Differential expression of CCR8 in tumors versus normal tissue allows specific depletion of tumor-infiltrating T regulatory cells by GS-1811, a novel Fc-optimized anti-CCR8 antibody

The presence of T regulatory (Treg) cells in the tumor microenvironment is associated with poor prognosis and resistance to therapies aimed at reactivating anti-tumor immune responses. Therefore, depletion of tumor-infiltrating Tregs is a potential approach to overcome resistance to immunotherapy. However, identifying Treg-specific targets to drive such selective depletion is challenging. CCR8 has recently emerged as one of these potential targets. Here, we describe GS-1811, a novel therapeutic monoclonal antibody that specifically binds to human CCR8 and is designed to selectively deplete tumor-infiltrating Tregs. We validate previous findings showing restricted expression of CCR8 on tumor Tregs, and precisely quantify CCR8 receptor densities on tumor and normal tissue T cell subsets, demonstrating a window for selective depletion of Tregs in the tumor. Importantly, we show that GS-1811 depleting activity is limited to cells expressing CCR8 at levels comparable to tumor-infiltrating Tregs. Targeting CCR8 in mouse tumor models results in robust anti-tumor efficacy, which is dependent on Treg depleting activity, and synergizes with PD-1 inhibition to promote anti-tumor responses in PD-1 resistant models. Our data support clinical development of GS-1811 to target CCR8 in cancer and drive tumor Treg depletion in order to promote anti-tumor immunity.

Factors that influence the thymic selection of CD8αα intraepithelial lymphocytes

Thymocytes bearing αβ T cell receptors (TCRαβ) with high affinity for self-peptide-MHC complexes undergo negative selection or are diverted to alternate T cell lineages, a process termed agonist selection. Among thymocytes bearing TCRs restricted to MHC class I, agonist selection can lead to the development of precursors that can home to the gut and give rise to CD8αα-expressing intraepithelial lymphocytes (CD8αα IELs). The factors that influence the choice between negative selection versus CD8αα IEL development remain largely unknown. Using a synchronized thymic tissue slice model that supports both negative selection and CD8αα IEL development, we show that the affinity threshold for CD8αα IEL development is higher than for negative selection. We also investigate the impact of peptide presenting cells and cytokines, and the migration patterns associated with these alternative cell fates. Our data highlight the roles of TCR affinity and the thymic microenvironments on T cell fate.

Thymic regulatory T cell niche size is dictated by limiting IL-2 from antigen-bearing dendritic cells and feedback competition

The thymic production of regulatory T cells (Treg cells) requires interleukin 2 (IL-2) and agonist T cell antigen receptor (TCR) ligands and is controlled by competition for a limited developmental niche, but the thymic sources of IL-2 and the factors that limit access to the niche are poorly understood. Here we found that IL-2 produced by antigen-bearing dendritic cells (DCs) had a key role in Treg cell development and that existing Treg cells limited new development of Treg cells by competing for IL-2. Our data suggest that antigen-presenting cells (APCs) that can provide both IL-2 and a TCR ligand constitute the thymic niche and that competition by existing Treg cells for a limited supply of IL-2 provides negative feedback for new production of Treg cells.

Complement protein C1q bound to apoptotic cells suppresses human macrophage and dendritic cell-mediated Th17 and Th1 T cell subset proliferation

A complete genetic deficiency of the complement protein C1q results in SLE with nearly 100% penetrance in humans, but the molecular mechanisms responsible for this association have not yet been fully determined. C1q opsonizes ACs for enhanced ingestion by phagocytes, such as Mϕ and iDCs, avoiding the extracellular release of inflammatory DAMPs upon loss of the membrane integrity of the dying cell. We previously showed that human monocyte-derived Mϕ and DCs ingesting autologous, C1q-bound LALs (C1q-polarized Mϕ and C1q-polarized DCs), enhance the production of anti-inflammatory cytokines, and reduce proinflammatory cytokines relative to Mϕ or DC ingesting LAL alone. Here, we show that C1q-polarized Mϕ have elevated PD-L1 and PD-L2 and suppressed surface CD40, and C1q-polarized DCs have higher surface PD-L2 and less CD86 relative to Mϕ or DC ingesting LAL alone, respectively. In an MLR, C1q-polarized Mϕ reduced allogeneic and autologous Th17 and Th1 subset proliferation and demonstrated a trend toward increased Treg proliferation relative to Mϕ ingesting LAL alone. Moreover, relative to DC ingesting AC in the absence of C1q, C1q-polarized DCs decreased autologous Th17 and Th1 proliferation. These data demonstrate that a functional consequence of C1q-polarized Mϕ and DC is the regulation of Teff activation, thereby "sculpting" the adaptive immune system to avoid autoimmunity, while clearing dying cells. It is noteworthy that these studies identify novel target pathways for therapeutic intervention in SLE and other autoimmune diseases.

MHC mismatch results in neural progenitor cell rejection following spinal cord transplantation in a model of viral-induced demyelination

Transplantation of syngeneic neural progenitor cells (NPCs) into mice persistently infected with the JHM strain of mouse hepatitis virus (JHMV) results in enhanced differentiation into oligodendrocyte progenitor cells that is associated with remyelination, axonal sparing, and clinical improvement. Whether allogeneic NPCs are tolerated or induce immune-mediated rejection is controversial and poorly defined under neuroinflammatory demyelinating conditions. We have used the JHMV-induced demyelination model to evaluate the antigenicity of transplanted allogeneic NPCs within the central nervous system (CNS) of mice with established immune-mediated demyelination. Cultured NPCs constitutively expressed the costimulatory molecules CD80/CD86, and IFN-γ treatment induced expression of MHC class I and II antigens. Injection of allogeneic C57BL/6 NPCs (H-2b background) led to a delayed type hypersensitivity response in BALB/c (H-2d background) mice associated with T-cell proliferation and IFN-γ secretion following coculture with allogeneic NPCs. Transplantation of MHC-mismatched NPCs into JHMV-infected mice resulted in increased transcripts encoding the T-cell chemoattractant chemokines CXCL9 and CXCL10 that correlated with increased T-cell infiltration that was associated with NPC rejection. Treatment of MHC-mismatched mice with T-cell subset-specific depleting antibodies increased survival of allogeneic NPCs without affecting commitment to an oligodendrocyte lineage. Collectively, these results show that allogeneic NPCs are antigenic, and T-cells contribute to rejection following transplantation into an inflamed CNS suggesting that immunomodulatory treatments may be necessary to prolong survival of allogeneic cells.

DRAK2 participates in a negative feedback loop to control TGF-β/Smads signaling by binding to type I TGF-β receptor

TGF-β1 is a multifunctional cytokine that mediates diverse biological processes. However, the mechanisms by which the intracellular signals of TGF-β1 are terminated are not well understood. Here, we demonstrate that DRAK2 serves as a TGF-β1-inducible antagonist of TGF-β signaling. TGF-β1 stimulation rapidly induces DRAK2 expression and enhances endogenous interaction of the type I TGF-β receptor with DRAK2, thereby blocking R-Smads recruitment. Depletion of DRAK2 expression markedly augmented the intensity and the extent of TGF-β1 responses. Furthermore, a high level of DRAK2 expression was observed in basal-like and HER2-enriched breast tumors and cell lines, and depletion of DRAK2 expression suppressed the tumorigenic ability of breast cancer cells. Thus, these studies define a function for DRAK2 as an intrinsic intracellular antagonist participating in the negative feedback loop to control TGF-β1 responses, and aberrant expression of DRAK2 increases tumorigenic potential, in part, through the inhibition of TGF-β1 tumor suppressor activity.

Loss of DRAK2 signaling enhances allogeneic transplant survival by limiting effector and memory T cell responses

Here, we demonstrate that loss of DRAK2 signaling significantly promotes the acceptance of allogeneic engraftment in two separate transplant models without promoting generalized immunosuppression. Drak2-/- T cells failed to reject allogeneic tumors, and were defective in rejecting Balb/C allogeneic skin grafts on C57BL6/J recipients. A significant fraction of alloreactive Drak2-/- T cells underwent apoptosis following activation, whereas enforced expression of Bcl-xL in Drak2-/- T cells restored allograft rejection. Formation of allogeneic memory was also greatly hampered in T cells lacking the Drak2 gene. Adoptive transfer of memory T cells from Drak2-/- mice failed to promote the rejection of allogeneic tumors, and such cells led to significantly delayed rejection of skin allografts in the Balb/C->C57BL/6J model. Costimulatory blockade by in vivo administration of Cytotoxic T-Lymphocyte Antigen 4 fusion protein (CTLA4-Ig) synergized with the DRAK2 deficiency and led to long-term allogeneic skin graft acceptance. Overall, these results demonstrate that DRAK2 plays an important role in primary and memory T cell responsiveness to allografts. Because previous studies have demonstrated that a loss of DRAK2 does not negatively impact antiviral immunity, the studies here underscore the potential utility of pharmacological blockade of DRAK2 to achieve transplant maintenance without the imposition of generalized immunosuppression.

Non-apoptotic caspase-8 activation balances T lymphocyte autophagy (82.14)

Apoptosis and autophagy are critical mediators of a functional immune response. FADD and caspase-8 (casp8) were originally discovered to transduce apoptotic signals delivered through TNF family members. Paradoxically, lymphocytes lacking FADD or casp8 function have impaired clonal expansion following antigen receptor crosslinking and succumb to a caspase-independent cell death. We show that T cells lacking FADD or casp8 are subject to hyperactive autophagy. T cell autophagy induces casp8 activation through its interaction with FADD:Atg5:Atg12 complexes. Blockade of autophagic signaling with shRNA knockdown of the autophagy-dependent protein Atg7 leads to a rescue of T cells lacking FADD and casp8. Similarly, inhibition of RIPK1 kinase activity with necrostatin-1 completely restored T cell cycle and survival. We hypothesize that the non-apoptotic casp8 activity serves to dampen the autophagic response of proliferating T cells through direct cleavage of RIPK1 or the casp8 homolog, c-FLIP, which may secondarily inhibit RIPK1-dependent authophagic signaling. While rapidly dividing T cells require autophagy, this negative feedback loop can prevent hyperactive autophagy. Linking apoptotic proteins to this signaling paradigm may serve to prevent an inflammatory response driven by autophagic cell death in lymphocytes during an immune response. (Funded by NIH T32CA09054 to B.D.B. and R01AI63419 to C.M.W.)

The role of Drak2 in T-cells during an alloresponse (141.7)

Drak2 is a serine threonine kinase that has previously been shown to negatively regulate activation of T cells through the T cell receptor. Additionally, T cells lacking Drak2 are hypersensitive to low levels of stimulation, but are surprisingly resistant to autoimmunity in an EAE model. Since it has been hypothesized that Drak2 deficient T-cells die after activation, I tested the ability of these T cells to respond to allogeneic stimulation, which would mimic transplantation from an allogeneic donor. Using in vitro mixed lymphocyte reactions, Drak2-/- CD8+ T-cells responded less to allogeneic stimulation compared to wildtype T-cells when analyzed by CFSE dilution. Additionally, allogeneic tumors were used to determine the role of Drak2 during an in vivo allogeneic transplant. Contrary to in vitro results, Drak2-/- mice were able to clear the allogeneic tumor in vivo. Subsequently, T cells isolated from Drak2-/- mice which had cleared an allogeneic tumor displayed proliferation defects when re-challenged in vitro. In the future adoptive transfers of Drak2-/- T-cells into mice lacking B, T, and NK cells will be done to address the specific role of Drak2 in T cell mediated allorejection.

A Fas-associated death domain protein/caspase-8-signaling axis promotes S-phase entry and maintains S6 kinase activity in T cells responding to IL-2

Fas-associated death domain protein (FADD) constitutes an essential component of TNFR-induced apoptotic signaling. Paradoxically, FADD has also been shown to be crucial for lymphocyte development and activation. In this study, we report that FADD is necessary for long-term maintenance of S6 kinase (S6K) activity. S6 phosphorylation at serines 240 and 244 was only observed after long-term stimulation of wild-type cells, roughly corresponding to the time before S-phase entry, and was poorly induced in T cells expressing a dominantly interfering form of FADD (FADDdd), viral FLIP, or possessing a deficiency in caspase-8. Defects in S6K1 phosphorylation were also observed. However, defective S6K1 phosphorylation was not a consequence of a wholesale defect in mammalian target of rapamycin function, because 4E-BP1 phosphorylation following T cell activation was unaffected by FADDdd expression. Although cyclin D3 up-regulation and retinoblastoma hypophosphorylation occurred normally in FADDdd T cells, cyclin E expression and cyclin-dependent kinase 2 activation were markedly impaired in FADDdd T cells. These results demonstrate that a FADD/caspase-8-signaling axis promotes T cell cycle progression and sustained S6K activity.

NON-APOPTOTIC INDUCTION OF CASPASE-8 IN T-CELLS DEPENDS ON FADD AND CELL CYCLE PROGRESSION (87.1)

Upon triggering of apoptosis by tumor necrosis factor (TNF) receptor family member engagement, FADD (Fas-Associated Death Domain-containing protein) acts as an adaptor protein by recruiting caspase-8 and promoting its autocatalytic activation. Surprisingly, it was found that FADD and caspase-8 are also critical for T lymphocytes proliferation. Indeed, T cell specific overexpression of a FADD dominant-negative mutant (FADDdd) or conditional deletion of caspase-8 or FADD in T cells leads to profound defects in proliferation. While it is known that FADD transmits an apoptotic signal by recruiting caspase-8 within the cytoplasm, it is unclear how FADD and caspase-8 promote T cell proliferation. Since it has been shown that both proteins can localize within the nucleus, we investigated caspase-8 catalytic activity in the nucleus of proliferating T cells and detected an increasing level of caspase-8 activity upon stimulation with anti-CD3 and anti-CD28. Using FADDdd T cells, we determined that this nuclear catalytic activity was dependent on normal FADD function. Blocking cell cycle progression at different stages also drastically reduced caspase-8 activation in non-apoptotic T cells, whereas inhibiting TNF receptor activation did not appreciably diminish caspase-8 activity. We are currently attempting to identify caspase-8 nuclear substrates involved in T-cell proliferation. Taken together, our results suggest that specific subcellular compartmentalization is associated with a distinct magnitude in caspase-8 activity, possibly leading to new substrate selectivity during cell cycle progression. These findings may explain how two apoptotic regulatory proteins, FADD and caspase-8, alternatively promote cell death or proliferation in T cells.

A non-apoptotic FADD/caspase-8 dependent mechanism essential for T cell clonal expansion (87.48)

FADD constitutes an essential component of TNF receptor-induced apoptotic signaling. Paradoxically, FADD has also been shown to be crucial for lymphocyte development and activation. Using mice expressing a T cell specific transgene encoding the death domain of FADD (FADDdd), we have found that FADD participates in T cell clonal expansion. Based on BrdU uptake assays, we have found that FADD signaling is required for transition between the late G1 and S-phase of cell cycle. The defective progression through cycle is marked by a defect in cyclin E expression and CDK2 activation. We have also identified a defect in the maintenance of S6K1 activity and ribosomal S6 phosphorylation, two essential intermediaries of proliferative signaling in T cells. We have found that caspase-8 is essential in this FADD dependent process, suggesting that FADD promotes the assembly of a non-apoptotic caspase-8 complex that serves to relieve inhibition of cell cycle progression during the G1/S transition. In line with this hypothesis, we have found that caspase-8 activity is induced in T cells in a cell-cycle dependent manner in normal T cells. The potential that this signaling axis may modulate autophagy is presently under investigation. Since TNF receptor family members play key roles in regulating adaptive immunity, these results suggest that FADD and caspase-8 serve multiple functions to maintain immune homeostasis.

Modulation of DRAK2 autophosphorylation by antigen receptor signaling in primary lymphocytes

Death-associated protein-related apoptotic kinase-2 (DRAK2), a member of the death-associated protein-like family of serine/threonine kinases, is highly expressed in lymphoid organs and is a negative regulator of T cell activation. To investigate the regulation of DRAK2 activity in primary lymphocytes, we employed mass spectrometry to identify sites of autophosphorylation on DRAK2. These studies have revealed a key site of autophosphorylation on serine 12. Using a phospho-specific antibody to detect Ser(12) phosphorylation, we found that autophosphorylation is induced by antigen receptor stimulation in T and B cells. In Jurkat T cells, resting B cells and thymocytes, DRAK2 was hypophosphorylated on Ser(12) but rapidly phosphorylated with antigen receptor ligation. This increase in phosphorylation was dependent on intracellular calcium mobilization, because BAPTA-AM blocked DRAK2 kinase activity, whereas the SERCA inhibitor thapsigargin promoted Ser(12) phosphorylation. Our results show that DRAK2 kinase activity is regulated in a calcium-dependent manner and that Ser(12) phosphorylation is necessary for optimal suppression of T cell activation by this kinase, suggesting a potential feedback loop may act to modulate the activity of this kinase following antigen receptor signaling.