Delayed clinical appearance of GvHD by a period of several weeks follows progressive tissue damage inflicted by inflammation and is frequently associated with infection, which may trigger and intensify GvH and reciprocally, GvH-mediated injury perpetuates infection by disruption of the mucosal barriers. stimulation of alloreactivity by exposure of donor T cells to host antigens and depletion of the reactive responders, a conceptual frame that awards dual selectivity: responsiveness to host antigens of 5-Bromo Brassinin a fraction 5-Bromo Brassinin of donor clones and selective depletion restricted to activated T cells (Figure ?(Figure1).1). Characteristics of T cell activation targeted for selective depletion include fast-cycling (13), sensitivity to fludarabine (14) metabolic mitochondrial activity (15), and photoactivation of synthetic psoralen (16). Superior outcome attained by depletion of the 5-Bromo Brassinin chain CD25 IL-2 receptor (IL-2R) in conjunction with CD69 (17) and CD71 (transferrin receptor) (18) emphasizes phenotypic variability of activated T cells where neither one can be considered as universal marker of activation. IL-2R is an attractive target of activation because internalization of the receptor/ligand complex introduces toxic moieties, such as IL-2R monoclonal antibodies conjugated to ricin and diphtheria toxins (19, 20), and IL-2 fusion proteins encoding apoptotic moieties such as caspase-3 (21). A fundamental characteristic of immune cell activation is upregulation of TNF family receptors rendering them susceptible to negative regulation by activation-induced cell death (AICD), where Fas cross-linking by membrane-bound Fas-ligand (FasL) is the common executioner of apoptosis (22). depletion of host-sensitized donor T cells with agonistic Fas antibodies (23), cross-linking by soluble FasL oligomers (24), and expression of the ligand in dendritic cells (DC) (25) in murine models and human mobilized peripheral blood (MPB) cells (26) has reduced GvHD severity. Open in a separate window Figure 1 Differential time axis and procedures for GvHD prophylaxis. simulation of GvHD by exposure of isolated donor T cells to Rabbit Polyclonal to STK17B irradiated host stimulators followed by depletion of the sensitized T cells, as compared to elimination of apoptosis-sensitive donor T cells in whole grafts without antigen-specific stimulation. All procedures of fractional depletion of host-primed donor T cells have documented significant advantages of add back of insensitive T cells: support engraftment, sustain reactivity against tumors (24), and infections in 5-Bromo Brassinin the early post-transplant period (26), due to persistence of effector/memory cells that are relatively insensitive to AICD (22). However, the main drawback of this technique is the relatively slow sensitization process that requires mixed lymphocyte cultures of ~3?days, imposing laborious isolation of T cells and cryopreservation of progenitors. Because transduction of apoptotic signals is very effective, this approach to GvHD prophylaxis has been improved through increased proficiency of stimulation using non-selective T cell stimulation with CD3 antibodies (23), and DC to amplify antigen presentation (25) and boost T cell proliferation (13). Although GvH simulation by donor T cell sensitization to the host is intuitive, it has been long recognized that cytotoxic T cell assays correlate poorly with GvH reactivity against minor antigens (27), possibly because gradual transition to apoptosis-insensitive effector/memory phenotypes in culture may cause persistent recollection of alloresponses in residual T cells. Early post-transplant administration of cytotoxic agents such as cyclophosphamide may be more effective in concomitant suppression of reciprocal sensitization of donor GvH effectors and host versus graft (HvG) rejection (28). T Cell Depletion Without Host-Specific Sensitization The GvH reaction is effectively prevented, on the one hand, by non-selective depletion of donor T cells using phenotypic markers (9), and on the other hand, by selective depletion of host-primed donor T cells (13C21, 23C26). We reasoned that elimination of apoptosis-sensitive donor T cells without host-specific priming may be effective in GvHD prevention. Exposure of murine splenocytes and bone marrow cells (BMC) to FasL reduced significantly the clinical.