Engagement of Fcγ-receptors triggers a range of downstream signalling events resulting in a diverse array of immune functions. As a result, blockade of Fc-mediated function is an important strategy for the control of several autoimmune and inflammatory conditions. We have generated a hexameric-Fc fusion protein (hexameric-Fc) and tested the consequences of multi-valent Fcγ-receptor engagement in in vitro and in vivo systems. In vitro engagement of hexameric-Fc with FcγRs showed complex binding interactions that altered with receptor density and triggered the internalisation and degradation of Fcγ-receptors. This caused a disruption of Fc-binding and phagocytosis. In vivo, in a mouse ITP model we observed a short half-life of hexameric-Fc but were nevertheless able to observe inhibition of platelet phagocytosis several days after hexameric-Fc dosing. In cynomolgus monkeys, we again observed a short half-life, but were able to demonstrate effective FcγR blockade. These findings demonstrate the ability of multi-valent Fc-based therapeutics to interfere with FcγR function and a potential mechanism through which they could have a sustained effect; the internalisation and degradation of FcγRs.

Allogeneic blood or BM transplantation (BMT) is the most commonly applied form of adoptive cellular therapy for cancer. In this context, the ability of donor T cells to respond to recipient antigens is coopted to generate graft-versus-tumor (GVT) responses. The major reason for treatment failure is tumor recurrence, which is linked to the eventual loss of functional, host-specific CTLs. In this study, we have explored the role of recipient antigen expression by nonhematopoietic cells in the failure to sustain effective CTL immunity. Using clinically relevant models, we found that nonhematopoietic antigen severely disrupts the formation of donor CD8 + T cell memory at 2 distinct levels that operate in the early and late phases of the response. First, initial and direct encounters between donor CD8 + T cells and nonhematopoietic cells blocked the programming of memory precursors essential for establishing recall immunity. Second, surviving CD8 + T cells became functionally exhausted with heightened expression of the coinhibitory receptor programmed death-1 (PD-1). These 2 factors acted together to induce even more profound failure in long-term immunosurveillance. Crucially, the functions of exhausted CD8 + T cells could be partially restored by late in vivo blockade of the interaction between PD-1 and its ligand, PD-L1, without induction of graft-versus-host disease, suggestive of a potential clinical strategy to prevent or treat relapse following allogeneic BMT.

We have examined how the host environment influences the graft-vs-leukemia (GVL) response following transfer of donor T cells to allogeneic chimeras. Donor T cells induce significant GVL when administered in large numbers to established mixed chimeras (MC). However, when using limiting numbers of T cells, we found that late transfer to MC induced less GVL than did early transfer to freshly irradiated allogeneic recipients. Late donor T cell transfer to MC was associated with marked accumulation of anti-host CD8 cells within the spleen, but delayed kinetics of differentiation, reduced expression of effector molecules including IFN-␥, impaired cytotoxicity, and higher rates of sustained apoptosis. Furthermore, in contrast to the spleen, we observed a significant delay in donor CD8 cell recruitment to the bone marrow, a key location for hematopoietic tumors. donor T cell alloreactivity can be exploited to generate a powerful anti-leukemia effect, a phenomenon termed the graft-vs-leukemia (GVL) response (1, 2). Unfortunately, under conditions in which donor T cells are transferred immediately to lethally irradiated recipients, GVL responses are often associated with the development of graft-vs-host disease (GVHD). Delaying the timing of donor T cell administration by 2-3 wk may reduce the risk of GVHD in full allogeneic chimeras and still permit the induction of GVL, but by 4 -5 wk, graft-vs-host (GVH) reactivity cannot be induced (3, 4). In sharp contrast, the GVL response remains intact when donor T cells are infused (at Ͼ8 wk) into established mixed chimeras (MC), where hematopoietic elements from both the donor and recipient are present at the time of transfer (5-7). This finding relates to the continued presence of large numbers of host hematopoietic APCs, which are required for maximal priming of donor T cells recognizing recipient class I (7) and class II (5) following MHC-mismatched transplantation.Although high numbers of donor T cells activated in MC do not cause GVHD, they do so readily upon transfer to secondary, irradiated allogeneic recipients (8). Moreover, the application of a local or systemic TLR stimulus allows such donor T cells to cause local or systemic GVHD, respectively (8). These studies indicate that donor leukocyte infusion (DLI)-derived GVH-reactive T cell populations activated in MC have no absolute, intrinsic functional defect. Rather, these and other data (9 -14) argue that extrinsic factors such as inflammation within the host environment are critical for the recruitment of activated T cells to the epithelial target tissues and hence the development of GVHD. It is still not known, however, whether antihost CTL arising in established MC or freshly irradiated allogeneic (TBI-allo) recipients are fully equivalent in terms of functional activity and their capacity to induce GVL. Potentially important differences between the two host environments include the duration of direct Ag presentation by professional APC, the levels of lymphopenia-induced proliferation, the extent of suppression m...

OX40 is a member of the TNFR superfamily that has potent costimulatory properties. Although the impact of blockade of the OX40-OX40L pathway has been well documented in models of autoimmune disease, its effect on the rejection of allografts is less well defined. Here we show that the alloantigen-mediated activation of naïve and memory CD4+ T cells results in the induction of OX40 expression and that blockade of OX40-OX40L interactions prevents skin allograft rejection mediated by either subset of T cells. Moreover, a blocking anti-OX40 was found to have no effect on the activation and proliferation of T cells, but rather effector T cells failed to accumulate in peripheral lymph nodes and subsequently migrate to skin allografts. This was found to be the result of an enhanced degree of cell death amongst proliferating effector cells. In clear contrast, blockade of OX40-OX40L interactions at the time of exposure to alloantigen enhanced the ability of regulatory T cells to suppress T cell responses to alloantigen by supporting rather than diminishing regulatory T cell survival. These data show that OX40-OX40L signalling contributes to the evolution of the adaptive immune response to an allograft via the differential control of alloreactive effector and regulatory T cell survival. Moreover, these data serve to further highlight OX40 and OX40L as therapeutic targets to assist the induction of tolerance to allografts and self-antigens.

Langerhans cells (LCs) are a distinct population of dendritic cells that form a contiguous network in the epidermis of the skin. Although LCs possess many of the properties of highly proficient dendritic cells, recent studies have indicated that they are not necessary to initiate cutaneous immunity. In this study, we used a tractable model of cutaneous GVHD, induced by topical application of a Toll-like receptor agonist, to explore the role of LCs in the development of tissue injury. By adapting this model to permit inducible and selective depletion of host LCs, we found that GVHD was significantly reduced when LCs were absent. However, LCs were not required either for CD8 T-cell activation within the draining lymph node or subsequent homing of effector cells to the epidermis. Instead, we found that LCs were necessary for inducing transcription of IFN-γ and other key effector molecules by donor CD8 cells in the epidermis, indicating that they license CD8 cells to induce epithelial injury. These data demonstrate a novel regulatory role for epidermal LCs during the effector phase of an inflammatory immune response in the skin.