytes express α4β1 integrin which bind not only to VCAM-1 but to specific sites on the fibronectin molecule, the connecting segment-1 (CS-1) motif present in alternatively spliced variants .
Enhanced adhesion of α4β1+ T lymphocytes to CS-1 fibronectin in vitro has been shown in patients with vasculitis and rheumatoid arthritis and this molecule was selectively expressed in synovial endothelium from biopsies with rheumatoid arthritis but not in normal synovium [32, 36].
CS-1 fibronectin and CCL17 are probably most important in the rapid adhesion step, because of their presentation by endothelial cells. Indeed they trigger rapid adhesion to VCAM-1 and ICAM-1 of monocytes and T cells rolling on E-selectin. Thus CS-1 fibronectin may work together with CCL17 inducing adhesion of passing cutaneous α4β1+/CCR4+ mononuclear cells under shear.
We also report here that CCL17 is also produced by scattered keratinocytes in patients with ACD which since to be similar to the finding of Vestergaard C et al in skin biopsies from patients with atopic dermatitis . This chemokine may work together with CCL27 and MCP-1/CCL2 which are also produced by activated keratinocytes , and may subsequently attract the adherent cells into the epidermis of patients with ACD.
Since induction of CS-1 fibronectin production has also been reported by us in lesional skin from patients with atopic dermatitis and irritative contact dermatitis [Martín AP; Ortiz S; Cabalier MED; Frede S; Burgos E; Hliba E; Serra, HM. In press Journal Cutaneous Pathology], it is inferred that this early inflammatory molecule is expressed in skin's BEC upon different types of stimulus in a non antigen-specific manner.
So a useful therapeutic strategy in dermatitis may be to block α4β1 and CCR4 interactions with its ligands on inflamed endothelial cell surfaces, by specific antibodies or antagonists. In this regard, CS-1 analogue peptide have been efficiently used to diminish the efferent phases of Th2 and Th1 mediated inflammatory responses in animal models [[37, 38], Serra et al, in preparation].
T lymphocytes with polarized cytokine production (Th1 and Th2) show a different distribution of inflammatory CR, with CXCR3 and CCR5 transcripts markedly enhanced in Th1 cells and increased amounts of transcripts for CCR3, CCR4 and CCR8 in Th2 cells . Moreover a large body of evidences indicate that the chemokine receptor CXCR3 and CCR5 are markers for T cells associated with certain inflammatory reactions, particularly TH-1 type reactions [40, 41].
In our kinetic study over a 48 hours course of provoked human allergic contact dermatitis we found a gradual and higher accumulation of CXCR3+ than CCR5+ cells which was directly associated with CD3+ T cells. Flier J et al have demonstrated differences in chemokine expression between ACD and ICD reactions being CXCL9, CXCL10 and CXCL11 only detected in the allergic patch test lesions . Interestingly the proportion of cells in our allergic patch test reactions expressing CCR5 was lower than those that were CXCR3+, providing new evidences to the findings of Yamamoto et al. who has demonstrated that CXCR3 is the best marker for identification of circulating Th1 effector population because Th1-type cytokine-producing cells resided not only in CCR5-positive CD4+ T cells but also in those that were CCR5-negative .
IL-17 is an important player of CD4+ T cell-mediated skin inflammation in ACD with synergistic (ICAM-1, IL-8) or antagonist (RANTES) effects on IFN-gamma-stimulated keratinocyte activation . Our results also confirmed the finding of Albanesi et al. who have shown that the majority of infiltrating cells in ACD are recruited by CXCR3 agonistic chemokines released by keratinocytes activated with IFN-gamma and TNF-alpha or IFN-gamma and IL-4 . But here IL-4 exerts a proinflammatory function on keratinocytes by potentiating IFN-gamma and TNF-alpha induction of IP-10, Mig, and I-TAC, which in turn may determine a prominent recruitment of CXCR3+ T lymphocytes at inflammatory reaction sites.
Our findings of a small number of CCR3+ cells infiltrating the skin at 48 hours could be due to the present of T cells releasing high levels of IL-10, low IFN-γ, and undetectable IL-4 (Th IL-10) which have been reported to represent an important mechanism for terminating Th1-mediated allergic reactions and limiting excessive tissue damage. They have a broad array of functional T1- and T2-associated chemokine receptors, but also express high levels of CCR2 and CCR8 which allow resting as well as activated Th IL-10 cells to display a significant migratory capacity to CCL2 and CCL1 .