Farnaz Fallah-Arani

Nonhematopoietic antigen blocks memory programming of alloreactive CD8 + T cells and drives their eventual exhaustion in mouse models of bone marrow transplantation

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.

Langerhans cells regulate cutaneous injury by licensing CD8 effector cells recruited to the skin

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.

The Host Environment Regulates the Function of CD8+ Graft-versus-Host-Reactive Effector Cells

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-γ, 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. Increasing the numbers of T cells transferred to MC led to the enhancement of CTL activity and detectable increases in absolute numbers of IFN-γ+ cells without inducing graft-vs-host disease (GVHD). TLR-induced systemic inflammation accelerated differentiation of functional CTL in MC but was associated with severe GVHD. In the absence of inflammation, both recipient T and non-T cell populations impeded the full development of GVHD-inducing effector function. We conclude that per-cell deficits in the function of donor CD8 cells activated in MC may be overcome by transferring larger numbers of T cells without inducing GVHD.

Specificity for the tumor-associated self-antigen WT1 drives the development of fully functional memory T cells in the absence of vaccination

Recently, vaccines against the Wilms Tumor antigen 1 (WT1) have been tested in cancer patients. However, it is currently not known whether physiologic levels of WT1 expression in stem and progenitor cells of normal tissue result in the deletion or tolerance induction of WT1-specific T cells. Here, we used an human leukocyte antigen-transgenic murine model to study the fate of human leukocyte antigen class-I restricted, WT1-specific T cells in the thymus and in the periphery. Thymocytes expressing a WT1-specific T-cell receptor derived from high avidity human CD8 T cells were positively selected into the single-positive CD8 population. In the periphery, T cells specific for the WT1 antigen differentiated into CD44-high memory phenotype cells, whereas T cells specific for a non-self-viral antigen retained a CD44(low) naive phenotype. Only the WT1-specific T cells, but not the virus-specific T cells, displayed rapid antigen-specific effector function without prior vaccination. Despite long-term persistence of WT1-specific memory T cells, the animals did not develop autoimmunity, and the function of hematopoietic stem and progenitor cells was unimpaired. This is the first demonstration that specificity for a tumor-associated self-antigen may drive differentiation of functionally competent memory T cells.

Cell-intrinsic regulation of murine dendritic cell function and survival by prereceptor amplification of glucocorticoid

Although the inhibitory effects of therapeutic glucocorticoids (GCs) on dendritic cells (DCs) are well established, the roles of endogenous GCs in DC homeostasis are less clear. A critical element regulating endogenous GC concentrations involves local conversion of inactive substrates to active 11-hydroxyglucocorticoids, a reduction reaction catalyzed within the endoplasmic reticulum by an enzyme complex containing 11β-hydroxysteroid dehydrogenase type 1 (11βHSD1) and hexose-6-phosphate dehydrogenase (H6PDH). In this study, we found that this GC amplification pathway operates both constitutively and maximally in steady state murine DC populations and is unaffected by additional inflammatory stimuli. Under physiologic conditions, 11βHSD1-H6PDH increases the sensitivity of plasmacytoid DCs (pDCs) to GC-induced apoptosis and restricts the survival of this population through a cell-intrinsic mechanism. Upon CpG activation, the effects of enzyme activity are overridden, with pDCs becoming resistant to GCs and fully competent to release type I interferon. CD8α(+) DCs are also highly proficient in amplifying GC levels, leading to impaired maturation following toll-like receptor-mediated signaling. Indeed, pharmacologic inhibition of 11βHSD1 synergized with CpG to enhance specific T-cell responses following vaccination targeted to CD8α(+) DCs. In conclusion, amplification of endogenous GCs is a critical cell-autonomous mechanism for regulating the survival and functions of DCs in vivo.