Comparison of the pathogenesis of the highly passaged MCMV Smith strain with that of the low passaged MCMV HaNa1 isolate in BALB/c mice upon oronasal inoculation

Late-life Attenuation of Cytomegalovirus-mediated CD8 T Cell Memory Inflation: Shrinking of the Cytomegalovirus Latency Niche

CMV drives the accumulation of virus-specific, highly differentiated CD8 memory T cells (memory inflation [MI]). In mice, MI was shown to directly correlate with the CMV infection dose, yet the CMV-associated CD8 MI plateaus over time. It is unclear how MI is regulated with aging. We infected young mice with 102, 104, and 106 PFU of murine CMV and confirmed that MI magnitude was directly proportional to the infectious dose, reaching a setpoint by midlife. By old age, MI subsided, most prominently in mice infected with 106 PFU, and reached statistical parity between groups in 26-mo-old mice. This corresponded to an age-related loss in lymphatic endothelial cells in lymph nodes, recently shown to be sufficient to drive MI in mice. We propose that MI size and persistence over the lifespan is controlled by the size of the lymphatic endothelial cell niche, whose shrinking leads to reduced MI with aging.

Oncolytic α-herpesvirus and myeloid-tropic cytomegalovirus cooperatively enhance systemic antitumor responses

Oncolytic virotherapy aims to activate host antitumor immunity. In responsive tumors, intratumorally injected herpes simplex viruses (HSVs) have been shown to lyse tumor cells, resulting in local inflammation, enhanced tumor antigen presentation, and boosting of antitumor cytotoxic lymphocytes. In contrast to HSV, cytomegalovirus (CMV) is nonlytic and reprograms infected myeloid cells, limiting their antigen-presenting functions and protecting them from recognition by natural killer (NK) cells. Here, we show that when co-injected into mouse tumors with an oncolytic HSV, mouse CMV (mCMV) preferentially targeted tumor-associated myeloid cells, promoted the local release of proinflammatory cytokines, and enhanced systemic antitumor immune responses, leading to superior control of both injected and distant contralateral tumors. Deletion of mCMV genes m06, which degrades major histocompatibility complex class I (MHC class I), or m144, a viral MHC class I homolog that inhibits NK activation, was shown to diminish the antitumor activity of the HSV/mCMV combination. However, an mCMV recombinant lacking the m04 gene, which escorts MHC class I to the cell surface, showed superior HSV adjuvanticity. CMV is a potentially promising agent with which to reshape and enhance antitumor immune responses following oncolytic HSV therapy.

Pathogenesis of wild-type-like rhesus cytomegalovirus strains following oral exposure of immune-competent rhesus macaques

Rhesus cytomegalovirus (RhCMV) infection of rhesus macaques (Macaca mulatta) is a valuable nonhuman primate model of human CMV (HCMV) persistence and pathogenesis. In vivo studies predominantly use tissue culture-adapted variants of RhCMV that contain multiple genetic mutations compared to wild-type (WT) RhCMV. In many studies, animals have been inoculated by non-natural routes (e.g., subcutaneous, intravenous) that do not recapitulate disease progression via the normative route of mucosal exposure. Accordingly, the natural history of RhCMV would be more accurately reproduced by infecting macaques with strains of RhCMV that reflect the WT genome using natural routes of mucosal transmission. Herein, we tested two WT-like RhCMV strains, UCD52 and UCD59, and demonstrated that systemic infection and frequent, high-titer viral shedding in bodily fluids occurred following oral inoculation. RhCMV disseminated to a broad range of tissues, including the central nervous system and reproductive organs. Commonly infected tissues included the thymus, spleen, lymph nodes, kidneys, bladder, and salivary glands. Histological examination revealed prominent nodular hyperplasia in spleens and variable levels of lymphoid lymphofollicular hyperplasia in lymph nodes. One of six inoculated animals had limited viral dissemination and shedding, with commensurately weak antibody responses to RhCMV antigens. These data suggest that long-term RhCMV infection parameters might be restricted by local innate factors and/or de novo host immune responses in a minority of primary infections. Together, we have established an oral RhCMV infection model that mimics natural HCMV infection. The virological and immunological parameters characterized in this study will greatly inform HCMV vaccine designs for human immunization. IMPORTANCE Human cytomegalovirus (HCMV) is globally ubiquitous with high seroprevalence rates in all communities. HCMV infections can occur vertically following mother-to-fetus transmission across the placenta and horizontally following shedding of virus in bodily fluids in HCMV infected hosts and subsequent exposure of susceptible individuals to virus-laden fluids. Intrauterine HCMV has long been recognized as an infectious threat to fetal growth and development. Since vertical HCMV infections occur following horizontal HCMV transmission to the pregnant mother, the nonhuman primate model of HCMV pathogenesis was used to characterize the virological and immunological parameters of infection following primary mucosal exposures to rhesus cytomegalovirus.

A Systematic Review of the Neuropathologic Findings of Post-Viral Olfactory Dysfunction: Implications and Novel Insight for the COVID-19 Pandemic

BACKGROUND

Post-viral olfactory dysfunction is a common cause of both short- and long-term smell alteration. The coronavirus pandemic further highlights the importance of post-viral olfactory dysfunction. Currently, a comprehensive review of the neural mechanism underpinning post-viral olfactory dysfunction is lacking.

OBJECTIVES

To synthesize the existing primary literature related to olfactory dysfunction secondary to viral infection, detail the underlying pathophysiological mechanisms, highlight relevance for the current COVID-19 pandemic, and identify high impact areas of future research.

METHODS

PubMed and Embase were searched to identify studies reporting primary scientific data on post-viral olfactory dysfunction. Results were supplemented by manual searches. Studies were categorized into animal and human studies for final analysis and summary.

RESULTS

A total of 38 animal studies and 7 human studies met inclusion criteria and were analyzed. There was significant variability in study design, experimental model, and outcome measured. Viral effects on the olfactory system varies significantly based on viral substrain but generally include damage or alteration in components of the olfactory epithelium and/or the olfactory bulb.

CONCLUSIONS

The mechanism of post-viral olfactory dysfunction is highly complex, virus-dependent, and involves a combination of insults at multiple levels of the olfactory pathway. This will have important implications for future diagnostic and therapeutic developments for patients infected with COVID-19.

Hematopoietic cell-mediated dissemination of murine cytomegalovirus is regulated by NK cells and immune evasion

Cytomegalovirus (CMV) causes clinically important diseases in immune compromised and immune immature individuals. Based largely on work in the mouse model of murine (M)CMV, there is a consensus that myeloid cells are important for disseminating CMV from the site of infection. In theory, such dissemination should expose CMV to cell-mediated immunity and thus necessitate evasion of T cells and NK cells. However, this hypothesis remains untested. We constructed a recombinant MCMV encoding target sites for the hematopoietic specific miRNA miR-142-3p in the essential viral gene IE3. This virus disseminated poorly to the salivary gland following intranasal or footpad infections but not following intraperitoneal infection in C57BL/6 mice, demonstrating that dissemination by hematopoietic cells is essential for specific routes of infection. Remarkably, depletion of NK cells or T cells restored dissemination of this virus in C57BL/6 mice after intranasal infection, while dissemination occurred normally in BALB/c mice, which lack strong NK cell control of MCMV. These data show that cell-mediated immunity is responsible for restricting MCMV to hematopoietic cell-mediated dissemination. Infected hematopoietic cells avoided cell-mediated immunity via three immune evasion genes that modulate class I MHC and NKG2D ligands (m04, m06 and m152). MCMV lacking these 3 genes spread poorly to the salivary gland unless NK cells were depleted, but also failed to replicate persistently in either the nasal mucosa or salivary gland unless CD8⁺ T cells were depleted. Surprisingly, CD8⁺ T cells primed after intranasal infection required CD4⁺ T cell help to expand and become functional. Together, our data suggest that MCMV can use both hematopoietic cell-dependent and -independent means of dissemination after intranasal infection and that cell mediated immune responses restrict dissemination to infected hematopoietic cells, which are protected from NK cells during dissemination by viral immune evasion. In contrast, viral replication within mucosal tissues depends on evasion of T cells.

Cytomegalovirus (CMV) is a common cause of disease in immune compromised individuals as well as a common cause of congenital infections leading to disease in newborns. The virus is thought to enter primarily via mucosal barrier tissues, such as the oral and nasal mucosa. However, it is not clear how the virus escapes these barrier tissues to reach distant sites. In this study, we used a mouse model of CMV infection. Our data illustrate a complex balance between the immune system and viral infection of “myeloid cells”, which are most commonly thought to carry the virus around the body after infection. In particular, our data suggest that robust immune responses at the site of infection force the virus to rely on myeloid cells to escape the site of infection. Moreover, viral genes designed to evade these immune responses were needed to protect the virus during and after its spread to distant sites. Together, this work sheds light on the mechanisms of immune control and viral survival during CMV infection of mucosal tissues and spread to distant sites of the body.

Persistent viral replication and the development of T-cell responses after intranasal infection by MCMV

Natural transmission of cytomegalovirus (CMV) has been difficult to observe. However, recent work using the mouse model of murine (M)CMV demonstrated that MCMV initially infects the nasal mucosa after transmission from mothers to pups. We found that intranasal (i.n.) inoculation of C57BL/6J mice resulted in reliable recovery of replicating virus from the nasal mucosa as assessed by plaque assay. After i.n. inoculation, CD8⁺ T-cell priming occurred in the mandibular, deep-cervical, and mediastinal lymph nodes within 3 days of infection. Although i.n. infection induced "memory inflation" of T cells specific for the M38_316-323 epitope, there were no detectable CD8⁺ T-cell responses against the late-appearing IE3_416-423 epitope, which contrasts with intraperitoneal (i.p.) infection. MCMV-specific T cells migrated into the nasal mucosa where they developed a tissue-resident memory (T_RM) phenotype and this could occur independently of local virus infection or antigen. Strikingly however, virus replication was poorly controlled in the nasal mucosa and MCMV was detectable by plaque assay for at least 4 months after primary infection, making the nasal mucosa a second site for MCMV persistence. Unlike in the salivary glands, the persistence of MCMV in the nasal mucosa was not modulated by IL-10. Taken together, our data characterize the development of local and systemic T-cell responses after intranasal infection by MCMV and define the nasal mucosa, a natural site of viral entry, as a novel site of viral persistence.

RHIM-based protein:protein interactions in microbial defence against programmed cell death by necroptosis

The Receptor-interacting protein kinase Homotypic Interaction Motif (RHIM) is an amino acid sequence that mediates multiple protein:protein interactions in the mammalian programmed cell death pathway known as necroptosis. At least one key RHIM-based complex has been shown to have a functional amyloid fibril structure, which provides a stable hetero-oligomeric platform for downstream signaling. RHIMs and related motifs are present in immunity-related proteins across nature, from viruses to fungi to metazoans. Necroptosis is a hallmark feature of cellular clearance of infection. For this reason, numerous pathogens, including viruses and bacteria, have developed varied methods to modulate necroptosis, focusing on inhibiting RHIM:RHIM interactions, and thus their downstream cell death effects. This review will discuss current understanding of RHIM:RHIM interactions in normal cellular activation of necroptosis, from a structural and cell biology perspective. It will compare the mechanisms by which pathogens subvert these interactions in order to maintain their replicative and infective cycles and consider the similarities between RHIMs and other functional amyloid-forming proteins associated with cell death and innate immunity. It will discuss the implications of the heteromeric nature and structure of RHIM-based amyloid complexes in the context of other functional amyloids.

Lytic viral replication and immunopathology in a cytomegalovirus-induced mouse model of secondary hemophagocytic lymphohistiocytosis

Background

Hemophagocytic lymphohistiocytosis (HLH) is a rare immunological disorder caused by unbridled activation of T cells and macrophages, culminating in a life-threatening cytokine storm. A genetic and acquired subtype are distinguished, termed primary and secondary HLH, respectively. Clinical manifestations of both forms are frequently preceded by a viral infection, predominantly with herpesviruses. The exact role of the viral infection in the development of the hemophagocytic syndrome remains to be further elucidated.

Methods

We utilized a recently developed murine model of cytomegalovirus-associated secondary HLH and dissected the respective contributions of lytic viral replication and immunopathology in its pathogenesis.

Results

HLH-like disease only developed in cytomegalovirus-susceptible mouse strains unable to clear the virus, but the severity of symptoms was not correlated to the infectious viral titer. Lytic viral replication and sustained viremia played an essential part in the pathogenesis since abortive viral infection was insufficient to induce a full-blown HLH-like syndrome. Nonetheless, a limited set of symptoms, in particular anemia, thrombocytopenia and elevated levels of soluble CD25, appeared less dependent of the viral replication but rather mediated by the host’s immune response, as corroborated by immunosuppressive treatment of infected mice with dexamethasone.

Conclusion

Both virus-mediated pathology and immunopathology cooperate in the pathogenesis of full-blown virus-associated secondary HLH and are closely entangled. A certain level of viremia appears necessary to elicit the characteristic HLH-like symptoms in the model.

Modulation of Microglial Cell Fcγ Receptor Expression Following Viral Brain Infection

Fcγ receptors (FcγRs) for IgG couple innate and adaptive immunity through activation of effector cells by antigen-antibody complexes. We investigated relative levels of activating and inhibitory FcγRs on brain-resident microglia following murine cytomegalovirus (MCMV) infection. Flow cytometric analysis of microglial cells obtained from infected brain tissue demonstrated that activating FcγRs were expressed maximally at 5 d post-infection (dpi), while the inhibitory receptor (FcγRIIB) remained highly elevated during both acute and chronic phases of infection. The highly induced expression of activating FcγRIV during the acute phase of infection was also noteworthy. Furthermore, in vitro analysis using cultured primary microglia demonstrated the role of interferon (IFN)γ and interleukin (IL)-4 in polarizing these cells towards a M1 or M2 phenotype, respectively. Microglial cell-polarization correlated with maximal expression of either FcγRIV or FcγRIIB following stimulation with IFNγ or IL-4, respectively. Finally, we observed a significant delay in polarization of microglia towards an M2 phenotype in the absence of FcγRs in MCMV-infected Fcer1g and FcgR2b knockout mice. These studies demonstrate that neuro-inflammation following viral infection increases expression of activating FcγRs on M1-polarized microglia. In contrast, expression of the inhibitory FcγRIIB receptor promotes M2-polarization in order to shut-down deleterious immune responses and limit bystander brain damage.

Cytomegalovirus Reinfections Stimulate CD8 T-Memory Inflation

Cytomegalovirus (CMV) has been shown to induce large populations of CD8 T-effector memory cells that unlike central memory persist in large quantities following infection, a phenomenon commonly termed “memory inflation”. Although murine models to date have shown very large and persistent CMV-specific T-cell expansions following infection, there is considerable variability in CMV-specific T-memory responses in humans. Historically such memory inflation in humans has been assumed a consequence of reactivation events during the life of the host. Because basic information about CMV infection/re-infection and reactivation in immune competent humans is not available, we used a murine model to test how primary infection, reinfection, and reactivation stimuli influence memory inflation. We show that low titer infections induce “partial” memory inflation of both mCMV specific CD8 T-cells and antibody. We show further that reinfection with different strains can boost partial memory inflation. Finally, we show preliminary results suggesting that a single strong reactivation stimulus does not stimulate memory inflation. Altogether, our results suggest that while high titer primary infections can induce memory inflation, reinfections during the life of a host may be more important than previously appreciated.

Productive replication of nephropathogenic infectious bronchitis virus in peripheral blood monocytic cells, a strategy for viral dissemination and kidney infection in chickens

In the present study, the replication kinetics of nephropathogenic (B1648) and respiratory (Massachusetts-M41) IBV strains were compared in vitro in respiratory mucosa explants and blood monocytes (KUL01⁺ cells), and in vivo in chickens to understand why some IBV strains have a kidney tropism. B1648 was replicating somewhat better than M41 in the epithelium of the respiratory mucosa explants and used more KUL01⁺ cells to penetrate the deeper layers of the respiratory tract. B1648 was productively replicating in KUL01⁺ monocytic cells in contrast with M41. In B1648 inoculated animals, 10^2.7–6.8 viral RNA copies/100 mg were detected in tracheal secretions at 2, 4, 6, 8, 10 and 12 days post inoculation (dpi), 10^2.4–4.5 viral RNA copies/mL in plasma at 2, 4, 6, 8, 10 and 12 dpi and 10^1.8–4.4 viral RNA copies/10⁶ mononuclear cells in blood at 2, 4, 6 and 8 dpi. In M41 inoculated animals, 10^2.6–7.0 viral RNA copies/100 mg were detected in tracheal secretions at 2, 4, 6, 8 and 10 dpi, but viral RNA was not demonstrated in plasma and mononuclear cells (except in one chicken at 6 dpi). Infectious virus was detected only in plasma and mononuclear cells of the B1648 group. At euthanasia (12 dpi), viral RNA and antigen positive cells were detected in lungs, liver, spleen and kidneys of only the B1648 group and in tracheas of both the B1648 and M41 group. In conclusion, only B1648 can easily disseminate to internal organs via a cell-free and -associated viremia with KUL01⁺ cells as important carrier cells.

Difference in replication of low-passage MCMV HaNa1 in BALB/c, C57BL/6 and NOD mice and role of different branches of immunity in susceptibility

Currently, murine cytomegalovirus (MCMV) infections have been studied extensively in inbred mice via intraperitoneal route with highly passaged strains. However, the question how a low-passage MCMV replicates in inbred mice via a natural route remained unanswered. Here, different inbred mice (BALB/c, C57BL/6 and NOD) were inoculated oronasally with a low-passage MCMV strain, HaNa1. Viral replication was evaluated by virus titration and quantitative real-time PCR, and antibody response was assessed by immunoperoxidase cell monolayer assay (IPMA). In BALB/c mice, virus persisted in nasal mucosa (from 3 dpi) and submandibular glands (from 7 dpi) until the end of experiment (49 dpi). In C57BL/6 mice, infectious virus was only detected in nasal mucosa from 3 dpi until 21 dpi; viral genome was still detectable in nasal mucosa until 49 dpi. Although infectious virus was not detected in submandibular glands of C57BL/6 mice, viral genome was detected from 7 dpi until 49 dpi. NOD mice appeared to be even more resistant with absence of any productive infection; viral genome was detected at low levels in nasal mucosa. We demonstrated that there was a strong correlation between on the one hand degree of productive replication and on the other hand the time of first appearance and titer of MCMV-specific IgG antibody. The deficiency of functional T and B cells and interleukin-2 (IL-2) common-γ chain (γc) did not increase the susceptibility to MCMV by the use of NOD.SCID and NSG mice. In addition, using monocytic cells from different inbred mice we found patterns of resistance similar to those seen in vivo, as assessed by viral antigen expression. Taken together, these results demonstrated that upon oronasal inoculation low-passage MCMV HaNa1 replication clearly differs between different inbred mice (BALB/c>C57BL/6>NOD); resistance in vivo to MCMV is partly due to less susceptibility of host target cells and is independent of T, B cells and γc signaling cytokine-dependent NK cell activities.

MCMV exploits the spleen as a transfer hub for systemic dissemination upon oronasal inoculation

Murine cytomegalovirus (MCMV) infection in mice is a commonly used animal model for studying human cytomegalovirus (HCMV) infections. In our previous studies, a mouse model based on an oronasal MCMV infection was set up for mimicking a natural infection, and the spleen was hypothesized to regulate viremia and virus dissemination to distal organs such as submandibular glands. Here, the role of the spleen during an MCMV infection was investigated by the comparison of intact and splenectomized Balb/c mice. Both highly passaged MCMV Smith and low passaged MCMV HaNa1 were used. Various samples were collected at 7, 14, and 21 days post inoculation (dpi) for analyses by virus isolation/titration, co-cultivation and qPCR. The results showed that for both virus strains, 1) cell-associated virus in PBMC (determined by co-cultivation) was detected in intact mice but not in splenectomized mice; 2) the mean viral DNA load in PBMC of splenectomized mice was 4.4-(HaNa1)/2.7-(Smith) fold lower at the peak viremia (7dpi) in contrast to that of intact mice; and 3) infectious virus in the submandibular glands was detected later in splenectomized mice (14dpi) than in intact mice (7dpi). Moreover, the average virus titers in submandibular glands of splenectomized mice were 10-(HaNa1)/7.9-(Smith) fold lower at 14dpi and 1.7-(HaNa1)/2.1-(Smith) fold lower at 21dpi compared with that of intact mice. Upon inoculation with MCMV Smith, infectious virus was found in the kidneys and liver of intact mice, but not in splenectomized mice. Taken together, all these data clearly demonstrate that virus dissemination to distant organs is reduced in splenectomized mice, further confirming the importance of the spleen as a viremia booming site for a natural MCMV infection.

Pattern of circulation of MCMV mimicking natural infection upon oronasal inoculation

Cytomegaloviruses may infect mammals via oronasal route. However, up till now it remains unclear how this exposure leads to a general infection and shedding. To address this issue, BALB/c female mice were oronasally inoculated with either the highly passaged murine cytomegalovirus (MCMV) Smith or the low passaged MCMV HaNa1. Virus titration showed a productive virus replication of both strains in the nasal mucosa from 1 dpi until the end of the experiment (14 dpi), in lungs from 5 until 14 dpi, and in submandibular glands from 7 until 14 dpi. In contrast to MCMV HaNa1, MCMV Smith also established a low level productive infection in abdominal organs (spleen, liver and kidneys) from 5 dpi (spleen), 7 dpi (liver), and 10 dpi (kidneys) until the end of the experiment. Co-culture showed that for both strains, cell-associated virus was detected in a non-infectious form in nasopharynx-associated lymphoid tissues (NALT) from 1 until 14 dpi, in submandibular lymph nodes from 3 until 5 dpi, in deep cervical lymph nodes from 3 until 14 dpi, in mediastinal lymph nodes from 7 until 14 dpi, in spleen from 5 until at least 10 dpi and in the peripheral blood mononuclear cells (PBMC) at 7 and 10 dpi. The present study shows that upon oronasal exposure, MCMV first enters the nasal mucosa and NALT, from where the virus disseminates to the spleen possibly via the draining lymphatic system and blood; a subsequent cell-associated viremia transports MCMV to submandibular glands and for MCMV Smith also to liver and kidneys, where a second productive replication starts.

Infections of neonatal and adult mice with murine CMV HaNa1 strain upon oronasal inoculation: New insights in the pathogenesis of natural primary CMV infections

In healthy individuals, naturally acquired infections of human cytomegalovirus (HCMV) are generally asymptomatic. Animal models mimicking the natural primary HCMV infections in infants and adults are scarce. Here, neonatal and adult BALB/c mice were inoculated oronasally with a Belgian isolate HaNa1 of murine cytomegalovirus (MCMV). None of the mice showed clinical symptoms. In neonatal mice, a typical systemic infection occurred. In adult mice, viral replication was restricted to the nasal mucosa and submandibular glands. Infectious virus was not detected in trachea, oral mucosa, pharynx, esophagus, small intestines of both neonatal and adult mice at all time points. Nose was demonstrated to be the entry site. Double immunofluorescence staining showed that in nose infected cells were olfactory neurons and sustentacular cells in olfactory epithelium and were macrophages and dendritic cells in nasopharynx-associated lymphoid tissues (NALT). Neonatal and adult mice developed similar antibody response pattern, though former magnitude was lower. In summary, we have established intranasal (without anesthesia) infections of neonatal and adult mice with murine CMV HaNa1 strain, which mimic the range and extent of virus replication during natural primary HCMV infections in healthy infants and adults. These findings might bring new insights in the pathogenesis of natural primary CMV infections.