Virological, innate, and adaptive immune profiles shaped by variation in route and age of host in murine cytomegalovirus infection.

Human cytomegalovirus (hCMV) is a ubiquitous facultative pathogen, which establishes a characteristic latent and reactivating lifelong infection in immunocompetent hosts. Murine CMV (mCMV) infection is widely used as an experimental model of hCMV infection, employed to investigate the causal nature and extent of CMV's contribution to inflammatory, immunological, and health disturbances in humans. Therefore, mimicking natural human infection in mice would be advantageous to hCMV research. To assess the role of route and age at infection in modeling hCMV in mice, we infected prepubescent and young sexually mature C57BL/6 (B6) mice intranasally (i.n., a likely physiological route in humans) and intraperitoneally (i.p., a frequently used experimental route, possibly akin to transplant-mediated infection). In our hands, both routes led to comparable early viral loads and tissue spreads. However, they yielded differential profiles of innate and adaptive systemic immune activation. Specifically, the younger, prepubescent mice exhibited the strongest natural killer cell activation in the blood in response to i.p. infection. Further, the i.p. infected animals (particularly those infected at 12 weeks) exhibited larger anti-mCMV IgG and greater expansion of circulating CD8+ T cells specific for both acute (non-inflationary) and latent phase (inflationary) mCMV epitopes. By contrast, tissue immune responses were comparable between i.n. and i.p. groups. Our results illustrate a distinction in the bloodborne immune response profiles across infection routes and ages and are discussed in light of physiological parameters of interaction between CMV, immunity, inflammation, and health over the lifespan. IMPORTANCE: The majority of experiments modeling human cytomegalovirus (hCMV) infection in mice have been carried out using intraperitoneal infection in sexually mature adult mice, which stands in contrast to the large number of humans being infected with human CMV at a young age, most likely via bodily fluids through the nasopharyngeal/oral route. This study examined the impact of the choice of age and route of infection in modeling CMV infection in mice. By comparing young, prepubescent to older sexually mature counterparts, infected either via the intranasal or intraperitoneal route, we discovered substantial differences in deployment and response intensity of different arms of the immune system in systemic control of the virus; tissue responses, by contrast, appeared similar between ages and infection routes.

Cutting Edge: Characterization of Low Copy Number Human Angiotensin-Converting Enzyme 2-Transgenic Mice as an Improved Model of SARS-CoV-2 Infection.

A popular mouse model of COVID-19, the K18-hACE2 mouse, expresses the SARS-coronavirus entry receptor, human angiotensin-converting enzyme 2 (hACE2) driven by the keratin-18 promoter. SARS-CoV-2-infected K18-hACE2 mice exhibit neuropathology not representative of human infection. They contain eight transgene (Tg) copies, leading to excess hACE2 expression and rampant viral replication. We generated two new lines of K18-hACE2 mice encoding one and two copies of hACE2 (1-hACE2-Tg and 2-hACE2-Tg, respectively). Relative to the original strain (called 8-hACE2-Tg in this study), 2-hACE2-Tg mice exhibited lower mortality, with less viral replication in the lung and brain. Furthermore, 1-hACE2-Tg mice exhibited no mortality and had no detectable virus in the brain; yet, they exhibited clear viral replication in the lung. All three strains showed SARS-CoV-2-related weight loss commensurate with the mortality rates. 1-hACE2-Tg mice mounted detectable primary and memory T effector cell and Ab responses. We conclude that these strains provide improved models to study hACE2-mediated viral infections.

The aging of the immune system and its implications for transplantation.

By the last third of life, most mammals, including humans, exhibit a decline in immune cell numbers, immune organ structure, and immune defense of the organism, commonly known as immunosenescence. This decline leads to clinical manifestations of increased susceptibility to infections, particularly those caused by emerging and reemerging microorganisms, which can reach staggering levels-infection with SARS-CoV-2 has been 270-fold more lethal to older adults over 80 years of age, compared to their 18-39-year-old counterparts. However, while this would be expected to be beneficial to situations where hyporeactivity of the immune system may be desirable, this is not always the case. Here, we discuss the cellular and molecular underpinnings of immunosenescence as they pertain to outcomes of solid organ and hematopoietic transplantation.

Characterization of Retronasal Airflow Patterns during Intraoral Fluid Discrimination Using a Low-Cost, Open-Source Biosensing Platform.

Nasal airflow plays a critical role in olfactory processes, and both retronasal and orthonasal olfaction involve sensorimotor processes that facilitate the delivery of volatiles to the olfactory epithelium during odor sampling. Although methods are readily available for monitoring nasal airflow characteristics in laboratory and clinical settings, our understanding of odor sampling behavior would be enhanced by the development of inexpensive wearable technologies. Thus, we developed a method of monitoring nasal air pressure using a lightweight, open-source brain-computer interface (BCI) system and used the system to characterize patterns of retronasal airflow in human participants performing an oral fluid discrimination task. Participants exhibited relatively sustained low-rate retronasal airflow during sampling punctuated by higher-rate pulses often associated with deglutition. Although characteristics of post-deglutitive pulses did not differ across fluid conditions, the cumulative duration, probability, and estimated volume of retronasal airflow were greater during discrimination of perceptually similar solutions. These findings demonstrate the utility of a consumer-grade BCI system in assessing human olfactory behavior. They suggest further that sensorimotor processes regulate retronasal airflow to optimize the delivery of volatiles to the olfactory epithelium and that discrimination of perceptually similar oral fluids may be accomplished by varying the duration of optimal airflow rate.

Competent immune responses to SARS-CoV-2 variants in older adults following two doses of mRNA vaccination.

Aging is associated with a reduced magnitude of primary immune responses to vaccination. mRNA-based SARS-CoV-2 vaccines have shown efficacy in older adults but virus variant escape is still unclear. Here we analyze humoral and cellular immunity against an early-pandemic viral isolate and compare that to the P.1 (Gamma) and B.1.617.2 (Delta) variants in two cohorts (<50 and >55 age) of mRNA vaccine recipients. We further measure neutralizing antibody titers for B.1.617.1 (Kappa) and B.1.595, with the latter SARS-CoV-2 isolate bearing the spike mutation E484Q. Robust humoral immunity is measured following second vaccination, and older vaccinees manifest cellular immunity comparable to the adult group against early-pandemic SARS-CoV-2 and more recent variants. More specifically, the older cohort has lower neutralizing capacity at 7-14 days following the second dose but equilibrates with the younger cohort after 2-3 months. While long-term vaccination responses remain to be determined, our results implicate vaccine-induced protection in older adults against SARS-CoV-2 variants and inform thinking about boost vaccination.

Quantitative restoration of immune defense in old animals determined by naive antigen-specific CD8 T-cell numbers.

Older humans and animals often exhibit reduced immune responses to infection and vaccination, and this often directly correlates to the numbers and frequency of naive T (Tn) cells. We found such a correlation between reduced numbers of blood CD8+ Tn cells and severe clinical outcomes of West Nile virus (WNV) in both humans naturally exposed to, and mice experimentally infected with, WNV. To examine possible causality, we sought to increase the number of CD8 Tn cells by treating C57BL/6 mice with IL-7 complexes (IL-7C, anti-IL-7 mAb bound to IL-7), shown previously to efficiently increase peripheral T-cell numbers by homeostatic proliferation. T cells underwent robust expansion following IL-7C administration to old mice increasing the number of total T cells (>fourfold) and NS4b:H-2Db -restricted antigen-specific CD8 T cells (twofold). This improved the numbers of NS4b-specific CD8 T cells detected at the peak of the response against WNV, but not survival of WNV challenge. IL-7C-treated old animals also showed no improvement in WNV-specific effector immunity (neutralizing antibody and in vivo T-cell cytotoxicity). To test quantitative limits to which CD8 Tn cell restoration could improve protective immunity, we transferred graded doses of Ag-specific precursors into old mice and showed that injection of 5400 (but not of 1800 or 600) adult naive WNV-specific CD8 T cells significantly increased survival after WNV. These results set quantitative limits to the level of Tn reconstitution necessary to improve immune defense in older organisms and are discussed in light of targets of immune reconstitution.

IL-6 can singlehandedly drive many features of frailty in mice.

Frailty is a geriatric syndrome characterized by age-related declines in function and reserve resulting in increased vulnerability to stressors. The most consistent laboratory finding in frail subjects is elevation of serum IL-6, but it is unclear whether IL-6 is a causal driver of frailty. Here, we characterize a new mouse model of inducible IL-6 expression (IL-6TET-ON/+ mice) following administration of doxycycline (Dox) in food. In this model, IL-6 induction was Dox dose-dependent. The Dox dose that increased IL-6 levels to those observed in frail old mice directly led to an increase in frailty index, decrease in grip strength, and disrupted muscle mitochondrial homeostasis. Littermate mice lacking the knock-in construct failed to exhibit frailty after Dox feeding. Both naturally old mice and young Dox-induced IL-6TET-ON/+ mice exhibited increased IL-6 levels in sera and spleen homogenates but not in other tissues. Moreover, Dox-induced IL-6TET-ON/+ mice exhibited selective elevation in IL-6 but not in other cytokines. Finally, bone marrow chimera and splenectomy experiments demonstrated that non-hematopoietic cells are the key source of IL-6 in our model. We conclude that elevated IL-6 serum levels directly drive age-related frailty, possibly via mitochondrial mechanisms.

Immunity to acute virus infections with advanced age.

New infections in general, and new viral infections amongst them, represent a serious challenge to an older organism. This review discusses the age-related alterations in responsiveness to infection from the standpoint of virus:host relationship and the host physiological whole-organism and specific immune response to the virus. Changes with age in the innate and adaptive immune system homeostasis and function are reviewed briefly. This is followed by a review of specific alterations and defects in the response of older organisms (chiefly mice and humans) to acute (particularly emerging and re-emerging) viral infections, with a very brief summary of the response to latent persistent infections. Finally, we provide a brief summary of the perspectives for possible interventions to enhance antiviral immunity.