A critical role for neutralizing-antibody-producing B cells, CD4(+) T cells, and interferons in persistent and acute infections of mice with lymphocytic choriomeningitis virus: implications for adoptive immunotherapy of virus carriers

Immune correlates of protection following Rift Valley fever virus vaccination

Rift Valley fever virus (RVFV) is a hemorrhagic fever virus with the potential for significant economic and public health impact. Vaccination with an attenuated strain, DelNSsRVFV, provides protection from an otherwise lethal RVFV challenge, but mechanistic determinants of protection are undefined. In this study, a murine model was used to assess the contributions of humoral and cellular immunity to DelNSsRVFV-mediated protection. Vaccinated mice depleted of T cells were protected against subsequent challenge, and passive transfer of immune serum from vaccinated animals to naïve animals was also protective, demonstrating that T cells were dispensable in the presence of humoral immunity and that humoral immunity alone was sufficient. Animals depleted of B cells and then vaccinated were protected against challenge. Total splenocytes, but not T cells alone, B cells alone, or B + T cells harvested from vaccinated animals and then transferred to naïve animals were sufficient to confer protection, suggesting that multiple cellular interactions were required for effective cellular immunity. Together, these data indicate that humoral immunity is sufficient to confer vaccine-mediated protection and suggests that cellular immunity plays a role in protection that requires the interaction of various cellular components.

Adoptive B cell therapy for chronic viral infection

T cell-based therapies have been widely explored for the treatment of cancer and chronic infection, but B cell-based therapies have remained largely unexplored. To study the effect of B cell therapy, we adoptively transferred virus-specific B cells into mice that were chronically infected with lymphocytic choriomeningitis virus (LCMV). Adoptive transfer of virus-specific B cells resulted in increase in antibody titers and reduction of viral loads. Importantly, the efficacy of B cell therapy was partly dependent on antibody effector functions, and was improved by co-transferring virus-specific CD4 T cells. These findings provide a proof-of-concept that adoptive B cell therapy can be effective for the treatment of chronic infections, but provision of virus-specific CD4 T cells may be critical for optimal virus neutralization.

LCMV induced down-regulation of HVEM on anti-viral T cells is critical for an efficient effector response

T‐cell responses against tumors and pathogens are critically shaped by cosignaling molecules providing a second signal. Interaction of herpes virus entry mediator (HVEM, CD270, TNFRSF14) with multiple ligands has been proposed to promote or inhibit T‐cell responses and inflammation, dependent on the context. In this study, we show that absence of HVEM did neither affect generation of effector nor maintenance of memory antiviral T cells and accordingly viral clearance upon acute and chronic lymphocytic choriomeningitis virus (LCMV) infection, due to potent HVEM downregulation during infection. Notably, overexpression of HVEM on virus‐specific CD8⁺ T cells resulted in a reduction of effector cells, whereas numbers of memory cells were increased. Overall, this study indicates that downregulation of HVEM driven by LCMV infection ensures an efficient acute response at the price of impaired formation of T‐cell memory.

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.

New advances in immune components mediating viral control in the CNS

Protective immune responses in the central nervous system (CNS) must act efficiently but need to be tightly controlled to avoid excessive damage to this vital organ. Under homeostatic conditions, the immune surveillance of the CNS is mediated by innate immune cells together with subsets of memory lymphocytes accumulating over lifetime. Accordingly, a wide range of immune responses can be triggered upon pathogen infection that can be associated with devastating clinical outcomes, and which most frequently are due to neurotropic viruses. Here, we discuss recent advances about our understanding of anti-viral immune responses with special emphasis on mechanisms operating in the various anatomical compartments of the CNS.

Quantitative and Qualitative Analysis of Humoral Immunity Reveals Continued and Personalized Evolution in Chronic Viral Infection

Control of established chronic lymphocytic choriomeningitis virus (LCMV) infection requires the production of neutralizing antibodies, but it remains unknown how the ensemble of antibodies evolves during ongoing infection. Here, we analyze the evolution of antibody responses during acute or chronic LCMV infection, combining quantitative functional assays and time-resolved antibody repertoire sequencing. We establish that antibody responses initially converge in both infection types on a functional and repertoire level, but diverge later during chronic infection, showing increased clonal diversity, the appearance of mouse-specific persistent clones, and distinct phylogenetic signatures. Chronic infection is characterized by a longer-lasting germinal center reaction and a continuous differentiation of plasma cells, resulting in the emergence of higher-affinity plasma cells exhibiting increased antibody secretion rates. Taken together, our findings reveal the emergence of a personalized antibody response in chronic infection and support the concept that maintaining B cell diversity throughout chronic LCMV infection correlates with the development of infection-resolving antibodies.

IgM Antibody Repertoire Fingerprints in Mice Are Personalized but Robust to Viral Infection Status

Antibody repertoire sequencing provides a molecular fingerprint of current and past pathogens encountered by the immune system. Most repertoire studies in humans require measuring the B cell response in the blood, resulting in a large bias to the IgM isotype. The extent to which the circulating IgM antibody repertoire correlates to lymphoid tissue-resident B cells in the setting of viral infection remains largely uncharacterized. Therefore, we compared the IgM repertoires from both blood and bone marrow (BM) plasma cells (PCs) following acute or chronic lymphocytic choriomeningitis virus (LCMV) infection in mice. Despite previously reported serum alterations between acute and chronic infection, IgM repertoire signatures based on clonal diversity metrics, public clones, network, and phylogenetic analysis were largely unable to distinguish infection cohorts. Our findings, however, revealed mouse-specific repertoire fingerprints between the blood and PC repertoires irrespective of infection status.

Dysregulation of humoral immunity in chronic infection

Chronic viral infections disrupt the ability of the humoral immune response to produce neutralizing antibody or form effective immune memory, preventing viral clearance and making vaccine design difficult. Multiple components of the B-cell response are affected by pathogens that are not cleared from the host. Changes in the microenvironment shift production of B cells to short-lived plasma cells early in the response. Polyclonal B cells are recruited into both the plasma cell and germinal center compartments, inhibiting the formation of a targeted, high-affinity response. Finally, memory B cells shift toward an "atypical" phenotype, which may in turn result in changes to the functional properties of this population. While similar properties of B-cell dysregulation have been described across different types of persistent infections, key questions about the underlying mechanisms remain. This review will discuss the recent advances in this field, as well as highlight the critical questions about the interplay between viral load, microenvironment, the polyclonal response and atypical memory B cells that are yet to be answered. Design of new preventative treatments will rely on identifying the extrinsic and intrinsic modulators that push B cells toward an ineffective response, and thus identify new ways to guide them back onto the best path for clearance of virus and formation of effective immune memory.

What if protective immunity is antigen-driven and not due to so-called "memory" B and T cells?

Vaccines or early childhood exposure to infection mediate immunity, that is, improved resistance against disease and death caused by a second infection with the same agent. This has been explained by and equaled to immunological memory, that is, an "altered immune system behavior" that is maintained in a presumably antigen-independent fashion. This review summarizes epidemiological and experimental data, that largely falsify this idea and that show that periodic re-exposure to antigen either, artificially as vaccines or naturally as low-level persisting antigens or infections, or immune complexes on follicular dendritic cells or endemic re-exposure is necessary for protection. Both, the huge success of vaccines in controlling childhood infections, the reduction in clinical disease and the chance of endemically re-exposure, have gradually reduced periodical re-exposure to infections and thereby endangered protective herd immunity. In parallel, vaccine deniers have created susceptibility islands even in an otherwise well vaccinated population, thereby creating a very new situation when compared to the later parts of the 20th century. If protective Immunity is-as emphasized here-antigen driven, then increasingly frequent revaccinations will be necessary (even more so with too much attenuated vaccines) to maintain both herd immunity and individual resistance to acute infections. Of course, this rule also applies to tumor vaccines.

The Role of Interferon in Persistent Viral Infection: Insights from Murine Norovirus

Persistent viral infections result from evasion or avoidance of sterilizing immunity, extend the timeframe of virus transmission, and can trigger disease. Prior studies in mouse models of persistent infection have suggested that ineffective adaptive immune responses are necessary for persistent viral infection. However, recent work in the murine norovirus (MNV) model of persistent infection demonstrates that innate immunity can control both early and persistent viral replication independently of adaptive immune effector functions. Interferons (IFNs) are central to the innate control of persistent MNV, apart from a role in modulating adaptive immunity. Furthermore, subtypes of IFN play distinct tissue-specific roles in innate control of persistent MNV infection. Type I IFN (IFN-α/β) controls systemic replication, and type III IFN (IFN-λ) controls MNV persistence in the intestinal epithelium. In this article, we review recent findings in the MNV model, highlighting the role of IFNs and innate immunity in clearing persistent viral infection, and discussing the broader implications of these findings for control of persistent human infections.

CD4+ T Cell Differentiation in Chronic Viral Infections: The Tfh Perspective

CD4+ T cells play a critical role in the response to chronic viral infections during the acute phase and in the partial containment of infections once chronic infection is established. As infection persists, the virus-specific CD4+ T cell response begins to shift in phenotype. The predominant change described in both mouse and human studies of chronic viral infection is a decrease in detectable T helper type (Th)1 responses. Some Th1 loss is due to decreased proliferative potential and decreased cytokine production in the setting of chronic antigen exposure. However, recent data suggest that Th1 dysfunction is accompanied by a shift in the differentiation pathway of virus-specific CD4+ T cells, with enrichment for cells with a T follicular helper cell (Tfh) phenotype. A Tfh-like program during chronic infection has now been identified in virus-specific CD8+ T cells as well. In this review, we discuss what is known about CD4+ T cell differentiation in chronic viral infections, with a focus on the emergence of the Tfh program and the implications of this shift with respect to Tfh function and the host-pathogen interaction.

Immunological tolerance to LCMV antigens differently affects control of acute and chronic virus infection in mice

Cytotoxic T lymphocytes (CTLs) play a key role in the control of lymphocytic choriomeningitis virus (LCMV) infection. In C57BL/6 mice (H-2^b ), the CTL response is mainly directed against epitopes from the LCMV glycoprotein (GP) and the nucleoprotein (NP) which represent the two major viral proteins. The role of GP- versus NP-derived epitopes for viral clearance was examined using transgenic (tg) mice ubiquitously expressing LCMV GP and NP, respectively. These mice lack GP- or NP-specific CTLs and show decreased levels of GP- or NP-specific antibodies as a result of tolerance induction. During acute LCMV infection, CTLs specific for GP- and NP-derived epitopes are generated with similar frequencies. Nonetheless, we found that lack of GP- but not of NP-specific CTLs abolished control of acute LCMV infection. In contrast, after high-dose or chronic LCMV infection, virus elimination was delayed to a similar extent in GP- and NP-tg mice. Thus, immunological tolerance to LCMV antigens differently affects virus clearance in acute and chronic infection settings. In addition, our data reveal that immunodominance of H-2^b -restricted LCMV-specific CTL epitopes and their antiviral activity do not strictly correlate.

Adoptive Transfer of Engineered Rhesus Simian Immunodeficiency Virus-Specific CD8+ T Cells Reduces the Number of Transmitted/Founder Viruses Established in Rhesus Macaques

AIDS virus infections are rarely controlled by cell-mediated immunity, in part due to viral immune evasion and immunodeficiency resulting from CD4+ T-cell infection. One likely aspect of this failure is that antiviral cellular immune responses are either absent or present at low levels during the initial establishment of infection. To test whether an extensive, timely, and effective response could reduce the establishment of infection from a high-dose inoculum, we adoptively transferred large numbers of T cells that were molecularly engineered with anti-simian immunodeficiency virus (anti-SIV) activity into rhesus macaques 3 days following an intrarectal SIV inoculation. To measure in vivo antiviral activity, we assessed the number of viruses transmitted using SIVmac239X, a molecularly tagged viral stock containing 10 genotypic variants, at a dose calculated to transmit 12 founder viruses. Single-genome sequencing of plasma virus revealed that the two animals receiving T cells expressing SIV-specific T-cell receptors (TCRs) had significantly fewer viral genotypes than the two control animals receiving non-SIV-specific T cells (means of 4.0 versus 7.5 transmitted viral genotypes; P = 0.044). Accounting for the likelihood of transmission of multiple viruses of a particular genotype, the calculated means of the total number of founder viruses transmitted were 4.5 and 14.5 in the experimental and control groups, respectively (P = 0.021). Thus, a large antiviral T-cell response timed with virus exposure can limit viral transmission. The presence of strong, preexisting T-cell responses, including those induced by vaccines, might help prevent the establishment of infection at the lower-exposure doses in humans that typically transmit only a single virus.

IMPORTANCE

The establishment of AIDS virus infection in an individual is essentially a race between the spreading virus and host immune defenses. Cell-mediated immune responses induced by infection or vaccination are important contributors in limiting viral replication. However, in human immunodeficiency virus (HIV)/SIV infection, the virus usually wins the race, irreversibly crippling the immune system before an effective cellular immune response is developed and active. We found that providing an accelerated response by adoptively transferring large numbers of antiviral T cells shortly after a high-dose mucosal inoculation, while not preventing infection altogether, limited the number of individual viruses transmitted. Thus, the presence of strong, preexisting T-cell responses, including those induced by vaccines, might prevent infection in humans, where the virus exposure is considerably lower.

Dendritic Cells Are Dispensable for T Cell Priming and Control of Acute Lymphocytic Choriomeningitis Virus Infection

Dendritic cells (DCs) are considered to be the major APCs with potent activity for priming of naive CD4 and CD8 T cells. However, T cell priming can also be achieved by other APCs including macrophages, B cells, or even nonhematopoietic cell types. Systemic low-dose infection of mice with lymphocytic choriomeningitis virus (LCMV) results in massive expansion of virus-specific CD4 and CD8 T cells. To determine the role of DCs as APCs and source of type I IFNs in this infection model, we used ΔDC mice in which DCs are constitutively ablated because of expression of the diphtheria toxin α subunit within developing DCs. ΔDC mice showed lower serum concentrations of IFN-β and IL-12p40, but normal IFN-α levels during the first days postinfection. No differences were found for proliferation of transferred TCR-transgenic cells during the early phase of infection, suggesting that T cell priming occurred with the same efficiency in wild-type and ΔDC mice. Expansion and cytokine expression of endogenous LCMV-specific T cells was comparable between wild-type and ΔDC mice during primary infection and upon rechallenge of memory mice. In both strains of infected mice the viral load was reduced below the limit of detection with the same kinetic. Further, germinal center formation and LCMV-specific Ab responses were not impaired in ΔDC mice. This indicates that DCs are dispensable as APCs for protective immunity against LCMV infection.

Innate and Adaptive Immune Regulation During Chronic Viral Infections

Chronic viral infections represent a unique challenge to the infected host. Persistently replicating viruses outcompete or subvert the initial antiviral response, allowing the establishment of chronic infections that result in continuous stimulation of both the innate and adaptive immune compartments. This causes a profound reprogramming of the host immune system, including attenuation and persistent low levels of type I interferons, progressive loss (or exhaustion) of CD8(+) T cell functions, and specialization of CD4(+) T cells to produce interleukin-21 and promote antibody-mediated immunity and immune regulation. Epigenetic, transcriptional, posttranscriptional, and metabolic changes underlie this adaptation or recalibration of immune cells to the emerging new environment in order to strike an often imperfect balance between the host and the infectious pathogen. In this review we discuss the common immunological hallmarks observed across a range of different persistently replicating viruses and host species, the underlying molecular mechanisms, and the biological and clinical implications.

T Follicular Helper Cell-Dependent Clearance of a Persistent Virus Infection Requires T Cell Expression of the Histone Demethylase UTX

Epigenetic changes, including histone methylation, control T cell differentiation and memory formation, though the enzymes that mediate these processes are not clear. We show that UTX, a histone H3 lysine 27 (H3K27) demethylase, supports T follicular helper (Tfh) cell responses that are essential for B cell antibody generation and the resolution of chronic viral infections. Mice with a T cell-specific UTX deletion had fewer Tfh cells, reduced germinal center responses, lacked virus-specific immunoglobulin G (IgG), and were unable to resolve chronic lymphocytic choriomeningitis virus infections. UTX-deficient T cells showed decreased expression of interleukin-6 receptor-α and other Tfh cell-related genes that were associated with increased H3K27 methylation. Additionally, Turner Syndrome subjects, who are predisposed to chronic ear infections, had reduced UTX expression in immune cells and decreased circulating CD4(+) CXCR5(+) T cell frequency. Thus, we identify a critical link between UTX in T cells and immunity to infection.

Therapeutic antiviral T cells noncytopathically clear persistently infected microglia after conversion into antigen-presenting cells

Clearance of neurotropic infections is challenging because the CNS is relatively intolerant of immunopathological reactions. Herz et al. use a model of persistent viral infection in mice to demonstrate therapeutic antiviral T cells can purge the CNS infection without causing tissue damage resulting from limited recruitment of inflammatory innate immune cells and conversion of microglia into APCs.

Several viruses can infect the mammalian nervous system and induce neurological dysfunction. Adoptive immunotherapy is an approach that involves administration of antiviral T cells and has shown promise in clinical studies for the treatment of peripheral virus infections in humans such as cytomegalovirus (CMV), Epstein-Barr virus (EBV), and adenovirus, among others. In contrast, clearance of neurotropic infections is particularly challenging because the central nervous system (CNS) is relatively intolerant of immunopathological reactions. Therefore, it is essential to develop and mechanistically understand therapies that noncytopathically eradicate pathogens from the CNS. Here, we used mice persistently infected from birth with lymphocytic choriomeningitis virus (LCMV) to demonstrate that therapeutic antiviral T cells can completely purge the persistently infected brain without causing blood–brain barrier breakdown or tissue damage. Mechanistically, this is accomplished through a tailored release of chemoattractants that recruit antiviral T cells, but few pathogenic innate immune cells such as neutrophils and inflammatory monocytes. Upon arrival, T cells enlisted the support of nearly all brain-resident myeloid cells (microglia) by inducing proliferation and converting them into CD11c⁺ antigen-presenting cells (APCs). Two-photon imaging experiments revealed that antiviral CD8⁺ and CD4⁺ T cells interacted directly with CD11c⁺ microglia and induced STAT1 signaling but did not initiate programmed cell death. We propose that noncytopathic CNS viral clearance can be achieved by therapeutic antiviral T cells reliant on restricted chemoattractant production and interactions with apoptosis-resistant microglia.

Requirement of immune system heterogeneity for protective immunity

Although our knowledge on the immune system and immunological memory has expanded enormously during the last decades, the development of strategies to induce robust protective memory against infections and tumors remains challenging. Intense efforts and immense resources have been put into the development of vaccines. However, effective tools to assess protective immunity, beyond neutralizing antibody titers and cytotoxic T cell activity, are still missing. Previous trials have primarily focused on individual cell subsets to induce and maintain protection while current research emphasizes the importance of functional heterogeneity and necessity of efficient communication within the immunological network. In this review, established knowledge as well as current perspectives on protective immunological memory will be discussed comprehensively.

NK cells inhibit humoral immunity by reducing the abundance of CD4+ T follicular helper cells during a chronic virus infection

There is a need to understand better how to improve B cell responses and immunity to persisting virus infections, which often cause debilitating illness or death. People with chronic virus infection show evidence of improved virus control when there is a strong neutralizing antibody response, and conversely, B cell dysfunction is associated with higher viral loads. We showed previously that NK cells inhibit CD4(+) and CD8(+) T cell responses to disseminating LCMV infection and that depletion of NK cells attenuates chronic infection. Here, we examined the effect of NK cell depletion on B cell responses to LCMV infection in mice. Whereas mice infected acutely generated a peak level of antibody soon after the infection was resolved, mice infected chronically showed a continued increase in antibody levels that exceeded those after acute infection. We found that early NK cell depletion rapidly increased virus-specific antibody levels to chronic infection, and this effect depended on CD4(+) T cells and was associated with elevated numbers of CXCR5(+)CD4(+) TFH cells. However, the NK cell-depleted mice controlled the infection and by 1 mo pi, had lower TFH cell numbers and antibody levels compared with mice with sustained infection. Finally, we show that NK cell depletion improved antiviral CD8(+) T cell responses only when B cells and virus-specific antibody were present. Our data indicate that NK cells diminish immunity to chronic infection, in part, by suppressing TFH cell and antibody responses.

T cell lipid peroxidation induces ferroptosis and prevents immunity to infection

Matsushita et al. investigated the role of the selenoenzyme glutathione peroxidae 4 (Gpx4) in T cell responses and found that loss of Gpx4 results in an intrinsic T cell developmental defect in the periphery, which leads to a failure to expand and protect from acute viral and parasitic infection.The defects were rescued with dietary supplementation of vitamin E. The Gp4^−/− T cells accumulate membrane lipid peroxides and undergo cell death by ferroptosis.

The selenoenzyme glutathione peroxidase 4 (Gpx4) is a major scavenger of phospholipid hydroperoxides. Although Gpx4 represents a key component of the reactive oxygen species-scavenging network, its relevance in the immune system is yet to be defined. Here, we investigated the importance of Gpx4 for physiological T cell responses by using T cell–specific Gpx4-deficient mice. Our results revealed that, despite normal thymic T cell development, CD8⁺ T cells from T^{ΔGpx4/ΔGpx4} mice had an intrinsic defect in maintaining homeostatic balance in the periphery. Moreover, both antigen-specific CD8⁺ and CD4⁺ T cells lacking Gpx4 failed to expand and to protect from acute lymphocytic choriomeningitis virus and Leishmania major parasite infections, which were rescued with diet supplementation of high dosage of vitamin E. Notably, depletion of the Gpx4 gene in the memory phase of viral infection did not affect T cell recall responses upon secondary infection. Ex vivo, Gpx4-deficient T cells rapidly accumulated membrane lipid peroxides and concomitantly underwent cell death driven by ferroptosis but not necroptosis. These studies unveil an essential role of Gpx4 for T cell immunity.

T-cell STAT3 is required for the maintenance of humoral immunity to LCMV

STAT3 is a critical transcription factor activated downstream of cytokine signaling and is integral for the function of multiple immune cell types. Human mutations in STAT3 cause primary immunodeficiency resulting in impaired control of a variety of infections, including reactivation of latent viruses. In this study, we investigate how T-cell functions of STAT3 contribute to responses to viral infection by inducing chronic lymphocytic choriomeningitis virus (LCMV) infection in mice lacking STAT3 specifically in T cells. Although mice with conditional disruption of STAT3 in T cells were able to mount early responses to viral infection similar to control animals, including expansion of effector T cells, we found generation of T-follicular helper (Tfh) cells to be impaired. As a result, STAT3 T cell deficient mice produced attenuated germinal center reactions, and did not accumulate bone marrow virus specific IgG-secreting cells, resulting in failure to maintain levels of virus-specific IgG or mount neutralizing responses to LCMV in the serum. These effects were associated with reduced control of viral replication and prolonged infection. Our results demonstrate the importance of STAT3 in T cells for the generation of functional long-term humoral immunity to viral infections.

Plasticity and complexity of B cell responses against persisting pathogens

Vaccines against acute infections execute their protective effects almost exclusively via the induction of antibodies. Development of protective vaccines against persisting pathogens lags behind probably because standard immunogens and application regimen do not sufficiently stimulate those circuits in B cell activation that mediate protection. In general, B cell responses against pathogen derived-antigens are generated through complex cellular interactions requiring the coordination of innate and adaptive immune mechanisms. In this review, we summarize recent findings from prototypic infection models to exemplify how generation of protective antibodies against persisting pathogens is imprinted by particular pathogen-derived factors and how distinct CD4(+) T cell populations determine the quality of these antibodies. Clearly, it is the high plasticity of these processes that is instrumental to drive tailored B cell responses that protect the host. In sum, application of novel knowledge on B cell plasticity and complexity can guide the development of rationally designed vaccines that elicit protective antibodies against persisting pathogens.

Genetic variants in human leukocyte antigen-DP influence both hepatitis C virus persistence and hepatitis C virus F protein generation in the Chinese Han population

Chronic hepatitis C is a serious liver disease that often results in cirrhosis or hepatocellular carcinoma. The aim of this study was to assess the association of human leukocyte antigen-DP (HLA-DP) variants with risk of chronic hepatitis C virus (HCV) or anti-F antibody generation. We selected two single nucleotide polymorphisms (SNPs) in a region including HLA-DPA1 (rs3077) and HLA-DPB1 (rs9277534) and genotyped SNPs in 702 cases and 342 healthy controls from the Chinese population using TaqMan SNP genotyping assay. Moreover, the exon 2 of the HLA-DPA1 and HLA-DPB1 genes were amplified and determined by sequencing-based typing (SBT). The results showed that rs3077 significantly increased the risk of chronic HCV infection in additive models and dominant models (odds ratio (OR) = 1.32 and 1.53). The rs3077 also contributed to decrease the risk of anti-F antibody generation in additive models and dominant models (OR = 0.46 and 0.56). Subsequent analyses revealed the risk haplotypes (DPA1*0103-DPB1*0501 and DPA1*0103-DPB1*0201) and protective haplotypes (DPA1*0202-DPB1*0501 and DPA1*0202-DPB1*0202) to chronic HCV infection. Moreover, we also found that the haplotype of DPA1*0103-DPB1*0201 and DPA1*0202-DPB1*0202 were associated with the anti-F antibody generation. Our findings show that genetic variants in HLA-DP gene are associated with chronic HCV infection and anti-F antibody generation.

IK induced by coxsackievirus B3 infection transiently downregulates expression of MHC class II through increasing cAMP

Major histocompatibility complex (MHC) class II expression is critical for the presentation of antigens in the immune response to viral infection. Consequently, some viruses regulate the MHC class II-mediated presentation of viral antigens as a mechanism of immune escape. In this study, we found that Coxsackievirus B3 (CVB3) infection transiently increased IK expression, which reduced the expression of MHC class II (I-A/I-E) on splenic B cells. Interestingly, CVB3-induced IK elevated cAMP, a downstream molecule of the G protein-coupled receptors, which inhibited MHC class II presentation on B cells. Transgenic mice expressing truncated IK showed lower expression of MHC class II on B cells than did wild-type mice after CVB3 infection. Taken together, these results imply that IK plays a role in downregulating MHC class II expression on B cells during CVB3 infection through the induction of cAMP.

Nucleoprotein-specific nonneutralizing antibodies speed up LCMV elimination independently of complement and FcγR

CD8(+) T cells have an essential role in controlling lymphocytic choriomeningitis virus (LCMV) infection in mice. Here, we examined the contribution of humoral immunity, including nonneutralizing antibodies (Abs), in this infection induced by low virus inoculation doses. Mice with impaired humoral immunity readily terminated infection with the slowly replicating LCMV strain Armstrong but showed delayed virus elimination after inoculation with the faster replicating LCMV strain WE and failed to clear the rapidly replicating LCMV strain Docile, which is in contrast to the results obtained with wild-type mice. Thus, the requirement for adaptive humoral immunity to control the infection was dependent on the replication speed of the LCMV strains used. Ab transfers further showed that LCMV-specific IgG Abs isolated from LCMV immune serum accelerated virus elimination. These Abs were mainly directed against the viral nucleoprotein (NP) and completely lacked virus neutralizing activity. Moreover, mAbs specific for the LCMV NP were also able to decrease viral titers after transfer into infected hosts. Intriguingly, neither C3 nor Fcγ receptors were required for the antiviral activity of the transferred Abs. In conclusion, our study suggests that rapidly generated nonneutralizing Abs specific for the viral NP speed up virus elimination and thereby may counteract T-cell exhaustion.

Antigen experience shapes phenotype and function of memory Th1 cells

Primary and secondary (boosted) memory CD8 T cells exhibit differences in gene expression, phenotype and function. The impact of repeated antigen stimulations on memory CD4 T cells is largely unknown. To address this issue, we utilized LCMV and Listeria monocytogenes infection of mice to characterize primary and secondary antigen (Ag)-specific Th1 CD4 T cell responses. Ag-specific primary memory CD4 T cells display a CD62L^loCCR7^hi CD27^hi CD127^hi phenotype and are polyfunctional (most produce IFNγ, TNFα and IL-2). Following homologous prime-boost immunization we observed pathogen-specific differences in the rate of CD62L and CCR7 upregulation on memory CD4 T cells as well as in IL-2+IFNγco-production by secondary effectors. Phenotypic and functional plasticity of memory Th1 cells was observed following heterologous prime-boost immunization, wherein secondary memory CD4 T cells acquired phenotypic and functional characteristics dictated by the boosting agent rather than the primary immunizing agent. Our data also demonstrate that secondary memory Th1 cells accelerated neutralizing Ab formation in response to LCMV infection, suggesting enhanced capacity of this population to provide quality help for antibody production. Collectively these data have important implications for prime-boost vaccination strategies that seek to enhance protective immune responses mediated by Th1 CD4 T cell responses.

CD4 T cell responses in latent and chronic viral infections

The spectrum of tasks which is fulfilled by CD4 T cells in the setting of viral infections is large, ranging from support of CD8 T cells and humoral immunity to exertion of direct antiviral effector functions. While our knowledge about the differentiation pathways, plasticity, and memory of CD4 T cell responses upon acute infections or immunizations has significantly increased during the past years, much less is still known about CD4 T cell differentiation and their beneficial or pathological functions during persistent viral infections. In this review we summarize current knowledge about the differentiation, direct or indirect antiviral effector functions, and the regulation of virus-specific CD4 T cells in the setting of persistent latent or active chronic viral infections with a particular emphasis on herpes virus infections for the former and chronic lymphocytic choriomeningitis virus infection for the latter.

Alternative serotype adenovirus vaccine vectors elicit memory T cells with enhanced anamnestic capacity compared to Ad5 vectors

The failure of the adenovirus serotype 5 (Ad5) vector-based human immunodeficiency virus type 1 (HIV-1) vaccine in the STEP study has led to the development of adenovirus vectors derived from alternative serotypes, such as Ad26, Ad35, and Ad48. We have recently demonstrated that vaccines using alternative-serotype Ad vectors confer partial protection against stringent simian immunodeficiency virus (SIV) challenges in rhesus monkeys. However, phenotypic differences between the T cell responses elicited by Ad5 and those of alternative-serotype Ad vectors remain unexplored. Here, we report the magnitude, phenotype, functionality, and recall capacity of memory T cell responses elicited in mice by Ad5, Ad26, Ad35, and Ad48 vectors expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP). Our data demonstrate that memory T cells elicited by Ad5 vectors were high in magnitude but exhibited functional exhaustion and decreased anamnestic potential following secondary antigen challenge compared to Ad26, Ad35, and Ad48 vectors. These data suggest that vaccination with alternative-serotype Ad vectors offers substantial immunological advantages over Ad5 vectors, in addition to circumventing high baseline Ad5-specific neutralizing antibody titers.

Polyfunctional type-1, -2, and -17 CD8⁺ T cell responses to apoptotic self-antigens correlate with the chronic evolution of hepatitis C virus infection

Caspase-dependent cleavage of antigens associated with apoptotic cells plays a prominent role in the generation of CD8⁺ T cell responses in various infectious diseases. We found that the emergence of a large population of autoreactive CD8⁺ T effector cells specific for apoptotic T cell-associated self-epitopes exceeds the antiviral responses in patients with acute hepatitis C virus infection. Importantly, they endow mixed polyfunctional type-1, type-2 and type-17 responses and correlate with the chronic progression of infection. This evolution is related to the selection of autoreactive CD8⁺ T cells with higher T cell receptor avidity, whereas those with lower avidity undergo prompt contraction in patients who clear infection. These findings demonstrate a previously undescribed strict link between the emergence of high frequencies of mixed autoreactive CD8⁺ T cells producing a broad array of cytokines (IFN-γ, IL-17, IL-4, IL-2…) and the progression toward chronic disease in a human model of acute infection.

Toll-like receptor 7 is required for effective adaptive immune responses that prevent persistent virus infection

TLR7 is an innate signaling receptor that recognizes single-stranded viral RNA and is activated by viruses that cause persistent infections. We show that TLR7 signaling dictates either clearance or establishment of life-long chronic infection by lymphocytic choriomeningitis virus (LCMV) Cl 13 but does not affect clearance of the acute LCMV Armstrong 53b strain. TLR7(-/-) mice infected with LCMV Cl 13 remained viremic throughout life from defects in the adaptive antiviral immune response-notably, diminished T cell function, exacerbated T cell exhaustion, decreased plasma cell maturation, and negligible antiviral antibody production. Adoptive transfer of TLR7(+/+) LCMV immune memory cells that enhanced clearance of persistent LCMV Cl 13 infection in TLR7(+/+) mice failed to purge LCMV Cl 13 infection in TLR7(-/-) mice, demonstrating that a TLR7-deficient environment renders antiviral responses ineffective. Therefore, methods that promote TLR7 signaling are promising treatment strategies for chronic viral infections.

Immunological memory ≠ protective immunity

So-called 'immunological memory' is, in my view, a typical example where a field of enquiry, i.e. to understand long-term protection to survive reexposure to infection, has been overtaken by 'l'art pour l'art' of 'basic immunology'. The aim of this critical review is to point out some key differences between academic text book-defined immunological memory and protective immunity as viewed from a co-evolutionary point of view, both from the host and the infectious agents. A key conclusion is that 'immunological memory' of course exists, but only in particular experimental laboratory models measuring 'quicker and better' responses after an earlier immunization. These often do correlate with, but are not the key mechanisms of, protection. Protection depends on pre-existing neutralizing antibodies or pre-activated T cells at the time of infection-as documented by the importance of maternal antibodies around birth for survival of the offspring. Importantly, both high levels of antibodies and of activated T cells are antigen driven. This conclusion has serious implications for our thinking about vaccines and maintaining a level of protection in the population to deal with old and new infectious diseases.

Modification of the carboxy-terminal flanking region of a universal influenza epitope alters CD4⁺ T-cell repertoire selection

Human CD4(+) αβ T cells are activated via T-cell receptor recognition of peptide epitopes presented by major histocompatibility complex (MHC) class II (MHC-II). The open ends of the MHC-II binding groove allow peptide epitopes to extend beyond a central nonamer core region at both the amino- and carboxy-terminus. We have previously found that these non-bound C-terminal residues can alter T cell activation in an MHC allele-transcending fashion, although the mechanism for this effect remained unclear. Here we show that modification of the C-terminal peptide-flanking region of an influenza hemagglutinin (HA(305-320)) epitope can alter T-cell receptor binding affinity, T-cell activation and repertoire selection of influenza-specific CD4(+) T cells expanded from peripheral blood. These data provide the first demonstration that changes in the C-terminus of the peptide-flanking region can substantially alter T-cell receptor binding affinity, and indicate a mechanism through which peptide flanking residues could influence repertoire selection.

Evaluation of the immunomodulatory and antiviral effects of the cytokine combination IFN-α and IL-7 in the lymphocytic choriomeningitis virus and Friend retrovirus mouse infection models

Existing therapies for chronic viral infections are still suboptimal or have considerable side effects, so new therapeutic strategies need to be developed. One option is to boost the host's immune response with cytokines. We have recently shown in an acute ex vivo HIV infection model that co-administration of interferon (IFN)-α and interleukin (IL)-7 allows us to combine the potent anti-HIV activity of IFN-α with the beneficial effects of IL-7 on T-cell survival and function. Here we evaluated the effect of combining IFN-α and IL-7 on viral replication in vivo in the chronic lymphocytic choriomeningitis virus (LCMV) and acute Friend retrovirus (FV) infection models. In the chronic LCMV model, cytokine treatment was started during the early replication phase (i.e., on day 7 post-infection [pi]). Under the experimental conditions used, exogenous IFN-α inhibited FV replication, but had no effect on viral replication in the LCMV model. There was no therapeutic benefit of IL-7 either alone or in combination with IFN-α in either of the two infection models. In the LCMV model, dose-dependent effects of the cytokine combination on T-cell phenotype/function were observed. It is possible that these effects would translate into antiviral activity in re-challenged mice. It is also possible that another type of IFN-α/β or induction of endogenous IFN-α/β alone or in combination with IL-7 would have antiviral activity in the LCMV model. Furthermore, we cannot exclude that some effect on viral titers would have been seen at later time points not investigated here (i.e., beyond day 34 pi). Finally, IFN-α/IL-7 may inhibit the replication of other viruses. Thus it might be worth testing these cytokines in other in vivo models of chronic viral infections.

Vaccination against lymphocytic choriomeningitis virus infection in MHC class II-deficient mice

The impact of prophylactic vaccination against acute and chronic infection in a Th-deficient host has not been adequately addressed because of difficulties in generating protective immunity in the absence of CD4(+) T cell help. In this study, we demonstrated that a broad CD8(+) T cell immune response could be elicited in MHC class II-deficient mice by vaccination with adenovirus encoding lymphocytic choriomeningitis virus (LCMV) glycoprotein tethered to MHC class II-associated invariant chain. Moreover, the response induced conferred significant cytolytic CD8(+) T cell-mediated protection against challenge with a high dose of the invasive clone 13 strain of LCMV. In contrast, vaccination with adenovirus encoding unlinked LCMV glycoprotein induced weak virus control in the absence of CD4(+) T cells, and mice may die of increased immunopathology associated with incomplete protection. Acute mortality was not observed in any vaccinated mice following infection with the less-invasive Traub strain. However, LCMV Traub infection caused accelerated late mortality in unvaccinated MHC class II-deficient mice; in this case, we observed a strong trend toward delayed mortality in vaccinated mice, irrespective of the nature of the vaccine. These results indicated that optimized vaccination may lead to efficient protection against acute viral infection, even in Th-deficient individuals, but that the duration of such immunity is limited. Nevertheless, for select immunodeficiencies in which CD4(+) T cell deficiency is incomplete or transient, these results are very encouraging.

Characterization of the specific CD4+ T cell response against the F protein during chronic hepatitis C virus infection

BACKGROUND

The hepatitis C virus (HCV) Alternate Reading Frame Protein (ARFP or F protein) presents a double-frame shift product of the HCV core gene. We and others have previously reported that the specific antibodies against the F protein could be raised in the sera of HCV chronically infected patients. However, the specific CD4(+) T cell responses against the F protein during HCV infection and the pathological implications remained unclear. In the current study, we screened the MHC class II-presenting epitopes of the F protein through HLA-transgenic mouse models and eventually validated the specific CD4(+) T cell responses in HCV chronically infected patients.

METHODOLOGY

DNA vaccination in HLA-DR1 and-DP4 transgenic mouse models, proliferation assay to test the F protein specific T cell response, genotyping of Chronic HCV patients and testing the F-peptide stimulated T cell response in the peripheral blood mononuclear cell (PBMC) by in vitro expansion and interferon (IFN)- γ intracellular staining.

PRINCIPAL FINDINGS

At least three peptides within HCV F protein were identified as HLA-DR or HLA-DP4 presenting epitopes by the proliferation assays in mouse models. Further study with human PBMCs evidenced the specific CD4(+) T cell responses against HCV F protein as well in patients chronically infected with HCV.

CONCLUSION

The current study provided the evidence for the first time that HCV F protein could elicit specific CD4(+) T cell response, which may provide an insight into the immunopathogenesis during HCV chronic infection.

Correlates of antibody-mediated protection against HIV infection

PURPOSE OF REVIEW

The only unequivocal correlate of protection against primate immunodeficiency virus infection is the presence of neutralizing antibody at sufficient titre. This correlate has been determined experimentally using animal models, and the data are reproducible and robust. Recent advances have added further depth to this knowledge by moving us closer to understanding how antibodies neutralize HIV-1, and what effects they may have in vivo with regard to protection from infection and disease.

RECENT FINDINGS

This review will cover recent advances in our understanding of the structural basis of HIV-1 neutralization by antibody and how this understanding may relate to vaccine design, and incorporate this into the broader context of how antibodies may influence viral transmission, replication and disease.

SUMMARY

The sum of these findings provides a strong rationale for designing an HIV-1 vaccine on the principle of induction of neutralizing antibodies, although other effector functions of antibodies such as complement and antibody-mediated cellular immunity should also be borne in mind, as should CD4 and CD8 T cell responses.

Impaired antibody response causes persistence of prototypic T cell-contained virus

CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor gamma chain or Fc gamma receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell-controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.

Identification of HIV-1 epitopes that induce the synthesis of a R5 HIV-1 suppression factor by human CD4+ T cells isolated from HIV-1 immunized hu-PBL SCID mice

We have previously reported that immunization of the severe combined immunodeficiency (SCID) mice reconstituted with human peripheral blood mononuclear cells (PBMC) (hu-PBL-SCID mice) with inactivated human immunodeficiency virus type-1 (HIV-1)-pulsed-autologous dendritic cells (HIV-DC) elicits HIV-1-reactive CD4(+) T cells that produce an as yet to be defined novel soluble factor in vitro with anti-viral properties against CCR5 tropic (R5) HIV-1 infection. These findings led us to perform studies designed to identify the lineage of the cell that synthesizes such a factor in vivo and define the epitopes of HIV-1 protein that have specificity for the induction of such anti-viral factor. Results of our studies show that this property is a function of CD4(+) but not CD8(+) T cells. Human CD4(+) T cells were thus recovered from the HIV-DC-immunized hu-PBL-SCID mice and were re-stimulated in vitro by co-culture for 2 days with autologous adherent PBMC as antigen presenting cells, APC previously pulsed with inactivated HIV in IL-2-containing medium to expand HIV-1-reactive CD4(+) T cells. Aliquots of these re-stimulated CD4(+) T cells were then co-cultured with similar APC's that were previously pulsed with 10 microg/ml of a panel of HIV peptides for an additional 2 days, and their culture supernatants were examined for the production of both the R5 HIV-1 suppression factor and IFN-gamma. The data presented herein show that the HIV-1 primed CD4(+) T cells produced the R5 suppression factor in response to a wide variety of HIV-1 gag, env, pol, nef or vif peptides, depending on the donor of the CD4(+) T cells. Simultaneous production of human interferon (IFN)-gamma was observed in some cases. These results indicate that human CD4(+) T cells in PBMC of HIV-1 naive donors have a wide variety of HIV-1 epitope-specific CD4(+) T cell precursors that are capable of producing the R5 HIV-1 suppression factor upon DC-based vaccination with whole inactivated HIV-1.

CD4+ T cell help improves CD8+ T cell memory by retained CD27 expression

CD4+ T cell help during the priming of CD8+ T lymphocytes imprints the capacity for optimal secondary expansion upon re-encounter with antigen. Helped memory CD8+ T cells rapidly expand in response to a secondary antigen exposure, even in the absence of T cell help and, are most efficient in protection against a re-infection. In contrast, helpless memory CTL can mediate effector function, but secondary expansion is reduced. How CD4+ T cells instruct CD8+ memory T cells during priming to undergo efficient secondary expansion has not been resolved in detail. Here, we show that memory CTL after infection with lymphocytic choriomeningitis virus are CD27(high) whereas memory CTL primed in the absence of CD4+ T cell have a reduced expression of CD27. Helpless memory CTL produced low amounts of IL-2 and did not efficiently expand after restimulation with peptide in vitro. Blocking experiments with monoclonal antibodies and the use of CD27(-/-) memory CTL revealed that CD27 ligation during restimulation increased autocrine IL-2 production and secondary expansion. Therefore, regulating CD27 expression on memory CTL is a novel mechanism how CD4+ T cells control CTL memory.

A novel virus carrier state to evaluate immunotherapeutic regimens: regulatory T cells modulate the pathogenicity of antiviral memory cells

Restrictions in the diversity of an adaptive immune repertoire can facilitate viral persistence. Because a host afflicted with an immune deficiency is not likely to purge a persistent infection using endogenous mechanisms, it is important to explore adoptive therapies to supplement the host with a functional immune defense. In this study, we describe a virus carrier state that results from introducing lymphocytic choriomeningitis virus (LCMV) into adult mice possessing a restricted T cell repertoire. On infection of these mice, LCMV establishes systemic persistence, and within the CNS the virus infects astrocytes (and later oligodendrocytes) rather than its traditional parenchymal target neurons. To determine whether LCMV could be purged from a novel target selection in the absence of an endogenous immune repertoire, we adoptively transferred virus-specific memory cells into adult carrier mice. The memory cells purged virus from the periphery as well as the CNS, but they induced fatalities not typically associated with adoptive immunotherapy. When the repertoire of the recipient mice was examined, a deficiency in natural regulatory T cells was noted. We therefore supplemented carrier mice with regulatory T cells and simultaneously performed adoptive immunotherapy. Cotransfer of regulatory T cells significantly reduced mortality while still permitting the antiviral memory cells to purge the persistent infection. These data indicate that regulatory T cells can be used therapeutically to lessen the pathogenicity of virus-specific immune cells in an immunodeficient host. We also propose that the novel carrier state described herein will facilitate the study of immunotherapeutic regimens.

Lymphocytic choriomeningitis infection of the central nervous system

Viral infection of the central nervous system (CNS) can result in a multitude of responses including pathology, persistence or immune clearance. Lymphocytic choriomeningitis virus (LCMV) is a powerful model system to explore these potential outcomes of CNS infection due to the diversity of responses that can be achieved after viral inoculation. Several factors including tropism, timing, dose and variant of LCMV in combination with the development or suppression of the corresponding immune response dictates whether lethal meningitis, chronic infection or clearance of LCMV in the CNS will occur. Importantly, the functionality and positioning of the LCMV-specific CD8+ T cell response are critical in directing the subsequent outcome of CNS LCMV infection. Although a basic understanding of LCMV and immune interactions in the brain exists, the molecular machinery that shapes the balance between pathogenesis and clearance in the LCMV-infected CNS remains to be elucidated. This review covers the various outcomes of LCMV infection in the CNS and what is currently known about the impact of the virus itself versus the immune response in the development of disease or clearance.

Extralymphatic virus sanctuaries as a consequence of potent T-cell activation

T helper cells can support the functions of CD8(+) T cells against persistently infecting viruses such as murine lymphocytic choriomeningitis virus (LCMV), cytomegalovirus, hepatitis C virus and HIV. These viruses often resist complete elimination and remain detectable at sanctuary sites, such as the kidneys and other extralymphatic organs. The mechanisms underlying this persistence are not well understood. Here we show that mice with potent virus-specific T-cell responses have reduced levels and delayed formation of neutralizing antibodies, and these mice fail to clear LCMV from extralymphatic epithelia. Transfer of virus-specific B cells but not virus-specific T cells augmented virus clearance from persistent sites. Virus elimination from the kidneys was associated with the formation of IgG deposits in the interstitial space, presumably from kidney-infiltrating B cells. CD8(+) T cells in the kidneys of mice that did not clear virus from this site were activated but showed evidence of exhaustion. Thus, we conclude that in this model of infection, site-specific virus persistence develops as a consequence of potent immune activation coupled with reductions in virus-specific neutralizing antibodies. Our results suggest that sanctuary-site formation depends both on organ anatomy and on the induction of different adaptive immune effector mechanisms. Boosting T-cell responses alone may not reduce virus persistence.

Chronic lymphocytic choriomeningitis virus infection actively down-regulates CD4+ T cell responses directed against a broad range of epitopes

Activation of CD4(+) T cells helps establish and sustain CD8(+) T cell responses and is required for the effective clearance of acute infection. CD4-deficient mice are unable to control persistent infection and CD4(+) T cells are usually defective in chronic and persistent infections. We investigated the question of how persistent infection impacted pre-existing lymphocytic choriomeningitis virus (LCMV)-specific CD4(+) T cell responses. We identified class II-restricted epitopes from the entire set of open reading frames from LCMV Armstrong in BALB/c mice (H-2(d)) acutely infected with LCMV Armstrong. Of nine epitopes identified, six were restricted by I-A(d), one by I-E(d) and two were dually restricted by both I-A(d) and I-E(d) molecules. Additional experiments revealed that CD4(+) T cell responses specific for these epitopes were not generated following infection with the immunosuppressive clone 13 strain of LCMV. Most importantly, in peptide-immunized mice, established CD4(+) T cell responses to these LCMV CD4 epitopes as well as nonviral, OVA-specific responses were actively suppressed following infection with LCMV clone 13 and were undetectable within 12 days after infection, suggesting an active inhibition of established helper responses. To address this dysfunction, we performed transfer experiments using both the Smarta and OT-II systems. OT-II cells were not detected after clone 13 infection, indicating physical deletion, while Smarta cells proliferated but were unable to produce IFN-gamma, suggesting impairment of the production of this cytokine. Thus, multiple mechanisms may be involved in the impairment of helper responses in the setting of early persistent infection.

Fully MHC-Disparate Mixed Hemopoietic Chimeras Show Specific Defects in the Control of Chronic Viral Infections

The establishment of mixed allogeneic chimerism can induce donor-specific transplantation tolerance across full MHC barriers. However, a theoretical disadvantage of this approach is the possibility that the state of mixed chimerism might negatively affect the recipient's immune competence to control pathogens. Previous studies using murine models have not supported this hypothesis, because they indicate that acute viral infections are cleared by chimeric animals with similar kinetics to that of unmanipulated controls. However, chronic or persistent viral infections often require a more complex and sustained response with cooperation between CD4 Th cells, CTL, and B cells for effective control. The current study indicates that profound defects become manifest in the control of chronic pathogenic infections in MHC-disparate mixed allogeneic chimeric mice. Furthermore, we show that ineffective priming of the donor-restricted CTL response leads to virus persistence, as well as severe T cell exhaustion. Our results further suggest that either T cell adoptive immunotherapy or selected MHC haplotype matching partially restore immune competence. These approaches may facilitate the translation of mixed chimerism therapeutic regimens.

Relative importance of T-cell subsets in monocytotropic ehrlichiosis: a novel effector mechanism involved in Ehrlichia-induced immunopathology in murine ehrlichiosis

Infection with gram-negative monocytotropic Ehrlichia strains results in a fatal toxic shock-like syndrome characterized by a decreased number of Ehrlichia-specific CD4(+) Th1 cells, the expansion of tumor necrosis factor alpha (TNF-alpha)-producing CD8(+) T cells, and the systemic overproduction of interleukin-10 (IL-10) and TNF-alpha. Here, we investigated the role of CD4(+) and CD8(+) T cells in immunity to Ehrlichia and the pathogenesis of fatal ehrlichiosis caused by infection with low- and high-dose (10(3) and 10(5) bacterial genomes/mouse, respectively) ehrlichial inocula. The CD4(+) T-cell-deficient mice showed exacerbated susceptibility to a lethal high- or low-dose infection and harbored higher bacterial numbers than did wild-type (WT) mice. Interestingly, the CD8(+) T-cell-deficient mice were resistant to a low dose but succumbed to a high dose of Ehrlichia. The absence of CD8(+) T cells abrogated TNF-alpha and IL-10 production, reduced tissue injury and bacterial burden, restored splenic CD4(+) T-cell numbers, and increased the frequency of Ehrlichia-specific CD4(+) Th1 cells in comparison to infected WT mice. Although fatal disease is perforin independent, our data suggested that perforin played a critical role in controlling bacterial burden and mediating liver injury. Similar to WT mice, mortality of infected perforin-deficient mice was associated with CD4(+) T-cell apoptosis and a high serum concentration of IL-10. Depletion of IL-10 restored the number of CD4(+) and CD8(+) T cells in infected WT mice. Our data demonstrate a novel mechanism of immunopathology in which CD8(+) T cells mediate Ehrlichia-induced toxic shock, which is associated with IL-10 overproduction and CD4(+) T-cell apoptosis.

Clearance of an immunosuppressive virus from the CNS coincides with immune reanimation and diversification

Once a virus infection establishes persistence in the central nervous system (CNS), it is especially difficult to eliminate from this specialized compartment. Therefore, it is of the utmost importance to fully understand scenarios during which a persisting virus is ultimately purged from the CNS by the adaptive immune system. Such a scenario can be found following infection of adult mice with an immunosuppressive variant of lymphocytic choriomeningitis virus (LCMV) referred to as clone 13. In this study we demonstrate that following intravenous inoculation, clone 13 rapidly infected peripheral tissues within one week, but more slowly inundated the entire brain parenchyma over the course of a month. During the establishment of persistence, we observed that genetically tagged LCMV-specific cytotoxic T lymphocytes (CTL) progressively lost function; however, the severity of this loss in the CNS was never as substantial as that observed in the periphery. One of the most impressive features of this model system is that the peripheral T cell response eventually regains functionality at ~60–80 days post-infection, and this was associated with a rapid decline in virus from the periphery. Coincident with this "reanimation phase" was a massive influx of CD4 T and B cells into the CNS and a dramatic reduction in viral distribution. In fact, olfactory bulb neurons served as the last refuge for the persisting virus, which was ultimately purged from the CNS within 200 days post-infection. These data indicate that a functionally revived immune response can prevail over a virus that establishes widespread presence both in the periphery and brain parenchyma, and that therapeutic enhancement of an existing response could serve as an effective means to thwart long term CNS persistence.

Clarification of how HIV-1 DNA and protein immunizations may be better used to obtain HIV-1-specific mucosal and systemic immunity

More focused research on a mucosal HIV-1 vaccine is needed urgently. An increasing amount of collected data, using heterologous multimodality prime-booster strategies, suggest that an efficient and protective HIV-1 vaccine must generate broad, long-lasting HIV-specific CD8(+) cytotoxic T-lymphocyte and neutralizing antibody responses. In the mucosa, these responses would be most effective if a preferential stimulus of HIV-1 neutralizing secretory immunoglobulin A and G were obtained. The attractive property of mucosal immunization is the obtained mucosal and systemic immunity, whereas systemic immunization induces a more limited immunity, predominantly in systemic sites. These objectives will require new vaccine regimens, such as multiclade HIV DNA and protein vaccines (nef, tat, gag and env expressed in DNA plasmids) delivered onto mucosal surfaces with needle-free delivery methods, such as nasal drop, as well as oral and rectal/vaginal delivery, and should merit clinical trials.

Immunologic evidence for lack of heterologous protection following resolution of HCV in patients with non-genotype 1 infection

Chronic hepatitis C virus (HCV) infection is typically characterized by a lack of virus-specific CD4(+) T-cell-proliferative responses, but strong responses have been described in a subset of persons with persistent viremia. One possible explanation for these responses is that they were primed by an earlier resolved infection and do not recognize the current circulating virus. We defined all targeted epitopes using overlapping peptides corresponding to a genotype 1a strain in 44 patients chronically infected with different HCV genotypes (GT). Surprisingly, more HCV-specific CD4(+) T-cell responses were detected in patients with chronic non-GT1 infection compared with patients with chronic GT1 infection (P = .017). Notably, we found serologic evidence of a previous exposure to GT1 in 4 patients with non-GT1 infection, and these persons also demonstrated significantly more responses than non-GT1 patients in whom genotype and HCV serotype were identical (P < .001). Comparison of recognition of GT1-specific peptides to peptides representing autologous virus revealed the absence of cross-recognition of the autologous circulating virus. These data indicate that persistent HCV infection can occur in the presence of an HCV-specific T-cell response primed against a heterologous HCV strain, and suggest that clearance of 1 GT does not necessarily protect against subsequent exposure to a second GT.