Similar events may be initiated in T cells by ICs and complement activation in autoimmune disorders. Syk has been a target for therapeutic intervention for autoimmune diseases. Syk-mediated signalling contributes to the altered T cell signalling . In this report, we demonstrate that the FcγRIIIA/B receptor engagement by ICs on CD4+
T cells leads to the recruitment of the signalling subunit, the FcRγ chain, thus resulting in Syk activation. The learn more presence of soluble TCC enhances this signalling event. TCC in fluid phase by associating with vitronectin (S protein) becomes cytolytically inactive and is regarded as irrelevant. However, recent reports have shown that TCC induces functional activities such as kinin-dependent vascular leakage, activation of endothelial cells and induction of osteoprotegerin [16,32,33]. Vitronectin facilitates the cellular adhesion of soluble TCC, providing a mechanism to trigger cellular responses . Previously, we have shown elevated levels of vitronectin associated with membrane attack complex (MAC) in lupus nephritis patients . Our results point to a synergistic
role for TCC in IC-mediated Syk activation in CD4+ T cells. Such synergistic action of ICs and MAC in chemokine secretion during lung tissue injury has also been reported previously . Binding of AHG (Fig. 1) and ICs purified from SLE Bcl-w to the peripheral CD4+ T cells establishes the interaction and a possible role of ICs in T cell responses. Previously, activation-dependent selleck screening library expression of FcγRII and FcγRIII receptors in the human T lymphocyte subpopulation has been observed . This study showed a four- to 10-fold increase in the FcγRIII+ CD8+ T cell population in response to phytohaemagglutinin (PHA) treatment on day 3 post-stimulation . Our results also point to a similar phenomenon, where FcγRIII+CD4+
T cells expanded in vitro using anti-CD3 and CD28, a total of more than 40% cells stained for FcγRIIIA/B in comparison to 10% directly from the PBMC. To explore whether ICs can influence the T cell physiology, we investigated the role of these complexes in Syk activation. Syk is a homologue of non-receptor tyrosine kinase ZAP-70. Syk is activated by FcRγ chain upon ITAM phosphorylation. Syk is expressed widely in both immune and non-immune cells [37,38]. Both DAP-12 and FcγR associate with Syk and mediate β-2 integrin signalling in neutrophils and macrophages . Syk phosphorylation also occurs upon engagement of pathogen recognition receptors such as FcγR, CR3 and Dectin-1 . Accumulating evidence points to Syk expression in subsets of T lymphocytes such as thymocytes, naive αβ T cells and intraepithelial γδ T cells, but not in proliferating and mature T cells [31,40].