Pathogenic and Apathogenic Strains of Lymphocytic Choriomeningitis Virus Have Distinct Entry and Innate Immune Activation Pathways.

Lymphocytic choriomeningitis virus (LCMV) and Lassa virus (LASV) share many genetic and biological features including subtle differences between pathogenic and apathogenic strains. Despite remarkable genetic similarity, the viscerotropic WE strain of LCMV causes a fatal LASV fever-like hepatitis in non-human primates (NHPs) while the mouse-adapted Armstrong (ARM) strain of LCMV is deeply attenuated in NHPs and can vaccinate against LCMV-WE challenge. Here, we demonstrate that internalization of WE is more sensitive to the depletion of membrane cholesterol than ARM infection while ARM infection is more reliant on endosomal acidification. LCMV-ARM induces robust NF-κB and interferon response factor (IRF) activation while LCMV-WE seems to avoid early innate sensing and failed to induce strong NF-κB and IRF responses in dual-reporter monocyte and epithelial cells. Toll-like receptor 2 (TLR-2) signaling appears to play a critical role in NF-κB activation and the silencing of TLR-2 shuts down IL-6 production in ARM but not in WE-infected cells. Pathogenic LCMV-WE infection is poorly recognized in early endosomes and failed to induce TLR-2/Mal-dependent pro-inflammatory cytokines. Following infection, Interleukin-1 receptor-associated kinase 1 (IRAK-1) expression is diminished in LCMV-ARM- but not LCMV-WE-infected cells, which indicates it is likely involved in the LCMV-ARM NF-κB activation. By confocal microscopy, ARM and WE strains have similar intracellular trafficking although LCMV-ARM infection appears to coincide with greater co-localization of early endosome marker EEA1 with TLR-2. Both strains co-localize with Rab-7, a late endosome marker, but the interaction with LCMV-WE seems to be more prolonged. These findings suggest that LCMV-ARM's intracellular trafficking pathway may facilitate interaction with innate immune sensors, which promotes the induction of effective innate and adaptive immune responses.

Genome-Wide Knockout Screen Identifies Human Sialomucin CD164 as an Essential Entry Factor for Lymphocytic Choriomeningitis Virus.

Lymphocytic choriomeningitis virus (LCMV) is a well-studied mammarenavirus that can be fatal in congenital infections. However, our understanding of LCMV and its interactions with human host factors remains incomplete. Here, host determinants affecting LCMV infection were investigated through a genome-wide CRISPR knockout screen in A549 cells, a human lung adenocarcinoma line. We identified and validated a variety of novel host factors that play a functional role in LCMV infection. Among these, knockout of the sialomucin CD164, a heavily glycosylated transmembrane protein, was found to ablate infection with multiple LCMV strains but not other hemorrhagic mammarenaviruses in several cell types. Further characterization revealed a dependency of LCMV entry on the cysteine-rich domain of CD164, including an N-linked glycosylation site at residue 104 in that region. Given the documented role of LCMV with respect to transplacental human infections, CD164 expression was investigated in human placental tissue and placental cell lines. CD164 was found to be highly expressed in the cytotrophoblast cells, an initial contact site for pathogens within the placenta, and LCMV infection in placental cells was effectively blocked using a monoclonal antibody specific to the cysteine-rich domain of CD164. Together, this study identifies novel factors associated with LCMV infection of human tissues and highlights the importance of CD164, a sialomucin that previously had not been associated with viral infection. IMPORTANCE Lymphocytic choriomeningitis virus (LCMV) is a human-pathogenic mammarenavirus that can be fatal in congenital infections. Although frequently used in the study of persistent infections in the field of immunology, aspects of this virus's life cycle remain incomplete. For example, while viral entry has been shown to depend on a cell adhesion molecule, DAG1, genetic knockout of this gene allows for residual viral infection, implying that additional receptors can mediate cell entry. The significance of our study is the identification of host factors important for successful infection, including the sialomucin CD164, which had not been previously associated with viral infection. We demonstrated that CD164 is essential for LCMV entry into human cells and can serve as a possible therapeutic target for treatment of congenital infection.

CD164 is a host factor for lymphocytic choriomeningitis virus entry.

Lymphocytic choriomeningitis virus (LCMV) is a rodent-borne zoonotic arenavirus that causes congenital abnormalities and can be fatal for transplant recipients. Using a genome-wide loss-of-function screen, we identify host factors required for LCMV entry into cells. We identify the lysosomal mucin CD164, glycosylation factors, the heparan sulfate biosynthesis machinery, and the known receptor alpha-dystroglycan (α-DG). Biochemical analysis revealed that the LCMV glycoprotein binds CD164 at acidic pH and requires a sialylated glycan at residue N104. We demonstrate that LCMV entry proceeds by the virus switching binding from heparan sulfate or α-DG at the plasma membrane to CD164 prior to membrane fusion, thus identifying additional potential targets for therapeutic intervention.

Clonally Expanded Virus-Specific CD8 T Cells Acquire Diverse Transcriptional Phenotypes During Acute, Chronic, and Latent Infections.

CD8+ T cells play a crucial role in the control and resolution of viral infections and can adopt a wide range of phenotypes and effector functions depending on the inflammatory context and the duration and extent of antigen exposure. Similarly, viral infections can exert diverse selective pressures on populations of clonally related T cells. Technical limitations have nevertheless made it challenging to investigate the relationship between clonal selection and transcriptional phenotypes of virus-specific T cells. We therefore performed single-cell T cell receptor (TCR) repertoire and transcriptome sequencing of virus-specific CD8 T cells in murine models of acute, chronic and latent infection. We observed clear infection-specific populations corresponding to memory, effector, exhausted, and inflationary phenotypes. We further uncovered a mouse-specific and polyclonal T cell response, despite all T cells sharing specificity to a single viral epitope, which was accompanied by stereotypic TCR germline gene usage in all three infection types. Persistent antigen exposure during chronic and latent viral infections resulted in a higher proportion of clonally expanded T cells relative to acute infection. We furthermore observed a relationship between transcriptional heterogeneity and clonal expansion for all three infections, with highly expanded clones having distinct transcriptional phenotypes relative to less expanded clones. Together our work relates clonal selection to gene expression in the context of viral infection and further provides a dataset and accompanying software for the immunological community.

Induction of thymic atrophy and loss of thymic output by type-I interferons during chronic viral infection.

Type-I interferon (IFN-I) signals exert a critical role in disease progression during viral infections. However, the immunomodulatory mechanisms by which IFN-I dictates disease outcomes remain to be fully defined. Here we report that IFN-I signals mediate thymic atrophy in viral infections, with more severe and prolonged loss of thymic output and unique kinetics and subtypes of IFN-α/ß expression in chronic infection compared to acute infection. Loss of thymic output was linked to inhibition of early stages of thymopoiesis (DN1-DN2 transition, and DN3 proliferation) and pronounced apoptosis during the late DP stage. Notably, infection-associated thymic defects were largely abrogated upon ablation of IFNαßR and partially mitigated in the absence of CD8 T cells, thus implicating direct as well as indirect effects of IFN-I on thymocytes. These findings provide mechanistic underpinnings for immunotherapeutic strategies targeting IFN-1 signals to manipulate disease outcomes during chronic infections and cancers.

Dynamic CD4+ T cell heterogeneity defines subset-specific suppression and PD-L1-blockade-driven functional restoration in chronic infection.

Inhibiting PD-1:PD-L1 signaling has transformed therapeutic immune restoration. CD4+ T cells sustain immunity in chronic infections and cancer, yet little is known about how PD-1 signaling modulates CD4+ helper T (TH) cell responses or the ability to restore CD4+ TH-mediated immunity by checkpoint blockade. We demonstrate that PD-1:PD-L1 specifically suppressed CD4+ TH1 cell amplification, prevents CD4+ TH1 cytokine production and abolishes CD4+ cytotoxic killing capacity during chronic infection in mice. Inhibiting PD-L1 rapidly restored these functions, while simultaneously amplifying and activating TH1-like T regulatory cells, demonstrating a system-wide CD4-TH1 recalibration. This effect coincided with decreased T cell antigen receptor signaling, and re-directed type I interferon (IFN) signaling networks towards dominant IFN-γ-mediated responses. Mechanistically, PD-L1 blockade specifically targeted defined populations with pre-established, but actively suppressed proliferative potential, with limited impact on minimally cycling TCF-1+ follicular helper T cells, despite high PD-1 expression. Thus, CD4+ T cells require unique differentiation and functional states to be targets of PD-L1-directed suppression and therapeutic restoration.

Heparan sulfate proteoglycans serve as alternative receptors for low affinity LCMV variants.

Members of the Old World Arenaviruses primarily utilize α-dystroglycan (α-DAG1) as a cellular receptor for infection. Mutations within the glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV) reduce or abrogate the binding affinity to α-DAG1 and thus influence viral persistence, kinetics, and cell tropism. The observation that α-DAG1 deficient cells are still highly susceptible to low affinity variants, suggests the use of an alternative receptor(s). In this study, we used a genome-wide CRISPR Cas9 knockout screen in DAG1 deficient 293T cells to identify host factors involved in α-DAG1-independent LCMV infection. By challenging cells with vesicular stomatitis virus (VSV), pseudotyped with the GP of LCMV WE HPI (VSV-GP), we identified the heparan sulfate (HS) biosynthesis pathway as an important host factor for low affinity LCMV infection. These results were confirmed by a genetic approach targeting EXTL3, a key factor in the HS biosynthesis pathway, as well as by enzymatic and chemical methods. Interestingly, a single point mutation within GP1 (S153F or Y155H) of WE HPI is sufficient for the switch from DAG1 to HS binding. Furthermore, we established a simple and reliable virus-binding assay, using directly labelled VSV-GP by intramolecular fusion of VSV-P and mWasabi, demonstrating the importance of HS for virus attachment but not entry in Burkitt lymphoma cells after reconstitution of HS expression. Collectively, our study highlights the essential role of HS for low affinity LCMV infection in contrast to their high affinity counterparts. Residual LCMV infection in double knockouts indicate the use of (a) still unknown entry receptor(s).

Proinflammatory cytokines promote TET2-mediated DNA demethylation during CD8 T cell effector differentiation.

To gain insight into the signaling determinants of effector-associated DNA methylation programming among CD8 T cells, we explore the role of interleukin (IL)-12 in the imprinting of IFNg expression during CD8 T cell priming. We observe that anti-CD3/CD28-mediated stimulation of human naive CD8 T cells is not sufficient to induce substantial demethylation of the IFNg promoter. However, anti-CD3/CD28 stimulation in the presence of the inflammatory cytokine, IL-12, results in stable demethylation of the IFNg locus that is commensurate with IFNg expression. IL-12-associated demethylation of the IFNg locus is coupled to cell division through TET2-dependent demethylation in an ex vivo human chimeric antigen receptor T cell model system and an in vivo immunologically competent murine system. Collectively, these data illustrate that IL-12 signaling promotes TET2-mediated effector DNA demethylation programming in CD8 T cells and serve as proof of concept that cytokines can guide induction of epigenetically regulated traits for T cell-based immunotherapies.

PD-1+ Tcf1+ CD8+ T cells from established chronic infection can form memory while retaining a stableimprint of persistent antigen exposure.

Virus-specific PD1+ Tcf1+ memory-like CD8+ T cells (TMLs) maintain the CD8+ T cell response during chronic viral infection. However, the fate of these cells following cessation of persistent antigen exposure has been unclear. Here, we find that TMLs persist upon transfer into antigen-free hosts and form memory following recall stimulation. Phenotypic, functional, and transcriptome analyses show that TML-derived memory cells resemble those arising in response to acute, resolved infection, but they retain features of chronically stimulated cells, including elevated PD-1 and Tox and reduced cytokine expression. This chronic infection imprint is largely accounted for by constitutive Tox expression. Virus-specific Tcf1+ CD8+ T cells that persist after clearance of systemic infection also display a chronic infection imprint. Notwithstanding, renewed virus exposure induces a recall response, which controls virus infection in part. Thus, cessation of chronic antigen exposure yields a memory CD8+ T cell compartment that reflects prior stimulation.

Brd4 Regulates the Homeostasis of CD8+ T-Lymphocytes and Their Proliferation in Response to Antigen Stimulation.

CD8+ T cells are major components of adaptive immunity and confer robust protective cellular immunity, which requires adequate T-cell numbers, targeted migration, and efficient T-cell proliferation. Altered CD8+ T-cell homeostasis and impaired proliferation result in dysfunctional immune response to infection or tumorigenesis. However, intrinsic factors controlling CD8+ T-cell homeostasis and immunity remain largely elusive. Here, we demonstrate the prominent role of Brd4 on CD8+ T cell homeostasis and immune response. By upregulating Myc and GLUT1 expression, Brd4 facilitates glucose uptake and energy production in mitochondria, subsequently supporting naïve CD8+ T-cell survival. Besides, Brd4 promotes the trafficking of naïve CD8+ T cells partially through maintaining the expression of homing receptors (CD62L and LFA-1). Furthermore, Brd4 is required for CD8+ T cell response to antigen stimulation, as Brd4 deficiency leads to a severe defect in clonal expansion and terminal differentiation by decreasing glycolysis. Importantly, as JQ1, a pan-BRD inhibitor, severely dampens CD8+ T-cell immune response, its usage as an anti-tumor agent or latency-reversing agent for human immunodeficiency virus type I (HIV-1) should be more cautious. Collectively, our study identifies a previously-unexpected role of Brd4 in the metabolic regulation of CD8+ T cell-mediated immune surveillance and also provides a potential immunomodulation target.

Persistent RNA virus infection is short-lived at the single-cell level but leaves transcriptomic footprints.

Several RNA viruses can establish life-long persistent infection in mammalian hosts, but the fate of individual virus-infected cells remains undefined. Here we used Cre recombinase-encoding lymphocytic choriomeningitis virus to establish persistent infection in fluorescent cell fate reporter mice. Virus-infected hepatocytes underwent spontaneous noncytolytic viral clearance independently of type I or type II interferon signaling or adaptive immunity. Viral clearance was accompanied by persistent transcriptomic footprints related to proliferation and extracellular matrix remodeling, immune responses, and metabolism. Substantial overlap with persistent epigenetic alterations in HCV-cured patients suggested a universal RNA virus-induced transcriptomic footprint. Cell-intrinsic clearance occurred in cell culture, too, with sequential infection, reinfection cycles separated by a period of relative refractoriness to infection. Our study reveals that systemic persistence of a prototypic noncytolytic RNA virus depends on continuous spread and reinfection. Yet undefined cell-intrinsic mechanisms prevent viral persistence at the single-cell level but give way to profound transcriptomic alterations in virus-cleared cells.

Distinct mechanisms govern populations of myeloid-derived suppressor cells in chronic viral infection and cancer.

Myeloid-derived suppressor cells (MDSCs) are major negative regulators of immune responses in cancer and chronic infections. It remains unclear if regulation of MDSC activity in different conditions is controlled by similar mechanisms. We compared MDSCs in mice with cancer and lymphocytic choriomeningitis virus (LCMV) infection. Chronic LCMV infection caused the development of monocytic MDSCs (M-MDSCs) but did not induce polymorphonuclear MDSCs (PMN-MDSCs). In contrast, both MDSC populations were present in cancer models. An acquisition of immune-suppressive activity by PMN-MDSCs in cancer was controlled by IRE1α and ATF6 pathways of the endoplasmic reticulum (ER) stress response. Abrogation of PMN-MDSC activity by blockade of the ER stress response resulted in an increase in tumor-specific immune response and reduced tumor progression. In contrast, the ER stress response was dispensable for suppressive activity of M-MDSCs in cancer and LCMV infection. Acquisition of immune-suppressive activity by M-MDSCs in spleens was mediated by IFN-γ signaling. However, it was dispensable for suppressive activity of M-MDSCs in tumor tissues. Suppressive activity of M-MDSCs in tumors was retained due to the effect of IL-6 present at high concentrations in the tumor site. These results demonstrate disease- and population-specific mechanisms of MDSC accumulation and the need for targeting different pathways to achieve inactivation of these cells.

Mast Cells Modulate Antigen-Specific CD8+ T Cell Activation During LCMV Infection.

Mast cells (MCs), strategically localized at mucosal surfaces, provide first-line defense against pathogens and shape innate and adaptive immune responses. Recent studies have shown that MCs are involved in pathogenic responses to several viruses including herpes simplex viruses, dengue virus, vaccinia virus and influenza virus. However, the underlying mechanisms of MCs in the activation of CD8+ T cells during viral infections are not fully understood. Therefore, we investigate the role of MCs in the development of virus-specific CD8+ T cell responses using the well-characterized murine lymphocytic choriomeningitis virus (LCMV) model and the transgenic MasTRECK mice that contain the human diphtheria toxin receptor as an inducible MC-deficient model. Here, we report that MCs are essential for the activation and expansion of virus-specific CD8+ T cells. After MC depletion and subsequent intradermal LCMV infection, the CD8 + T cell effector phenotype and antiviral cytokine production were impaired at the peak of infection (day 8 p.i.). Importantly, MC-deficient mice were unable to control the infection and exhibited significantly higher viral loads in the spleen and in the ear draining lymph nodes compared to that of wild type control mice. In the absence of MCs, dendritic cell (DC) activation was impaired upon LCMV infection. In addition, type-I interferon (IFN) levels in the serum and in the spleen of MC-deficient mice were reduced during the first days of infection. Interestingly, depletion of MCs after intradermal LCMV infection did not impair virus-specific CD8+ T cell expansion, activation or antiviral cytokine production. In summary, our results indicate that MCs play a pivotal role in the activation and antiviral functions of CD8+ T cells through proper DC activation. A better understanding of the impact of MCs on CD8+ T cell responses is mandatory to improve antiviral immune responses.

Lymphocytic Choriomeningitis Virus Alters the Expression of Male Mouse Scent Proteins.

Mature male mice produce a particularly high concentration of major urinary proteins (MUPs) in their scent marks that provide identity and status information to conspecifics. Darcin (MUP20) is inherently attractive to females and, by inducing rapid associative learning, leads to specific attraction to the individual male's odour and location. Other polymorphic central MUPs, produced at much higher abundance, bind volatile ligands that are slowly released from a male's scent marks, forming the male's individual odour that females learn. Here, we show that infection of C57BL/6 males with LCMV WE variants (v2.2 or v54) alters MUP expression according to a male's infection status and ability to clear the virus. MUP output is substantially reduced during acute adult infection with LCMV WE v2.2 and when males are persistently infected with LCMV WE v2.2 or v54. Infection differentially alters expression of darcin and, particularly, suppresses expression of a male's central MUP signature. However, following clearance of acute v2.2 infection through a robust virus-specific CD8 cytotoxic T cell response that leads to immunity to the virus, males regain their normal mature male MUP pattern and exhibit enhanced MUP output by 30 days post-infection relative to uninfected controls. We discuss the likely impact of these changes in male MUP signals on female attraction and mate selection. As LCMV infection during pregnancy can substantially reduce embryo survival and lead to lifelong infection in surviving offspring, we speculate that females use LCMV-induced changes in MUP expression both to avoid direct infection from a male and to select mates able to develop immunity to local variants that will be inherited by their offspring.

A unified atlas of CD8 T cell dysfunctional states in cancer and infection.

CD8 T cells play an essential role in defense against viral and bacterial infections and in tumor immunity. Deciphering T cell loss of functionality is complicated by the conspicuous heterogeneity of CD8 T cell states described across experimental and clinical settings. By carrying out a unified analysis of over 300 assay for transposase-accessible chromatin sequencing (ATAC-seq) and RNA sequencing (RNA-seq) experiments from 12 studies of CD8 T cells in cancer and infection, we defined a shared differentiation trajectory toward dysfunction and its underlying transcriptional drivers and revealed a universal early bifurcation of functional and dysfunctional T cell states across models. Experimental dissection of acute and chronic viral infection using single-cell ATAC (scATAC)-seq and allele-specific single-cell RNA (scRNA)-seq identified state-specific drivers and captured the emergence of similar TCF1+ progenitor-like populations at an early branch point, at which functional and dysfunctional T cells diverge. Our atlas of CD8 T cell states will facilitate mechanistic studies of T cell immunity and translational efforts.

Effector and stem-like memory cell fates are imprinted in distinct lymph node niches directed by CXCR3 ligands.

T cells dynamically interact with multiple, distinct cellular subsets to determine effector and memory differentiation. Here, we developed a platform to quantify cell location in three dimensions to determine the spatial requirements that direct T cell fate. After viral infection, we demonstrated that CD8+ effector T cell differentiation is associated with positioning at the lymph node periphery. This was instructed by CXCR3 signaling since, in its absence, T cells are confined to the lymph node center and alternatively differentiate into stem-like memory cell precursors. By mapping the cellular sources of CXCR3 ligands, we demonstrated that CXCL9 and CXCL10 are expressed by spatially distinct dendritic and stromal cell subsets. Unlike effector cells, retention of stem-like memory precursors in the paracortex is associated with CCR7 expression. Finally, we demonstrated that T cell location can be tuned, through deficiency in CXCL10 or type I interferon signaling, to promote effector or stem-like memory fates.

BACH2 enforces the transcriptional and epigenetic programs of stem-like CD8+ T cells.

During chronic infection and cancer, a self-renewing CD8+ T cell subset maintains long-term immunity and is critical to the effectiveness of immunotherapy. These stem-like CD8+ T cells diverge from other CD8+ subsets early after chronic viral infection. However, pathways guarding stem-like CD8+ T cells against terminal exhaustion remain unclear. Here, we show that the gene encoding transcriptional repressor BACH2 is transcriptionally and epigenetically active in stem-like CD8+ T cells but not terminally exhausted cells early after infection. BACH2 overexpression enforced stem-like cell fate, whereas BACH2 deficiency impaired stem-like CD8+ T cell differentiation. Single-cell transcriptomic and epigenomic approaches revealed that BACH2 established the transcriptional and epigenetic programs of stem-like CD8+ T cells. In addition, BACH2 suppressed the molecular program driving terminal exhaustion through transcriptional repression and epigenetic silencing. Thus, our study reveals a new pathway that enforces commitment to stem-like CD8+ lineage and prevents an alternative terminally exhausted cell fate.

T cells in the brain enhance neonatal mortality during peripheral LCMV infection.

In adult mice the severity of disease from viral infections is determined by the balance between the efficiency of the immune response and the magnitude of viral load. Here, the impact of this dynamic is examined in neonates. Newborns are highly susceptible to infections due to poor innate responses, lower numbers of T cells and Th2-prone immune responses. Eighty-percent of 7-day old mice, immunologically equivalent to human neonates, succumbed to extremely low doses (5 PFU) of the essentially non-lethal lymphocytic choriomeningitis virus (LCMV-Armstrong) given intraperitoneally. This increased lethality was determined to be dependent upon poor early viral control, as well as, T cells and perforin as assessed in knockout mice. By day 3, these neonatal mice had 400-fold higher viral loads as compared to adults receiving a 10,000-fold (5X104 PFU) higher dose of LCMV. The high viral load in combination with the subsequent immunological defect of partial CD8 T cell clonal exhaustion in the periphery led to viral entry and replication in the brain. Within the brain, CD8 T cells were protected from exhaustion, and thus were able to mediate lethal immunopathology. To further delineate the role of early viral control, neonatal mice were infected with Pichinde virus, a less virulent arenavirus, or LCMV was given to pups of LCMV-immune mothers. In both cases, peak viral load was at least 29-fold lower, leading to functional CD8 T cell responses and 100% survival.

TCRα reporter mice reveal contribution of dual TCRα expression to T cell repertoire and function.

It is known that a subpopulation of T cells expresses two T cell receptor (TCR) clonotypes, though the extent and functional significance of this is not established. To definitively evaluate dual TCRα cells, we generated mice with green fluorescent protein and red fluorescent protein reporters linked to TCRα, revealing that ∼16% of T cells express dual TCRs, notably higher than prior estimates. Importantly, dual TCR expression has functional consequences, as dual TCR cells predominated response to lymphocytic choriomeningitis virus infection, comprising up to 60% of virus-specific CD4+ and CD8+ T cells during acute responses. Dual receptor expression selectively influenced immune memory, as postinfection memory CD4+ populations contained significantly increased frequencies of dual TCR cells. These data reveal a previously unappreciated contribution of dual TCR cells to the immune repertoire and highlight their potential effects on immune responses.

Role of LFA-1 integrin in the control of a lymphocytic choriomeningitis virus (LCMV) infection.

Leukocyte function-associated antigen 1 (LFA-1) is the most widely expressed member of the ß2 integrin family of cell-cell adhesion molecules. Although LFA-1 is thought to regulate multiple aspects of T cell immunity, its role in the response of CD8+ T cells to viral infections remains unclear. Indeed, compelling clinical evidence shows that loss of LFA-1 function predisposes to infection in humans but animal models show limited to no susceptibility to infection. Here, we addressed this conundrum in a mouse model of infection with lymphocytic choriomeningitis virus (LCMV), where CD8+ T cells are necessary and sufficient to confer protection. To this end, we followed the fate and function of wild-type and LFA-1 deficient virus-specific CD8+ T cells and assessed the effect of blocking anti-LFA-1 monoclonal antibody in the outcome of infection. Our analysis of viral clearance and T cell responses using transcriptome profiling reveals a role for LFA-1 as a gatekeeper of effector T cell survival and dysfunction that when defective can predispose to LCMV infection.