Immunity to cytomegalovirus in early life

Repertoire-based mapping and time-tracking of T helper cell subsets in scRNA-Seq

Introduction

The functional programs of CD4+ T helper (Th) cell clones play a central role in shaping immune responses to different challenges. While advances in single-cell RNA sequencing (scRNA-Seq) have significantly improved our understanding of the diversity of Th cells, the relationship between scRNA-Seq clusters and the traditionally characterized Th subsets remains ambiguous.

Methods

In this study, we introduce TCR-Track, a method leveraging immune repertoire data to map phenotypically sorted Th subsets onto scRNA-Seq profiles.

Results and discussion

This approach accurately positions the Th1, Th1-17, Th17, Th22, Th2a, Th2, T follicular helper (Tfh), and regulatory T-cell (Treg) subsets, outperforming mapping based on CITE-Seq. Remarkably, the mapping is tightly focused on specific scRNA-Seq clusters, despite 4-year interval between subset sorting and the effector CD4+ scRNA-Seq experiment. These findings highlight the intrinsic program stability of Th clones circulating in peripheral blood. Repertoire overlap analysis at the scRNA-Seq level confirms that the circulating Th1, Th2, Th2a, Th17, Th22, and Treg subsets are clonally independent. However, a significant clonal overlap between the Th1 and cytotoxic CD4+ T-cell clusters suggests that cytotoxic CD4+ T cells differentiate from Th1 clones. In addition, this study resolves a longstanding ambiguity: we demonstrate that, while CCR10+ Th cells align with a specific Th22 scRNA-Seq cluster, CCR10-CCR6+CXCR3-CCR4+ cells, typically classified as Th17, represent a mixture of bona fide Th17 cells and clonally unrelated CCR10low Th22 cells. The clear distinction between the Th17 and Th22 subsets should influence the development of vaccine- and T-cell-based therapies. Furthermore, we show that severe acute SARS-CoV-2 infection induces systemic type 1 interferon (IFN) activation of naive Th cells. An increased proportion of effector IFN-induced Th cells is associated with a moderate course of the disease but remains low in critical COVID-19 cases. Using integrated scRNA-Seq, TCR-Track, and CITE-Seq data from 122 donors, we provide a comprehensive Th scRNA-Seq reference that should facilitate further investigation of Th subsets in fundamental and clinical studies.

In utero human cytomegalovirus infection expands NK-like FcγRIII+CD8+ T cells that mediate Fc antibody functions

Human cytomegalovirus (HCMV) profoundly impacts host T and NK cells across the lifespan, yet how this common congenital infection modulates developing fetal immune cell compartments remains underexplored. Using cord blood from neonates with and without congenital HCMV (cCMV) infection, we identify an expansion of Fcγ receptor III-expressing (FcγRIII-expressing) CD8+ T cells following HCMV exposure in utero. Most FcγRIII+CD8+ T cells express the canonical αβ T cell receptor (TCR), but a proportion express noncanonical γδ TCR. FcγRIII+CD8+ T cells are highly differentiated and have increased expression of NK cell markers and cytolytic molecules. Transcriptional analysis reveals FcγRIII+CD8+ T cells upregulate T-bet and downregulate BCL11B, known transcription factors that govern T/NK cell fate. We show that FcγRIII+CD8+ T cells mediate antibody-dependent IFN-γ production and degranulation against IgG-opsonized target cells, similar to NK cell antibody-dependent cellular cytotoxicity (ADCC). FcγRIII+CD8+ T cell Fc effector functions were further enhanced by IL-15, as has been observed in neonatal NK cells. Our study reveals that FcγRIII+CD8+ T cells elicited in utero by HCMV infection can execute Fc-mediated effector functions bridging cellular and humoral immunity and may be a promising target for antibody-based therapeutics and vaccination in early life.

Immune surveillance of cytomegalovirus in tissues

Cytomegalovirus (CMV), a representative member of the Betaherpesvirinae subfamily of herpesviruses, is common in the human population, but immunocompetent individuals are generally asymptomatic when infected with this virus. However, in immunocompromised individuals and immunologically immature fetuses and newborns, CMV can cause a wide range of often long-lasting morbidities and even death. CMV is not only widespread throughout the population but it is also widespread in its hosts, infecting and establishing latency in nearly all tissues and organs. Thus, understanding the pathogenesis of and immune responses to this virus is a prerequisite for developing effective prevention and treatment strategies. Multiple arms of the immune system are engaged to contain the infection, and general concepts of immune control of CMV are now reasonably well understood. Nonetheless, in recent years, tissue-specific immune responses have emerged as an essential factor for resolving CMV infection. As tissues differ in biology and function, so do immune responses to CMV and pathological processes during infection. This review discusses state-of-the-art knowledge of the immune response to CMV infection in tissues, with particular emphasis on several well-studied and most commonly affected organs.

Age and ligand specificity influence the outcome of pathogen engagement on preleukemic and leukemic B-cell precursor populations

Common infections have long been proposed to play a role in the development of pediatric B-cell acute lymphoblastic leukemia (B-ALL). However, epidemiologic studies report contradictory effects of infection exposure on subsequent B-ALL risk, and no specific pathogen has been definitively linked to the disease. A unifying mechanism to explain the divergent outcomes could inform disease prevention strategies. We previously reported that the pattern recognition receptor (PRR) ligand Poly(I:C) exerted effects on B-ALL cells that were distinct from those observed with other nucleic acid-based PRR ligands. Here, using multiple double-stranded RNA (dsRNA) moieties, we show that the overall outcome of exposure to Poly(I:C) reflects the balance of opposing responses induced by its ligation to endosomal and cytoplasmic receptors. This PRR response biology is shared between mouse and human B-ALL and can increase leukemia-initiating cell burden in vivo during the preleukemia phase of B-ALL, primarily through tumor necrosis factor α signaling. The age of the responding immune system further influences the impact of dsRNA exposure on B-ALL cells in both mouse and human settings. Overall, our study demonstrates that potentially proleukemic and antileukemic effects can each be generated by the stimulation of pathogen recognition pathways and indicates a mechanistic explanation for the contrasting epidemiologic associations reported for infection exposure and B-ALL.

A novel gut microbiome-immune axis influencing pathology in HCMV infected infants with neonatal cholestasis

The interplay of active HCMV infection with gut dysbiosis in the immunopathology of cholestasis in neonates and infants remains unexplored. In this study, we evaluated gut microbiome profiles and immune dysfunction in a cohort of HCMV infected cholestatic infants (IgM positive, N = 21; IgM negative, N = 25) compared to healthy infants, N = 10. HCMV infected IgM positive individuals exhibited increased clinical severity in terms of liver dysfunction, altered CD4+: CD8+ ratio, and elevated Granzyme B levels in cellular immune subsets. Gut microbiome analysis revealed distinct and differential diversity and composition within infected groups aligned with clinical severity reflected through the increased abundance of Gammaproteobacteria, reduced Bifidobacteria, and a unique signature mapping to the HCMV infected IgM negative group. Correlation analyses revealed associations between Bifidobacterium breve, Gammaproteobacteria, Firmicutes, Clostridia, Finegoldia magna, Veillonella dispar, and Granzyme B expressing immune cell subsets. Our study describes a novel gut microbiome-immune axis that may influence disease severity in cholestatic infants with active HCMV infection.

Prospects of Cytomegalovirus-Specific T-Cell Receptors in Clinical Diagnosis and Therapy

Human cytomegalovirus (HCMV) is responsible for widespread infections worldwide. In immunocompetent individuals it is typically latent, while infection or reactivation in immunocompromised individuals can result in severe clinical symptoms or even death. Although there has been significant progress in the treatment and diagnosis of HCMV infection in recent years, numerous shortcomings and developmental limitations persist. There is an urgent need to develop innovative, safe, and effective treatments, as well as to explore early and timely diagnostic strategies for HCMV infection. Cell-mediated immune responses are the primary factor controlling HCMV infection and replication, but the protective role of humoral immune responses remains controversial. T-cells, key effector cells of the cellular immune system, are critical for clearing and preventing HCMV infection. The T-cell receptor (TCR) lies at the heart of T-cell immune responses, and its diversity enables the immune system to differentiate between self and non-self. Given the significant influence of cellular immunity on human health and the indispensable role of the TCR in T-cell immune responses, we posit that the impact of TCR on the development of novel diagnostic and prognostic methods, as well as on patient monitoring and management of clinical HCMV infection, will be far-reaching and profound. High-throughput and single-cell sequencing technologies have facilitated unprecedented quantitative detection of TCR diversity. With these current sequencing technologies, researchers have already obtained a vast number of TCR sequences. It is plausible that in the near future studies on TCR repertoires will be instrumental in assessing vaccine efficacy, immunotherapeutic strategies, and the early diagnosis of HCMV infection.

Does congenital cytomegalovirus infection contribute to the development of acute lymphoblastic leukemia in children?

Cytomegalovirus (CMV) is a ubiquitous herpesvirus that has a profound impact on the host immune system. Congenital cytomegalovirus (cCMV) infection modulates neonatal immune cell compartments, yet the full impact of in utero exposure on developing fetal immune cells remains poorly characterized. A series of recent studies have identified a potential link between cCMV infection and the development of acute lymphoblastic leukemia (ALL) in childhood. Here, we review the emerging evidence linking CMV and ALL risk, discuss what is known about the causes of childhood ALL, and propose how CMV infection in early life may confer increased ALL risk.

Antiviral prophylaxis or preemptive therapy for cytomegalovirus after liver transplantation?: A systematic review and meta-analysis

BACKGROUND

To conduct a meta-analysis with the aim of comparing the outcomes of antiviral prophylaxis and preemptive therapy for the prevention of cytomegalovirus (CMV) infection in liver transplant (LT) recipients.

METHODS

We searched databases for qualified studies up until March 2022. Finally, a meta-analysis was carried out using a fixed-effect or random-effect model based on the heterogeneity.

RESULTS

With a total of 1834 LT patients, the pooled incidence of CMV infection and CMV disease in the overall LT recipients using antiviral prophylaxis and preemptive therapy were 24.7% vs. 40.4% and 6.4% vs. 9.4%, respectively. Our meta-analysis exhibited a significant reduction in the incidence of CMV infection due to antiviral prophylaxis when compared to preemptive therapy in the high-risk group (OR: 6.67, 95% CI: 1.73, 25.66; p = 0.006). In contrast, there was a significant reduction in the incidence of late-onset of CMV disease in preemptive therapy compared to antiviral prophylaxis in the high-risk group (OR: 0.29, 95% CI: 0.12, 0.74; p = 0.009). However, the incidence of CMV disease, allograft rejection, graft loss, drug related adverse effects, opportunistic infections and mortality did not differ significantly between both the interventions (all p> 0.05).

CONCLUSIONS

We found the use of antiviral prophylaxis, compared with preemptive therapy, is superior in controlling CMV infection and prolonging the time to CMV disease in LT recipients without an increased risk of opportunistic infections, allograft rejection, graft loss, drug related adverse effects, development of drug resistance, and mortality.

Novel NHEJ1 pathogenic variant linked to severe combined immunodeficiency, microcephaly, and abnormal T and B cell receptor repertoires

BACKGROUND

During the process of generating diverse T and B cell receptor (TCR and BCR, respectively) repertoires, double-strand DNA breaks are produced. Subsequently, these breaks are corrected by a complex system led by the non-homologous end-joining (NHEJ). Pathogenic variants in genes involved in this process, such as the NHEJ1 gene, cause severe combined immunodeficiency syndrome (SCID) along with neurodevelopmental disease and sensitivity to ionizing radiation.

OBJECTIVE

To provide new clinical and immunological insights on NHEJ1 deficiency arising from a newly diagnosed patient with severe immunodeficiency.

MATERIALS AND METHODS

A male infant, born to consanguineous parents, suspected of having primary immunodeficiency underwent immunological and genetic workup. This included a thorough assessment of T cell phenotyping and lymphocyte activation by mitogen stimulation tests, whole-exome sequencing (WES), TCR repertoire Vβ repertoire via flow cytometry analysis, and TCR and BCR repertoire analysis via next-generation sequencing (NGS).

RESULTS

Clinical findings included microcephaly, recurrent pneumonia, and failure to thrive. An immune workup revealed lymphopenia, reduced T cell function, and hypogammaglobulinemia. Skewed TCR Vβ repertoire, TCR gamma (TRG) repertoire, and BCR repertoire were determined in the patient. Genetic analysis identified a novel homozygous missense pathogenic variant in XLF/Cernunnos: c.A580Ins.T; p.M194fs. The patient underwent a successful hematopoietic stem cell transplantation (HSCT).

CONCLUSION

A novel NHEJ1 pathogenic variant is reported in a patient who presented with SCID phenotype that displayed clonally expanded T and B cells. An adjusted HSCT was safe to ensure full T cell immune reconstitution.

Early Life Inflammation and the Developing Hematopoietic and Immune Systems: The Cochlea as a Sensitive Indicator of Disruption

Emerging evidence indicates that perinatal infection and inflammation can influence the developing immune system and may ultimately affect long-term health and disease outcomes in offspring by perturbing tissue and immune homeostasis. We posit that perinatal inflammation influences immune outcomes in offspring by perturbing (1) the development and function of fetal-derived immune cells that regulate tissue development and homeostasis, and (2) the establishment and function of developing hematopoietic stem cells (HSCs) that continually generate immune cells across the lifespan. To disentangle the complexities of these interlinked systems, we propose the cochlea as an ideal model tissue to investigate how perinatal infection affects immune, tissue, and stem cell development. The cochlea contains complex tissue architecture and a rich immune milieu that is established during early life. A wide range of congenital infections cause cochlea dysfunction and sensorineural hearing loss (SNHL), likely attributable to early life inflammation. Furthermore, we show that both immune cells and bone marrow hematopoietic progenitors can be simultaneously analyzed within neonatal cochlear samples. Future work investigating the pathogenesis of SNHL in the context of congenital infection will therefore provide critical information on how perinatal inflammation drives disease susceptibility in offspring.

Immune Landscape of CMV Infection in Cancer Patients: From "Canonical" Diseases Toward Virus-Elicited Oncomodulation

Human Cytomegalovirus (HCMV) is an immensely pervasive herpesvirus, persistently infecting high percentages of the world population. Despite the apparent robust host immune responses, HCMV is capable of replicating, evading host defenses, and establishing latency throughout life by developing multiple immune-modulatory strategies. HCMV has coexisted with humans mounting various mechanisms to evade immune cells and effectively win the HCMV-immune system battle mainly through maintaining its viral genome, impairing HLA Class I and II molecule expression, evading from natural killer (NK) cell-mediated cytotoxicity, interfering with cellular signaling, inhibiting apoptosis, escaping complement attack, and stimulating immunosuppressive cytokines (immune tolerance). HCMV expresses several gene products that modulate the host immune response and promote modifications in non-coding RNA and regulatory proteins. These changes are linked to several complications, such as immunosenescence and malignant phenotypes leading to immunosuppressive tumor microenvironment (TME) and oncomodulation. Hence, tumor survival is promoted by affecting cellular proliferation and survival, invasion, immune evasion, immunosuppression, and giving rise to angiogenic factors. Viewing HCMV-induced evasion mechanisms will play a principal role in developing novel adapted therapeutic approaches against HCMV, especially since immunotherapy has revolutionized cancer therapeutic strategies. Since tumors acquire immune evasion strategies, anti-tumor immunity could be prominently triggered by multimodal strategies to induce, on one side, immunogenic tumor apoptosis and to actively oppose the immune suppressive microenvironment, on the other side.

Effector Vγ9Vδ2 T cell response to congenital Toxoplasma gondii infection

A major γδ T cell population in human adult blood are the Vγ9Vδ2 T cells that are activated and expanded in a TCR-dependent manner by microbe-derived and endogenously derived phosphorylated prenyl metabolites (phosphoantigens). Vγ9Vδ2 T cells are also abundant in human fetal peripheral blood, but compared with their adult counterparts they have a distinct developmental origin, are hyporesponsive toward in vitro phosphoantigen exposure, and do not possess a cytotoxic effector phenotype. In order to obtain insight into the role of Vγ9Vδ2 T cells in the human fetus, we investigated their response to in utero infection with the phosphoantigen-producing parasite Toxoplasma gondii (T. gondii). Vγ9Vδ2 T cells expanded strongly when faced with congenital T. gondii infection, which was associated with differentiation toward potent cytotoxic effector cells. The Vγ9Vδ2 T cell expansion in utero resulted in a fetal footprint with public germline-encoded clonotypes in the Vγ9Vδ2 TCR repertoire 2 months after birth. Overall, our data indicate that the human fetus, from early gestation onward, possesses public Vγ9Vδ2 T cells that acquire effector functions following parasite infections.

Severe Postnatal Cytomegalovirus Enterocolitis in Immunocompetent Term Infants Requiring Total Parenteral Nutrition

Postnatal cytomegalovirus enterocolitis is uncommon in immunocompetent infants. We report a 10-week-old term boy with severe and prolonged secretory diarrhea, leading to dependence on total parenteral nutrition and a 10-week hospitalization. Cytomegalovirus enterocolitis was diagnosed based on duodenal biopsy in the context of marked viremia, and the child recovered promptly on initiation of ganciclovir. Collated case reports reveal delayed diagnoses as the norm but rapid improvement with antiviral treatment. Cytomegalovirus enterocolitis should be considered early as a differential diagnosis in infants with refractory diarrhea.

Understanding Early-Life Adaptive Immunity to Guide Interventions for Pediatric Health

Infants are capable of mounting adaptive immune responses, but their ability to develop long-lasting immunity is limited. Understanding the particularities of the neonatal adaptive immune system is therefore critical to guide the design of immune-based interventions, including vaccines, in early life. In this review, we present a thorough summary of T cell, B cell, and humoral immunity in early life and discuss infant adaptive immune responses to pathogens and vaccines. We focus on the differences between T and B cell responses in early life and adulthood, which hinder the generation of long-lasting adaptive immune responses in infancy. We discuss how knowledge of early life adaptive immunity can be applied when developing vaccine strategies for this unique period of immune development. In particular, we emphasize the use of novel vaccine adjuvants and optimization of infant vaccine schedules. We also propose integrating maternal and infant immunization strategies to ensure optimal neonatal protection through passive maternal antibody transfer while avoiding hindering infant vaccine responses. Our review highlights that the infant adaptive immune system is functionally distinct and uniquely regulated compared to later life and that these particularities should be considered when designing interventions to promote pediatric health.

Deciphering the Immunological Phenomenon of Adaptive Natural Killer (NK) Cells and Cytomegalovirus (CMV)

Natural killer (NK) cells play a significant and vital role in the first line of defense against infection through their ability to target cells without prior sensitization. They also contribute significantly to the activation and recruitment of both innate and adaptive immune cells through the production of a range of cytokines and chemokines. In the context of cytomegalovirus (CMV) infection, NK cells and CMV have co-evolved side by side to employ several mechanisms to evade one another. However, during this co-evolution the discovery of a subset of long-lived NK cells with enhanced effector potential, increased antibody-dependent responses and the potential to mediate immune memory has revolutionized the field of NK cell biology. The ability of a virus to imprint on the NK cell receptor repertoire resulting in the expansion of diverse, highly functional NK cells to this day remains a significant immunological phenomenon that only occurs in the context of CMV. Here we review our current understanding of the development of these NK cells, commonly referred to as adaptive NK cells and their current role in transplantation, infection, vaccination and cancer immunotherapy to decipher the complex role of CMV in dictating NK cell functional fate.

B Cells in Multiple Sclerosis and Virus-Induced Neuroinflammation

Neuroinflammation can be defined as an inflammatory response within the central nervous system (CNS) mediated by a complex crosstalk between CNS-resident and infiltrating immune cells from the periphery. Triggers for neuroinflammation not only include pathogens, trauma and toxic metabolites, but also autoimmune diseases such as neuromyelitis optica spectrum disorders and multiple sclerosis (MS) where the inflammatory response is recognized as a disease-escalating factor. B cells are not considered as the first responders of neuroinflammation, yet they have recently gained focus as a key component involved in the disease pathogenesis of several neuroinflammatory disorders like MS. Traditionally, the prime focus of the role of B cells in any disease, including neuroinflammatory diseases, was their ability to produce antibodies. While that may indeed be an important contribution of B cells in mediating disease pathogenesis, several lines of recent evidence indicate that B cells are multifunctional players during an inflammatory response, including their ability to present antigens and produce an array of cytokines. Moreover, interaction between B cells and other cellular components of the immune system or nervous system can either promote or dampen neuroinflammation depending on the disease. Given that the interest in B cells in neuroinflammation is relatively new, the precise roles that they play in the pathophysiology and progression of different neuroinflammatory disorders have not yet been well-elucidated. Furthermore, the possibility that they might change their function during the course of neuroinflammation adds another level of complexity and the puzzle remains incomplete. Indeed, advancing our knowledge on the role of B cells in neuroinflammation would also allow us to tackle these disorders better. Here, we review the available literature to explore the relationship between autoimmune and infectious neuroinflammation with a focus on the involvement of B cells in MS and viral infections of the CNS.

Neonatal and Children’s Immune System and COVID-19: Biased Immune Tolerance versus Resistance Strategy

The recent outbreak of COVID-19 has emerged as a major global health concern. Although susceptible to infection, recent evidence indicates mostly asymptomatic or mild presentation of the disease in infants, children, and adolescents. Similar observations were made for acute respiratory infections caused by other coronaviruses (severe acute respiratory syndrome and Middle East respiratory syndrome). These observations suggest that the immune system behaves differently in children than adults. Recent developments in the field demonstrated fundamental differences in the neonatal immune system as compared with adults, whereby infants respond to microorganisms through biased immune tolerance rather than resistance strategies. Similarly, more frequent/recent vaccinations in children and younger populations may result in trained immunity. Therefore, the physiological abundance of certain immunosuppressive cells, a tightly regulated immune system, and/or exposure to attenuated vaccines may enhance trained immunity to limit excessive immune reaction to COVID-19 in the young.

Single-cell RNA sequencing unveils the shared and the distinct cytotoxic hallmarks of human TCRVδ1 and TCRVδ2 γδ T lymphocytes

γδ T lymphocytes represent ∼1% of human peripheral blood mononuclear cells and even more cells in most tissues of vertebrates. Although they have important anticancer functions, most current single-cell RNA sequencing (scRNA-seq) studies do not identify γδ T lymphocytes because their transcriptomes at the single-cell level are unknown. Here we show that high-resolution clustering of large scRNA-seq datasets and a combination of gene signatures allow the specific detection of human γδ T lymphocytes and identification of their T cell receptor (TCR)Vδ1 and TCRVδ2 subsets in large datasets from complex cell mixtures. In t-distributed stochastic neighbor embedding plots from blood and tumor samples, the few γδ T lymphocytes appear collectively embedded between cytotoxic CD8 T and NK cells. Their TCRVδ1 and TCRVδ2 subsets form close yet distinct subclusters, respectively neighboring NK and CD8 T cells because of expression of shared and distinct cytotoxic maturation genes. Similar pseudotime maturation trajectories of TCRVδ1 and TCRVδ2 γδ T lymphocytes were discovered, unveiling in both subsets an unattended pool of terminally differentiated effector memory cells with preserved proliferative capacity, a finding confirmed by in vitro proliferation assays. Overall, the single-cell transcriptomes of thousands of individual γδ T lymphocytes from different CMV+ and CMV- donors reflect cytotoxic maturation stages driven by the immunological history of donors. This landmark study establishes the rationale for identification, subtyping, and deep characterization of human γδ T lymphocytes in further scRNA-seq studies of complex tissues in physiological and disease conditions.

CXCR6+ NK Cells in Human Fetal Liver and Spleen Possess Unique Phenotypic and Functional Capabilities

Tissue-resident Natural Killer (NK) cells vary in phenotype according to tissue origin, but are typically CD56^bright, CXCR6⁺, and CD69⁺. NK cells appear very early in fetal development, but little is known about when markers of tissue residency appear during gestation and whether the expression of these markers, most notably the chemokine receptor CXCR6, are associated with differences in functional capability. Using multi-parametric flow cytometry, we interrogated fetal liver and spleen NK cells for the expression of a multitude of extracellular markers associated with NK cell maturation, differentiation, and migration. We analyzed total NK cells from fetal liver and spleen and compared them to their adult liver and spleen counterparts, and peripheral blood (PB) NK. We found that fetal NK cells resemble each other and their adult counterparts more than PB NK. Maturity markers including CD16, CD57, and KIR are lower in fetal NK cells than PB, and markers associated with an immature phenotype are higher in fetal liver and spleen NK cells (NKG2A, CD94, and CD27). However, T-bet/EOMES transcription factor profiles are similar amongst fetal and adult liver and spleen NK cells (T-bet⁻/EOMES⁺) but differ from PB NK cells (T-bet⁺EOMES⁻). Further, donor-matched fetal liver and spleen NK cells share similar patterns of expression for most markers as a function of gestational age. We also performed functional studies including degranulation, cytotoxicity, and antibody-dependent cellular cytotoxicity (ADCC) assays. Fetal liver and spleen NK cells displayed limited cytotoxic effector function in chromium release assays but produced copious amounts of TNFα and IFNγ, and degranulated efficiently in response to stimulation with PMA/ionomycin. Further, CXCR6⁺ NK cells in fetal liver and spleen produce more cytokines and degranulate more robustly than their CXCR6⁻ counterparts, even though CXCR6⁺ NK cells in fetal liver and spleen possess an immature phenotype. Major differences between CXCR6⁻ and + NK cell subsets appear to occur later in development, as a distinct CXCR6⁺ NK cell phenotype is much more clearly defined in PB. In conclusion, fetal liver and spleen NK cells share similar phenotypes, resemble their adult counterparts, and already possess a distinct CXCR6⁺ NK cell population with discrete functional capabilities.

Maturation of the immune system in the fetus and the implications for congenital CMV

Congenital cytomegalovirus (CMV) infection is the most prevalent and consequential congenital infection, among others, that affects approximately 0.6% of all live births worldwide. Timing of maternal infection and maternal immune status largely determine the likelihood of a symptomatic infection. However, recent studies suggest that the fetal immune system, long perceived as naïve and immature, may also play a role in deciding the outcome of congenital CMV infection. Here, we review the development of four immune cells most pertinent to CMV control in the human fetus. αβT cells, B cells, natural killer (NK) cells, and γδT cells are all present, mature and partially functional in utero, and are capable of mounting some form of response to congenital CMV infection. Whether this response is negligible, effective, or harmful remains an open question. Expanding our knowledge of normal and abnormal immune development could provide clinicians with more accurate tools for the detection, monitoring, and treatment of congenital CMV infection in fetuses.

Comparison of the Motor Performance and Vestibular Function in Infants with a Congenital Cytomegalovirus Infection or a Connexin 26 Mutation: A Preliminary Study

OBJECTIVES

Hearing-impaired children are at risk for vestibular damage and delayed motor development. Two major causes of congenital hearing loss are cytomegalovirus (CMV) infection and connexin (Cx) 26 mutations. Comparison of the motor performance and vestibular function between these specific groups is still underexplored. The objective of this study was to investigate the impact of congenital (c)CMV and Cx26 on the motor performance and vestibular function in 6 months old infants.

DESIGN

Forty children (mean age 6.7 months; range 4.8 to 8.9 months) participated in this cross-sectional design and were recruited from the Flemish CMV registry. They were divided into five age-matched groups: normal-hearing control, asymptomatic cCMV, normal-hearing symptomatic cCMV, hearing-impaired symptomatic cCMV, and hearing-impaired Cx26. Children were examined with the Peabody Developmental Motor Scales-2 and cervical vestibular-evoked myogenic potential (cVEMP) test.

RESULTS

Symptomatic hearing-impaired cCMV children demonstrated a significantly lower gross motor performance compared with the control group (p = 0.005), the asymptomatic cCMV group (p = 0.034), and the Cx26 group (0.016). In this symptomatic hearing-impaired cCMV group, 4 out of 8 children had absent cVEMP responses that were related to the weakest gross motor performance. The Cx26 children showed no significant delay in motor development compared with the control children and none of these children had absent cVEMP responses.

CONCLUSIONS

The weakest gross motor performance was found in symptomatic hearing-impaired cCMV-infected children with absent cVEMP responses. These results suggest that abnormal saccular responses are a major factor for this delayed motor development, although more work is needed including comprehensive vestibular function testing to verify this.

Immunodominant cytomegalovirus-specific CD8+ T-cell responses in sub-Saharan African populations

More than 90% of children in Africa are infected with cytomegalovirus (CMV) by the age of 12 months. However, the high-frequency, immunodominant CD8+ T-cell responses that control CMV infection have not been well studied in African populations. We therefore sought to define the immunodominant CMV-specific CD8+ T-cell responses within sub-Saharan African study subjects. Among 257 subjects, we determined the CD8+ T-cell responses to overlapping peptides spanning three of the most immunogenic CMV proteins, pp65, IE-1 and IE-2, using IFN-γ ELISpot assays. A bioinformatics tool was used to predict optimal epitopes within overlapping peptides whose recognition was statistically associated with expression of particular HLA class I molecules. Using this approach, we identified 16 predicted novel CMV-specific epitopes within CMV-pp65, IE-1 and IE-2. The immunodominant pp65-specific, IE-1, IE-2 responses were all either previously well characterised or were confirmed using peptide-MHC tetramers. The novel epitopes identified included an IE-2-specific epitope restricted by HLA*B*44:03 that induced high-frequency CD8+ T-cell responses (mean 3.4% of CD8+ T-cells) in 95% of HLA-B*44:03-positive subjects tested, in one individual accounting for 18.8% of all CD8+ T-cells. These predicted novel CMV-specific CD8+ T-cell epitopes identified in an African cohort will facilitate future analyses of immune responses in African populations where CMV infection is almost universal during infancy.

γδ T Cell-Mediated Immunity to Cytomegalovirus Infection

γδ T lymphocytes are unconventional immune cells, which have both innate- and adaptive-like features allowing them to respond to a wide spectrum of pathogens. For many years, we and others have reported on the role of these cells in the immune response to human cytomegalovirus in transplant patients, pregnant women, neonates, immunodeficient children, and healthy people. Indeed, and as described for CD8+ T cells, CMV infection leaves a specific imprint on the γδ T cell compartment: (i) driving a long-lasting expansion of oligoclonal γδ T cells in the blood of seropositive individuals, (ii) inducing their differentiation into effector/memory cells expressing a TEMRA phenotype, and (iii) enhancing their antiviral effector functions (i.e., cytotoxicity and IFNγ production). Recently, two studies using murine CMV (MCMV) have corroborated and extended these observations. In particular, they have illustrated the ability of adoptively transferred MCMV-induced γδ T cells to protect immune-deficient mice against virus-induced death. In vivo, expansion of γδ T cells is associated with the clearance of CMV infection as well as with reduced cancer occurrence or leukemia relapse risk in kidney transplant patients and allogeneic stem cell recipients, respectively. Taken together, all these studies show that γδ T cells are important immune effectors against CMV and cancer, which are life-threatening diseases affecting transplant recipients. The ability of CMV-induced γδ T cells to act independently of other immune cells opens the door to the development of novel cellular immunotherapies that could be particularly beneficial for immunocompromised transplant recipients.

Chronic infections with viruses or parasites: breaking bad to make good

Eukaryotic forms of life have been continually invaded by microbes and larger multicellular parasites, such as helminths. Over a billion years ago bacterial endosymbionts permanently colonized eukaryotic cells leading to recognized organelles with a distinct genetic lineage, such as mitochondria and chloroplasts. Colonization of our skin and mucosal surfaces with bacterial commensals is now known to be important for host health. However, the contribution of chronic virus and parasitic infections to immune homeostasis is being increasingly questioned. Persistent infection does not necessarily equate to exhibiting a chronic illness: healthy hosts (e.g. humans) have chronic viral and parasitic infections with no evidence of disease. Indeed, there are now examples of complex interactions between these microbes and hosts that seem to confer an advantage to the host at a particular time, suggesting that the relationship has progressed along an axis from parasitic to commensal to one of a mutualistic symbiosis. This concept is explored using examples from viruses and parasites, considering how the relationships may be not only detrimental but also beneficial to the human host.

Evaluation of T Cell Immunity against Human Cytomegalovirus: Impact on Patient Management and Risk Assessment of Vertical Transmission

Cytomegalovirus (CMV) is one of the most common infectious agents, infecting the general population at an early age without causing morbidity most of the time. However, on particular occasions, it may represent a serious risk, as active infection is associated with rejection and disease after solid organ transplantation or fetal transmission during pregnancy. Several methods for CMV diagnosis are available on the market, but because infection is so common, careful selection is needed to discriminate primary infection from reactivation. This review focuses on methods based on CMV-specific T cell reactivity to help monitor the consequences of CMV infection/reactivation in specific categories of patients. This review makes an attempt at discussing the pros and cons of the methods available.

T and B Cell Markers in Dried Blood Spots of Neonates with Congenital Cytomegalovirus Infection: B Cell Numbers at Birth Are Associated with Long-Term Outcomes

Congenital CMV infection (cCMV) is the most common congenital infection that can cause long-term impairment (LTI). The pathogenesis of LTI is not completely understood. Fetal immunity may play a role in controlling the infection and preventing LTI, although immune activation may also contribute to fetal immunopathology. In this study, we analyzed various molecular markers of T and B cell numbers in neonatal dried blood spots of 99 children with cCMV and 54 children without cCMV: δRec-ψJα signal joints on TCR excision circles, intron recombination signal sequence k-deleting element signal joints on Igκ-deleting recombination excision circles, genomic intron recombination signal sequence k-deleting element coding joint, genomic Vδ1-Jδ1, and Vδ2-Jδ1 rearrangements. Of this cohort, clinical symptoms at birth and LTI at 6 y of age were recorded. Neonates with cCMV had fewer TCR excision circles in their blood than non-infected controls. Furthermore, cCMV infection was associated with increased numbers of γδ T cells and B cells, and these numbers were positively correlated with CMV viral load in the dried blood spots. Infected children with a better long-term outcome had higher numbers of B cells at birth than those who developed LTI; no difference in B cell replication was observed. The potential protective role of B cells in controlling cCMV-related disease and the clinical value of this marker as a predictor of long-term outcome merit further evaluation.

T cell responses to cytomegalovirus

Human cytomegalovirus (HCMV) establishes a latent infection that generally remains asymptomatic in immune-competent hosts for decades but can cause serious illness in immune-compromised individuals. The long-term control of CMV requires considerable effort from the host immune system and has a lasting impact on the profile of the immune system. One hallmark of CMV infection is the maintenance of large populations of CMV-specific memory CD8(+) T cells - a phenomenon termed memory inflation - and emerging data suggest that memory inflation is associated with impaired immunity in the elderly. In this Review, we discuss the molecular triggers that promote memory inflation, the idea that memory inflation could be considered a natural pathway of T cell maturation that could be harnessed in vaccination, and the broader implications of CMV infection and the T cell responses it elicits.

Antiviral Host Defence Peptides

The ongoing global mortality and morbidity associated with viral pathogens highlights the need for the continued development of effective, novel antiviral molecules. The antiviral activity of cationic host defence peptides is of significant interest as novel therapeutics for treating viral infection and predominantly due to their broad spectrum antiviral activity. These peptides also display powerful immunomodulatory activity and are key mediators of inflammation. Therefore, they offer a significant opportunity to inform the development of novel therapeutics for treating viral infections by either directly targeting the pathogen or by enhancing the innate immune response. In this chapter, we review the antiviral activity of cathelicidins and defensins, and examine the potential for these peptides to be used as novel antiviral agents.

Cytomegalovirus- and Epstein-Barr Virus-Induced T-Cell Expansions in Young Children Do Not Impair Naive T-cell Populations or Vaccination Responses: The Generation R Study

BACKGROUND

Cytomegalovirus (CMV) and Epstein-Barr virus (EBV) induce effector memory T-cell expansions, which are variable and potentially depend on the age at primary exposure and coinfections. We evaluated the T-cell compartment and herpesvirus infections in 6-year-old children.

METHODS

T-cell subsets and immunoglobulin G seropositivity for CMV, EBV, herpes-simplex virus 1, and varicella-zoster virus were studied in 1079 6-year-old children. A random subgroup of 225 children was evaluated for CMV and EBV seropositivity before 2 years of age and for vaccination responses against measles and tetanus.

RESULTS

CMV and EBV infections were associated with significant expansions of CD27(-) and CD27(+) effector memory T cells, respectively. These expansions were enhanced in CMV-EBV-coinfected children and were independent of varicella-zoster virus or herpes-simplex virus 1 coinfection. Naive and central memory T-cell numbers were not affected, nor were anti-tetanus and anti-measles immunoglobulin G levels. Children infected before 2 years of age showed smaller effector memory T-cell expansions than those infected between 2 and 6 years of age.

CONCLUSIONS

CMV- and EBV-related T-cell expansions do not impair naive T-cell numbers or maintenance of protective responses against nonrelated pathogens. Duration of infection was not directly related to larger expansions of effector memory T cells in children, suggesting that other mechanisms affect these expansions at later age.