The CD80/CD86:CD28/CTLA-4 (cytotoxic T lymphocyte-associated antigen 4) pathway is the best-characterized inhibitory pathway for T-cell activation [58, 59]. CD28 is constitutively expressed on naïve and activated T cells. CD80 is expressed at low levels on resting

antigen-presenting cells (APCs) and is upregulated with prolonged interaction with T cells, whereas CD86 is constitutively expressed and rapidly upregulated on APCs. Thus, CD86 is likely to be mainly involved in mediating initial T-cell activation, while CD80 may play an important role in propagating the immune responses. After activation, T cells express CTLA-4 (CD152). Engagement of CTLA-4 delivers BTK inhibitor negative signal into T cells, resulting in inhibition and/or termination of T-cell responses. Taking advantage of the fact that CTLA-4

binds CD80 and CD86 with much higher affinity than CD28 does, a fusion protein consisting of the extracellular domain of Vemurafenib research buy CTLA-4 and the constant region of IgG (CTLA-Ig) has been developed to block the interaction between CD80-CD86 and CD28 and thereby inhibit T-cell activation [39]. Such a fusion protein would preferentially inhibit lymphocytes that are in the process of responding to self-antigens without affecting resting T cells that recognize other antigens. After the encouraging results of in vivo studies in animal models, including PBC models [60], the efficacy of the CTLA-4 Ig (Abatacept) has been examined in patients with autoimmune diseases. Abatacept has shown efficacy in a broad spectrum of RA patients from early stage to refractory

diseases that are resistant to TNF blockers [61, 62] and in patients with psoriasis in a phase I trial [63]. Blockade of costimulation between T cells and APCs through CD80 could represent an important therapeutic FER approach for the treatment of refractory PBC. TNF-α is an activating factor for a number of intracellular pathways that determine the fate of hepatocytes, and thus plays a key role in liver homeostasis [64]. Interactions between specific members of the TNF pathway lead to the induction of apoptosis as well as the activation of NF-κB signaling, which is antiapoptotic and proinflammatory [65]. GWAS in PBC identified three loci containing genes in TNF-α signaling pathways: TNFRSF1A, DENND1B [21], and TNFAIP2 [21, 22]. TNFRSF1A is one of two receptors for TNF-α; TNFRSF1A−/− mice show attenuated liver fibrosis when compared with wild-type mice after administration of a potent hepatotoxin [66]. DENND1B interacts directly with TNFRSF1A [67] and has previously been associated with asthma [68]. TNF-α signaling also directly induces TNFAIP2 expression [69]. Macrophages from PBC patients, when stimulated with apoptotic bodies from cholangiocytes, produce high levels of TNF-α [70]. Furthermore, serum levels of TNF-α reflect the severity of morphological liver changes in PBC [71].