MHC I tetramer staining tends to overestimate the number of functionally relevant self-reactive CD8 T cells in the preimmune repertoire.

Previous studies that used peptide-MHC (pMHC) tetramers (tet) to identify self-specific T cells have questioned the effectiveness of thymic-negative selection. Here, we used pMHCI tet to enumerate CD8 T cells specific for the immunodominant gp33 epitope of lymphocytic choriomeningitis virus glycoprotein (GP) in mice transgenically engineered to express high levels of GP as a self-antigen in the thymus. In GP-transgenic mice (GP+ ), monoclonal P14 TCR+ CD8 T cells that express a GP-specific TCR could not be detected by gp33/Db -tet staining, indicative of their complete intrathymic deletion. By contrast, in the same GP+ mice, substantial numbers of polyclonal CD8 T cells identifiable by gp33/Db -tet were present. The gp33-tet staining profiles of polyclonal T cells from GP+ and GP-negative (GP- ) mice were overlapping, but mean fluorescence intensities were ∼15% lower in cells from GP+ mice. Remarkably, the gp33-tet+ T cells in GP+ mice failed to clonally expand after lymphocytic choriomeningitis virus infection, whereas those of GP- mice did so. In Nur77GFP -reporter mice, dose-dependent responses to gp33 peptide-induced TCR stimulation revealed that gp33-tet+ T cells with high ligand sensitivity are lacking in GP+ mice. Hence, pMHCI tet staining identifies self-specific CD8 T cells but tends to overestimate the number of truly self-reactive cells.

The alarmin interleukin-33 promotes the expansion and preserves the stemness of Tcf-1+ CD8+ T cells in chronic viral infection.

T cell factor 1 (Tcf-1) expressing CD8+ T cells exhibit stem-like self-renewing capacity, rendering them key for immune defense against chronic viral infection and cancer. Yet, the signals that promote the formation and maintenance of these stem-like CD8+ T cells (CD8+SL) remain poorly defined. Studying CD8+ T cell differentiation in mice with chronic viral infection, we identified the alarmin interleukin-33 (IL-33) as pivotal for the expansion and stem-like functioning of CD8+SL as well as for virus control. IL-33 receptor (ST2)-deficient CD8+ T cells exhibited biased end differentiation and premature loss of Tcf-1. ST2-deficient CD8+SL responses were restored by blockade of type I interferon signaling, suggesting that IL-33 balances IFN-I effects to control CD8+SL formation in chronic infection. IL-33 signals broadly augmented chromatin accessibility in CD8+SL and determined these cells' re-expansion potential. Our study identifies the IL-33-ST2 axis as an important CD8+SL-promoting pathway in the context of chronic viral infection.

Vectored antibody gene delivery restores host B and T cell control of persistent viral infection.

Passive antibody therapy and vectored antibody gene delivery (VAGD) in particular offer an innovative approach to combat persistent viral diseases. Here, we exploit a small animal model to investigate synergies of VAGD with the host's endogenous immune defense for treating chronic viral infection. An adeno-associated virus (AAV) vector delivering the lymphocytic choriomeningitis virus (LCMV)-neutralizing antibody KL25 (AAV-KL25) establishes protective antibody titers for >200 days. When therapeutically administered to chronically infected immunocompetent wild-type mice, AAV-KL25 affords sustained viral load control. In contrast, viral mutational escape thwarts therapeutic AAV-KL25 effects when mice are unable to mount LCMV-specific antibody responses or lack CD8+ T cells. VAGD augments antiviral germinal center B cell and antibody-secreting cell responses and reduces inhibitory receptor expression on antiviral CD8+ T cells. These results indicate that VAGD fortifies host immune defense and synergizes with B cell and CD8 T cell responses to restore immune control of chronic viral infection.

Development and Characterization of a Novel Non-Lytic Cancer Immunotherapy Using a Recombinant Arenavirus Vector Platform.

Engineered viral vectors represent a promising strategy to trigger antigen-specific antitumor T cell responses. Arenaviruses have been widely studied because of their ability to elicit potent and protective T cell responses. Here, we provide an overview of a novel intravenously administered, replication-competent, non-lytic arenavirus-based vector technology that delivers tumor antigens to induce antigen-specific anti-cancer T cell responses. Preclinical studies in mice and cell culture experiments with human peripheral blood mononuclear cells demonstrate that arenavirus vectors preferentially infect antigen-presenting cells. This, in conjunction with a non-lytic functional activation of the infected antigen-presenting cells, leads to a robust antigen-specific CD8+ T cell response. T cell migration to, and infiltration of, the tumor microenvironment has been demonstrated in various preclinical tumor models with vectors encoding self- and non-self-antigens. The available data also suggest that arenavirus-based vector therapy can induce immunological memory protecting from tumor rechallenge. Based on promising preclinical data, a phase 1/2 clinical trial was initiated and is currently ongoing to test the activity and safety of arenavirus vectors, HB-201 and HB-202, created using lymphocytic choriomeningitis virus and Pichinde virus, respectively. Both vectors have been engineered to deliver non-oncogenic versions of the human papilloma virus 16 (HPV16) antigens E7 and E6 and will be injected intravenously with or without an initial intratumoral dose. This dose escalation/expansion study is being conducted in patients with recurrent or metastatic HPV16+ cancers. Promising preliminary data from this ongoing clinical study have been reported. Immunogenicity data from several patients demonstrate that a single injection of HB-201 or HB-202 monotherapy is highly immunogenic, as evidenced by an increase in inflammatory cytokines/chemokines and the expansion of antigen-specific CD8+ T cell responses. This response can be further enhanced by alternating injections of HB-202 and HB-201, which has resulted in frequencies of circulating HPV16 E7/E6-specific CD8+ T cells of up to 40% of the total CD8+ T cell compartment in peripheral blood in analyses to date. Treatment with intravenous administration also resulted in a disease control rate of 73% among 11 evaluable patients with head and neck cancer dosed every three weeks, including 2 patients with a partial response.

Vaccine-elicited CD4 T cells prevent the deletion of antiviral B cells in chronic infection.

Chronic viral infections subvert protective B cell immunity. An early type I interferon (IFN-I)-driven bias to short-lived plasmablast differentiation leads to clonal deletion, so-called "decimation," of antiviral memory B cells. Therefore, prophylactic countermeasures against decimation remain an unmet need. We show that vaccination-induced CD4 T cells prevented the decimation of naïve and memory B cells in chronically lymphocytic choriomeningitis virus (LCMV)-infected mice. Although these B cell responses were largely T independent when IFN-I was blocked, preexisting T help assured their sustainability under conditions of IFN-I-driven inflammation by instructing a germinal center B cell transcriptional program. Prevention of decimation depended on T cell-intrinsic Bcl6 and Tfh progeny formation. Antigen presentation by B cells, interactions with antigen-specific T helper cells, and costimulation by CD40 and ICOS were also required. Importantly, B cell-mediated virus control averted Th1-driven immunopathology in LCMV-challenged animals with preexisting CD4 T cell immunity. Our findings show that vaccination-induced Tfh cells represent a cornerstone of effective B cell immunity to chronic virus challenge, pointing the way toward more effective B cell-based vaccination against persistent viral diseases.

Heterologous arenavirus vector prime-boost overrules self-tolerance for efficient tumor-specific CD8 T cell attack.

Therapeutic vaccination regimens inducing clinically effective tumor-specific CD8+ T lymphocyte (CTL) responses are an unmet medical need. We engineer two distantly related arenaviruses, Pichinde virus and lymphocytic choriomeningitis virus, for therapeutic cancer vaccination. In mice, life-replicating vector formats of these two viruses delivering a self-antigen in a heterologous prime-boost regimen induce tumor-specific CTL responses up to 50% of the circulating CD8 T cell pool. This CTL attack eliminates established solid tumors in a significant proportion of animals, accompanied by protection against tumor rechallenge. The magnitude of CTL responses is alarmin driven and requires combining two genealogically distantly related arenaviruses. Vector-neutralizing antibodies do not inhibit booster immunizations by the same vector or by closely related vectors. Rather, CTL immunodominance hierarchies favor vector backbone-targeted responses at the expense of self-reactive CTLs. These findings establish an arenavirus-based immunotherapy regimen that allows reshuffling of immunodominance hierarchies and breaking self-directed tolerance for efficient tumor control.

Live-attenuated lymphocytic choriomeningitis virus-based vaccines for active immunotherapy of HPV16-positive cancer.

Infection with human papillomavirus (HPV) is associated with a variety of cancer types and limited therapy options. Therapeutic cancer vaccines targeting the HPV16 oncoproteins E6 and E7 have recently been extensively explored as a promising immunotherapy approach to drive durable antitumor T cell immunity and induce effective tumor control. With the goal to achieve potent and lasting antitumor T cell responses, we generated a novel lymphocytic choriomeningitis virus (LCMV)-based vaccine, TT1-E7E6, targeting HPV16 E6 and E7. This replication-competent vector was stably attenuated using a three-segmented viral genome packaging strategy. Compared to wild-type LCMV, TT1-E7E6 demonstrated significantly reduced viremia and CNS immunopathology. Intravenous vaccination of mice with TT1-E7E6 induced robust expansion of HPV16-specific CD8+ T cells producing IFN-γ, TNF-α and IL-2. In the HPV16 E6 and E7-expressing TC-1 tumor model, mice immunized with TT1-E7E6 showed significantly delayed tumor growth or complete tumor clearance accompanied with prolonged survival. Tumor control by TT1-E7E6 was also achieved in established large-sized tumors in this model. Furthermore, a combination of TT1-E7E6 with anti-PD-1 therapy led to enhanced antitumor efficacy with complete tumor regression in the majority of tumor-bearing mice that were resistant to anti-PD-1 treatment alone. TT1-E7E6 vector itself did not exhibit oncolytic properties in TC-1 cells, while the antitumor effect was associated with the accumulation of HPV16-specific CD8+ T cells with reduced PD-1 expression in the tumor tissues. Together, our results suggest that TT1-E7E6 is a promising therapeutic vaccine for HPV-positive cancers.

Memory CD8+ T Cell Protection From Viral Reinfection Depends on Interleukin-33 Alarmin Signals.

Memory CD8+ cytotoxic T lymphocytes (CTLs) can protect against viral reinfection. However, the signals driving rapid memory CTL reactivation have remained ill-defined. Viral infections can trigger the release of the alarmin interleukin-33 (IL-33) from non-hematopoietic cells. IL-33 signals through its unique receptor ST2 to promote primary effector expansion and activation of CTLs. Here, we show that the transcription factor STAT4 regulated the expression of ST2 on CTLs in vitro and in vivo in primary infections with lymphocytic choriomeningitis virus (LCMV). In the primary antiviral response, IL-33 enhanced effector differentiation and antiviral cytokine production in a CTL-intrinsic manner. Further, using sequential adoptive transfers of LCMV-specific CD8+ T cells, we deciphered the IL-33 dependence of circulating memory CTLs at various stages of their development. IL-33 was found dispensable for the formation and maintenance of memory CTLs, and its absence during priming did not affect their recall response. However, in line with the CTL-boosting role of IL-33 in primary LCMV infections, circulating memory CTLs required IL-33 for efficient secondary expansion, enhanced effector functions, and virus control upon challenge infection. Thus, beyond their effector-promoting activity in primary immune reactions, innate alarmin signals also drive memory T cell recall responses, which has implications for immunity to recurrent diseases.

Recombinant LCMV Vectors Induce Protective Immunity following Homologous and Heterologous Vaccinations.

Successful vaccination against cancer and infectious diseases relies on the induction of adaptive immune responses that induce high-titer antibodies or potent cytoxic T cell responses. In contrast to humoral vaccines, the amplification of cellular immune responses is often hampered by anti-vector immunity that either pre-exists or develops after repeated homologous vaccination. Replication-defective lymphocytic choriomeningitis virus (LCMV) vectors represent a novel generation of vaccination vectors that induce potent immune responses while escaping recognition by neutralizing antibodies. Here, we characterize the CD8 T cell immune response induced by replication-defective recombinant LCMV (rLCMV) vectors with regard to expansion kinetics, trafficking, phenotype, and function and we perform head-to-head comparisons of the novel rLCMV vectors with established vectors derived from adenovirus, vaccinia virus, or Listeria monocytogenes. Our results demonstrate that replication-deficient rLCMV vectors are safe and ideally suited for both homologous and heterologous vaccination regimens to achieve optimal amplification of CD8 T cell immune responses in vivo.

Convergent immunological solutions to Argentine hemorrhagic fever virus neutralization.

Transmission of hemorrhagic fever New World arenaviruses from their rodent reservoirs to human populations poses substantial public health and economic dangers. These zoonotic events are enabled by the specific interaction between the New World arenaviral attachment glycoprotein, GP1, and cell surface human transferrin receptor (hTfR1). Here, we present the structural basis for how a mouse-derived neutralizing antibody (nAb), OD01, disrupts this interaction by targeting the receptor-binding surface of the GP1 glycoprotein from Junín virus (JUNV), a hemorrhagic fever arenavirus endemic in central Argentina. Comparison of our structure with that of a previously reported nAb complex (JUNV GP1-GD01) reveals largely overlapping epitopes but highly distinct antibody-binding modes. Despite differences in GP1 recognition, we find that both antibodies present a key tyrosine residue, albeit on different chains, that inserts into a central pocket on JUNV GP1 and effectively mimics the contacts made by the host TfR1. These data provide a molecular-level description of how antibodies derived from different germline origins arrive at equivalent immunological solutions to virus neutralization.

Replicating viral vector platform exploits alarmin signals for potent CD8+ T cell-mediated tumour immunotherapy.

Viral infections lead to alarmin release and elicit potent cytotoxic effector T lymphocyte (CTLeff) responses. Conversely, the induction of protective tumour-specific CTLeff and their recruitment into the tumour remain challenging tasks. Here we show that lymphocytic choriomeningitis virus (LCMV) can be engineered to serve as a replication competent, stably-attenuated immunotherapy vector (artLCMV). artLCMV delivers tumour-associated antigens to dendritic cells for efficient CTL priming. Unlike replication-deficient vectors, artLCMV targets also lymphoid tissue stroma cells expressing the alarmin interleukin-33. By triggering interleukin-33 signals, artLCMV elicits CTLeff responses of higher magnitude and functionality than those induced by replication-deficient vectors. Superior anti-tumour efficacy of artLCMV immunotherapy depends on interleukin-33 signalling, and a massive CTLeff influx triggers an inflammatory conversion of the tumour microenvironment. Our observations suggest that replicating viral delivery systems can release alarmins for improved anti-tumour efficacy. These mechanistic insights may outweigh safety concerns around replicating viral vectors in cancer immunotherapy.

Additive Protection against Congenital Cytomegalovirus Conferred by Combined Glycoprotein B/pp65 Vaccination Using a Lymphocytic Choriomeningitis Virus Vector.

Subunit vaccines for prevention of congenital cytomegalovirus (CMV) infection based on glycoprotein B (gB) and pp65 are in clinical trials, but it is unclear whether simultaneous vaccination with both antigens enhances protection. We undertook evaluation of a novel bivalent vaccine based on nonreplicating lymphocytic choriomeningitis virus (rLCMV) vectors expressing a cytoplasmic tail-deleted gB [gB(dCt)] and full-length pp65 from human CMV in mice. Immunization with the gB(dCt) vector alone elicited a comparable gB-binding antibody response and a superior neutralizing response to that elicited by adjuvanted subunit gB. Immunization with the pp65 vector alone elicited robust T cell responses. Comparable immunogenicity of the combined gB(dCt) and pp65 vectors with the individual monovalent formulations was demonstrated. To demonstrate proof of principle for a bivalent rLCMV-based HCMV vaccine, the congenital guinea pig cytomegalovirus (GPCMV) infection model was used to compare rLCMV vectors encoding homologs of pp65 (GP83) and gB(dCt), alone and in combination versus Freund's adjuvanted recombinant gB. Both vectors elicited significant immune responses, and no loss of gB immunogenicity was noted with the bivalent formulation. Combined vaccination with rLCMV-vectored GPCMV gB(dCt) and pp65 (GP83) conferred better protection against maternal viremia than subunit or either monovalent rLCMV vaccine. The bivalent vaccine also was significantly more effective in reducing pup mortality than the monovalent vaccines. In summary, bivalent vaccines with rLCMV vectors expressing gB and pp65 elicited potent humoral and cellular responses and conferred protection in the GPCMV model. Further clinical trials of LCMV-vectored HCMV vaccines are warranted.

Interferon-driven deletion of antiviral B cells at the onset of chronic infection.

Inadequate antibody responses and perturbed B cell compartments represent hallmarks of persistent microbial infections, but the mechanisms whereby persisting pathogens suppress humoral immunity remain poorly defined. Using adoptive transfer experiments in the context of a chronic lymphocytic choriomeningitis virus (LCMV) infection of mice, we have documented rapid depletion of virus-specific B cells that coincided with the early type I interferon response to infection. We found that the loss of activated B cells was driven by type I interferon (IFN-I) signaling to several cell types including dendritic cells, T cells and myeloid cells. Intriguingly, this process was independent of B cell-intrinsic IFN-I sensing and resulted from biased differentiation of naïve B cells into short-lived antibody-secreting cells. The ability to generate robust B cell responses was restored upon IFN-I receptor blockade or, partially, when experimentally depleting myeloid cells or the IFN-I-induced cytokines interleukin 10 and tumor necrosis factor alpha. We have termed this IFN-I-driven depletion of B cells "B cell decimation". Strategies to counter "B cell decimation" should thus help us better leverage humoral immunity in the combat against persistent microbial diseases.

Specific fibroblastic niches in secondary lymphoid organs orchestrate distinct Notch-regulated immune responses.

Fibroblast-like cells of secondary lymphoid organs (SLO) are important for tissue architecture. In addition, they regulate lymphocyte compartmentalization through the secretion of chemokines, and participate in the orchestration of appropriate cell-cell interactions required for adaptive immunity. Here, we provide data demonstrating the functional importance of SLO fibroblasts during Notch-mediated lineage specification and immune response. Genetic ablation of the Notch ligand Delta-like (DL)1 identified splenic fibroblasts rather than hematopoietic or endothelial cells as niche cells, allowing Notch 2-driven differentiation of marginal zone B cells and of Esam(+) dendritic cells. Moreover, conditional inactivation of DL4 in lymph node fibroblasts resulted in impaired follicular helper T cell differentiation and, consequently, in reduced numbers of germinal center B cells and absence of high-affinity antibodies. Our data demonstrate previously unknown roles for DL ligand-expressing fibroblasts in SLO niches as drivers of multiple Notch-mediated immune differentiation processes.

Neuroprotective intervention by interferon-γ blockade prevents CD8+ T cell-mediated dendrite and synapse loss.

Neurons are postmitotic and thus irreplaceable cells of the central nervous system (CNS). Accordingly, CNS inflammation with resulting neuronal damage can have devastating consequences. We investigated molecular mediators and structural consequences of CD8(+) T lymphocyte (CTL) attack on neurons in vivo. In a viral encephalitis model in mice, disease depended on CTL-derived interferon-γ (IFN-γ) and neuronal IFN-γ signaling. Downstream STAT1 phosphorylation and nuclear translocation in neurons were associated with dendrite and synapse loss (deafferentation). Analogous molecular and structural alterations were also found in human Rasmussen encephalitis, a CTL-mediated human autoimmune disorder of the CNS. Importantly, therapeutic intervention by IFN-γ blocking antibody prevented neuronal deafferentation and clinical disease without reducing CTL responses or CNS infiltration. These findings identify neuronal IFN-γ signaling as a novel target for neuroprotective interventions in CTL-mediated CNS disease.

Gene-based vaccination with a mismatched envelope protects against simian immunodeficiency virus infection in nonhuman primates.

The RV144 trial demonstrated that an experimental AIDS vaccine can prevent human immunodeficiency virus type 1 (HIV-1) infection in humans. Because of its limited efficacy, further understanding of the mechanisms of preventive AIDS vaccines remains a priority, and nonhuman primate (NHP) models of lentiviral infection provide an opportunity to define immunogens, vectors, and correlates of immunity. In this study, we show that prime-boost vaccination with a mismatched SIV envelope (Env) gene, derived from simian immunodeficiency virus SIVmac239, prevents infection by SIVsmE660 intrarectally. Analysis of different gene-based prime-boost immunization regimens revealed that recombinant adenovirus type 5 (rAd5) prime followed by replication-defective lymphocytic choriomeningitis virus (rLCMV) boost elicited robust CD4 and CD8 T-cell and humoral immune responses. This vaccine protected against infection after repetitive mucosal challenge with efficacies of 82% per exposure and 62% cumulatively. No effect was seen on viremia in infected vaccinated monkeys compared to controls. Protection correlated with the presence of neutralizing antibodies to the challenge viruses tested in peripheral blood mononuclear cells. These data indicate that a vaccine expressing a mismatched Env gene alone can prevent SIV infection in NHPs and identifies an immune correlate that may guide immunogen selection and immune monitoring for clinical efficacy trials.

The alarmin interleukin-33 drives protective antiviral CD8⁺ T cell responses.

Pathogen-associated molecular patterns decisively influence antiviral immune responses, whereas the contribution of endogenous signals of tissue damage, also known as damage-associated molecular patterns or alarmins, remains ill defined. We show that interleukin-33 (IL-33), an alarmin released from necrotic cells, is necessary for potent CD8(+) T cell (CTL) responses to replicating, prototypic RNA and DNA viruses in mice. IL-33 signaled through its receptor on activated CTLs, enhanced clonal expansion in a CTL-intrinsic fashion, determined plurifunctional effector cell differentiation, and was necessary for virus control. Moreover, recombinant IL-33 augmented vaccine-induced CTL responses. Radio-resistant cells of the splenic T cell zone produced IL-33, and efficient CTL responses required IL-33 from radio-resistant cells but not from hematopoietic cells. Thus, alarmin release by radio-resistant cells orchestrates protective antiviral CTL responses.

Multiparameter telemetry as a sensitive screening method to detect vaccine reactogenicity in mice.

Refined vaccines and adjuvants are urgently needed to advance immunization against global infectious challenges such as HIV, hepatitis C, tuberculosis and malaria. Large-scale screening efforts are ongoing to identify adjuvants with improved efficacy profiles. Reactogenicity often represents a major hurdle to the clinical use of new substances. Yet, irrespective of its importance, this parameter has remained difficult to screen for, owing to a lack of sensitive small animal models with a capacity for high throughput testing. Here we report that continuous telemetric measurements of heart rate, heart rate variability, body core temperature and locomotor activity in laboratory mice readily unmasked systemic side-effects of vaccination, which went undetected by conventional observational assessment and clinical scoring. Even minor aberrations in homeostasis were readily detected, ranging from sympathetic activation over transient pyrogenic effects to reduced physical activity and apathy. Results in real-time combined with the potential of scalability and partial automation in the industrial context suggest multiparameter telemetry in laboratory mice as a first-line screen for vaccine reactogenicity. This may accelerate vaccine discovery in general and may further the success of vaccines in combating infectious disease and cancer.

Homing and adhesion patterns determine the cellular composition of the bone marrow plasma cell niche.

According to commonly held concepts, plasma cell (PC) longevity in bone marrow (BM) depends upon their access to survival niches. These are thought to exist in nursery cell types, which support PCs by secreting PC survival factors. To better define PC survival niches and their functioning, we adoptively transferred traceable Blimp-1-(GFP) PCs into recipient mice lacking a proliferation-inducing ligand (APRIL), IL-6, or macrophage migration inhibitory factor. Transferred BMPCs were preferentially associated with Ly-6C(high) monocytes (normalized colocalization index: 9.84), eosinophils (4.29), and megakaryocytes (2.12). Although APRIL was essential for BMPC survival, PC recruitment into the proximity of nursery cells was unimpaired in APRIL-deficient mice, questioning the concept that the same factors account for attraction/retention of PCs as for their local survival. Rather, the order of colocalization with BMPCs (monocytes > eosinophils > megakaryocytes) reflected these cells' relative expression of CXCR4, VLA-4, and LFA-1, the homing and adhesion molecules that direct/retain PCs in the BM. This suggests a scenario wherein the cellular composition of the BMPC niche is defined by a common pattern of attraction/retention on CXCL12-abundant reticular docking cells. Thereby, PCs are directed to associate in a functional BM niche with hematopoietic CXCR4(+)VLA-4(+)LFA-1(+) nursery cells, which provide PC survival factors.

Impact of sphingosine 1-phosphate modulation on immune outcomes.

Viral infections may have an important role in the precipitation or relapse of multiple sclerosis (MS) and its treatment. This review describes the normal immune response to viral infection, the possible associations between viral infections and MS therapy, and the impact of sphingosine 1-phosphate (S1P) receptor (S1PR) modulation with fingolimod (FTY720) on the immune responses to viral infection. The physiologic immune response to viral infection involves lymphocyte activation and control of the circulation of subsets of lymphocytes with different functions between the lymph nodes, vascular system, and tissues, under the control of the S1P/S1PR signaling mechanism. In MS, it has been postulated that viral infections may play a role in triggering MS relapses, with virus-specific T cells being responsible for the demyelinating lesions within the CNS. Fingolimod-an S1PR modulator approved for the treatment of relapsing MS in some countries-is thought to act by downmodulating lymphatic S1P subtype 1 receptors. This retains naïve T cells and central memory T cells, but not effector memory T cells, within the lymph nodes and prevents their circulation to the CNS. Evidence from infection models supports that the selective effects of fingolimod on T cell subsets allows key immune responses to be preserved during therapy. However, in patients, long-term observation is important as both the risk of cancer and infection is potentially increased by the use of any immunomodulatory agent.