Memory generation and maintenance of CD8+ T cell function during viral persistence

Understanding the complexities of SARS-CoV2 infection and its immunology: A road to immune-based therapeutics

Emerging infectious diseases always pose a threat to humans along with plant and animal life. SARS-CoV2 is the recently emerged viral infection that originated from Wuhan city of the Republic of China in December 2019. Now, it has become a pandemic. Currently, SARS-CoV2 has infected more than 27.74 million people worldwide, and taken 901,928 human lives. It was named first 'WH 1 Human CoV' and later changed to 2019 novel CoV (2019-nCoV). Scientists have established it as a zoonotic viral disease emerged from Chinese horseshoe bats, which do not develop a severe infection. For example, Rhinolophus Chinese horseshoe bats harboring severe acute respiratory syndrome-related coronavirus (SARSr-CoV) or SARSr-Rh-BatCoV appear healthy and clear the virus within 2-4 months period. The article introduces first the concept of EIDs and some past EIDs, which have affected human life. Next section discusses mysteries regarding SARS-CoV2 origin, its evolution, and human transfer. Third section describes COVID-19 clinical symptoms and factors affecting susceptibility or resistance. The fourth section introduces the SARS-CoV2 entry in the host cell, its replication, and the establishment of productive infection. Section five describes the host's immune response associated with asymptomatic, symptomatic, mild to moderate, and severe COVID-19. The subsequent seventh and eighth sections mention the immune status in COVID-19 convalescent patients and re-emergence of COVID-19 in them. Thereafter, the eighth section describes viral strategies to hijack the host antiviral immune response and generate the "cytokine storm". The ninth section describes about transgenic humane ACE2 (hACE2) receptor expressing mice to study immunity, drugs, and vaccines. The article ends with the development of different immunomodulatory and immunotherapeutics strategies, including vaccines waiting for their approval in humans as prophylaxis or treatment measures.

Proteome-wide analysis of T-cell response to BK polyomavirus in healthy virus carriers and kidney transplant recipients reveals a unique transcriptional and functional profile

Objectives

Cellular immunity against BK polyomavirus (BKV)‐encoded antigens plays a crucial role in long‐term protection against virus‐associated pathogenesis in transplant recipients. However, in‐depth understanding on dynamics of these cellular immune responses is required to develop better immune monitoring and immunotherapeutic strategies.

Methods

Here, we have conducted a proteome‐wide analysis of BKV‐specific T‐cell responses in a cohort of 53 healthy individuals and 26 kidney transplant recipients to delineate the functional and transcriptional profile of these effector cells and compared these characteristics to T cells directed against cytomegalovirus, which is also known to cause significant morbidity in transplant recipients.

Results

Profiling of BKV‐specific CD4⁺ and CD8⁺ T cells revealed that kidney transplant recipients with high levels of circulating viraemia showed significantly reduced T‐cell reactivity against large T and/or small T antigens when compared to healthy donors. Interestingly, T cells specific for these antigens showed strong cross‐recognition to orthologous JC virus (JCV) peptides, including those exhibiting varying degrees of sequence identity. Ex vivo functional and phenotypic characterisation revealed that the majority of BKV‐specific T cells from renal transplant recipients expressed low levels of the key transcriptional regulators T‐bet and eomesodermin, which was coincident with undetectable expression of granzyme B and perforin. However, in vitro stimulation of T cells with BKV epitopes selectively enhanced the expression of T‐bet, granzyme B and cellular trafficking molecules (CCR4, CD49d and CD103) with minimal change in eomesodermin and perforin.

Conclusions

These observations provide an important platform for the future development of immune monitoring and adoptive T‐cell therapy strategies for BKV‐associated diseases in transplant recipients, which may also be exploited for similar therapeutic value in JCV‐associated clinical complications.

Increased frequencies of circulating CCR5+ memory T cells are correlated to chronic chagasic cardiomyopathy progression

The infection with the protozoan parasite Trypanosoma cruzi causes Chagas disease, a neglected tropical disease in Latin America and an imported emerging disease worldwide. Chronic Chagasic cardiomyopathy (CCC), a progressive inflammatory and fibrosing disease, is the most prominent clinical form of Chagas disease, culminating in heart failure and high rates of sudden death. CCC pathogenesis is influenced by both host and parasite factors and is proposed to be mostly immune-driven. Chemokines are crucial players in orchestrating immune cell recruitment to infected tissues and inflammation. Herein, we investigated inflammatory chemokine receptor expression on circulating T cells in patients stratified by CCC severity. Compared to asymptomatic individuals, we found increased percentages of effector CD4⁺ T cells and central memory CD4⁺ and CD8⁺ T cells expressing CCR5 in patients with structural cardiopathy, but normal global ventricular function and no symptoms of chronic heart failure. Even naïve T cells expressed CCR5 in these patients. In contrast, reduced frequencies of CD4⁺ and CD8⁺ effector T cells expressing CXCR3 were observed in patients presenting with severe heart disease. Patients with increased left ventricular diameter, heart enlargement, and insufficiency had higher frequencies of CCR5⁺ effector and effector memory CD8⁺ T cells. Moreover, the percentage of effector CCR5⁺ CD8⁺ T cells was increased in patients with a reduced ejection fraction. Our results show that high expression CCR5 and low expression of CXCR3 on circulating T cells are associated with worse prognosis, possibly reflecting immune-mediated cardiac remodeling of CCC.

Unsupervised learning techniques reveal heterogeneity in memory CD8+ T cell differentiation following acute, chronic and latent viral infections

CD8⁺ T lymphocytes are critical for the control of gammaherpesvirus latency. To determine how memory CD8⁺ T cells generated during latency differ from those primed during acute or chronic viral infection, we adoptively transferred naive P14 CD8⁺ T cells into uninfected recipients, and examined surface proteins, cytokines and transcription factors following infection with the Armstrong (acute) or Clone 13 (chronic) strains of lymphocytic choriomeningitis virus (LCMV), or murine gammaherpesvirus 68 (MHV68) expressing the LCMV epitope D^bGP33-41. By performing k-means clustering and generating self organizing maps (SOM), we observed increased short-lived effector-like, CD27^lo CD62L^lo and Bcl-6^lo CD8⁺ T cells following latent infection. In addition, we found that memory CD8⁺ T cells from latent primed mice underwent less expansion following adoptive transfer and antigen rechallenge. Data from cluster models were combined and visualized by principal component analysis (PCA) demonstrating memory CD8⁺ T cells from latent infection occupy an intermediate differentiation space.

TCR independent suppression of CD8(+) T cell cytokine production mediated by IFNγ in vivo

CD8(+) memory T cells produce IFNγ within hours of secondary infection, but this is quickly terminated in vivo despite the presence of stimulatory viral antigen, suggesting that active suppression occurs. Herein, we investigated the in vivo effector function of CD8(+) memory T cells during successive encounters with viral antigen. CD8(+) T cells in immune mice receiving prior viral or peptide challenge failed to reproduce IFNγ during LCMV rechallenge. Surprisingly, this refractory state was induced even in memory cells that had not encountered their cognate antigen, indicating that the silencing of CD8(+) T cell responses is TCR-independent. Direct injection of IFNγ also suppressed the ability of virus-specific memory cells to respond to subsequent viral challenge. We propose the existence of a negative feedback loop whereby IFNγ, produced by memory CD8(+) T cells to combat viral challenge, acts - directly or indirectly - to limit its further production.

Early Decision: Effector and Effector Memory T Cell Differentiation in Chronic Infection

As effector memory T cells (Tem) are the predominant population elicited by chronic parasitic infections, increasing our knowledge of their function, survival and derivation, as phenotypically and functionally distinct from central memory and effector T cells will be critical to vaccine development for these diseases. In some infections, memory T cells maintain increased effector functions, however; this may require the presence of continued antigen, which can also lead to T cell exhaustion. Alternatively, in the absence of antigen, only the increase in the number of memory cells remains, without enhanced functionality as central memory. In order to understand the requirement for antigen and the potential for longevity or protection, the derivation of each type of memory must be understood. A thorough review of the data establishes the existence of both memory (Tmem) precursors and effector T cells (Teff) from the first hours of an immune response. This suggests a new paradigm of Tmem differentiation distinct from the proposition that Tmem only appear after the contraction of Teff. Several signals have been shown to be important in the generation of memory T cells, such as the integrated strength of “signals 1-3” of antigen presentation (antigen receptor, co-stimulation, cytokines) as perceived by each T cell clone. Given that these signals integrated at antigen presentation cells have been shown to determine the outcome of Teff and Tmem phenotypes and numbers, this decision must be made at a very early stage. It would appear that the overwhelming expansion of effector T cells and the inability to phenotypically distinguish memory T cells at early time points has masked this important decision point. This does not rule out an effect of repeated stimulation or chronic inflammatory milieu on populations generated in these early stages. Recent studies suggest that Tmem are derived from early Teff, and we suggest that this includes Tem as well as Tcm. Therefore, we propose a testable model for the pathway of differentiation from naïve to memory that suggests that Tem are not fully differentiated effector cells, but derived from central memory T cells as originally suggested by Sallusto et al. in 1999, but much debated since.

Activation of monocytic cells by immunostimulatory lipids conjugated to peptide antigens

Bacterial derived lipoproteins constitute potent macrophage activators in vivo and are effective stimuli, enhancing the immune response especially with respect to low or non-immunogenic compounds. In the present study we have prepared branched lipopeptide constructs in which different (B- and T-cell) epitopes of Herpes simplex virus type 1, derived from glycoproteins B (gB) and D (gD), are linked to a synthetic lipid core. The ability of the lipid core peptide (LCP) constructs (LCP-gB and LCP-gD) to induce cytokine expression and activate the mitogen-activated protein kinase cascade has been evaluated and compared with the behaviour of the isolated epitopes and the lipid core. In this respect, the use of LCP technology coupled with the use of three different gB or gD peptide epitopes in the same branched constructs could represent an interesting approach in order to obtain efficient delivery systems in the development of a synthetic multiepitopic vaccine for the prevention of viral infections.

Recovered patients with Stevens-Johson syndrome and toxic epidermal necrolysis maintain long-lived IFN-γ and sFasL memory response

There is evidence that drug-specific T cells are involved in inducing keratinocyte apoptosis in acute stage of Steven-Johson syndrome (SJS) and Toxic epidermal necrolysis (TEN). However, there are few studies that have attempted to examine T cell memory responses over time. We sought to determine the duration of IFN-γ and sFasL memory response to causal drugs in patients with SJS and TEN after remission. Eight patients with previous SJS and TEN were enrolled. Memory T cells were measured by 10-day cultured IFN-γ enzyme-linked immunosorbent spot-forming cell (ELISpot) assay. Effector T-cell responses were measured by ex vivo IFN-γ ELISpot assay and sFasL ELISA. The sFasL-mediated toxicities of drug-stimulated PBMC supernatants against keratinocyte line were further investigated by MTT proliferation assay and Annexin-V staining. We observed significant cultured and ex vivo IFN-γ ELISpot responses against causal drugs in all 8 patients. In addition, the sFasL levels were specifically increased in the supernatant of PBMCs cultured with causal drugs from 6 of 8 patients. Drug-stimulated PBMC supernatants were cytotoxic against keratinocyte line, which was inhibited by anti-FasL mAb in a dose-dependent manner. Our findings confirmed that drug-specific IFN-γ and sFasL memory response against causal drugs could be sustained over several years and further suggest that patients should avoid causal drug re-exposure after the recovery of TEN and SJS.

CD8(+) T cells from mice transnuclear for a TCR that recognizes a single H-2K(b)-restricted MHV68 epitope derived from gB-ORF8 help control infection

To study the CD8(+) T cell response against a mouse γ-herpes virus, we generated K(b)-MHV-68-ORF8(604-612)RAG(-/-) CD8(+) T cell receptor transnuclear (TN) mice as a source of virus-specific CD8(+) T cells. K(b)-ORF8-Tet(+) CD8(+) T cells, expanded in the course of a resolving MHV-68 infection, served as a source of nucleus donors. Various in vivo and ex vivo assay criteria demonstrated the fine specificity and functionality of TN cells. TN cells proliferated extensively in response to viral infection, helped control viral burden, and exhibited a phenotype similar to that of endogenous K(b)-ORF8-Tet(+) cells. When compared to OT-1 cells, TN cells displayed distinct properties in response to lymphopenia and cognate antigen stimulation, which may be attributable to the affinity of the TCR expressed by the TN cells. The availability of MHV-68-specific CD8(+) TCR TN mice provides a new tool for investigating aspects of host-pathogen interactions unique to γ-herpes viruses.

Protective antifungal memory CD8(+) T cells are maintained in the absence of CD4(+) T cell help and cognate antigen in mice

Individuals who are immunocompromised, including AIDS patients with few CD4(+) T cells, are at increased risk for opportunistic fungal infections. The incidence of such infections is increasing worldwide, meaning that the need for antifungal vaccines is increasing. Although CD4(+) T cells play a dominant role in resistance to many pathogenic fungal infections, we have previously shown that vaccination can induce protective antifungal CD8(+) T cell immunity in the absence of CD4(+) T cells. However, it has not been determined whether vaccine-induced antifungal CD8(+) T cell memory can be maintained in the absence of CD4(+) T cell help. Here, we have shown in a mouse model of vaccination against blastomycosis that antifungal memory CD8(+) T cells are maintained in the absence of CD4(+) T cells without loss of numbers or function for at least 6 months and that the cells protect against infection. Using a system that enabled us to induce and track antigen-specific, antifungal CD8(+) T cells, we found that such cells were maintained for at least 5 months upon transfer into naive mice lacking both CD4(+) T cells and persistent fungal antigen. Additionally, fungal vaccination induced a profile of transcription factors functionally linked with persistent memory in CD8(+) T cells. Thus, unlike bacteria and viruses, fungi elicit long-term CD8(+) T cell memory that is maintained without CD4(+) T cell help or persistent antigen. This has implications for the development of novel antifungal vaccine strategies effective in immunocompromised patients.

Optimizing vaccine-induced CD8(+) T-cell immunity: focus on recombinant adenovirus vectors

Recombinant adenoviruses have emerged as promising viral vectors for CD8(+) T-cell vaccines. Our studies have indicated that unlike most acute infections, the CD8(+) T-cell memory population elicited by recombinant human adenovirus serotype 5 (rHuAd5) displays a dominant effector memory phenotype. Persistent, low-level transgene expression from the rHuAd5 vector sustains the CD8(+) T-cell memory population and a nonhematopoietic cell compartment appears to be involved in long-term presentation of adenoviral antigens. Although we are beginning to learn more about the factors that control the maintenance and functionality of memory CD8(+) T cells, we do not yet fully understand what comprises a protective CD8(+) T-cell response. Results from upcoming Phase II clinical trials will be important for determining whether rHuAd5 T-cell vaccines are effective in humans and should help identify correlates of CD8(+) T-cell protection.

Cutting edge: activation of virus-specific CD4 T cells throughout γ-herpesvirus latency

CD4 T cells are essential for immune control of γ-herpesvirus latency. We previously identified a murine MHC class II-restricted epitope in γ-herpesvirus-68 gp150 (gp150(67-83)I-A(b)) that elicits CD4 T cells that are maintained throughout long-term infection. However, it is unknown whether naive cells can be recruited into the antiviral CD4 T cell pool during latency. In this study, we generate a mouse transgenic for a gp150-specific TCR and show epitope-specific activation of transgenic CD4 T cells during acute and latent infections. Furthermore, although only dendritic cells can stimulate virus-specific CD8 T cells during latency, we show that both dendritic cells and B cells stimulate transgenic CD4 T cells. These studies demonstrate that naive CD4 T cells specific for a viral glycoprotein can be stimulated throughout infection, even during quiescent latency, suggesting that CD4 T cell memory is maintained in part by the continual recruitment of naive cells.

Pathogenesis and host control of gammaherpesviruses: lessons from the mouse

Gammaherpesviruses are lymphotropic viruses that are associated with the development of lymphoproliferative diseases, lymphomas, as well as other nonlymphoid cancers. Most known gammaherpesviruses establish latency in B lymphocytes. Research on Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV68/γHV68/MHV4) has revealed a complex relationship between virus latency and the stage of B cell differentiation. Available data support a model in which gammaherpesvirus infection drives B cell proliferation and differentiation. In general, the characterized gammaherpesviruses exhibit a very narrow host tropism, which has severely limited studies on the human gammaherpesviruses EBV and Kaposi's sarcoma-associated herpesvirus. As such, there has been significant interest in developing animal models in which the pathogenesis of gammaherpesviruses can be characterized. MHV68 represents a unique model to define the effects of chronic viral infection on the antiviral immune response.

Autoimmune melanocyte destruction is required for robust CD8+ memory T cell responses to mouse melanoma

A link between autoimmunity and improved antitumor immunity has long been recognized, although the exact mechanistic relationship between these two phenomena remains unclear. In the present study we have found that vitiligo, the autoimmune destruction of melanocytes, generates self antigen required for mounting persistent and protective memory CD8+ T cell responses to melanoma. Vitiligo developed in approximately 60% of mice that were depleted of regulatory CD4+ T cells and then subjected to surgical excision of large established B16 melanomas. Mice with vitiligo generated 10-fold larger populations of CD8+ memory T cells specific for shared melanoma/melanocyte antigens. CD8+ T cells in mice with vitiligo acquired phenotypic and functional characteristics of effector memory, suggesting that they were supported by ongoing antigen stimulation. Such responses were not generated in melanocyte-deficient mice, indicating a requirement for melanocyte destruction in maintaining CD8+ T cell immunity to melanoma. Vitiligo-associated memory CD8+ T cells provided durable tumor protection, were capable of mounting a rapid recall response to melanoma, and did not demonstrate phenotypic or functional signs of exhaustion even after many months of exposure to antigen. This work establishes melanocyte destruction as a key determinant of lasting melanoma-reactive immune responses, thus illustrating that immune-mediated destruction of normal tissues can perpetuate adaptive immune responses to cancer.

KLRG1+NKG2A+ CD8 T cells mediate protection and participate in memory responses during γ-herpesvirus infection

Functional CD8 T cell effector and memory responses are generated and maintained during murine γ-herpesvirus 68 (γHV68) persistent infection despite continuous presentation of viral lytic Ags. However, the identity of the CD8 T cell subpopulations that mediate effective recall responses and that can participate in the generation of protective memory to a γ-herpesvirus infection remains unknown. During γHV68 persistence, ∼75% of γHV68-specific CD8 T cells coexpress the NK receptors killer cell lectin-like receptor G1 (KLRG1) and NKG2A. In this study, we take advantage of this unique phenotype to analyze the capacity of CD8 T cells expressing or not expressing KLRG1 and NKG2A to mediate effector and memory responses. Our results show that γHV68-specific KLRG1(+)NKG2A(+) CD8 T cells have an effector memory phenotype as well as characteristics of polyfunctional effector cells such us IFN-γ and TNF-α production, killing capacity, and are more efficient at protecting against a γHV68 challenge than their NKG2A(-)KLRG1(-) counterparts. Nevertheless, γHV68-specific NKG2A(+)KLRG1(+) CD8 T cells express IL-7 and IL-15 receptors, can survive long-term without cognate Ag, and subsequently mount a protective response during antigenic recall. These results highlight the plasticity of the immune system to generate protective effector and proliferative memory responses during virus persistence from a pool of KLRG1(+)NKG2A(+) effector memory CD8 T cells.

Effector memory Th1 CD4 T cells are maintained in a mouse model of chronic malaria

Protection against malaria often decays in the absence of infection, suggesting that protective immunological memory depends on stimulation. Here we have used CD4⁺ T cells from a transgenic mouse carrying a T cell receptor specific for a malaria protein, Merozoite Surface Protein-1, to investigate memory in a Plasmodium chabaudi infection. CD4⁺ memory T cells (CD44^hiIL-7Rα⁺) developed during the chronic infection, and were readily distinguishable from effector (CD62L^loIL-7Rα⁻) cells in acute infection. On the basis of cell surface phenotype, we classified memory CD4⁺ T cells into three subsets: central memory, and early and late effector memory cells, and found that early effector memory cells (CD62L^loCD27⁺) dominated the chronic infection. We demonstrate a linear pathway of differentiation from central memory to early and then late effector memory cells. In adoptive transfer, CD44^hi memory cells from chronically infected mice were more effective at delaying and reducing parasitemia and pathology than memory cells from drug-treated mice without chronic infection, and contained a greater proportion of effector cells producing IFN-γ and TNFα, which may have contributed to the enhanced protection. These findings may explain the observation that in humans with chronic malaria, activated effector memory cells are best maintained in conditions of repeated exposure.

Protective immunity against malaria develops only after several infections and can be lost on leaving an area in which malaria is transmitted. This suggests that the chronic infection may maintain the protective immune response. In this paper we have used a mouse model of a blood-stage malaria infection to examine the memory response of CD4⁺ T cells during chronic infection. These T cells are required for protective immunity, and also play a part in the inflammatory response that gives rise to malaria disease. Understanding what constitutes a protective CD4⁺ T cell may help us design more protective vaccines. We show that these memory CD4⁺ T cells persist in an activated state, produce the inflammatory cytokines TNFα and IFN-γ, and are more protective than “resting” memory CD4⁺ T cells obtained from mice in which the infection has been eliminated. This may explain why people are better protected against malaria disease when they are infected frequently.

Mononucleosis and antigen-driven T cell responses have different requirements for interleukin-2 signaling in murine gammaherpesvirus infection

Interleukin-2 (IL-2) has been implicated as being necessary for the optimal formation of primary CD8(+) T cell responses against various pathogens. Here we have examined the role that IL-2 signaling plays in several aspects of a CD8(+) T cell response against murine gammaherpesvirus 68 (MHV-68). Exposure to MHV-68 causes a persistent infection, along with infectious mononucleosis, providing a model for studying these processes in mice. Our study indicates that CD25 is necessary for optimal expansion of the antigen-specific CD8(+) T cell response but not for the long-term memory response. Contrastingly, IL-2 signaling through CD25 is absolutely required for CD8(+) T cell mononucleosis.

Dendritic cells loaded with tumor B cells elicit broad immunity against murine gammaherpesvirus 68 but fail to prevent long-term latency

It is still unknown whether a noninfectious gammaherpesvirus vaccine is able to prevent or reduce virus persistence. This led us to use dendritic cells loaded with tumor B cells as a vaccine approach for the murine gammaherpesvirus 68 (gammaHV68) model of infection. Dendritic cells loaded with UV-irradiated latently infected tumor B cells induce broad, strong, and long-lasting immunity against gammaHV68. Dendritic cell vaccination prevents the enlargement of lymph nodes and severely limits acute infection and early latency but does not prevent gammaHV68 from establishing long-term latency. Our findings support the concept that attenuated viruses may be the best vaccine option for preventing gammaherpesvirus persistence.

Persistence of viral infection despite similar killing efficacy of antiviral CD8(+) T cells during acute and chronic phases of infection

Why some viruses establish chronic infections while others do not is poorly understood. One possibility is that the host's immune response is impaired during chronic infections and is unable to clear the virus from the host. In this report, we use a recently proposed framework to estimate the per capita killing efficacy of CD8(+) T cells, specific for the polyoma virus (PyV), which establishes a chronic infection in mice. Surprisingly, the estimated per cell killing efficacy of PyV-specific effector CD8(+) T cells during the acute phase of the infection was very similar to the efficacy of effector CD8(+) T cells specific to lymphocytic choriomeningitis virus (LCMV-Armstrong), which is cleared from the host. Our results suggest that persistence of PyV does not result from the generation of an inefficient PyV-specific CD8(+) T cell response, and that other host or viral factors are responsible for the ability of PyV to establish chronic infection.

Evaluation of the memory CD4+ and CD8+ T cells homeostasis during chronic venous disease of lower limbs

More and more is known about the role of venous wall abnormalities and valvular incompetence in the development of chronic venous disorders (CVD). Unfortunately detailed mechanisms of CVD pathophysiology are not well understood. Recent studies focus on involvement of the inflammatory process in the structural remodeling of venous valves and venous wall. The aim of this study is to investigate and to document the memory T cells homeostasis in CVD patients. In this study we present lymphocytic changes in blood from varicose veins in terms of total CD4+ and CD8+ T cells and their particular subsets of memory T cells: TN, TCM and TEM. Results suggest that immunological memory may be involved in the CVD development.

Plasticity in programming of effector and memory CD8 T-cell formation

CD8(+) T cells (also called cytotoxic T lymphocytes) play a major role in protective immunity against many infectious pathogens and can eradicate malignant cells. The path from naive precursor to effector and memory CD8(+) T-cell development begins with interactions between matured antigen-bearing dendritic cells (DCs) and antigen-specific naive T-cell clonal precursors. By integrating differences in antigenic, costimulatory, and inflammatory signals, a developmental program is established that governs many key parameters associated with the ensuing response, including the extent and magnitude of clonal expansion, the functional capacities of the effector cells, and the size of the memory pool that survives after the contraction phase. In this review, we discuss the multitude of signals that drive effector and memory CD8(+) T-cell differentiation and how the differences in the nature of these signals contribute to the diversity of CD8(+) T-cell responses.

Persistent gamma-herpesvirus infection induces a CD4 T cell response containing functionally distinct effector populations

The direct effector mechanisms of CD4 T cells during gamma-herpesvirus 68 (gammaHV68)-persistent infection are less well understood than those of their CD8 T cell counterparts, although there is substantial evidence that CD4 T cells are critical for the control of persistent gamma-herpesvirus infection. Our results show that in gammaHV68-persistently infected mice, CD4 T cells are not cytokine polyfunctional, but there is a division of labor in the CD4 T cell compartment in which CD4 T cells polarize toward two distinct populations with different effector functions: IFN-gamma producers and CD107(+) cytolytic effectors. These two CD4 T cell effector populations degranulate and produce IFN-gamma during steady state without need for exogenous antigenic restimulation, which is fundamentally different from that observed with gammaHV68-specific CD8 T cells. By using anti-IFN-gamma Ab depletions and IFN-gamma-deficient mice, we show that CD4 T cell-mediated cytotoxicity in vivo is not dependent on IFN-gamma activity. In addition, our data show that purified CD4 T cells isolated from gammaHV68-latently infected mice have the capacity to inhibit gammaHV68 reactivation from latency. Our results support the concept that CD4 T cells are critical effectors for the control of gamma-herpesvirus latent infection, and they mediate this effect by two independent mechanisms: IFN-gamma production and cytotoxicity.

Differential impact of ageing on cellular and humoral immunity to a persistent murine gamma-herpesvirus

Background

Oncogenic γ-herpesviruses establish life-long infections in their hosts and control of these latent infections is dependent on continual immune surveillance. Immune function declines with age, raising the possibility that immune control of γ-herpesvirus infection becomes compromised with increasing age, allowing viral reactivation and/or increased latent load, both of which are associated with the development of malignancies.

Results

In this study, we use the experimental mouse γ-herpesvirus model, γHV68, to investigate viral immunity in aged mice. We found no evidence of viral recrudescence or increased latent load in aged latently-infected mice, suggesting that effective immune control of γ-herpesvirus infection remains intact with ageing. As both cellular and humoral immunity have been implicated in host control of γHV68 latency, we independently examined the impact of ageing on γHV68-specific CD8 T cell function and antibody responses. Virus-specific CD8 T cell numbers and cytolytic function were not profoundly diminished with age. In contrast, whereas ELISA titers of virus-specific IgG were maintained over time, there was a progressive decline in neutralizing activity. In addition, although aged mice were able to control de novo acute infection with only slightly delayed viral clearance, serum titers of neutralizing antibody were reduced in aged mice as compared to young mice.

Conclusion

Although there is no obvious loss of immune control of latent virus, these data indicate that ageing has differential impacts on anti-viral cellular and humoral immune protection during persistent γHV68 infection. This observation has potential relevance for understanding γ-herpesvirus immune control during disease-associated or therapeutic immunosuppression.

Two kinetic patterns of epitope-specific CD8 T-cell responses following murine gammaherpesvirus 68 infection

Murine gammaherpesvirus 68 (gammaHV68) provides an important experimental model for understanding mechanisms of immune control of the latent human gammaherpesviruses. Antiviral CD8 T cells play a key role throughout three separate phases of the infection: clearance of lytic virus, control of the latency amplification stage, and prevention of reactivation of latently infected cells. Previous analyses have shown that T-cell responses to two well-characterized epitopes derived from ORF6 and ORF61 progress with distinct kinetics. ORF6(487)-specific cells predominate early in infection and then decline rapidly, whereas ORF61(524)-specific cells continue to expand through early latency, due to sustained epitope expression. However, the paucity of identified epitopes to this virus has limited our understanding of the overall complexities of CD8 T-cell immune control throughout infection. Here we screened 1,383 predicted H-2(b)-restricted peptides and identified 33 responses, of which 21 have not previously been reported. Kinetic analysis revealed a spectrum of T-cell responses based on the rapidity of their decline after the peak acute response that generally corresponded to the expression patterns of the two previously characterized epitopes. The slowly declining responses that were maintained during latency amplification proliferated more rapidly and underwent maturation of functional avidity over time. Furthermore, the kinetics of decline was accelerated following infection with a latency-null mutant virus. Overall, the data show that gammaHV68 infection elicits a highly heterogeneous CD8 T-cell response that segregates into two distinctive kinetic patterns controlled by differential epitope expression during the lytic and latency amplification stages of infection.

The role of inflammation in the generation and maintenance of memory T cells

Following infection or vaccination, antigen-specific T cells undergo enormous expansion in numbers and differentiate into effector cells that control infection and modulate other aspects of innate and adaptive immunity. The effector T-cell expansion phase is followed by an abrupt period of contraction, during which 90-95% of antigen-specific T cells are eliminated. The surviving pool of T cells subsequently differentiates into long-lived memory populations that can persist for the life of the host and mediate enhanced protective immunity following pathogen re-infection. The generation and maintenance of memory T-cell populations are influenced by a multitude of factors, including inflammatory cytokines that can act on T cells at various points during their differentiation. Herein, we discuss our current understanding of how inflammation shapes not only the quantity and quality of memory T cells, but also the rate at which functional memory T-cell populations develop.

Persistent infection contributes to heterologous protective immunity against fatal ehrlichiosis

Human monocytotropic ehrlichiosis (HME), an emerging and often life-threatening tick-transmitted disease, is caused by the obligately intracellular bacterium Ehrlichia chaffeensis. HME is modeled in C57BL/6 mice using Ehrlichia muris, which causes persistent infection, and Ixodes ovatus Ehrlichia (IOE), which is either acutely lethal or sublethal depending on the dose and route of inoculation. A persistent primary E. muris infection, but not a sublethal IOE infection, protects mice against an ordinarily lethal secondary IOE challenge. In the present study, we determined the role of persistent infection in maintenance of protective memory immune responses. E. muris-infected mice were treated with doxycycline or left untreated and then challenged with an ordinarily lethal dose of IOE. Compared to E. muris-primed mice treated with doxycycline, untreated mice persistently infected with E. muris had significantly greater numbers of antigen-specific gamma interferon-producing splenic memory T cells, significant expansion of CD4(+) CD25(+) T regulatory cells, and production of transforming growth factor beta1 in the spleen. Importantly, E. muris-primed mice treated with doxycycline showed significantly greater susceptibility to challenge infection with IOE compared to untreated mice persistently infected with E. muris. The study indicated that persistent ehrlichial infection contributes to heterologous protection by stimulating the maintenance of memory T-cell responses.

CD8+ T cells in Trypanosoma cruzi infection

CD8(+) T cells have emerged as crucial players in the control of a number of protozoan pathogens, including Trypanosoma cruzi, the agent of human Chagas disease. The recent identification of the dominant targets of T. cruzi-specific T cells has allowed investigators to follow the generation of and document the functionality of T cell responses in both mice and humans. Although slow to develop in the early stages of the infection, T. cruzi-specific CD8(+) T cells reach prodigious levels and remain highly functional throughout chronic infections in mice. Following drug-induced cure during either the acute or chronic stage, these immunodominant T cells persist as stable, antigen-independent memory populations. T. cruzi-specific CD8(+) T cells in humans are less-well-studied but appear to lose functionality and decline in numbers in these decades-long infections. Changes in the frequency of parasite-specific T cell upon therapeutic treatment in humans may provide a new metric for determining treatment efficacy.

IL-7 receptor expression provides the potential for long-term survival of both CD62Lhigh central memory T cells and Th1 effector cells during Leishmania major infection

Infection with the intracellular protozoan parasite Leishmania major induces a state of concomitant immunity wherein secondary immunity is dependent upon the persistence of the original pathogen. Our laboratory has described two populations of Leishmania-induced CD4(+) T cells that contribute to immunity: CD62L(high) central memory T (T(CM)) cells and CD62L(low) effector T cells. To determine whether the prosurvival cytokine IL-7 contributes to maintaining these T cells, we examined expression of the IL7R on CD4(+) T cells activated during L. major infection. We found that T(CM) cells present in chronically infected mice expressed high levels of the IL7R. However, in addition to the expression of the IL7R by T(CM) cells, CD62L(low) cells responding to L. major infection expressed the IL7R. Additional experiments revealed that a large percentage of the IL7R(high)CD62L(low) cells were Th1 cells, based on transcription at the IFN-gamma locus and up-regulation of the Th1-promoting transcription factor T-bet. The up-regulation of T-bet did not prevent IL7R expression by L. major-responding CD4(+) T cells, nor did the absence of T-bet result in increased IL7R expression. Finally, blockade of IL7R signaling decreased the number of T-bet(+)CD4(+) T cells, reduced IFN-gamma production, and inhibited delayed-type hypersensitivity responses in immune mice challenged with L. major, indicating that IL7R signaling contributes to the maintenance of Th1 effector cells. Thus, both T(CM) and Th1 effector cells can express the IL7R during chronic L. major infection, which provides a potential means for their long-term survival in addition to the presence of persisting parasites.

Protective antigen-independent CD8 T cell memory is maintained during {gamma}-herpesvirus persistence

Ag persistence during high-titer chronic viral infections induces CD8 T cell dysfunction and lack of Ag-independent CD8 T cell memory formation. However, we have a poor understanding of the generation and maintenance of CD8 T cell memory during asymptomatic persistent viral infections, particularly gamma-herpesvirus infections. In this study, we demonstrate that the continuous presence of cognate Ag in the host is not required for the maintenance of CD8 T cell memory during a persistent gamma-herpesvirus infection. Importantly, the Ag-independent CD8 T cell memory that is maintained during gamma-herpesvirus persistence has the capacity to survive long-term under homeostatic conditions and to mount a protective recall response to a secondary encounter with the pathogen. These data highlight the ability of the immune system to maintain a population of protective memory CD8 T cells with capacity for long-term Ag-independent survival in the presence of systemic virus persistence.

CD4 T cells mediate killing during persistent gammaherpesvirus 68 infection

CD4 T cells are critical for the control of gammaherpesvirus persistence, but their direct effector mechanisms of virus control in vivo are still poorly understood. In this study, we use murine gammaherpesvirus 68 (gammaHV68) in in vitro and in vivo cytotoxicity assays to show CD4-dependent killing of gammaHV68-loaded cells in mice persistently infected with gammaHV68. Our results underscore the cytotoxic capacity of CD4 T cells during gammaHV68 persistence.

Stable CD8+ T cell memory during persistent Trypanosoma cruzi infection

CD8(+) T cell responses to persistent infections caused by intracellular pathogens are dominated by resting T effectors and T effector memory cells, with little evidence suggesting that a T central memory (T(CM)) population is generated. Using a model of Trypanosoma cruzi infection, we demonstrate that in contrast to the T effector/T effector memory phenotype of the majority of T. cruzi-specific CD8(+) T cells, a population of cells displaying hallmark characteristics of T(CM) cells is also present during long-term persistent infection. This population expressed the T(CM) marker CD127 and a subset expressed one or more of three other T(CM) markers: CD62L, CCR7, and CD122. Additionally, the majority of CD127(high) cells were KLRG1(low), indicating that they have not been repetitively activated through TCR stimulation. These CD127(high) cells were better maintained than their CD127(low) counterparts following transfer into naive mice, consistent with their observed surface expression of CD127 and CD122, which confer the ability to self-renew in response to IL-7 and IL-15. CD127(high) cells were capable of IFN-gamma production upon peptide restimulation and expanded in response to challenge infection, indicating that these cells are functionally responsive upon Ag re-encounter. These results are in contrast to what is typically observed during many persistent infections and indicate that a stable population of parasite-specific CD8(+) T cells capable of Ag-independent survival is maintained in mice despite the presence of persistent Ag.

Natural killer cells as novel helpers in anti-herpes simplex virus immune response

Innate defenses help to eliminate infection, but some of them also play a major role in shaping the magnitude and efficacy of the adaptive immune response. With regard to influencing subsequent adaptive immunity, NK cells aided by dendritic cells may be the most relevant components of the innate reaction to herpes simplex virus (HSV) infection. We confirm that mice lacking or depleted of NK cells are susceptible to HSV-induced lesions. The quantity and quality of CD8(+) cytotoxic T lymphocytes generated in the absence of NK cells were diminished, thereby contributing to susceptibility to HSV-induced encephalitis. We demonstrate a novel helper role for NK cells, in that NK cells compensate for the loss of CD4 helper T cells and NK cell supplementation enhances the function of wild type anti-HSV CD8 T cells. In addition, NK cells were able to partially rescue the dysfunctional CD8(+) T cells generated in the absence of CD4 T helper cells, thereby performing a novel rescue function. Hence, NK cells may well be exploited for enhancing and rescuing the T-cell response in situations where the CD4 helper response is affected.

Drug-induced cure drives conversion to a stable and protective CD8+ T central memory response in chronic Chagas disease

In this study, we document the development of stable, antigen-independent CD8+ T cell memory after drug-induced cure of a chronic infection. By establishing a system for drug cure of chronic Trypanosoma cruzi infection, we present the first extensively documented case of total parasite clearance after drug treatment of this infection. Cure resulted in the emergence of a stable, parasite-specific CD8+ T cell population with the characteristics of central memory cells, based upon expression of CD62L, CCR7, CD127, CD122, Bcl-2 and a reduced immediate in vivo CTL function. CD8+ T cells from treated and cured mice also expanded more rapidly and provided greater protection following challenge than those from chronically infected mice. These results show that complete pathogen clearance results in stable, antigen-independent and protective T cell memory, despite the potentially exhausting effects of prior long-term exposure to antigen in this chronic infection.

Host immunity during RSV pathogenesis

Infection by respiratory syncytial virus (RSV) is the leading cause of childhood hospitalization as well as a major health and economic burden worldwide. Unfortunately, RSV infection provides only limited immune protection to reinfection, mostly due to inadequate immunological memory, which leads to an exacerbated inflammatory response in the respiratory tract promoting airway damage during virus clearance. This exacerbated and inefficient immune-inflammatory response triggered by RSV, has often been attributed to the induction of a Th2-biased immunity specific for some of the RSV antigens. These features of RSV infection suggest that the virus might possess molecular mechanisms to enhance allergic-type immunity in the host in order to prevent clearance by cytotoxic T cells and ensure survival and dissemination to other hosts. In this review, we discuss recent findings that contribute to explain the components of the innate and adaptive immune response that are involved in RSV-mediated disease exacerbation. Further, the virulence mechanisms used by RSV to avoid activation of protective immune responses are described.