As studies by Klocke showed, collagen-induced arthritis is more severe in mice with adult CTLA-4 deficiency than in wild-type mice [41]. T cell response to antigen. As is now well recognized, T cell activation requires two signals: TCR recognition of antigen and co-stimulation. For the first signal, antigen recognition occurs in the context of MHC molecules on antigen presenting cells (APCs). Co-stimulation occurs between membrane-bound molecules on T cells and APCs, with the interaction of CD28 molecules on T cells with CD80/86 molecules on APCs a key Quarfloxin (CX-3543) event in co-stimulation (Fig.?1) [25, 26]. Open in a separate window Fig. 1 Two-step signalling process for activation of na?ve T cells Antigen presenting cells (APCs) such as dendritic cells (DCs) or B cells present antigen Mapkap1 to T cells via MHC class I or II molecules (signal 1). The co-stimulatory signal occurs with binding of CD80/86 on an APC (A) to the CD28 receptor on the CD25+CD4+ T cell resulting in upregulation of immune responses (signal 2). Alternatively, a co-inhibitory signal can occur with binding of the cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) receptor on the CD25+CD4+ T cell to CD80/86 (B) or binding of PD-1 on the peripheral T cell to PD-L1 or PD-L2 on an APC (B); both pathways result in downregulation of immune responses. Tumour cells can evade immune system recognition via upregulation of PD-L1 or PD-L2 on the tumour cell surface (C) to bind with CD8+ T cells resulting in downregulation Quarfloxin (CX-3543) of immune response. DC: dendritic cell; MHC: major histocompatibility complex. Following activation of T cells, the expression of CTLA-4 is induced. CTLA-4 is expressed on both activated T cells and on a subset of CD25+CD4+ T cells called T-regulatory (T-reg) cells [26]. A member of the immunoglobulin supergene family, CTLA-4 is 30% homologous with CD28; CTLA-4 binds CD80/86 with higher affinity and avidity than CD28. The binding of CTLA-4 by CD80/86 decreases T cell-mediated immune responses by reducing IL-2 and IL-2 receptor expression [27]. Another mechanism by which CTLA-4 can regulate immunity is via its effects on T regulatory (T-reg) cells [28]. While anti-CTLA-4 antibodies are termed checkpoint inhibitors, these agents may have Quarfloxin (CX-3543) other actions that may manifest in certain locales (i.e. tumour microenviroment) and involve other immune cell types [29C31]. Thus, treatment with anti-CTLA-4 can eliminate T-reg cells in a tumour microenvironment via Fc-receptor-mediated interactions. The relationship between a local reduction of T-reg cells and the emergence of irAEs is not clear since this mechanism seems most relevant for an established site of inflammation. While the PD-1CPD-L1 axis also regulates T cells, the outcome is distinct from that of CTLA-4. PD-1 is a member of the immunoglobulin supergene family, with activation of peripheral T cells and B cells inducing its expression. The main action of PD-1 appears to be the maintenance of peripheral tolerance [32]. PD-1 interacts with two ligands in the peripheral tissues: PD-L1 and PD-L2. PD-L1 is expressed on resting B cells, T cells, macrophages and dendritic cells Quarfloxin (CX-3543) [33]. PD-L2 is uncommonly expressed on resting immune cells, but its production can be induced by pro-inflammatory cytokines [33]. Signalling via both CTLA-4 and PD-1 converges on Akt, although the pathways and consequences of antibody inhibition are distinct [34]. Akt is a serine threonine kinase that plays a key role in the regulation of processes such as metabolism, apoptosis and proliferation. For T cells, ligation of CD28 leads to activation of phosphatidylinositol 3-kinase (PI3K) whose products bind to Akt, promoting its phosphorylation. Whereas PD-1 signalling can antagonize PI3K directly, the effects of CTLA-4 occur via the phosphatase called PP2A. As such, anti-CTLA-4 and anti-PD-1 act differently suggesting that combination therapy may lead to more global effects that are not observed with either therapy only; this situation could lead to increased effectiveness.