Immune profiling of age and adjuvant-specific activation of human blood mononuclear cells in vitro.

Vaccination reduces morbidity and mortality due to infections, but efficacy may be limited due to distinct immunogenicity at the extremes of age. This raises the possibility of employing adjuvants to enhance immunogenicity and protection. Early IFNγ production is a hallmark of effective vaccine immunogenicity in adults serving as a biomarker that may predict effective adjuvanticity. We utilized mass cytometry (CyTOF) to dissect the source of adjuvant-induced cytokine production in human blood mononuclear cells (BMCs) from newborns (~39-week-gestation), adults (~18-63 years old) and elders (>65 years of age) after stimulation with pattern recognition receptors agonist (PRRa) adjuvants. Dimensionality reduction analysis of CyTOF data mapped the BMC compartment, elucidated age-specific immune responses and profiled PRR-mediated activation of monocytes and DCs upon adjuvant stimulation. Furthermore, we demonstrated PRRa adjuvants mediated innate IFNγ induction and mapped NK cells as the key source of TLR7/8 agonist (TLR7/8a) specific innate IFNγ responses. Hierarchical clustering analysis revealed age and TLR7/8a-specific accumulation of innate IFNγ producing γδ T cells. Our study demonstrates the application of mass cytometry and cutting-edge computational approaches to characterize immune responses across immunologically distinct age groups and may inform identification of the bespoke adjuvantation systems tailored to enhance immunity in distinct vulnerable populations.

Mitogenic and progenitor gene programmes in single pilocytic astrocytoma cells.

Pilocytic astrocytoma (PA), the most common childhood brain tumor, is a low-grade glioma with a single driver BRAF rearrangement. Here, we perform scRNAseq in six PAs using methods that enabled detection of the rearrangement. When compared to higher-grade gliomas, a strikingly higher proportion of the PA cancer cells exhibit a differentiated, astrocyte-like phenotype. A smaller proportion of cells exhibit a progenitor-like phenotype with evidence of proliferation. These express a mitogen-activated protein kinase (MAPK) programme that was absent from higher-grade gliomas. Immune cells, especially microglia, comprise 40% of all cells in the PAs and account for differences in bulk expression profiles between tumor locations and subtypes. These data indicate that MAPK signaling is restricted to relatively undifferentiated cancer cells in PA, with implications for investigational therapies directed at this pathway.

Development of an 8-color antibody panel for functional phenotyping of human CD8+ cytotoxic T cells from peripheral blood mononuclear cells.

The study of CD8 positive cells in peripheral blood has become an essential part of research in the field of cancer immunotherapies, vaccine development, inflammation, autoimmune disease, etc. In this study, an 8-color flow cytometry panel, containing lineage and functional markers, was developed for the identification of CD8+ cytotoxic T cells in previously cryopreserved peripheral blood mononuclear cells from healthy human donors. By studying functional markers in naïve and CD3/CD28 activated T cells we demonstrate that the panel is capable of detecting protein markers corresponding to different T cell activation statuses. Data generated by flow cytometry were corroborated by different antibody based assay technologies to detect soluble cytokines. Our findings suggest that there is an inter donor variability in both baseline and activation responses. We have also successfully developed an antibody panel for flow cytometry that could be used to study cytotoxic function of CD8 T cells in clinical immunology research areas.

Characterization of midostaurin as a dual inhibitor of FLT3 and SYK and potentiation of FLT3 inhibition against FLT3-ITD-driven leukemia harboring activated SYK kinase.

Oncogenic FLT3 kinase is a clinically validated target in acute myeloid leukemia (AML), and both multi-targeted and selective FLT3 inhibitors have been developed. Spleen tyrosine kinase (SYK) has been shown to be activated and increased in FLT3-ITD-positive AML patients, and has further been shown to be critical for transformation and maintenance of the leukemic clone in these patients. Further, over-expression of constitutively activated SYK causes resistance to highly selective FLT3 tyrosine kinase inhibitors (TKI). Up to now, the activity of the multi-targeted FLT3 inhibitor, midostaurin, against cells expressing activated SYK has not been explored in the context of leukemia, although SYK has been identified as a target of midostaurin in systemic mastocytosis. We compared the ability of midostaurin to inhibit activated SYK in mutant FLT3-positive AML cells with that of inhibitors displaying dual SYK/FLT3 inhibition, targeted SYK inhibition, and targeted FLT3 inhibition. Our findings suggest that dual FLT3/SYK inhibitors and FLT3-targeted drugs potently kill oncogenic FLT3-transformed cells, while SYK-targeted small molecule inhibition displays minimal activity. However, midostaurin and other dual FLT3/SYK inhibitors display superior anti-proliferative activity when compared to targeted FLT3 inhibitors, such as crenolanib and quizartinib, against cells co-expressing FLT3-ITD and constitutively activated SYK-TEL. Interestingly, additional SYK suppression potentiated the effects of dual FLT3/SYK inhibitors and targeted FLT3 inhibitors against FLT3-ITD-driven leukemia, both in the absence and presence of activated SYK. Taken together, our findings have important implications for the design of drug combination studies in mutant FLT3-positive patients and for the design of future generations of FLT3 inhibitors.

Myeloma-specific multiple peptides able to generate cytotoxic T lymphocytes: a potential therapeutic application in multiple myeloma and other plasma cell disorders.

PURPOSE: The efficacy of peptide vaccines may be enhanced by stimulating immune cells with multiple peptides derived from distinct tumor-associated antigens. We have evaluated the heteroclitic XBP1-US(184-192) (YISPWILAV), heteroclitic XBP1-SP(367-375) (YLFPQLISV), native CD138(260-268) (GLVGLIFAV), and native CS1(239-247) (SLFVLGLFL) peptides, which have strong HLA-A2 affinity and immunogenicity in combination, for their ability to elicit multiple myeloma antigen-specific responses. EXPERIMENTAL DESIGN: Multipeptide-specific cytotoxic T lymphocytes (MP-CTL) were generated by the stimulation of CD3(+) T lymphocytes from HLA-A2(+) individuals with either autologous mature dendritic cells or T2 cells pulsed with a cocktail of these four peptides. RESULTS: The peptide cocktail did not compromise tumor antigen-specific activity of CTLs. MP-CTLs displayed increased total, effector memory (CCR7(-)CD45RO(+)), and activated (CD69(+)) CD3(+)CD8(+) T lymphocytes. In addition, MP-CTL showed IFN-γ production, cell proliferation, and cytotoxicity against HLA-A2(+) multiple myeloma cells, including cells of HLA-A2(+) patients with multiple myeloma. Importantly, MP-CTLs showed specific responses in functional assays to each relevant peptide but not to an irrelevant HLA-A2-specific CMV pp65 (NLVPMVATV) peptide. CONCLUSIONS: These results highlight the potential therapeutic application of vaccination with a cocktail of HLA-A2-specific peptides to induce CTLs with a broad spectrum of immune responses against multiple myeloma antigens.

A novel immunogenic CS1-specific peptide inducing antigen-specific cytotoxic T lymphocytes targeting multiple myeloma.

The CS1 antigen provides a unique target for the development of an immunotherapeutic strategy to treat patients with multiple myeloma (MM). This study aimed to identify HLA-A2(+) immunogenic peptides from the CS1 antigen, which induce peptide-specific cytotoxic T lymphocytes (CTL) against HLA-A2(+) MM cells. We identified a novel immunogenic HLA-A2-specific CS1(239-247) (SLFVLGLFL) peptide, which induced CS1-specific CTL (CS1-CTL) to MM cells. The CS1-CTL showed a distinct phenotype, with an increased percentage of effector memory and activated CTL and a decreased percentage of naïve CTL. CS1(239-247) peptide-specific CD8(+) T cells were detected by DimerX analyses and demonstrated functional activities specific to the peptide. The CTL displayed HLA-A2-restricted and antigen-specific cytotoxicity, proliferation, degranulation and γ-interferon (IFN-γ) production against both primary MM cells and MM cell lines. In addition, the effector memory cells subset (CD45RO(+) CCR7(-) /CD3(+) CD8(+) ) within CS1-CTL showed a higher level of CD107a degranulation and IFN-γ production as compared to effector cells (CD45RO(-) CCR7(-) /CD3(+) CD8(+) ) against HLA-A2(+) primary MM cells or MM cell lines. In conclusion, this study introduced a novel immunogenic HLA-A2-specific CS1(239-247) peptide capable of inducing antigen-specific CTL against MM cells that will provide a framework for its application as a novel MM immunotherapy.

Reversible resistance induced by FLT3 inhibition: a novel resistance mechanism in mutant FLT3-expressing cells.

OBJECTIVES: Clinical responses achieved with FLT3 kinase inhibitors in acute myeloid leukemia (AML) are typically transient and partial. Thus, there is a need for identification of molecular mechanisms of clinical resistance to these drugs. In response, we characterized MOLM13 AML cell lines made resistant to two structurally-independent FLT3 inhibitors. METHODS: MOLM13 cells were made drug resistant via prolonged exposure to midostaurin and HG-7-85-01, respectively. Cell proliferation was determined by Trypan blue exclusion. Protein expression was assessed by immunoblotting, immunoprecipitation, and flow cytometry. Cycloheximide was used to determine protein half-life. RT-PCR was performed to determine FLT3 mRNA levels, and FISH analysis was performed to determine FLT3 gene expression. RESULTS AND CONCLUSIONS: We found that MOLM13 cells readily developed cross-resistance when exposed to either midostaurin or HG-7-85-01. Resistance in both lines was associated with dramatically elevated levels of cell surface FLT3 and elevated levels of phosphor-MAPK, but not phospho-STAT5. The increase in FLT3-ITD expression was at least in part due to reduced turnover of the receptor, with prolonged half-life. Importantly, the drug-resistant phenotype could be rapidly reversed upon withdrawal of either inhibitor. Consistent with this phenotype, no significant evidence of FLT3 gene amplification, kinase domain mutations, or elevated levels of mRNA was observed, suggesting that protein turnover may be part of an auto-regulatory pathway initiated by FLT3 kinase activity. Interestingly, FLT3 inhibitor resistance also correlated with resistance to cytosine arabinoside. Over-expression of FLT3 protein in response to kinase inhibitors may be part of a novel mechanism that could contribute to clinical resistance.

Anti-tumor activity and signaling events triggered by the isothiocyanates, sulforaphane and phenethyl isothiocyanate, in multiple myeloma.

BACKGROUND: Isothiocyanates, a family of phytochemicals found in cruciferous vegetables, have cytotoxic effects against several types of tumor cells. Multiple myeloma is a fatal disease characterized by clonal proliferation of plasma cells in the bone marrow. The growing body of preclinical information on the anti-cancer activity of isothiocyanates led us to investigate their anti-myeloma properties. DESIGN AND METHODS: We evaluated the anti-myeloma activity of the isothiocyanates, sulforaphane and phenethyl isothiocyanate, on a panel of human myeloma cell lines as well as primary myeloma tumor cells. Cell viability, apoptosis, cell cycle alterations and cell proliferation were then analyzed in vitro and in a xenograft mouse model in vivo. The molecular sequelae of isothiocyanate treatment in multiple myeloma cells were evaluated by multiplex analyses using bead arrays and western blotting. RESULTS: We observed that sulforaphane and phenylethyl isothiocyanate have activity against myeloma cell lines and patients' myeloma cells both in vitro and in vivo using a myeloma xenograft mouse model. Isothiocyanates induced apoptotic death of myeloma cells; depletion of mitochondrial membrane potential; cleavage of PARP and caspases-3 and -9; as well as down-regulation of anti-apoptotic proteins including Mcl-1, X-IAP, c-IAP and survivin. Isothiocyanates induced G(2)/M cell cycle arrest accompanied by mitotic phosphorylation of histone H3. Multiplex analysis of phosphorylation of diverse components of signaling cascades revealed changes in MAPK activation; increased phosphorylation of c-jun and HSP27; as well as changes in the phosphorylation of Akt, and GSK3α/ß and p53. Isothiocyanates suppressed proliferation of myeloma cells alone and when co-cultured with HS-5 stromal cells. Sulforaphane and phenylethyl isothiocyanate enhanced the in vitro anti-myeloma activity of several conventional and novel therapies used in multiple myeloma. CONCLUSIONS: Our study shows that isothiocyanates have potent anti-myeloma activities and may enhance the activity of other anti-multiple myeloma agents. These results indicate that isothiocyanates may have therapeutic potential in multiple myeloma and provide the preclinical framework for future clinical studies of isothiocyanates in multiple myeloma.

Lenalidomide targets clonogenic side population in multiple myeloma: pathophysiologic and clinical implications.

Recurrence of multiple myeloma (MM) after therapy suggests the presence of tumor-initiating subpopulations. In our study, we performed flow cytometry-based Hoechst 33342 staining to evaluate the existence of a MM population with stem-like features known as side population (SP) cells. SP cells exhibit substantial heterogeneity in MM cell lines and primary MM cells; express CD138 antigen in MM cell lines; display higher mRNA expression and functional activity of ABCG2 transporter; and have a higher proliferation index compared with non-SP cells. We observed evidence for clonogenic potential of SP cells, as well as the ability of SP cells to regenerate original population. Moreover, SP cells revealed higher tumorigenicity compared with non-SP cells. Importantly, lenalidomide decreased the percentage and clonogenicity of SP cells, and also induced phosphorylation changes in Akt, GSK-3α/ß, MEK1, c-Jun, p53, and p70S6K in SP cells. Adherence to bone marrow stromal cells (BMSCs) increased the percentage, viability, and proliferation potential of SP cells. Lenalidomide and thalidomide abrogated this stimulatory effect of BMSCs and significantly decreased the percentage of SP cells. Our studies demonstrate a novel mechanism of action for lenalidomide, namely targeting SP fraction, providing the framework for new therapeutic strategies targeting subpopulations of MM cells including presumptive stem cells.

Elevated IL-17 produced by TH17 cells promotes myeloma cell growth and inhibits immune function in multiple myeloma.

Elevated cytokines in bone marrow (BM) micro-environment (interleukin-6 [IL-6], transforming growth factor-beta [TGF-beta], and IL-1beta) may play an important role in observed immune dysfunction in multiple myeloma (MM). As IL-6 and TGF-beta are important for the generation of T-helper 17 (T(H)17) cells, we evaluated and observed a significantly elevated baseline and induced frequency of T(h)17 cells in peripheral blood mononuclear cells (PBMCs) and BM mononuclear cells (BMMCs) from MM patients compared with healthy donors. We observed significant increase in levels of serum IL-17, IL-21, IL-22, and IL-23 in blood and BM in MM compared with healthy donors. We also observed that myeloma PBMCs after T(H)17 polarization significantly induced IL-1alpha, IL-13, IL-17, and IL-23 production compared with healthy donor PBMCs. We next observed that IL-17 promotes myeloma cell growth and colony formation via IL-17 receptor, adhesion to bone marrow stromal cells (BMSCs) as well as increased growth in vivo in murine xenograft model of human MM. Additionally, we have observed that combination of IL-17 and IL-22 significantly inhibited the production of T(H)1-mediated cytokines, including interferon-gamma (IFN-gamma), by healthy donor PBMCs. In conclusion, IL-17-producing T(h)17 cells play an important role in MM pathobiology and may be an important therapeutic target for anti-MM activity and to improve immune function.

Antileukemic effects of the novel, mutant FLT3 inhibitor NVP-AST487: effects on PKC412-sensitive and -resistant FLT3-expressing cells.

An attractive target for therapeutic intervention is constitutively activated, mutant FLT3, which is expressed in a subpopulation of patients with acute myelocyic leukemia (AML) and is generally a poor prognostic indicator in patients under the age of 65 years. PKC412 is one of several mutant FLT3 inhibitors that is undergoing clinical testing, and which is currently in late-stage clinical trials. However, the discovery of drug-resistant leukemic blast cells in PKC412-treated patients with AML has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to override drug resistance. Here, we report the potent and selective antiproliferative effects of the novel mutant FLT3 inhibitor NVP-AST487 on primary patient cells and cell lines expressing FLT3-ITD or FLT3 kinase domain point mutants. NVP-AST487, which selectively targets mutant FLT3 protein kinase activity, is also shown to override PKC412 resistance in vitro, and has significant antileukemic activity in an in vivo model of FLT3-ITD(+) leukemia. Finally, the combination of NVP-AST487 with standard chemotherapeutic agents leads to enhanced inhibition of proliferation of mutant FLT3-expressing cells. Thus, we present a novel class of FLT3 inhibitors that displays high selectivity and potency toward FLT3 as a molecular target, and which could potentially be used to override drug resistance in AML.

SYK-dependent tonic B-cell receptor signaling is a rational treatment target in diffuse large B-cell lymphoma.

The role of B-cell receptor (BCR)-mediated survival signals in diffuse large B-cell lymphoma (DLBCL) remains undefined. Ligand-induced BCR signaling induces receptor oligomerization, Igalpha/beta immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation, and activation of the spleen tyrosine kinase (SYK), which initiates downstream events and amplifies the initial BCR signal. BCRs also transmit low-level tonic survival signals in the absence of receptor engagement. Herein, we assess the role of SYK-dependent tonic BCR survival signals in DLBCL cell lines and primary tumors and evaluate the efficacy of an ATP-competitive inhibitor of SYK, R406, in vitro. R406 induced apoptosis of the majority of examined DLBCL cell lines. In R406-sensitive DLBCL cell lines, R406 specifically inhibited both tonic- and ligand-induced BCR signaling (autophosphorylation of SYK525/526 and SYK-dependent phosphorylation of the B-cell linker protein [BLNK]). The majority of examined primary DLBCLs also exhibited tonic- and ligand-induced BCR signaling; in these primary tumors, BCR signaling was also inhibited by R406. Of note, BCR-dependent and R406-sensitive DLBCL cell lines were independently identified as "BCR-type" tumors by transcriptional profiling. Therefore, SYK-dependent tonic BCR signaling is an important and potentially targetable survival pathway in some, but not all, DLBCLs. In addition, R406-sensitive DLBCLs can be identified by their transcriptional profiles.

Protein kinase C inhibitor enzastaurin induces in vitro and in vivo antitumor activity in Waldenstrom macroglobulinemia.

Waldenström macroglobulinemia (WM) is an incurable lymphoplasmacytic lymphoma with limited options of therapy. Protein kinase Cbeta (PKCbeta) regulates cell survival and growth in many B-cell malignancies. In this study, we demonstrate up-regulation of PKCbeta protein in WM using protein array techniques and immunohistochemistry. Enzastaurin, a PKCbeta inhibitor, blocked PKCbeta activity and induced a significant decrease of proliferation at 48 hours in WM cell lines (IC(50), 2.5-10 muM). Similar effects were demonstrated in primary CD19(+) WM cells, without cytotoxicity on peripheral blood mononuclear cells. In addition, enzastaurin overcame tumor cell growth induced by coculture of WM cells with bone marrow stromal cells. Enzastaurin induced dose-dependent apoptosis at 48 hours mediated via induction of caspase-3, caspase-8, caspase-9, and PARP cleavage. Enzastaurin inhibited Akt phosphorylation and Akt kinase activity, as well as downstream p-MARCKS and ribosomal p-S6. Furthermore, enzastaurin demonstrated additive cytotoxicity in combination with bortezomib, and synergistic cytotoxicity in combination with fludarabine. Finally, in an in vivo xenograft model of human WM, significant inhibition of tumor growth was observed in the enzastaurin-treated mice (P = .028). Our studies therefore show that enzastaurin has significant antitumor activity in WM both in vitro and in vivo, providing the framework for clinical trials to improve patient outcome in WM.

Dysfunctional T regulatory cells in multiple myeloma.

Multiple myeloma (MM) is characterized by the production of monoclonal immunoglobulin and is associated with suppressed uninvolved immunoglobulins and dysfunctional T-cell responses. The biologic basis of this dysfunction remains ill defined. Because T regulatory (T(reg)) cells play an important role in suppressing normal immune responses, we evaluated the potential role of T(reg) cells in immune dysfunction in MM. We observed a significant increase in CD4+ CD25+ T cells in patients with monoclonal gammopathy of undetermined significance (MGUS) and in patients with MM compared with healthy donors (25% and 26%, respectively, vs 14%); however, T(reg) cells as measured by FOXP3 expression are significantly decreased in patients with MGUS and MM compared with healthy donors. Moreover, even when they are added in higher proportions, T(reg) cells in patients with MM and MGUS are unable to suppress anti-CD3-mediated T-cell proliferation. This decreased number and function of T(reg) cells in MGUS and in MM may account, at least in part, for the nonspecific increase in CD4+ CD25+ T cells, thereby contributing to dysfunctional T-cell responses.

Poliovirus replication and spread in primary neuron cultures.

While some neurotropic viruses cause rapid central nervous system (CNS) disease upon entry into the brain parenchyma, other viruses that are cytolytic in the periphery either result in little neuropathology or are associated with a protracted course of CNS disease consistent with persistent infection. One such virus, poliovirus (PV), is an extremely lytic RNA virus that requires the expression of CD155, the poliovirus receptor (PVR), for infection. To compare the kinetics of PV infection in neuronal and non-neuronal cell types, primary hippocampal neurons and fibroblasts were isolated from CD155+ transgenic embryos and infected with the Mahoney and Sabin strains of PV. Despite similar levels of infection in these ex vivo cultures, PV-infected neurons produced 100-fold fewer infectious particles as compared to fibroblasts throughout infection, and death of PV-infected neurons was delayed approximately 48 h. Spread in neurons occurred primarily by trans-synaptic transmission and was CD155-dependent. Together, these results demonstrate that the magnitude and speed with which PV replication, spread, and subsequent cell death occur in neurons is decreased as compared to non-neuronal cells, implicating cell-specific effects on replication that may then influence viral pathogenesis.

Measles virus infection induces chemokine synthesis by neurons.

The role that neurons play in the induction of the immune response following CNS viral infection is poorly understood, largely owing to the belief that these cells are immunologically quiescent. In this report, we show that virus infection of neurons results in the synthesis of proinflammatory chemokines, which are early and important mediators of leukocyte recruitment to sites of viral infection. For these studies, a transgenic mouse model of neuron-restricted measles virus (MV) infection was used. Inoculation of immunocompetent and immunodeficient transgenic adult mice resulted in CNS induction of the mRNAs encoding IFN-gamma inducible protein of 10 kD, monokine inducible by gamma and RANTES. Colocalization of chemokine proteins with MV-infected neurons was detected by immunofluorescence in infected brain sections. Both IFN-gamma inducible protein 10 kD and RANTES were also induced in MV-infected primary hippocampal neurons cultured from transgenic embryos, as shown by RNase protection assay, confocal microscopy, and ELISA. Interestingly, neuronal infection with another RNA virus (lymphocytic choriomeningitis virus) was not associated with induction of these chemokines. In immunocompetent mice, chemokine synthesis preceded the infiltration of T lymphocytes, and chemokine ablation by neutralizing Abs resulted in a 20-50% reduction in the number of infiltrating lymphocytes. Collectively, these data indicate that neurons play an important role in the recruitment of a protective antiviral response to the CNS following viral infection, although such a role may be virus type-dependent.

Comparison of the predischarge exercise thallium-201 perfusion defect after myocardial infarction with myocardium at risk measured during acute infarction with technetium-99m sestamibi imaging.

BACKGROUND: Exercise thallium-201 imaging provides a noninvasive estimate of the amount of myocardium presumed to be at risk of infarcting should a complete occlusion of the coronary stenosis occur. The relationship between the size of the exercise thallium perfusion defect and the extent of myocardium supplied by a diseased coronary artery has not been established. This study evaluates that presumed correlation. METHODS: Patients were injected intravenously with technetium-99m sestamibi during acute myocardial infarction before thrombolysis or conventional therapy to quantify the myocardium at risk. Twenty-six patients who underwent risk-area assessment subsequently underwent clinically driven, predischarge, submaximal exercise imaging with thallium-201. The exercise testing was performed on day 7 +/- 2 days. A conventional polar map display was used to quantify the perfusion defect. RESULTS: The myocardium at risk determined by technetium-99m sestamibi at the time of infarction was 30% +/- 20% of the left ventricle. The mean exercise thallium-201 defect was 34% +/- 22% of the left ventricle. The exercise defect tended to be slightly larger than the myocardium at risk (4% +/- 10% of the left ventricle, P =.05). There was a close correlation between the 2 measurements (r = 0.89, SE = 9.4, P <.0001). CONCLUSIONS: This study shows a close correlation between the myocardium "at risk" assessed acutely by technetium-99m sestamibi and the "presumed at-risk area" determined by thallium-201 imaging on predischarge exercise testing. This finding supports the concept that the size of the exercise thallium defect caused by coronary stenosis indicates the likely size of a myocardial infarction resulting from occlusion of that stenosis.

Neuronal survival strategies in the face of RNA viral infection.

Neurons of the mammalian central nervous system (CNS) are an essential and largely nonrenewable cell population. Thus, viral infections that result in neuronal depletion, either by viral lysis or by induction of the cytolytic immune response, would likely lead to profound neurologic impairment. However, many viral infections that result in tissue destruction elsewhere in the host produce few overt symptoms in the CNS, despite readily detectable virus expression. This observation has lead to the speculation that neurons possess strategies to limit the replication and spread of otherwise cytopathic viruses. These strategies either favor the clearance of virus in the absence of appreciable neuronal loss or promote the establishment of noncytolytic persistent infections. This review discusses some of these strategies, with an emphasis on how such survival techniques lessen the potential for CNS neuropathology.