Tanner M. Johanns

Regulatory T Cell Suppressive Potency Dictates the Balance between Bacterial Proliferation and Clearance during Persistent Salmonella Infection

The pathogenesis of persistent infection is dictated by the balance between opposing immune activation and suppression signals. Herein, virulent Salmonella was used to explore the role and potential importance of Foxp3-expressing regulatory T cells in dictating the natural progression of persistent bacterial infection. Two distinct phases of persistent Salmonella infection are identified. In the first 3–4 weeks after infection, progressively increasing bacterial burden was associated with delayed effector T cell activation. Reciprocally, at later time points after infection, reductions in bacterial burden were associated with robust effector T cell activation. Using Foxp3 GFP reporter mice for ex vivo isolation of regulatory T cells, we demonstrate that the dichotomy in infection tempo between early and late time points is directly paralleled by drastic changes in Foxp3+ Treg suppressive potency. In complementary experiments using Foxp3 DTR mice, the significance of these shifts in Treg suppressive potency on infection outcome was verified by enumerating the relative impacts of regulatory T cell ablation on bacterial burden and effector T cell activation at early and late time points during persistent Salmonella infection. Moreover, Treg expression of CTLA-4 directly paralleled changes in suppressive potency, and the relative effects of Treg ablation could be largely recapitulated by CTLA-4 in vivo blockade. Together, these results demonstrate that dynamic regulation of Treg suppressive potency dictates the course of persistent bacterial infection.

Glutamate-induced protease-mediated loss of plasma membrane Ca2+ pump activity in rat hippocampal neurons

Ca2+ dysregulation is a hallmark of excitotoxicity, a process that underlies multiple neurodegenerative disorders. The plasma membrane Ca2+ ATPase (PMCA) plays a major role in clearing Ca2+ from the neuronal cytoplasm. Here, we show that the rate of PMCA‐mediated Ca2+ efflux from rat hippocampal neurons decreased following treatment with an excitotoxic concentration of glutamate. PMCA‐mediated Ca2+ extrusion following a brief train of action potentials exhibited an exponential decay with a mean time constant (τ) of 8.8 ± 0.2 s. Four hours following the start of a 30 min treatment with 200 µm glutamate, a second population of cells emerged with slowed recovery kinetics (τ = 16.5 ± 0.3 s). Confocal imaging of cells expressing an enhanced green fluorescent protein (EGFP)‐PMCA4b fusion protein revealed that glutamate treatment internalized EGFP and that cells with reduced plasma membrane fluorescence had impaired Ca2+ clearance. Treatment with inhibitors of the Ca2+‐activated protease calpain protected PMCA function and prevented EGFP‐PMCA internalization. PMCA internalization was triggered by activation of NMDA receptors and was less pronounced for a non‐toxic concentration of glutamate relative to one that produces excitotoxicity. PMCA isoform 2 also internalized following exposure to glutamate, although the Na+/K+ ATPase did not. These data suggest that glutamate exposure initiated protease‐mediated internalization of PMCAs with a corresponding loss of function that may contribute to the Ca2+ dysregulation that accompanies excitotoxicity.

Immunogenomics of Hypermutated Glioblastoma: A Patient with Germline POLE Deficiency Treated with Checkpoint Blockade Immunotherapy

We present the case of a patient with a left frontal glioblastoma with primitive neuroectodermal tumor features and hypermutated genotype in the setting of a POLE germline alteration. During standard-of-care chemoradiation, the patient developed a cervical spine metastasis and was subsequently treated with pembrolizumab. Shortly thereafter, the patient developed an additional metastatic spinal lesion. Using whole-exome DNA sequencing and clonal analysis, we report changes in the subclonal architecture throughout treatment. Furthermore, a persistently high neoantigen load was observed within all tumors. Interestingly, following initiation of pembrolizumab, brisk lymphocyte infiltration was observed in the subsequently resected metastatic spinal lesion and an objective radiographic response was noted in a progressive intracranial lesion, suggestive of active central nervous system (CNS) immunosurveillance following checkpoint blockade therapy. SIGNIFICANCE It is unclear whether hypermutated glioblastomas are susceptible to checkpoint blockade in adults. Herein, we provide proof of principle that glioblastomas with DNA-repair defects treated with checkpoint blockade may result in CNS immune activation, leading to clinically and immunologically significant responses. These patients may represent a genomically stratified group for whom immunotherapy could be considered. Cancer Discov; 6(11); 1230-6. ©2016 AACR.See related commentary by Snyder and Wolchok, p. 1210This article is highlighted in the In This Issue feature, p. 1197.

Selective Priming and Expansion of Antigen-Specific Foxp3−CD4+ T Cells during Listeria monocytogenes Infection

The Foxp3-expressing subset of regulatory CD4+ T cells have defined Ag specificity and play essential roles in maintaining peripheral tolerance by suppressing the activation of self-reactive T cells. Similarly, during chronic infection, pathogen-specific Foxp3-expressing CD4+ T cells expand and actively suppress pathogen-specific effector T cells. Herein, we used MHC class II tetramers and Foxp3gfp knockin mice to track the kinetics and magnitude whereby pathogen-specific Foxp3+CD4+ and Foxp3−CD4+ cells are primed and expand after acute infection with recombinant Listeria monocytogenes (Lm) expressing the non-“self”-Ag 2W1S52–68. We demonstrate that Lm infection selectively primes proliferation, expansion, and subsequent contraction of Lm-specific Foxp3− effector CD4+ cells, while the numbers of Lm-specific Foxp3+CD4+ regulatory cells remain essentially unchanged. In sharp contrast, purified 2W1S52–68 peptide primes coordinated expansion of both Foxp3+ regulatory and Foxp3− effector T cells with the same Ag specificity. Taken together, these results indicate selective priming and expansion of Foxp3− CD4 T cells is a distinguishing feature for acute bacterial infection.

Restricted STAT5 activation dictates appropriate thymic B versus T cell lineage commitment.

The molecular mechanisms regulating lymphocyte lineage commitment remain poorly characterized. To explore the role of the IL7R in this process, we generated transgenic mice that express a constitutively active form of STAT5 (STAT5b-CA), a key downstream IL7R effector, throughout lymphocyte development. STAT5b-CA mice exhibit a 40-fold increase in pro-B cells in the thymus. As documented by BrdU labeling studies, this increase is not due to enhanced B cell proliferation. Thymic pro-B cells in STAT5b-CA mice show a modest increase in cell survival (∼4-fold), which correlates with bcl-xL expression. However, bcl-xL transgenic mice do not show increases in thymic B cell numbers. Thus, STAT5-dependent bcl-xL up-regulation and enhanced B cell survival are not sufficient to drive the thymic B cell development observed in STAT5b-CA mice. Importantly, thymic pro-B cells in STAT5b-CA mice are derived from early T cell progenitors (ETPs), suggesting that STAT5 acts by altering ETP lineage commitment. Supporting this hypothesis, STAT5 binds to the pax5 promoter in ETPs from STAT5b-CA mice and induces pax5, a master regulator of B cell development. Conversely, STAT5b-CA mice exhibit a decrease in the DN1b subset of ETPs, demonstrating that STAT5 activation inhibits early T cell differentiation or lineage commitment. On the basis of these findings, we propose that the observed expression of the IL-7R on common lymphoid progenitors, but not ETPs, results in differential STAT5 signaling within these distinct progenitor populations and thus helps ensure appropriate development of B cells and T cells in the bone marrow and thymic environments, respectively.

PDL-1 Blockade Impedes T Cell Expansion and Protective Immunity Primed by Attenuated Listeria monocytogenes

Infection with attenuated Listeria monocytogenes (Lm) is a robust in vivo model for examining how Ag-specific T cells are primed, and subsequent challenge with virulent Lm allows for the protective effects of T cell priming to be quantified. Herein, we investigated the role of programmed death ligand 1 (PDL-1) in T cell priming and immunity conferred after primary infection with Lm ΔactA followed by virulent Lm challenge. In striking contrast to the inhibitory role of PDL-1 on T cell immunity in other infection models, marked reductions in the magnitude of T cell expansion and the kinetics of T cell proliferation were observed with PDL-1 blockade after primary Lm ΔactA infection. More importantly, PDL-1 blockade beginning before primary infection and maintained throughout the experiment resulted in delayed bacterial clearance and T cell expansion after secondary challenge with virulent Lm. These results indicate that for immunity to intracellular bacterial infection, PDL-1 plays an important stimulatory role for priming and expansion of protective T cells.

Activation of Protein Kinase C in Sensory Neurons Accelerates Ca2+ Uptake into the Endoplasmic Reticulum

The rate of Ca2+ clearance from the neuronal cytoplasm affects the amplitude, duration, and localization of Ca2+ signals and influences a variety of Ca2+-dependent functions. We reported previously that activation of protein kinase C (PKC) accelerates Ca2+ efflux in rat sensory neurons mediated by the plasma membrane Ca2+-ATPase isoform 4 (PMCA4). Here we show that sarco-endoplasmic reticulum Ca2+-ATPase (SERCA)-mediated Ca2+ uptake into intracellular stores is also accelerated by PKC activation. The rate of intracellular Ca2+ concentration ([Ca2+]i) clearance was studied after small (<350 nm) action potential-induced Ca2+ loads in rat dorsal root ganglion neurons. Under these conditions, mitochondrial Ca2+ uptake and Na+/Ca2+ exchange do not significantly influence [Ca2+]i recovery. Phorbol dibutyrate (PDBu) increased the rate of [Ca2+]i clearance by 71% in a manner sensitive to the selective PKC inhibitors GF109203x (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide) and calphostin. PKC-dependent acceleration was still observed (∼39%) when the PKC-sensitive PMCA isoform was knocked down by expression of an antisense PMCA4 cDNA (AS4). Direct measurement of Ca2+ in the endoplasmic reticulum (ER) lumen revealed that PKC increased the rate of store refilling more than twofold after depletion by treatment with cyclopiazonic acid. ER refilling was less complete in PDBu-treated cells, although, in AS4-expressing cells, PDBu accelerated the rate without reducing the ER capacity, suggesting that PMCA and SERCA compete for Ca2+. Thus, activation of PKC accelerates the clearance of Ca2+ from the cytoplasm by the concerted stimulation of Ca2+ sequestration and Ca2+ efflux.

Selective culling of high avidity antigen-specific CD4+ T cells after virulent Salmonella infection

Typhoid fever is a persistent infection caused by host‐adapted Salmonella strains adept at circumventing immune‐mediated host defences. Given the importance of T cells in protection, the culling of activated CD4+ T cells after primary infection has been proposed as a potential immune evasion strategy used by this pathogen. We demonstrate that the purging of activated antigen‐specific CD4+ T cells after virulent Salmonella infection requires SPI‐2 encoded virulence determinants, and is not restricted only to cells with specificity to Salmonella‐expressed antigens, but extends to CD4+ T cells primed to expand by co‐infection with recombinant Listeria monocytogenes. Unexpectedly, however, the loss of activated CD4+ T cells during Salmonella infection demonstrated using a monoclonal population of adoptively transferred CD4+ T cells was not reproduced among the endogenous repertoire of antigen‐specific CD4+ T cells identified with MHC class II tetramer. Analysis of T‐cell receptor variable segment usage revealed the selective loss and reciprocal enrichment of defined CD4+ T‐cell subsets after Salmonella co‐infection that is associated with the purging of antigen‐specific cells with the highest intensity of tetramer staining. Hence, virulent Salmonella triggers the selective culling of high avidity activated CD4+ T‐cell subsets, which re‐shapes the repertoire of antigen‐specific T cells that persist later after infection.

Endogenous Neoantigen-specific CD8 T Cells Identified in Two Glioblastoma Models Using a Cancer Immunogenomics Approach

Immunogenomics were used to identify tumor-specific neoantigens in two well characterized models of glioblastoma. Endogenous immune responses harbored neoantigen-specific T cells within the brain and lymph nodes, providing a tractable system for additional preclinical immunotherapeutic studies in these systems. The “cancer immunogenomics” paradigm has facilitated the search for tumor-specific antigens over the last 4 years by applying comprehensive cancer genomics to tumor antigen discovery. We applied this methodology to identify tumor-specific “neoantigens” in the C57BL/6-derived GL261 and VM/Dk-derived SMA-560 tumor models. Following DNA whole-exome and RNA sequencing, high-affinity candidate neoepitopes were predicted and screened for immunogenicity by ELISPOT and tetramer analyses. GL261 and SMA-560 harbored 4,932 and 2,171 nonsynonymous exome mutations, respectively, of which less than half were expressed. To establish the immunogenicities of H-2Kb and H-2Db candidate neoantigens, we assessed the ability of the epitopes predicted in silico to be the highest affinity binders to activate tumor-infiltrating T cells harvested from GL261 and SMA-560 tumors. Using IFNγ ELISPOT, we confirmed H-2Db–restricted Imp3D81N (GL261) and Odc1Q129L (SMA-560) along with H-2Kb–restricted E2f8K272R (SMA-560) as endogenous tumor-specific neoantigens that are functionally immunogenic. Furthermore, neoantigen-specific T cells to Imp3D81N and Odc1Q129L were detected within intracranial tumors as well as cervical draining lymph nodes by tetramer analysis. By establishing the immunogenicities of predicted high-affinity neoepitopes in these models, we extend the immunogenomics-based neoantigen discovery pipeline to glioblastoma models and provide a tractable system to further study the mechanism of action of T cell–activating immunotherapeutic approaches in preclinical models of glioblastoma. Cancer Immunol Res; 4(12); 1007–15. ©2016 AACR.

Early eradication of persistent Salmonella infection primes antibody-mediated protective immunity to recurrent infection

Typhoid fever is a systemic, persistent infection caused by host-specific strains of Salmonella. Although the use of antibiotics has reduced the complications associated with primary infection, recurrent infection remains an important cause of ongoing human morbidity and mortality. Herein, we investigated the impacts of antibiotic eradication of primary infection on protection against secondary recurrent infection. Using a murine model of persistent Salmonella infection, we demonstrate protection against recurrent infection is sustained despite early eradication of primary infection. In this model, protection is not mediated by CD4(+) or CD8(+) T cells because depletion of these cells either alone or in combination prior to rechallenge does not abrogate protection. Instead, infection followed by antibiotic-mediated clearance primes robust levels of Salmonella-specific antibody that can adoptively transfer protection to naïve mice. Thus, eradication of persistent Salmonella infection primes antibody-mediated protective immunity to recurrent infection.