Anti-cytomegalovirus antibody levels stratify human immune profiles across the lifespan

Human cytomegalovirus (hCMV) is a ubiquitous latent persistent herpesvirus infecting 60-90% of the population worldwide. hCMV carriage in immunocompetent people is asymptomatic; thus, hCMV can be considered a component of normative aging. However, hCMV powerfully modulates many features of the immune, and likely other, systems and organs. Questions remain as to how hCMV carriage affects the human host. We used anti-CMV antibody titers as a stratifying criterion to examine the impact of "intensity" of hCMV infection as a potential biomarker of aging, inflammation, and immune homeostasis in a cohort of 247 participants stratified into younger (21-40 years) and older (> 65 years of age) groups. We showed that anti-CMV antibody titers increased with age and directly correlated to increased levels of soluble tumor necrosis factor (sTNFR) I in younger but not older participants. CD8 + cell numbers were reduced in the older group due to the loss in CD8 + T naïve (Tn) cells. In CMV carriers and, in particular, in anti-CMV Ab-high participants, this loss was mitigated or reversed by an increase in the numbers of CD8 + T effector memory (Tem) and T effector memory reexpressing CD45RA (Temra) cells. Analysis of CD38, HLA-DR, and CD57 expression revealed subset (CD4 or CD8)-specific changes that correlated with anti-CMV Ab levels. In addition, anti-CMV Ab levels predicted anti-CMV CD8 T cell responsiveness to different CMV open reading frames (ORFs) selectively in older participants, which correlated to the transcriptional order of expression of specific CMV ORFs. Implications of these results for the potential predictive value of anti-CMV Ab titers during aging are discussed.

Inflammatory and immune markers in HIV-infected older adults on long-term antiretroviral therapy: Persistent elevation of sCD14 and of proinflammatory effector memory T cells

HIV-positive patients whose viral loads are successfully controlled by active antiretroviral therapy (ART) show no clinical signs of AIDS. However, their lifespan is shorter compared with individuals with no HIV infection and they prematurely exhibit a multitude of chronic diseases typically associated with advanced age. It was hypothesized that immune system aging may correlate with, and provide useful biomarkers for, this premature loss of healthspan in HIV-positive subjects. Here, we tested whether the immune correlates of aging, including cell numbers and phenotypes, inflammatory status, and control of human cytomegalovirus (hCMV) in HIV-positive subjects on long-term successful ART (HIV+) may reveal increased "immunological age" compared with HIV-negative, age-matched cohort (HIV-) in participants between 50 and 69 years of age. Specifically, we expected that younger HIV+ subjects may immunologically resemble older individuals without HIV. We found no evidence to support this hypothesis. While T cells from HIV+ participants displayed differential expression in several differentiation and/or inhibitory/exhaustion markers in different T cell subpopulations, aging by a decade did not pronounce these changes. Similarly, while the HIV+ participants exhibited higher T cell responses and elevated inflammatory marker levels in plasma, indicative of chronic inflammation, this trait was not age-sensitive. We did find differences in immune control of hCMV, and, more importantly, a sustained elevation of sCD14 and of proinflammatory CD4 and CD8 T cell responses across age groups, pointing towards uncontrolled inflammation as a factor in reduced healthspan in successfully treated older HIV+ patients.

Antibody therapy reverses biological signatures of COVID-19 progression

Understanding who is at risk of progression to severe coronavirus disease 2019 (COVID-19) is key to clinical decision making and effective treatment. We study correlates of disease severity in the COMET-ICE clinical trial that randomized 1:1 to placebo or to sotrovimab, a monoclonal antibody for the treatment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (ClinicalTrials.gov04545060). Laboratory parameters identify study participants at greater risk of severe disease, including a high neutrophil-to-lymphocyte ratio (NLR), a negative SARS-CoV-2 serologic test, and whole-blood transcriptome profiles. Sotrovimab treatment is associated with normalization of NLR and the transcriptomic profile and with a decrease of viral RNA in nasopharyngeal samples. Transcriptomics provides the most sensitive detection of participants who would go on to be hospitalized or die. To facilitate timely measurement, we identify a 10-gene signature with similar predictive accuracy. We identify markers of risk for disease progression and demonstrate that normalization of these parameters occurs with antibody treatment of established infection.

Infection-induced type I interferons critically modulate the homeostasis and function of CD8+ naïve T cells

Naïve T (Tn) cells require two homeostatic signals for long-term survival: tonic T cell receptor:self-peptide-MHC contact and IL-7 stimulation. However, how microbial exposure impacts Tn homeostasis is still unclear. Here we show that infections can lead to the expansion of a subpopulation of long-lived, Ly6C+ CD8+ Tn cells with accelerated effector function. Mechanistically, mono-infection with West Nile virus transiently, and polymicrobial exposure persistently, enhances Ly6C expression selectively on CD5hiCD8+ cells, which in the case of polyinfection translates into a numerical CD8+ Tn cell increase in the lymph nodes. This conversion and expansion of Ly6C+ Tn cells depends on IFN-I, which upregulates MHC class I expression and enhances tonic TCR signaling in differentiating Tn cells. Moreover, for Ly6C+CD8+ Tn cells, IFN-I-mediated signals optimize their homing to secondary sites, extend their lifespan, and enhance their effector differentiation and antibacterial function, particularly for low-affinity clones. Our results thus uncover significant regulation of Tn homeostasis and function via infection-driven IFN-I, with potential implications for immunotherapy.

Defects in Antiviral T Cell Responses Inflicted by Aging-Associated miR-181a Deficiency

Generation of protective immunity to infections and vaccinations declines with age. Studies in healthy individuals have implicated reduced miR-181a expression in T cells as contributing to this defect. To understand the impact of miR-181a expression on antiviral responses, we examined LCMV infection in mice with miR-181ab1-deficient T cells. We found that miR-181a deficiency delays viral clearance, thereby biasing the immune response in favor of CD4 over CD8 T cells. Antigen-specific CD4 T cells in mice with miR-181a-deficient T cells expand more and have a broader TCR repertoire with preferential expansion of high-affinity T cells than in wild-type mice. Importantly, generation of antigen-specific miR-181a-deficient CD8 effector T cells is particularly impaired, resulting in lower frequencies of CD8 T cells in the liver even at time points when the infection has been cleared. Consistent with the mouse model, CD4 memory T cells in individuals infected with West Nile virus at older ages tend to be more frequent and of higher affinity.

Defective Transcriptional Programming of Effector CD8 T Cells in Aged Mice Is Cell-Extrinsic and Can Be Corrected by Administration of IL-12 and IL-18

In response to infection with intracellular microorganisms, old mice mobilize decreased numbers of antigen-specific CD8 T cells with reduced expression of effector molecules and impaired cytolytic activity. Molecular mechanisms behind these defects and the cell-intrinsic (affecting naïve CD8 T cells themselves) vs. extrinsic, microenvironmental origin of such defects remain unclear. Using reciprocal transfer experiments of highly purified naïve T cells from adult and old transgenic OT-1 mice, we decisively show that the dominant effect is cell-extrinsic. Naïve adult OT-1 T cells failed to expand and terminally differentiate in the old organism infected with Listeria-OVA. This defect was preceded by blunted expression of the master transcription factor T-bet and impaired glycolytic switch when T cells are primed in the old organism. However, both old and adult naïve CD8 T cells proliferated and produced effector molecules to a similar extent when stimulated in vitro with polyclonal stimuli, as well as when transferred into adult recipients. Multiple inflammatory cytokines with direct effects on T cell effector differentiation were decreased in spleens of old animals, particularly IL-12 and IL-18. Of note, in vivo treatment of mice with IL-12 and IL-18 on days 4–6 of Listeria infection reconstituted cytotoxic T cell response of aged mice to the level of adult. Therefore, critical cytokine signals which are underproduced in the old priming environment can restore proper transcriptional programming of old naïve CD8 T cells and improve immune defense against intracellular microorganisms.

Lymph nodes as barriers to T-cell rejuvenation in aging mice and nonhuman primates

In youth, thymic involution curtails production of new naïve T cells, placing the onus of T-cell maintenance upon secondary lymphoid organs (SLO). This peripheral maintenance preserves the size of the T-cell pool for much of the lifespan, but wanes in the last third of life, leading to a dearth of naïve T cells in blood and SLO, and contributing to suboptimal immune defense. Both keratinocyte growth factor (KGF) and sex steroid ablation (SSA) have been shown to transiently increase the size and cellularity of the old thymus. It is less clear whether this increase can improve protection of old animals from infectious challenge. Here, we directly measured the extent to which thymic rejuvenation benefits the peripheral T-cell compartment of old mice and nonhuman primates. Following treatment of old animals with either KGF or SSA, we observed robust rejuvenation of thymic size and cellularity, and, in a reporter mouse model, an increase in recent thymic emigrants (RTE) in the blood. However, few RTE were found in the spleen and even fewer in the lymph nodes, and SSA-treated mice showed no improvement in immune defense against West Nile virus. In parallel, we found increased disorganization and fibrosis in old LN of both mice and nonhuman primates. These results suggest that SLO defects with aging can negate the effects of successful thymic rejuvenation in immune defense.

Diminished antibody response to influenza vaccination is characterized by expansion of an age-associated B-cell population with low PAX5

Individuals over the age of 65 comprise a substantial portion of the world population and become more susceptible to vaccine-preventable infections with age as vaccination response diminishes. The underlying reason for this impaired vaccine response in older individuals is not entirely clear. We evaluated potential differences in phenotypic and functional responses of B cells from healthy younger (22-45years) and older (64-95years) individuals that may associate with a diminished antibody response to influenza vaccination. We report that age is associated with expansion of atypical memory B cells (CD10^-CD20⁺CD21^-CD27^-) and an age-associated B cell (ABC, CD21^-T-bet⁺CD11c⁺) phenotype. Reduced expression of PAX5 was also seen in older individuals. Poor influenza-specific antibody production following vaccination was associated with low PAX5 expression and a distinct composition of the ABC compartment. Collectively, these findings demonstrate that the characteristics of the ABC populations of older individuals are associated with antibody production following influenza vaccination.

Known unknowns: how might the persistent herpesvirome shape immunity and aging?

The microbial community that colonizes all living organisms is gaining appreciation for its contributions to both physiologic and pathogenic processes. The virome, a subset of the overall microbiome, large and diverse, including viruses that persistently inhabit host cells, endogenous viral elements genomically or epigenomically integrated into cells, and viruses that infect the other (bacterial, protozoan, fungal, and archaeal) microbiome phylla. These viruses live in the organism for its life, and therefore are to be considered part of the aging process experienced by the organism. This review considers the impact of the persistent latent virome on immune aging. Specific attention will be devoted to the role of herpesviruses, and within them, the cytomegalovirus, as the key modulators of immune aging.

Functional and Homeostatic Impact of Age-Related Changes in Lymph Node Stroma

Adults over 65 years of age are more vulnerable to infectious disease and show poor responses to vaccination relative to those under 50. A complex set of age-related changes in the immune system is believed to be largely responsible for these defects. These changes, collectively termed immune senescence, encompass alterations in both the innate and adaptive immune systems, in the microenvironments where immune cells develop or reside, and in soluble factors that guide immune homeostasis and function. While age-related changes in primary lymphoid organs (bone marrow, and, in particular, the thymus, which involutes in the first third of life) have been long appreciated, changes affecting aging secondary lymphoid organs, and, in particular, aging lymph nodes (LNs) have been less well characterized. Over the last 20 years, LN stromal cells have emerged as key players in maintaining LN morphology and immune homeostasis, as well as in coordinating immune responses to pathogens. Here, we review recent progress in understanding the contributions of LN stromal cells to immune senescence. We discuss approaches to understand the mechanisms behind the decline in LN stromal cells and conclude by considering potential strategies to rejuvenate aging LN stroma to improve immune homeostasis, immune responses, and vaccine efficacy in the elderly.

Cutting Edge: The Aging Immune System Reveals the Biological Impact of Direct Antigen Presentation on CD8 T Cell Responses

The vertebrate immune system uses multiple, sometimes redundant, mechanisms to contain pathogenic microorganisms that are always evolving to evade host defenses. Thus, the cowpox virus (CPXV) uses genes encoding CPXV12 and CPXV203 to prevent direct MHC class I presentation of viral peptides by infected cells. However, CD8 T cells are effectively primed against CPXV by cross-presentation of viral Ags in young mice. Old mice accumulate defects in both CD8 T cell activation and cross-presentation. Using a double-deletion mutant (∆12∆203) of CPXV, we show that direct priming of CD8 T cells in old mice yields superior recall responses, establishing a key contribution of this mechanism to host antipoxvirus responses and enhancing our fundamental understanding of how viral manipulation of direct presentation impacts pathogenesis. This also provides a proof of principle that suboptimal CD8 T cell in old organisms can be optimized by manipulating Ag presentation, with implications for vaccine design.

Acute systemic DNA damage in youth does not impair immune defense with aging

Aging‐related decline in immunity is believed to be the main driver behind decreased vaccine efficacy and reduced resistance to infections in older adults. Unrepaired DNA damage is known to precipitate cellular senescence, which was hypothesized to be the underlying cause of certain age‐related phenotypes. Consistent with this, some hallmarks of immune aging were more prevalent in individuals exposed to whole‐body irradiation (WBI), which leaves no anatomical repository of undamaged hematopoietic cells. To decisively test whether and to what extent WBI in youth will leave a mark on the immune system as it ages, we exposed young male C57BL/6 mice to sublethal WBI (0.5–4 Gy), mimicking human survivor exposure during nuclear catastrophe. We followed lymphocyte homeostasis thorough the lifespan, response to vaccination, and ability to resist lethal viral challenge in the old age. None of the irradiated groups showed significant differences compared with mock‐irradiated (0 Gy) animals for the parameters measured. Even the mice that received the highest dose of sublethal WBI in youth (4 Gy) exhibited equilibrated lymphocyte homeostasis, robust T‐ and B‐cell responses to live attenuated West Nile virus (WNV) vaccine and full survival following vaccination upon lethal WNV challenge. Therefore, a single dose of nonlethal WBI in youth, resulting in widespread DNA damage and repopulation stress in hematopoietic cells, leaves no significant trace of increased immune aging in a lethal vaccine challenge model.

Lifespan-extending caloric restriction or mTOR inhibition impair adaptive immunity of old mice by distinct mechanisms

Aging of the world population and a concomitant increase in age-related diseases and disabilities mandates the search for strategies to increase healthspan, the length of time an individual lives healthy and productively. Due to the age-related decline of the immune system, infectious diseases remain among the top 5–10 causes of mortality and morbidity in the elderly, and improving immune function during aging remains an important aspect of healthspan extension. Calorie restriction (CR) and more recently rapamycin (rapa) feeding have both been used to extend lifespan in mice. Preciously few studies have actually investigated the impact of each of these interventions upon in vivo immune defense against relevant microbial challenge in old organisms. We tested how rapa and CR each impacted the immune system in adult and old mice. We report that each intervention differentially altered T-cell development in the thymus, peripheral T-cell maintenance, T-cell function and host survival after West Nile virus infection, inducing distinct but deleterious consequences to the aging immune system. We conclude that neither rapa feeding nor CR, in the current form/administration regimen, may be optimal strategies for extending healthy immune function and, with it, lifespan.

Histone deacetylation critically determines T cell subset radiosensitivity

Lymphocytes are sensitive to ionizing radiation and naive lymphocytes are more radiosensitive than their memory counterparts. Less is known about radiosensitivity of memory cell subsets. We examined the radiosensitivity of naive (TN), effector memory (TEM), and central memory (TCM) T cell subsets in C57BL/6 mice and found TEM to be more resistant to radiation-induced apoptosis than either TN or TCM. Surprisingly, we found no correlation between the extent of radiation-induced apoptosis in T cell subsets and 1) levels of pro- and antiapoptotic Bcl-2 family members or 2) the H2AX content and maximal γH2AX fold change. Rather, TEM cell survival correlated with higher levels of immediate γH2AX marking, immediate break binding and genome-wide open chromatin structure. T cells were able to mark DNA damage seemingly instantly (30 s), even if kept on ice. Relaxing chromatin with the histone deacetylase inhibitor valproic acid following radiation or etoposide treatment improved the survival of TCM and TN cells up to levels seen in the resistant TEM cells but did not improve survival from caspase-mediated apoptosis. We conclude that an open genome-wide chromatin state is the key determinant of efficient immediate repair of DNA damage in T cells, explaining the observed T cell subset radiosensitivity differences.

New advances in CMV and immunosenescence

Immunosenescence, defined as the age-associated dysregulation and dysfunction of the immune system, is characterized by impaired protective immunity and decreased efficacy of vaccines. An increasing number of immunological, clinical and epidemiological studies suggest that persistent Cytomegalovirus (CMV) infection is associated with accelerated aging of the immune system and with several age-related diseases. However, current evidence on whether and how human CMV (HCMV) infection is implicated in immunosenescence and in age-related diseases remains incomplete and many aspects of CMV involvement in immune aging remain controversial. The attendees of the 4th International Workshop on "CMV & Immunosenescence", held in Parma, Italy, 25-27th March, 2013, presented and discussed data related to these open questions, which are reported in this commentary.

Immune memory-boosting dose of rapamycin impairs macrophage vesicle acidification and curtails glycolysis in effector CD8 cells, impairing defense against acute infections

Direct mammalian target of rapamycin (Rapa) complex 1 inhibition by short-term low-dose Rapa treatment has recently been shown to improve CD8 T cell immunological memory. Whereas these studies focused on memory development, the impact of low-dose Rapa on the primary immune response, particularly as it relates to functional effector immunity, is far less clear. In this study, we investigated the impact of acute Rapa treatment on immune effector cell function during the primary immune response to several acute infections. We found that functional CD8 T cell and macrophage responses to both viral and intracellular bacterial pathogens were depressed in mice in vivo and in humans to phorbol ester and calcium ionophore stimulation in vitro in the face of low-dose Rapa treatment. Mechanistically, the CD8 defect was linked to impaired glycolytic switch in stimulated naive cells and the reduced formation of short-lived effector cells. Therefore, more than one cell type required for a protective effector immune response is impaired by Rapa in both mice and humans, at the dose shown to improve immune memory and extend lifespan. This urges caution with regard to the relative therapeutic costs and benefits of Rapa treatment as means to improve immune memory.

Two separate defects affecting true naive or virtual memory T cell precursors combine to reduce naive T cell responses with aging

Naive T cell responses are eroded with aging. We and others have recently shown that unimmunized old mice lose ≥ 70% of Ag-specific CD8 T cell precursors and that many of the remaining precursors acquire a virtual (central) memory (VM; CD44(hi)CD62L(hi)) phenotype. In this study, we demonstrate that unimmunized TCR transgenic (TCRTg) mice also undergo massive VM conversion with age, exhibiting rapid effector function upon both TCR and cytokine triggering. Age-related VM conversion in TCRTg mice directly depended on replacement of the original TCRTg specificity by endogenous TCRα rearrangements, indicating that TCR signals must be critical in VM conversion. Importantly, we found that VM conversion had adverse functional effects in both old wild-type and old TCRTg mice; that is, old VM, but not old true naive, T cells exhibited blunted TCR-mediated, but not IL-15-mediated, proliferation. This selective proliferative senescence correlated with increased apoptosis in old VM cells in response to peptide, but decreased apoptosis in response to homeostatic cytokines IL-7 and IL-15. Our results identify TCR as the key factor in differential maintenance and function of Ag-specific precursors in unimmunized mice with aging, and they demonstrate that two separate age-related defects--drastic reduction in true naive T cell precursors and impaired proliferative capacity of their VM cousins--combine to reduce naive T cell responses with aging.

Age-associated alterations in CD8α+ dendritic cells impair CD8 T-cell expansion in response to an intracellular bacterium

Age-associated decline in immunity to infection has been documented across multiple pathogens, yet the relative contributions of the aged priming environment and of lymphocyte-intrinsic defects remain unclear. To address the impact of the aging environment on T-cell priming, adult naïve OT-I TCR transgenic CD8 T cells, specific for the H-2K^b-restricted immunodominant OVA_257-264 epitope, were transferred into adult or old recipient mice infected with the recombinant intracellular bacterium Listeria monocytogenes carrying the chicken ovalbumin protein (Lm-OVA). We consistently found that adult OT-I CD8 expansion was reduced in aged recipient mice, and this correlated with numeric, phenotypic, and functional defects selectively affecting CD8α+ dendritic cells (DC). Following Lm-OVA infection, aged mice failed to accumulate CD8α+ DC in the spleen, and these cells expressed much lower levels of critical costimulatory molecules in the first three days following infection. Further, aged CD8α+ DC showed impaired uptake of the bacteria at very early time points following infection. Treatment of aged mice with Flt3 ligand (Flt3L) improved the number of DC present in the spleen prior to Lm-OVA infection, and improved, but did not reconstitute, OT-I expansion to Lm-OVA infection. These results suggest that age-associated changes in antigen uptake, pathogen sensing, and/or antigen presentation contribute to impaired adaptive immune responses to microbial pathogens with aging.

Lifelong persistent viral infection alters the naive T cell pool, impairing CD8 T cell immunity in late life

Persistent CMV infection has been associated with immune senescence. To address the causal impact of lifelong persistent viral infection on immune homeostasis and defense, we infected young mice systemically with HSV-1, murine CMV, or both viruses and studied their T cell homeostasis and function. Herpesvirus(+) mice exhibited increased all-cause mortality compared with controls. Upon Listeria-OVA infection, 23-mo-old animals that had experienced lifelong herpesvirus infections showed impaired bacterial control and CD8 T cell function, along with distinct alterations in the T cell repertoire both before and after Listeria challenge, compared with age-matched, herpesvirus-free controls. Herpesvirus infection was associated with reduced naive CD8 T cell precursors above the loss attributable to aging. Moreover, the OVA-specific CD8 T cell repertoire recruited after Listeria challenge was entirely nonoverlapping between control and herpesvirus(+) mice. To our knowledge, this study for the first time causally links lifelong herpesvirus infection to all-cause mortality in mice and to disturbances in the T cell repertoire, which themselves correspond to impaired immunity to a new infection in aging.

Age-related changes in CD8 T cell homeostasis and immunity to infection

Studies of CD8 T cell responses to vaccination or infection with various pathogens in both animal models and human subjects have revealed a markedly consistent array of age-related defects. In general, recent work shows that aged CD8 T cell responses are decreased in magnitude, and show poor differentiation into effector cells, with a reduced arsenal of effector functions. Here we review potential mechanisms underlying these defects. We specifically address phenotypic and numeric changes to the naïve CD8 T cell precursor pool, the impact of persistent viral infection(s) and inflammation, and contributions of the aging environment in which these cells are activated.

Increased apoptosis, curtailed expansion and incomplete differentiation of CD8+ T cells combine to decrease clearance of L. monocytogenes in old mice

Aging is accompanied by altered immunity, resulting in a variable state of poorly understood immunodeficiency. While both the numbers and the functionality of naïve T cells are decreased by aging, the impact of these changes upon immune defense against bacterial pathogens in vivo remains understudied. Using a model of Listeria monocytogenes (Lm), where the primary CD8(+) T-cell response is critically important for immune defense, we show that C57BL/6 (B6) mice exhibit an age-dependent reduction in survival, with delayed bacterial clearance in old animals. Kinetic analysis of antigen-specific CD8(+) T-cell expansion showed that CD8(+) effectors begin dividing at the same time in old and adult mice, but that the proliferative burst remained incomplete during discrete windows of time and was coupled with increased effector apoptosis in old mice. Further, antilisterial CD8(+) T cells in old mice showed altered expression of key phenotypic and effector molecules and diminished polyfunctionality, measured by the ability to simultaneously produce multiple effector molecules. These results suggest that defects in functional maturation of CD8(+) cells in aged mice, compounded by (or perhaps coupled to) their reduced expansion in response to infection, yield effector CD8(+) T-cell populations insufficient in size and capability to effectively clear newly encountered intracellular pathogens.

Diversity of the CD8+ T cell repertoire elicited against an immunodominant epitope does not depend on the context of infection

The diversity of the pathogen-specific T cell repertoire is believed to be important in allowing recognition of different pathogen epitopes and their variants and thereby reducing the opportunities for mutation-driven pathogen escape. However, the extent to which the TCR repertoire can be manipulated by different vaccine strategies so as to obtain broad diversity and optimal protection is incompletely understood. We have investigated the influence of the infectious/inflammatory context on the TCR diversity of the CD8(+) T cell response specific for the immunodominant epitope in C57BL/6 mice, derived from glycoprotein B of HSV-1. To that effect, we compared TCR V segment utilization, CDR3 length, and sequence diversity of the response to natural HSV-1 infection with those elicited by either Listeria monocytogenes or vaccinia virus expressing the immunodominant epitope in C57BL/6 mice. We demonstrate that although the type of infection in which the epitope was encountered can influence the magnitude of the CD8(+) T cell responses, TCR beta-chain repertoires did not significantly differ among the three infections. These results suggest that widely different live vaccine vectors may have little impact upon the diversity of the induced CTL response, which has important implications for the design of live CTL vaccine strategies against acute and chronic infections.

Aging increases susceptibility to systemic infection with L. monocytogenes (129.9)

Aging is accompanied by changes that affect components of both the innate and adaptive immune system, often resulting in a variable state of poorly-understood immunodeficiency. Using the murine model of systemic infection with the intracellular bacterium L. monocytogenes, we have investigated several parameters of the primary and secondary CD8 T-cell response to determine the impact of age on this arm of adaptive immunity. Comparison of these responses in young adult (8 week) and old (18 month) C57BL/6 mice revealed a dose-dependent decrease in survival, as well as a delayed ability to clear the bacterium in old animals. Kinetic analysis of the antigen-specific CD8 T-cell response revealed a markedly reduced magnitude of expansion in the primary effector T-cell population in old mice; however, antigen-specific CD8 T cells differentiating in old animals exhibited production of IFNγ and TNFα equivalent to those in adult counterparts. These results suggest that the quality of the CD8 T-cell response in old animals may be maintained with age. However, reduced primary expansion of Ag-specific CD8 T-cells and/or reduced precursor frequency of these cells may result in an effector CD8 T-cell population that is insufficient in size to protect against high-dose exposure to a new pathogen. Further experiments are in progress to test this hypothesis.

Stimulation of enhanced CD8 T cell responses following immunization with a hyper-antigen secreting intracytosolic bacterial pathogen

The intracytosolic niche for replication of Listeria monocytogenes (Lm) facilitates delivery of bacteria-derived Ags into the MHC class I pathway for subsequent stimulation of CD8 effector T cells. Using Lm strains that are equivalent for in vivo virulence yet express marked differences in the level of secretion of a protective target Ag, we have evaluated how these specific differences in secretion levels influences the magnitude and effector function of Ag-specific CD8 T cell responses following Lm injection. Immunization with low doses of a hyperantigen-secreting Lm strain stimulated enhanced target-Ag specific CD8 T cell responses compared with the magnitude stimulated following immunization with the same dose of wild-type Lm. The enhanced determinant-specific response was also evident by in vivo CTL activity, increased numbers of memory cells 4 wk following immunization, and enhanced antilisterial protection following a challenge infection. Initiation of antibiotic treatment 24 h following infection with wild-type Lm markedly reduced the magnitude of the effector CD8 T cell response. In contrast, antibiotic treatment initiated 24 h following immunization with the hyperantigen secreting strain of Lm did not impact the frequency of the target-Ag specific CD8 T cells. Thus, immunization with a low dose of a hyperantigen secreting Lm strain, followed by antibiotic treatment to limit the extent of the infection, may represent a safe strategy for the stimulation of enhanced effector CD8 T cell responses to specific Ag by a rLm vaccine.