Efbalropendekin Alfa enhances human natural killer cell cytotoxicity against tumor cell lines in vitro

IL-15 has shown preclinical activity by enhancing the functional maturation of natural killer (NK) cells. Clinical evaluation of the potential anticancer activity of most cytokines, including IL-15, has been limited by low tolerability and rapid in vivo clearance. Efbalropendekin Alfa (XmAb24306) is a soluble IL15/IL15-receptor alpha heterodimer complex fused to a half-life extended Fc domain (IL15/IL15Rα-Fc), engineered with mutations to reduce IL-15 affinity for CD122. Reduced affinity drives lower potency, leading to prolonged pharmacodynamic response in cynomolgus monkeys. We show that in vitro, human NK cells treated with XmAb24306 demonstrate enhanced cytotoxicity against various tumor cell lines. XmAb24306-treated NK cells also exhibit enhanced killing of 3D colorectal cancer spheroids. Daratumumab (dara), a monoclonal antibody (mAb) that targets CD38 results in antibody-dependent cellular cytotoxicity (ADCC) of both multiple myeloma (MM) cells and NK cells. Addition of XmAb24306 increases dara-mediated NK cell ADCC against various MM cell lines in vitro. Because NK cells express CD38, XmAb24306 increases dara-mediated NK cell fratricide, but overall does not negatively impact the ADCC activity against a MM cell line likely due to increased NK cell activity of the surviving cells. These data show that XmAb24306 increases direct and ADCC-mediated human NK cell cytotoxicity in vitro.

Site-specific development and progressive maturation of human tissue-resident memory T cells over infancy and childhood

Infancy and childhood are critical life stages for generating immune memory to protect against pathogens; however, the timing, location, and pathways for memory development in humans remain elusive. Here, we investigated T cells in mucosal sites, lymphoid tissues, and blood from 96 pediatric donors aged 0-10 years using phenotypic, functional, and transcriptomic profiling. Our results revealed that memory T cells preferentially localized in the intestines and lungs during infancy and accumulated more rapidly in mucosal sites compared with blood and lymphoid organs, consistent with site-specific antigen exposure. Early life mucosal memory T cells exhibit distinct functional capacities and stem-like transcriptional profiles. In later childhood, they progressively adopt proinflammatory functions and tissue-resident signatures, coincident with increased T cell receptor (TCR) clonal expansion in mucosal and lymphoid sites. Together, our findings identify staged development of memory T cells targeted to tissues during the formative years, informing how we might promote and monitor immunity in children.

Exhausted intratumoral Vδ2- γδ T cells in human kidney cancer retain effector function

Gamma delta (γδ) T cells reside within human tissues including tumors, but their function in mediating antitumor responses to immune checkpoint inhibition is unknown. Here we show that kidney cancers are infiltrated by Vδ2- γδ T cells, with equivalent representation of Vδ1+ and Vδ1- cells, that are distinct from γδ T cells found in normal human tissues. These tumor-resident Vδ2- T cells can express the transcriptional program of exhausted αβ CD8+ T cells as well as canonical markers of terminal T-cell exhaustion including PD-1, TIGIT and TIM-3. Although Vδ2- γδ T cells have reduced IL-2 production, they retain expression of cytolytic effector molecules and co-stimulatory receptors such as 4-1BB. Exhausted Vδ2- γδ T cells are composed of three distinct populations that lack TCF7, are clonally expanded and express cytotoxic molecules and multiple Vδ2- T-cell receptors. Human tumor-derived Vδ2- γδ T cells maintain cytotoxic function and pro-inflammatory cytokine secretion in vitro. The transcriptional program of Vδ2- T cells in pretreatment tumor biopsies was used to predict subsequent clinical responses to PD-1 blockade in patients with cancer. Thus, Vδ2- γδ T cells within the tumor microenvironment can contribute to antitumor efficacy.

Human T cells in barrier sites exhibit site-specific characteristics and clonal compartmentalization

The skin, gut, and lung are critical physical and immune barriers that protect from pathogen entry. Within these sites, T cell memory is largely maintained by populations of tissue resident memory T cells (TRMs), capable of rapid protective responses. The degree of interconnection between these TRMs from various sites of residence is poorly understood, particularly in humans. Here, we analyze how tissue specificity affects transcriptional heterogeneity and the T cell receptor (TCR) repertoire across 9 tissue sites within individual donors, including barrier sites (skin, gut, and lung) and associated lymph nodes, as well as blood and lymphoid organs (blood, bone marrow, spleen). Cytometry by time-of-flight (CyTOF) and single cell RNA sequencing reveal phenotypic and transcriptomic features unique to skin or gut T cells, which are distinct from T cells in lymphoid sites or blood. These results were reflected in both bulk sequencing of TRBV gene rearrangement and single cell TCR sequencing, where skin and gut T cells exhibited site-specific clonal expansions that were not present in other sites. Conversely, lung T cells showed clonal overlap and transcriptional similarity to T cells in lymphoid and blood rich sites. Site-specific expansions were mediated by TRMs, while clones disseminated across tissues exhibit large clonal expansions and a CD8 terminal effector (TEMRA) phenotype. Together, these results reveal that TRMs in barrier sites are maintained in situ, while memory T cells in the lung, blood-rich and lymphoid sites are more interconnected. Blood T cells are not representative of barrier T cell clonal space. These results have important implications for monitoring and promoting barrier immunity through site-specific targeting.

Supported by grants from NIH (P01 AI106697)

Immune and epithelial determinants of age-related risk and alveolar injury in fatal COVID-19

Respiratory failure in COVID-19 is characterized by widespread disruption of the lung’s alveolar gas exchange interface. To elucidate determinants of alveolar lung damage, we performed epithelial and immune cell profiling in lungs from 24 COVID-19 autopsies and 43 uninfected organ donors ages 18–92 years. We found marked loss of type 2 alveolar epithelial (T2AE) cells and increased perialveolar lymphocyte cytotoxicity in all fatal COVID-19 cases, even at early stages before typical patterns of acute lung injury are histologically apparent. In lungs from uninfected organ donors, there was also progressive loss of T2AE cells with increasing age, which may increase susceptibility to COVID-19–mediated lung damage in older individuals. In the fatal COVID-19 cases, macrophage infiltration differed according to the histopathological pattern of lung injury. In cases with acute lung injury, we found accumulation of CD4⁺ macrophages that expressed distinctly high levels of T cell activation and costimulation genes and strongly correlated with increased extent of alveolar epithelial cell depletion and CD8⁺ T cell cytotoxicity. Together, our results show that T2AE cell deficiency may underlie age-related COVID-19 risk and initiate alveolar dysfunction shortly after infection, and we define immune cell mediators that may contribute to alveolar injury in distinct pathological stages of fatal COVID-19.

Heterogeneity of human anti-viral immunity shaped by virus, tissue, age, and sex

The persistence of anti-viral immunity is essential for protection and exhibits profound heterogeneity across individuals. Here, we elucidate the factors that shape maintenance and function of anti-viral T cell immunity in the body by comprehensive profiling of virus-specific T cells across blood, lymphoid organs, and mucosal tissues of organ donors. We use flow cytometry, T cell receptor sequencing, single-cell transcriptomics, and cytokine analysis to profile virus-specific CD8+ T cells recognizing the ubiquitous pathogens influenza and cytomegalovirus. Our results reveal that virus specificity determines overall magnitude, tissue distribution, differentiation, and clonal repertoire of virus-specific T cells. Age and sex influence T cell differentiation and dissemination in tissues, while T cell tissue residence and functionality are highly correlated with the site. Together, our results demonstrate how the covariates of virus, tissue, age, and sex impact the anti-viral immune response, which is important for targeting, monitoring, and predicting immune responses to existing and emerging viruses.

SARS-CoV-2 infection generates tissue-localized immunological memory in humans

[Figure: see text].

Stealth Killing by Uterine NK Cells for Tolerance and Tissue Homeostasis

Human natural killer (NK) cells are critical for innate defense against pathogens through direct cytotoxicity of infected cells and are the predominant immune cell at the maternal-fetal interface. In this issue of Cell, Crespo et al. show that human NK cells in the decidual region of the uterus can clear a bacterial infection from the developing fetus by infusion of granulysin into placental trophoblast cells via nanotubes, thus removing the intracellular pathogen without damage to the placental cell. These findings reveal a mechanism for targeted immune protection of the developing fetus that maintains tolerance at the maternal-fetal interface.

Tissue localization and virus specificity shape the maintenance and function of human virus-specific memory T cells recognizing HCMV and influenza A

Generation and maintenance of memory T cells within tissue sites of infection is critical for long-term antiviral protection. Current knowledge of human antiviral responses is largely derived from studies sampling peripheral blood. Yet, little is known about human tissue-localized antiviral immunity. Utilizing our human donor tissue resource established in collaboration with LiveOnNY, we investigated how tissue, virus, and age shape T cell maintenance across diverse tissues and their function when presented with antigens derived from human cytomegalovirus (HCMV) or influenza A virus (flu). Using flow cytometry, we show that virus-specific CD8 T cells are maintained in diverse tissues with subset differentiation and distribution shaped by virus specificity and age. Compared to T cells recognizing HCMV, flu-specific T cells display higher levels of residency markers—CD69 and CD103—particularly in adult compared to pediatric donors. In contrast, HCMV-specific T cells are maintained in greater abundance as terminally-differentiated effector T cells. Sequencing of the T cell receptor CDR3b chain reveals that, while both HCMV- and flu-specific T cells demonstrate clonal overlap across multiple tissues, HCMV-specific T cells display greater clonality and less diversity. When presented with viral antigens, HCMV- and flu-specific T cells are polyfunctional. Additionally, single-cell transcriptome profiling reveals tissue localization is the primary determinant of antigen-driven responses. Together, these studies demonstrate the dynamics of T cell differentiation and maintenance throughout the human body, which is primarily driven by virus specificity, and the role of tissue localization in shaping antiviral response.

Signature of Enhanced Effector Function Defines Early Life Regulatory T Cells

The induction of self-tolerance and regulation of inflammatory immune responses is critical during early life when a myriad of primary antigenic challenges are encountered. Regulatory T Cells (Tregs) promote self-tolerance and function to modulate immune responses utilizing a variety of immunosuppressive mechanisms. Tregs are known to be disproportionately more abundant during early life, suggesting an essential role during immune development. We performed bulk RNA sequencing and multi-parameter flow cytometry on Tregs isolated from adult and pediatric (1 – 3 years of age) tissue (tonsils) and blood to discern the defining characteristics of early life Tregs. Early life Tregs were found to have an increased expression of genes associated with cell cycle (Ki67), effector and suppressive function (ICOS/CTLA4/IL10/GZMB/GZMA) and chemokine signaling (CCR5/CCR9) compared to adults. Importantly, distinctions in canonical effector function genes were apparent across all early life Tregs and not restricted to tissue site. Interestingly, CCR9, essential in gut-specific homing, was upregulated in both tissue and blood early life Tregs, suggesting a biasing towards intestinal immune homeostasis during childhood. We further explored the upregulation of genes related to cell cycle and found that early life Tregs exhibit higher proliferative capacity compared to both adult Tregs and early life CD4 conventional T cells, revealing a potential effector mechanism unique to early life. Early life Tregs are defined by a signature of augmented effector functionality enhancing their ability to respond to the abundant immunological challenges of infancy and childhood.

Longitudinal profiling of respiratory and systemic immune responses reveals myeloid cell-driven lung inflammation in severe COVID-19

Immune response dynamics in coronavirus disease 2019 (COVID-19) and their severe manifestations have largely been studied in circulation. Here, we examined the relationship between immune processes in the respiratory tract and circulation through longitudinal phenotypic, transcriptomic, and cytokine profiling of paired airway and blood samples from patients with severe COVID-19 relative to heathy controls. In COVID-19 airways, T cells exhibited activated, tissue-resident, and protective profiles; higher T cell frequencies correlated with survival and younger age. Myeloid cells in COVID-19 airways featured hyperinflammatory signatures, and higher frequencies of these cells correlated with mortality and older age. In COVID-19 blood, aberrant CD163+ monocytes predominated over conventional monocytes, and were found in corresponding airway samples and in damaged alveoli. High levels of myeloid chemoattractants in airways suggest recruitment of these cells through a CCL2-CCR2 chemokine axis. Our findings provide insights into immune processes driving COVID-19 lung pathology with therapeutic implications for targeting inflammation in the respiratory tract.

Analysis of respiratory and systemic immune responses in COVID-19 reveals mechanisms of disease pathogenesis

Immune responses to respiratory viruses like SARS-CoV-2 originate and function in the lung, yet assessments of human immunity are often limited to blood. Here, we conducted longitudinal, high-dimensional profiling of paired airway and blood samples from patients with severe COVID-19, revealing immune processes in the respiratory tract linked to disease pathogenesis. Survival from severe disease was associated with increased CD4 ⁺ T cells and decreased monocyte/macrophage frequencies in the airway, but not in blood. Airway T cells and macrophages exhibited tissue-resident phenotypes and activation signatures, including high level expression and secretion of monocyte chemoattractants CCL2 and CCL3 by airway macrophages. By contrast, monocytes in blood expressed the CCL2-receptor CCR2 and aberrant CD163 ⁺ and immature phenotypes. Extensive accumulation of CD163 ⁺ monocyte/macrophages within alveolar spaces in COVID-19 lung autopsies suggested recruitment from circulation. Our findings provide evidence that COVID-19 pathogenesis is driven by respiratory immunity, and rationale for site-specific treatment and prevention strategies.

Tissue-Resident Memory T Cells Mediate Immune Homeostasis in the Human Pancreas through the PD-1/PD-L1 Pathway

Non-recirculating tissue-resident memory T cells (TRMs) are the predominant T cell subset in diverse tissue sites, where they mediate protective immune responses in situ. Here, we reveal a role for TRM in maintaining immune homeostasis in the human pancreas through interactions with resident macrophages and the PD-1/PD-L1 inhibitory pathway. Using tissues obtained from organ donors, we identify that pancreas T cells comprise CD8⁺PD-1^hi TRMs, which are phenotypically, functionally, and transcriptionally distinct compared to TRMs in neighboring jejunum and lymph node sites. Pancreas TRMs cluster with resident macrophages throughout the exocrine areas; TRM effector functions are enhanced by macrophage-derived co-stimulation and attenuated by the PD-1/PD-L1 pathways. Conversely, in samples from chronic pancreatitis, TRMs exhibit reduced PD-1 expression and reduced interactions with macrophages. These findings suggest important roles for PD-1 and TRM-macrophage interactions in controlling tissue homeostasis and immune dysfunctions underlying inflammatory disease, with important implications for PD-1-based immunotherapies.

Antibody responses to SARS-CoV2 are distinct in children with MIS-C compared to adults with COVID-19

Clinical manifestations of COVID-19 caused by the novel coronavirus SARS-CoV-2 are associated with age. While children are largely spared from severe respiratory disease, they can present with a SARS-CoV-2-associated multisystem inflammatory syndrome (MIS-C) similar to Kawasaki's disease. Here, we show distinct antibody (Ab) responses in children with MIS-C compared to adults with severe COVID-19 causing acute respiratory distress syndrome (ARDS), and those who recovered from mild disease. There was a reduced breadth and specificity of anti-SARS-CoV-2-specific antibodies in MIS-C patients compared to the COVID patient groups; MIS-C predominantly generated IgG Abs specific for the Spike (S) protein but not for the nucleocapsid (N) protein, while both COVID-19 cohorts had anti-S IgG, IgM and IgA Abs, as well as anti-N IgG Abs. Moreover, MIS-C patients had reduced neutralizing activity compared to COVID-19 cohorts, indicating a reduced protective serological response. These results suggest a distinct infection course and immune response in children and adults who develop severe disease, with implications for optimizing treatments based on symptom and age.

Differentiation of antibody-secreting cells in the thymus of human neonates

Thymic resident B cells were reported for the first time more than 40 years ago. Yet, their phenotype and significance remain unclear. In this study, we investigated the heterogeneity of thymic B cells in neonates as an indication of their possible function. Using thymuses from 21 neonates, we observed two consistent subsets, CD19+CD21+CD35+ and CD19+CD21−CD35−, accounting for virtually all thymic B cells. ELISPOT assays revealed the presence of IgG, IgA, IgM and even IgE antibody-secreting cells within the CD21−CD35− subset but not the CD21+CD35+ subset. To characterize the heterogeneity of CD21−CD35− subset, we used a single-cell RNA-sequencing and identified 4 separate clusters, 3 of which included cells with a phenotype and transcriptome consistent with that of B cell activation and proliferation. The more distant cluster corresponded to plasma cells (PCs) expressing IGHG, IGHA, and IGHE, implying prior class-switch recombination. To assess their specificity, we generated recombinant monoclonal antibodies (rAbs) from 362 PCs using a paired IgH and lgL chain expression cloning strategy. For comparison, rAbs were also generated from 296 undifferentiated CD21+CD35+ thymic B cells. The PC contingent was highly enriched in clones reactive to apoptotic cells when compared to CD21+CD35+ naïve B cells, an essential characteristic of natural antibodies. Remarkably, we identified individual PC clones reactive to some bacterial species also recognized by natural antibodies. Taken together, we demonstrate the differentiation of plasma cells reactive to apoptotic cells and common bacteria within the thymus, suggesting that thymus is a source of natural antibodies, contributing to newborn humoral immunity.

Human influenza A-specific memory T cells reside within multiple tissues and elicit a tissue-specific antiviral response when presented with viral antigens

Generation of memory T cells specific for conserved viral epitopes provides heterosubtypic protection to influenza infection. Previous mouse studies have shown that tissue-resident memory T cells (TRM) are critical for optimal protection. However, knowledge of human antiviral response is largely derived from blood. With our unique human donor tissue resource established in collaboration with LiveOnNY, we investigate the maintenance of influenza-specific T cells across diverse tissue sites and their response when presented with viral antigens. Using multimer staining and flow cytometry, we show that influenza-specific CD8 T cells are maintained in the lung tissue site of infection, as well as in blood, bone marrow, multiple lymph nodes, and small intestines. Compared to T cells recognizing persistent cytomegalovirus infection, influenza-specific T cells display higher levels of canonical residency markers—CD69 and CD103. Additionally, deep sequencing of the T cell receptor CDR3b repertoire reveals that influenza-specific T cells isolated from human tissue donors without active infection are clonally expanded and display a high degree of clonal overlap across tissue sites. When presented with viral antigens, influenza-specific T cells in all sites are polyfunctional and rapidly respond through cytokine production. Together, these studies reveal the dynamics of immune memory maintenance and antiviral response across tissues in the human body in response to influenza A virus.

Dissecting Human Tissue Resident Memory T cell Heterogeneity on the Single Cell Level

The majority of T cells throughout the human body persist as non-circulating tissue-resident memory T cells (TRM) in lymphoid and mucosal sites, and are vital for orchestrating protective immune responses. However, given the difficulty of sampling healthy human tissues, a full understanding of TRM identity and function across tissue sites remains lacking. Here, we utilize a unique human tissue resource where tissues are obtained from organ donors to dissect CD4+ and CD8+ TRM heterogeneity using high-dimensional flow cytometry and single cell RNA-sequencing (scRNA-seq). We identify remarkable heterogeneity in expression of key surface markers that define TRM, namely CD103, CD101, CD49a, PD-1 and CXCR6, particularly in the lung and the intestines. These phenotypic groups of TRM denote different functional subsets after activation: CD103+ and CD101+ TRM preferentially produce IL-17A, CD49a+ and CXCR6+ TRM show enhanced IFNγ production, and PD-1+ TRM produce granzyme-B. To investigate the transcriptional programming underlying TRM, we profile ~20,000 resting and activated T cells from the lungs and intestines of human organ donors with scRNA-seq and identify TRM based on our previously defined tissue signature. In both tissues, we detect multiple clusters of TRM enriched for expression of individual or a combination of TRM marker genes. Importantly, these clusters show variable expression of genes coding for cytokines (IFNG, IL23A, IL26), transcription factors (TBX21, GATA3, BATF), and cytotoxic molecules (GNLY, GZMs), suggesting differentially regulated TRM subsets. Our results therefore highlight the heterogeneity of TRM across human tissue sites and define the transcriptional programming underlying TRM function.

Enhanced functional properties of in vitro expanded lymph node derived human NK cells

Natural killer (NK) cells are innate immune cells with the inherent ability to directly kill tumor and virus infected cells. Due to their ability to kill cancer cells without any prior priming, and their role in preventing metastasis NK cells have since long been the choice for autologous adoptive cell transfer therapy in cancer. However, poor expansion potential of PBMC derived NK cells in vitro is a major roadblock preventing the widespread use of NK cells in immunotherapy. We found that human NK cells isolated from lymph nodes (LNs) express higher levels of genes encoding for stem-like transcription factors (TCF7, LEF1, MYC) compared to NK cells from blood, spleen, bone marrow (BM) and lung. Therefore, we hypothesized that NK cells isolated from LNs will show superior expansion potential in vitro. Flow cytometric analysis shows that LN derived NK cells express high levels of TCF1 protein ex vivo, and show greater proliferation compared to NK cells isolated from blood, spleen and BM following stimulation with IL-2 and IL-15 in vitro. We also observed that a significant frequency of LN NK cells expressed TCF1 even after expansion, suggesting preserved proliferation potential of these cells. Additionally, the expanded NK cells from LN acquired properties of mature, highly functional NK cells such as increased expression of CD16, CD57 and higher Granzyme B expression. Lastly, LN derived NK cells also demonstrate enhanced cytolytic activity in vitro after expansion. Taken together our results suggest that in vitro expanded NK cells from LNs are potentially efficacious anti-tumor agents and could be leveraged for the development of future generation of NK cell directed immunotherapies.

Dynamic local T cell responses underlie associations in disease severity during infant RSV infection

Respiratory syncytial virus (RSV) infections are common throughout infancy, resulting in a wide range of clinical manifestations, with significant morbidity and mortality seen worldwide. The mechanisms leading to severe disease remain unknown. We sought to determine viral and immune mediated factors related to disease severity. Utilizing multi-parameter flow cytometry and single cell RNA sequencing, we correlated local T cell responses with dynamic changes to viral load in samples obtained from infants infected with RSV. Viral burden was highest at time of study enrollment and declined towards clearance in all subjects regardless of disease severity. Peak CD8 T cell response occurred several days after peak viral burden. Disease severity was associated with the accumulation of highly proliferative CD25+/Ki67+ CD8 T effector memory cells in the respiratory tract. Single cell RNA sequencing of T cells revealed distinct transcriptomic profiles of T cell populations. Respiratory tract CD8 T cells upregulated genes for granzyme, perforin, and tissue adhesion (CXCR6, CD49a) while CD4 T cells in the airway were enriched for regulatory T cell genes (FoxP3, CTLA4) compared to blood. Blood samples were defined primarily by gene signatures associated with naïve T cells and no significant upregulation of effector genes, emphasizing the compartmentalized nature of the immune response. Interestingly, clusters of gamma delta T cells upregulating AREG were found in airway samples, supporting a role for immune mediated tissue repair. These results show that disease severity during RSV infection in infancy is associated with localized immune responses and provide insights into how tissue repair may be mediated following severe disease.

Human T cell immunity in barrier sites is phenotypically distinct but clonally connected to other sites

The gut, lung, and skin are physical and immunological barriers that serve as the first line of defense against pathogens and foreign substances. Due to sampling limitations, previous studies have been limited to studying human barrier sites in isolation and often in the context of specific diseases. Our donor tissue resource established in collaboration with LiveOnNY offers a unique opportunity to examine human barrier T cell immunity as a network and elucidate how barrier sites are connected to each other and to lymphoid sites (lymph nodes, bone marrow, spleen). Here, using cytometry by time-of-flight (CyTOF) and T cell receptor (TCR) CDR3b sequencing, we investigate the unique defining characteristics and connectivity of T cells in barrier tissue – gut, lung, and skin – to lymphoid sites and blood. Through high-dimensional marker analysis by CyTOF, our results show that T cell subset composition and phenotype across 8 different sites comprise 33 clusters that are highly conserved between donors of different ages (22–70 yrs). Skin and gut T cells are distinct from each other and from lung, lymphoid sites, and blood, while lung T cells share features with lymphoid sites and blood. TCR repertoire analysis shows that, though majority of gut and skin T cell clones are unique to the tissue, expanded clones within these barrier sites share clonal overlap with other barrier and lymphoid sites. Moreover, T cells in the lung exhibit increased sharing with lymphoid sites and blood. Together, these results reveal how human T cells are compartmentalized in the skin and gut through site-specific adaptations while demonstrating that gut, lung, and skin barrier T cells are connected through shared specificities in a network of immune protection across the body.

Generation and persistence of human tissue-resident memory T cells in lung transplantation

Tissue-resident memory T cells (T_RM) maintain immunity in diverse sites as determined in mouse models, whereas their establishment and role in human tissues have been difficult to assess. Here, we investigated human lung T_RM generation, maintenance, and function in airway samples obtained longitudinally from human leukocyte antigen (HLA)-disparate lung transplant recipients, where donor and recipient T cells could be localized and tracked over time. Donor T cells persist specifically in the lungs (and not blood) of transplant recipients and express high levels of T_RM signature markers including CD69, CD103, and CD49a, whereas lung-infiltrating recipient T cells gradually acquire T_RM phenotypes over months in vivo. Single-cell transcriptome profiling of airway T cells reveals that donor T cells comprise two T_RM-like subsets with varying levels of expression of T_RM-associated genes, whereas recipient T cells comprised non-T_RM and similar T_RM-like subpopulations, suggesting de novo T_RM generation. Transplant recipients exhibiting higher frequencies of persisting donor T_RM experienced fewer adverse clinical events such as primary graft dysfunction and acute cellular rejection compared with recipients with low donor T_RM persistence, suggesting that monitoring T_RM dynamics could be clinically informative. Together, our results provide spatial and temporal insights into how human T_RM develop, function, persist, and affect tissue integrity within the complexities of lung transplantation.

Tissue Determinants of Human NK Cell Development, Function, and Residence

Immune responses in diverse tissue sites are critical for protective immunity and homeostasis. Here, we investigate how tissue localization regulates the development and function of human natural killer (NK) cells, innate lymphocytes important for anti-viral and tumor immunity. Integrating high-dimensional analysis of NK cells from blood, lymphoid organs, and mucosal tissue sites from 60 individuals, we identify tissue-specific patterns of NK cell subset distribution, maturation, and function maintained across age and between individuals. Mature and terminally differentiated NK cells with enhanced effector function predominate in blood, bone marrow, spleen, and lungs and exhibit shared transcriptional programs across sites. By contrast, precursor and immature NK cells with reduced effector capacity populate lymph nodes and intestines and exhibit tissue-resident signatures and site-specific adaptations. Together, our results reveal anatomic control of NK cell development and maintenance as tissue-resident populations, whereas mature, terminally differentiated subsets mediate immunosurveillance through diverse peripheral sites. VIDEO ABSTRACT.

Single-cell transcriptomics of human T cells reveals tissue and activation signatures in health and disease

Human T cells coordinate adaptive immunity in diverse anatomic compartments through production of cytokines and effector molecules, but it is unclear how tissue site influences T cell persistence and function. Here, we use single cell RNA-sequencing (scRNA-seq) to define the heterogeneity of human T cells isolated from lungs, lymph nodes, bone marrow and blood, and their functional responses following stimulation. Through analysis of >50,000 resting and activated T cells, we reveal tissue T cell signatures in mucosal and lymphoid sites, and lineage-specific activation states across all sites including distinct effector states for CD8+ T cells and an interferon-response state for CD4+ T cells. Comparing scRNA-seq profiles of tumor-associated T cells to our dataset reveals predominant activated CD8+ compared to CD4+ T cell states within multiple tumor types. Our results therefore establish a high dimensional reference map of human T cell activation in health for analyzing T cells in disease.

Single cell transcriptomics resolves activation dynamics and cellular states of human blood and tissue T cells

T cells persist as heterogeneous subsets throughout the body and are essential in mounting protective immune responses. In healthy humans, most of our knowledge of T cell activation derives from sampling the peripheral blood and therefore the transcriptional states of tissue T cells, their functional responses to stimulation, and how they relate to T cells in blood have been poorly defined. Here, we profile the activation dynamics of T cells isolated from human lungs (LG), lymph nodes (LN), bone marrow (BM) and blood following TCR-stimulation by single cell RNA-sequencing (scRNA-seq). Analysis of >50,000 individual resting and activated T cells using clustering and new factorization methods reveals lineage-specific gene expression signatures and discrete activation trajectories in all tissues. Between sites, T cells from LG and LN are most distinct, while blood T cells are most similar to those in BM but persist in a more activated basal state. We identify a common transcriptional profile of tissue T cells and detect trace numbers of these cells in the blood. We also define cellular states for resting and activated T cells across tissues, including an interferon-induced state in CD4+ T cells and distinct effector states specific to CD8+ T cells, and uncover new markers of T cell activation that may be central to T cell function. Importantly, we demonstrate that the T cell activation states resolved here serve as a new baseline for defining T cell dysfunction in disease, revealing novel insights into T cell states among tumor-infiltrating lymphocytes from previous studies. Our investigation couples scRNA-seq with new analysis methods to define the activation dynamics of human T cells from disparate anatomical sites on a single cell level.

Tissue driven influences on human NK cell development, function and residence

Natural killer (NK) cells are innate immune cells with the inherent ability to kill tumor and virus infected cells without any prior priming. Recent studies have demonstrated the ability of NK cells to develop adaptive memory and tissue residence. However, our understanding of human NK cells in tissues and their site-specific properties is limited. Here we use our unique tissue resource established in collaboration with LiveOnNY to elucidate the diversity of human NK cell phenotypic subsets, functional profile and transcriptional signatures in non-mucosal and mucosal tissue sites. We identified tissue-specific patterns of NK cell maturation and show that NK cell subset (immature and mature) and memory NK cell distribution is a function of tissue site which is not affected by age. Our results also show that while NK cells from different tissue sites express similar levels of granzyme B, NK cell antibody-dependent cellular cytotoxicity response and cytokine driven activation may be influenced by tissue site. Whole transcriptome profiling of NK cell subsets revealed that mature NK cells from all the sites (blood, bone marrow, spleen, lung and LN) have very similar transcriptional profiles. Interestingly, immature NK cells from all tissue sites have very different transcriptional profiles compared to their blood counterpart and are enriched for tissue resident memory T cell transcriptional signature. Furthermore, we show that immature NK cells show site-specific expression patterns of tissue residence surface markers. Overall, our data demonstrates the effects of tissue microenvironment on the developmental and function potential of tissue NK cells and provides insight into the development of future NK cell mediated immunotherapies.

Microanatomical dissection of human intestinal T-cell immunity reveals site-specific changes in gut-associated lymphoid tissues over life

Defining adaptive immunity with the complex structures of the human gastrointestinal (GI) tract over life is essential for understanding immune responses to ingested antigens, commensal and pathogenic microorganisms, and dysfunctions in disease. We present here an analysis of lymphocyte localization and T cell subset composition across the human GI tract including mucosal sites (jejunum, ileum, colon), gut-associated lymphoid tissues (isolated lymphoid follicles (ILFs), Peyer's patches (PPs), appendix), and mesenteric lymph nodes (MLNs) from a total of 68 donors spanning eight decades of life. In pediatric donors, ILFs and PP containing naïve T cells and regulatory T cells (Tregs) are prevalent in the jejunum and ileum, respectively; these decline in frequency with age, contrasting stable frequencies of ILFs and T cell subsets in the colon. In the mucosa, tissue resident memory T cells develop during childhood, and persist in high frequencies into advanced ages, while T cell composition changes with age in GALT and MLN. These spatial and temporal features of human intestinal T cell immunity define signatures that can be used to train predictive machine learning algorithms. Our findings demonstrate an anatomic basis for age-associated alterations in immune responses, and establish a quantitative baseline for intestinal immunity to define disease pathologies.

Abstract B153: Human NK cell distribution memory and residence in tissue sites

Natural killer (NK) cells are innate immune cells with the ability to kill tumor cells without prior exposure. NK cells express multiple activating and inhibitory receptors in addition to the low-affinity immunoglobulin G binding receptor CD16. Accumulating evidence implicates a role of NK cells in not only direct killing of tumor cells, but also in cancer immunosurveillance and preventing metastasis to tissues sites. However, at present the distribution, diversity and tissue driven differences in NK cell function are not well characterized which may have an implication on the anti-cancer potential of tissue NK cells. Through collaboration with LiveOnNY our local organ procurement organization, we receive blood, bone marrow (BM), lung, intestines, tonsil and associated lymph nodes (LN) from research consented organ donors. Here we used this unique tissue resource to investigate the distribution, phenotypic, functional and transcriptional diversity of NK cell subsets in human tissues. We found that NK cells are ubiquitously distributed in human tissues comprising up to 40% of the CD45+ CD14/19- cells in blood, BM, spleen and lung, while only a small fraction (up to 2%) in the intestine and the LN. While blood, BM, spleen and lung are dominated by mature NK cells (CD56dim CD16+), majority of the NK cells in intestine and LN are immature (CD56hi CD16-). Age, sex and CMV sero-status do not show any correlation with NK cell distribution or subset frequency in tissues. Notably, NK cell subset distribution seems to drive functional differences between tissue NK cells, with lymphoid site NK cells expressing lower levels of effector molecules granzyme B, TNFα, Prf, Ifnγ and displaying reduced degranulation compared with their counterparts from blood, BM and spleen. For an in-depth analysis of tissue-mediated effects on NK cell subset functionality, we performed whole-transcriptome profiling on immature and mature NK cells isolated from blood, BM, spleen, lung and LN. Our analysis identified several effector molecules and NK cell surface receptors being differentially expressed between immature and mature NK cells. Furthermore, while mature NK cells of blood and tissues have similar transcriptional profiles, the transcriptional profiles of immature blood and tissue NK cells show tissue driven heterogeneity with differential expression of transcription factors, metabolic enzymes and NK cell surface receptors. Interestingly, the transcriptional signature of immature NK cells is reminiscent of the transcriptional signature of tissue resident memory T-cells showing increased expression of CD103, CD49a, CXCR6. We validated the expression of these markers using multiparameter flow cytometry and found that subset of immature NK cells in mucosal sites (lung and intestine) do indeed express markers of tissue residence. Additionally, by applying trajectory projection algorithm on NK cells from tissue sites, we show that resident NK cells comprise a distinct population from immature NK cells. Our study has identified novel, previously unidentified diversity of tissue NK cells. Phenotypic and transcriptional profiling data provide evidence for putative resident NK cells being present in certain tissue sites. Blood NK cells differ from their counterparts in tissues; especially, immature NK cells in tissues may be specifically trained to function in the tissue environment. Finally, due to the uniqueness of phenotypic, functional and transcriptional features of tissue NK cells, they may be more suited to fight cancer in situ in tissues.

Citation Format: Pranay Dogra, Takashi Senda, Peter Szabo, Dustin Carpenter, Marta Toth, Puspa Thapa, Mark Snyder, Michelle Miron, Brahma Kumar, Donna L. Farber. Human NK cell distribution memory and residence in tissue sites [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr B153.

Abstract A26: De novo epigenetic programming restrains PD-1 blockade-mediated T cell rejuvenation

Immune checkpoint blockade (ICB)-mediated rejuvenation of exhausted T cells has emerged as a promising approach for treating various cancers and chronic infections. However, T cells that become fully exhausted during prolonged antigen exposure remain refractory to ICB-mediated rejuvenation. Given that many of the impaired effector properties of terminally exhausted CD8 T cells appear to be heritably maintained even in the absence of antigen, we investigated the role of de novo DNA methylation programming as a cell-intrinsic mechanism for establishing the ICB-nonresponsive state of T-cell exhaustion. We report that blocking de novo DNA methylation in activated CD8 T cells allows them to retain their effector functions despite chronic stimulation during a persistent viral infection. Whole-genome bisulfite sequencing of antigen-specific murine CD8 T cells at the effector and exhaustion stages of an immune response identified progressively acquired heritable de novo methylation programs that restrict T cell expansion and clonal diversity during PD-1 blockade treatment. Moreover, these exhaustion-associated DNA methylation programs were acquired in tumor-infiltrating PD-1hi CD8 T cells. Therapeutic approaches to reverse these programs can enhance ICB-mediated T cell rejuvenation and ultimately facilitate the control of chronic viral infections and tumor growth. These data establish de novo DNA methylation programming as a regulator of T cell exhaustion and barrier of ICB therapy.

Citation Format: Hazem E. Ghoneim, Yiping Fan, Ardiana Moustaki, Hossam Abdelsamed, Pradyot Dash, Pranay Dogra, Robert Carter, Walid Awad, Geoff Neale, Paul G. Thomas, Ben Youngblood. De novo epigenetic programming restrains PD-1 blockade-mediated T cell rejuvenation [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr A26.

Effector CD8 T cells dedifferentiate into long-lived memory cells

Memory CD8 T cells that circulate in the blood and are present in lymphoid organs are an essential component of long-lived T cell immunity. These memory CD8 T cells remain poised to rapidly elaborate effector functions upon re-exposure to pathogens, but also have many properties in common with naive cells, including pluripotency and the ability to migrate to the lymph nodes and spleen. Thus, memory cells embody features of both naive and effector cells, fuelling a long-standing debate centred on whether memory T cells develop from effector cells or directly from naive cells. Here we show that long-lived memory CD8 T cells are derived from a subset of effector T cells through a process of dedifferentiation. To assess the developmental origin of memory CD8 T cells, we investigated changes in DNA methylation programming at naive and effector cell-associated genes in virus-specific CD8 T cells during acute lymphocytic choriomeningitis virus infection in mice. Methylation profiling of terminal effector versus memory-precursor CD8 T cell subsets showed that, rather than retaining a naive epigenetic state, the subset of cells that gives rise to memory cells acquired de novo DNA methylation programs at naive-associated genes and became demethylated at the loci of classically defined effector molecules. Conditional deletion of the de novo methyltransferase Dnmt3a at an early stage of effector differentiation resulted in reduced methylation and faster re-expression of naive-associated genes, thereby accelerating the development of memory cells. Longitudinal phenotypic and epigenetic characterization of the memory-precursor effector subset of virus-specific CD8 T cells transferred into antigen-free mice revealed that differentiation to memory cells was coupled to erasure of de novo methylation programs and re-expression of naive-associated genes. Thus, epigenetic repression of naive-associated genes in effector CD8 T cells can be reversed in cells that develop into long-lived memory CD8 T cells while key effector genes remain demethylated, demonstrating that memory T cells arise from a subset of fate-permissive effector T cells.

Generating long-lived CD8(+) T-cell memory: Insights from epigenetic programs

T-cell-based immunological memory has the potential to provide the host with life-long protection against pathogen reexposure and thus offers tremendous promise for the design of vaccines targeting chronic infections or cancer. In order to exploit this potential in the design of new vaccines, it is necessary to understand how and when memory T cells acquire their poised effector potential, and moreover, how they maintain these properties during homeostatic proliferation. To gain insight into the persistent nature of memory T-cell functions, investigators have turned their attention to epigenetic mechanisms. Recent efforts have revealed that many of the properties acquired among memory T cells are coupled to stable changes in DNA methylation and histone modifications. Furthermore, it has recently been reported that the delineating features among memory T cells subsets are also linked to distinct epigenetic events, such as permissive and repressive histone modifications and DNA methylation programs, providing exciting new hypotheses regarding their cellular ancestry. Here, we review recent studies focused on epigenetic programs acquired during effector and memory T-cell differentiation and discuss how these data may shed new light on the developmental path for generating long-lived CD8(+) T-cell memory.

De Novo Epigenetic Programs Inhibit PD-1 Blockade-Mediated T Cell Rejuvenation

Immune-checkpoint-blockade (ICB)-mediated rejuvenation of exhausted T cells has emerged as a promising approach for treating various cancers and chronic infections. However, T cells that become fully exhausted during prolonged antigen exposure remain refractory to ICB-mediated rejuvenation. We report that blocking de novo DNA methylation in activated CD8 T cells allows them to retain their effector functions despite chronic stimulation during a persistent viral infection. Whole-genome bisulfite sequencing of antigen-specific murine CD8 T cells at the effector and exhaustion stages of an immune response identified progressively acquired heritable de novo methylation programs that restrict T cell expansion and clonal diversity during PD-1 blockade treatment. Moreover, these exhaustion-associated DNA-methylation programs were acquired in tumor-infiltrating PD-1hi CD8 T cells, and approaches to reverse these programs improved T cell responses and tumor control during ICB. These data establish de novo DNA-methylation programming as a regulator of T cell exhaustion and barrier of ICB-mediated T cell rejuvenation.

Human memory CD8 T cell effector potential is epigenetically preserved during in vivo homeostasis

Abdelsamed et al. demonstrate that the poised effector potential of human memory CD8 T cells is coupled to maintenance of effector-associated DNA methylation programs during in vitro and in vivo homeostatic proliferation.

Antigen-independent homeostasis of memory CD8 T cells is vital for sustaining long-lived T cell–mediated immunity. In this study, we report that maintenance of human memory CD8 T cell effector potential during in vitro and in vivo homeostatic proliferation is coupled to preservation of acquired DNA methylation programs. Whole-genome bisulfite sequencing of primary human naive, short-lived effector memory (T_EM), and longer-lived central memory (T_CM) and stem cell memory (T_SCM) CD8 T cells identified effector molecules with demethylated promoters and poised for expression. Effector-loci demethylation was heritably preserved during IL-7– and IL-15–mediated in vitro cell proliferation. Conversely, cytokine-driven proliferation of T_CM and T_SCM memory cells resulted in phenotypic conversion into T_EM cells and was coupled to increased methylation of the CCR7 and Tcf7 loci. Furthermore, haploidentical donor memory CD8 T cells undergoing in vivo proliferation in lymphodepleted recipients also maintained their effector-associated demethylated status but acquired T_EM-associated programs. These data demonstrate that effector-associated epigenetic programs are preserved during cytokine-driven subset interconversion of human memory CD8 T cells.

Tumor infiltrating T cells acquire exhaustion-associated epigenetic programs

Antigen-specific CD8 T cells play a critical role in controlling chronic infections and cancer, but progressively lose their effector functions during prolonged antigen exposure. Repression of CD8 T cell effector functions, commonly referred to as T cell exhaustion, limits the ability of the immune system to purge the chronic pathogen from the host. It has recently become recognized that CD8 T cell exhaustion programs can be reinforced and heritably maintained. Therefore in order to develop and/or improve current therapeutic approaches that utilize host antigen-specific T cells to treat chronic infections or cancer a major challenge for the field is to identify mechanisms that stabilize T cell exhaustion. Using the LCMV model system of chronic viral infection we determined that Dnmt3a mediated de novo DNA methylation plays a causal role in establishing CD8 T cell exhaustion. We then performed whole-genome methylation profiling of WT and Dnmt3a cKO CD8 T cells from chronically infected animals and identified Dnmt3a-dependent DNA methylation programs in genes, including interferon gamma and Tcf7, that are coupled to the progressive decline in effector function and developmental plasticity of the antigen specific cell. Lastly, we extended these findings to the tumor setting using a syngeneic mouse tumor model. We found that tumor-infiltrating PD-1hi CD8 T cells acquire exhaustion-associated de novo DNA methylation programs. These results have significant implications for therapeutic strategies that utilize reactivation of host pathogen-specific CD8 T cells to control chronic viral infections or cancer and provide a nucleotide-resolution map of epigenetic programs progressively acquired during T cell exhaustion.

A little cooperation helps murine cytomegalovirus (MCMV) go a long way: MCMV co-infection rescues a chemokine salivary gland defect

Cytomegaloviruses (CMVs) produce chemokines (vCXCLs) that have both sequence and functional homology to host chemokines. Assessment of vCXCL-1's role in CMV infection is limited to in vitro and in silico analysis due to CMVs species specificity. In this study, we used the murine CMV (MCMV) mouse model to evaluate the function of vCXCL-1 in vivo. Recombinant MCMVs expressing chimpanzee CMV vCXCL-1 (vCXCL-1CCMV) or host chemokine, mCXCL1, underwent primary dissemination to the popliteal lymph node, spleen and lung similar to the parental MCMV. However, neither of the recombinants expressing chemokines was recovered from the salivary gland (SG) at any time post-infection although viral DNA was detected. This implies that the virus does not grow in the SG or the overexpressed chemokine induces an immune response that leads to suppressed growth. Pointing to immune suppression of virus replication, recombinant viruses were isolated from the SG following infection of immune-ablated mice [i.e. SCID (severe combined immunodeficiency), NSG (non-obese diabetic SCID gamma) or cyclophosphamide treated]. Depletion of neutrophils or NK cells does not rescue the recovery of chemokine-expressing recombinants in the SG. Surprisingly we found that co-infection of parental virus and chemokine-expressing virus leads to the recovery of the recombinants in the SG. We suggest that parental virus reduces the levels of chemokine expression leading to a decrease in inflammatory monocytes and subsequent SG growth. Therefore, aberrant expression of the chemokines induces cells of the innate and adaptive immune system that curtail the growth and dissemination of the recombinants in the SG.

The inflammasome NLRP3 plays a protective role against a viral immunopathological lesion

Herpes simplex 1 infection of the eye can cause blindness with lesions in the corneal stroma largely attributable to inflammatory events that include components of both adaptive and innate immunity. Several innate immune responses are triggered by herpes simplex 1, but it is unclear how such innate events relate to the subsequent development of stromal keratitis. In this study, we compared the outcome of herpes simplex 1 ocular infection in mice unable to express NLRP3 because of gene knockout (NLRP3(-/-)) to that of wild-type mice. The NLRP3(-/-) mice developed more-severe and earlier stromal keratitis lesions and had higher angiogenesis scores than did infected wild-type animals. In addition, NLRP3(-/-) mice generated an increased early immune response with heightened chemokines and cytokines, including interleukin-1β and interleukin-18, and elevated recruitment of neutrophils. Increased numbers of CD4(+) T cells were seen at later stages of the disease in NLRP3(-/-) animals. Reduction in neutrophils prevented early onset of the disease in NLRP3(-/-) animals and lowered levels of bioactive interleukin-1β but did not lower bioactive interleukin-18. In conclusion, our results indicate that NLRP3 has a regulatory and beneficial role in herpetic stromal keratitis pathogenesis.

Novel heparan sulfate-binding peptides for blocking herpesvirus entry

Human cytomegalovirus (HCMV) infection can lead to congenital hearing loss and mental retardation. Upon immune suppression, reactivation of latent HCMV or primary infection increases morbidity in cancer, transplantation, and late stage AIDS patients. Current treatments include nucleoside analogues, which have significant toxicities limiting their usefulness. In this study we screened a panel of synthetic heparin-binding peptides for their ability to prevent CMV infection in vitro. A peptide designated, p5+14 exhibited ~ 90% reduction in murine CMV (MCMV) infection. Because negatively charged, cell-surface heparan sulfate proteoglycans (HSPGs), serve as the attachment receptor during the adsorption phase of the CMV infection cycle, we hypothesized that p5+14 effectively competes for CMV adsorption to the cell surface resulting in the reduction in infection. Positively charged Lys residues were required for peptide binding to cell-surface HSPGs and reducing viral infection. We show that this inhibition was not due to a direct neutralizing effect on the virus itself and that the peptide blocked adsorption of the virus. The peptide also inhibited infection of other herpesviruses: HCMV and herpes simplex virus 1 and 2 in vitro, demonstrating it has broad-spectrum antiviral activity. Therefore, this peptide may offer an adjunct therapy for the treatment of herpes viral infections and other viruses that use HSPGs for entry.

Novel Human Cytomegalovirus Viral Chemokines, vCXCL-1s, Display Functional Selectivity for Neutrophil Signaling and Function

Human CMV (HCMV) uses members of the hematopoietic system including neutrophils for dissemination throughout the body. HCMV encodes a viral chemokine, vCXCL-1, that is postulated to attract neutrophils for dissemination within the host. The gene encoding vCXCL-1, UL146, is one of the most variable genes in the HCMV genome. Why HCMV has evolved this hypervariability and how this affects the virus' dissemination and pathogenesis is unknown. Because the vCXCL-1 hypervariability maps to important binding and activation domains, we hypothesized that vCXCL-1s differentially activate neutrophils, which could contribute to HCMV dissemination, pathogenesis, or both. To test whether these viral chemokines affect neutrophil function, we generated vCXCL-1 proteins from 11 different clades from clinical isolates from infants infected congenitally with HCMV. All vCXCL-1s were able to induce calcium flux at a concentration of 100 nM and integrin expression on human peripheral blood neutrophils, despite differences in affinity for the CXCR1 and CXCR2 receptors. In fact, their affinity for CXCR1 or CXCR2 did not correlate directly with chemotaxis, G protein-dependent and independent (β-arrestin-2) activation, or secondary chemokine (CCL22) expression. Our data suggest that vCXCL-1 polymorphisms affect the binding affinity, receptor usage, and differential peripheral blood neutrophil activation that could contribute to HCMV dissemination and pathogenesis.

What we have learned from animal models of HCMV

Although human cytomegalovirus (HCMV) primary infection is generally asymptomatic, in immune-compromised patients HCMV increases morbidity and mortality. As a member of the betaherpesvirus family, in vivo studies of HCMV are limited due to its species specificity. CMVs from other species are often used as surrogates to express HCMV genes/proteins or used as models for inferring HCMV protein function in humans. Using innovative experiments, these animal models have answered important questions about CMV's life cycle, dissemination, pathogenesis, immune evasion, and host immune response. This chapter provides CMV biologists with an overview of the insights gained using these animal models. Subsequent chapters will provide details of the specifics of the experimental methods developed for each of the animal models discussed here.