Human cytomegalovirus (HCMV) encephalitis in an immunocompetent young person and diagnostic reliability of HCMV DNA PCR using cerebrospinal fluid of nonimmunosuppressed patients

The Wide Spectrum of Presentations of Cytomegalovirus Infection in Immunocompetent Hosts: An Exhaustive Narrative Review

CMV is a ubiquitous DNA virus that establishes infection and results in 40-100% seropositivity. Viral replication occurs following an acquired primary infection (or reinfection) or by the reactivation of life-long latency. In immunocompetent patients, CMV infection is mostly asymptomatic or mild and self-limited. However, an extensive review of the literature published up to April 2024 reveals that despite immunocompetence, CMV can cause a very large variety of clinical syndromes in any part of the gastrointestinal tract (the most common pattern), the central or peripheral nervous system, and the eyes, as well as hematological, pulmonary, cardiac, and cutaneous disease. Not uncommonly, more than one system is involved, and though the disease is often self-limited, treatment with intravenous ganciclovir or oral valganciclovir may be required, and in isolated cases, fatalities may occur. Thus, a potential CMV infection should be considered in the differential of myriad syndromes in non-immunocompromised patients. Associated systemic symptoms (fever, sweats, and weight loss), lymphocytosis, and hepatitis are not uncommon and can be a useful clue. Some populations, such as critically ill patients in intensive care, pregnant women, elderly patients, and those with inflammatory bowel disease, may be more susceptible. Moreover, the potential of past, latent CMV infection (i.e., CMV seropositivity) to be associated with significant cardiovascular morbidity and all-cause mortality years later is intriguing and requires further study. All these data indicate the outstanding importance of developing a vaccine against CMV, which hopefully will become available in the foreseeable future. Meanwhile, a solid diagnosis of active CMV infection can be quickly established (or ruled out) by widely available serology tests and PCR amplification, and clinicians in all disciplines need to be more aware of the diverse guises of CMV infection and remember to consider it in any host, including an immunocompetent one.

Severe cytomegalovirus encephalitis in an immunocompetent healthy young woman: A case report

Cytomegalovirus (CMV) causes a mild illness in immunocompetent patients. Conversely, it can be life-threatening in immunocompromised or critically ill patients. We present a 48-year-old immunocompetent woman presenting primary severe CMV encephalitis. She presented with a headache, fever, and drowsiness. She did not respond to empirical treatment. Her level of consciousness deteriorated, she was put on mechanical ventilation on day two. Bacterial culture, herpes simplex virus, and tuberculosis were negative in cerebrospinal fluid (CSF). After three weeks, the patient was transferred to our center due to financial matters. Brain magnetic resonance imaging (MRI) showed diffuse hydrocephalus, periventricular T2 hyperintensity, patchy basal ganglia, and diffuse leptomeningeal enhancement. CMV polymerase chain reaction (PCR) was positive in cerebrospinal fluid (CSF) specimen. Ganciclovir (5 mg/kg/IV q12h) was initiated. Subsequently, a brain shunt was inserted. Her level of consciousness raised, she was weaned from the ventilator. She was discharged after 52 days in a bedridden state, quadriplegic, and only able to speak words with a minor swallowing problem. She remained in the same condition for one year. She was expired one year later due to aspiration pneumonia after four weeks of hospitalization. Early diagnosis and treatment of severe CMV encephalitis are crucial to prevent neurological sequelae.

Viral Encephalitis

Viruses are a frequent cause of encephalitis. Common or important viruses causing encephalitis include herpesviruses, arboviruses, enteroviruses, parechoviruses, mumps, measles, rabies, Ebola, lymphocytic choriomeningitis virus, and henipaviruses. Other viruses may cause an encephalopathy. Host factors and clinical features of infection are important to consider in identifying the cause for encephalitis. Cerebrospinal fluid evaluation, serologic/polymerase chain reaction studies, and neuroimaging are cornerstones of diagnostic evaluation in encephalitis. Treatable forms of encephalitis are important to consider in all cases. Central nervous system inflammation may also occur because of postinfectious autoimmunity, such as acute disseminated encephalomyelitis or antibody-mediated encephalitis after herpes simplex virus encephalitis.

CMV encephalitis in an immune-competent patient

After being admitted to hospital with atypical chest pain, a 61-year-old woman was noted to become lethargic and confused. She also developed global dysphasia. MRI was suggestive of encephalitis, and lumbar puncture was positive for cytomegalovirus (CMV) PCR. The patient was treated with intravenous ganciclovir and subsequent oral valganciclovir and she gradually made a reasonable recovery. While this infection is usually closely linked to immunosuppression, the patient was found to be HIV negative, and was not on any immunosuppressive therapy. Going through the patient's medical history revealed two possible risk factors which might have led to the development of CMV encephalitis: immunosuppression secondary to underlying poorly controlled diabetes mellitus; and previous admission to the intensive care unit which might have lead to CMV reactivation.

Clinical evaluation of Roche COBAS® AmpliPrep/COBAS® TaqMan® CMV test using nonplasma samples

Cytomegalovirus (CMV) infection is a leading cause of loss of hearing, vision, and mental retardation in congenitally infected children. It is also associated with complications of organ transplant and opportunistic HIV coinfection. The Roche COBAS^® AmpliPrep/COBAS^® TaqMan^® CMV test is an FDA-approved test that measures CMV DNA viral load in plasma for the diagnosis and management of patients at risk of CMV-associated diseases. Besides plasma, CMV is often found in bronchoalveolar lavage (BAL), cerebrospinal fluid (CSF), and urine. Thus, monitoring of CMV for critical care of patients in these nonplasma samples becomes necessary. The objective of this study was to conduct an analytic and clinical feasibility study of the Roche CMV test in BAL, CSF, and urine. The lower limit of detection, analytic measurement range, assay sensitivity, specificity, and precision were determined. Results of this study showed that the lower limit of detections were 50, 100, and 300 IU/mL for BAL, CSF, or urine, respectively. The analytic measurement ranges were from log₁₀ 2.48 to log₁₀ 5.48. The assay specificity was 94.4% for BAL and 100% for CSF and urine. The assay precision was all within the acceptable range. The performance of Roche test was further compared with 2 comparators including the RealTime CMV assay (Abbott Molecular) and a CMV Quantitative Polymerase Chain Reaction test (Vela Diagnostics). There was a general positive correlation between the Roche method and the Abbott or the Vela method. Overall, this study suggests that the Roche CMV test is suitable for the quantification of CMV viral load DNA in the described nonplasma samples.

Human herpesvirus infections of the central nervous system: laboratory diagnosis based on DNA detection by nested PCR in plasma and cerebrospinal fluid samples

Infections of the central nervous systems (CNS) present a diagnostic problem for which an accurate laboratory diagnosis is essential. Invasive practices, such as cerebral biopsy, have been replaced by obtaining a polymerase chain reaction (PCR) diagnosis using cerebral spinal fluid (CSF) as a reference method. Tests on DNA extracted from plasma are noninvasive, thus avoiding all of the collateral effects and patient risks associated with CSF collection. This study aimed to determine whether plasma can replace CSF in nested PCR analysis for the detection of CNS human herpesvirus (HHV) diseases by analysing the proportion of patients whose CSF nested PCR results were positive for CNS HHV who also had the same organism identified by plasma nested PCR. In this study, CSF DNA was used as the "gold standard," and nested PCR was performed on both types of samples. Fifty-two patients with symptoms of nervous system infection were submitted to CSF and blood collection. For the eight HHV, one positive DNA result-in plasma and/or CSF nested PCR-was considered an active HHV infection, whereas the occurrence of two or more HHVs in the same sample was considered a coinfection. HHV infections were positively detected in 27/52 (51.9%) of the CSF and in 32/52 (61.5%) of the plasma, difference not significant, thus nested PCR can be performed on plasma instead of CSF. In conclusion, this findings suggest that plasma as a useful material for the diagnosis of cases where there is any difficulty to perform a CSF puncture.

Double encephalitis with herpes simplex virus type II and cytomegalovirus in an elder Chinese: a case report

Herpes simplex encephalitis is a rare disease. In adults, most of the reported cytomegalovirus (CMV) infections are seen in immunocompromised patients. We present a case of 67-year-old Chinese male with the coinfection of CMV and herpes simplex virus type II (HSV-II). He had no history of being treated with immunosuppressants, showed symptoms of psychosis and was scored 109 on the Positive and Negative Syndrome Scale. This patient presented with a rare case of coinfection of CMV and herpes simplex virus type II with psychotic symptoms.

Cytomegalovirus encephalitis in an immunocompetent child: a sceptic diagnosis

The authors present the case of an apparently immunocompetent 9-year-old child with probable cytomegalovirus encephalitis. The clinical picture was characterised by fever, frontal headache and behavioural changes, associated with visual and auditory hallucinations. Cerebrospinal fluid (CSF) biochemistry and brain CT were normal. Electroencephalography showed left temporal paroxysmal activity. Diagnosis was based on cytomegalovirus (CMV) DNA detection on the CSF by PCR. Acyclovir and ceftriaxone were given until herpes simplex virus (HSV) and bacterial encephalitis were ruled out. Rapid resolution of fever and complete clinical recovery was observed. Remarkably, anti-CMV serum antibodies were not detected on admission or until 6 months later. This discrepancy led us to question the presence of an impaired specific host humoral response, immune evasion by the virus or a false-positive result for CMV DNA in CSF.

CMV-associated encephalitis and antineuronal autoantibodies--a case report

Background

Human cytomegalovirus (CMV) is an ubiquitous pathogen capable of modulating the host immune system. Immune dysfunction is common during CMV infection and includes autoimmune phenomena. Here we focus on a case of primary CMV infection associated with encephalopathy in a patient with a rudimentary spleen. We discuss diagnostic challenges and immunological aspects as well as the hypothesis that CMV may break tolerance and induce potentially encephalitogenic autoantibodies.

Case presentation

A 33-year-old woman was admitted with features of encephalitis, rapidly progressing into a catatonic state. The patient tested negative for presence of herpes simplex virus DNA in cerebrospinal fluid (CSF), and had elevated liver enzymes and hepatomegaly at computed tomography scan (CT) examination. CT scan and magnetic resonance imaging (MRI) showed only a rudimentary spleen. Initially, serum was negative for anti-CMV IgM, but borderline for anti-CMV IgG by enzyme-linked immunosorbent assay. However, a more sensitive assay resulted in a positive specific IgM Western blot profile and low IgG avidity, suggesting primary CMV infection. Further, CMV DNA was retrospectively detected in a CSF sample collected at admission. We also detected antineuronal autoantibodies, which stained GAD-positive neurons in the hippocampus. The patient was treated by a combination of prednisone, intravenous immunoglobulins (IVIg) and antivirals, which resulted in a dramatic amelioration of the patient’s neurological status. One year after admission the patient exhibited a nearly complete recovery with mild deficits in attention and memory.

Conclusions

A possible reason for the critical course of CMV infection could be the lack of a functional spleen in this patient, a condition previously associated with severe CMV infection. Prompt treatment with antiviral drugs, steroids and IVIg was most likely important for the positive outcome in this case and should be considered for similar cases of severe primary CMV infection associated with immunopathological phenomena.

Erythema Multiforme Major Associated with CMV Infection in an Immunocompetent Patient

Background:

Cytomegalovirus (CMV) infection usually remains asymptomatic in immunocompetent adults, and few cases of complicated disease in nonimmunosuppressed patients have been reported. Erythema multiforme (EM), an acute and self-limiting skin eruption characterized by a typical targetoid lesion that may also affect the mucosa, is a hypersensitivity reaction that usually occurs after herpes simplex virus infection or use of certain drugs and resolves without complications in healthy individuals. To our knowledge, CMV infection has been associated with EM in only six patients.

Objective:

We present a case of an EM caused by CMV infection in a 35-year-old nonimmunosuppressed patient who was successfully treated with ganciclovir.

Conclusion:

Our report, like other similar reports found in the literature, suggests that CMV can trigger EM in apparently healthy individuals. Intravenous ganciclovir appears to be a good treatment option in these cases.

Limitations of polymerase chain reaction testing for diagnosing acute Epstein-Barr virus infections

Clinicians use molecular tests to detect Herpesviridae from blood without fully appreciating limitations of testing. Studies are needed to enhance our understanding of the impact of Herpesviridae latency on molecular testing. We retrospectively performed quantitative Epstein-Barr virus (EBV) on sera from patients between the ages of 1 and 30 who demonstrated serologic evidence of acute EBV (n = 50) or remote EBV (n = 50) infection. Epstein-Barr virus DNA was detected in 70% of acutely infected and 4% of remotely infected patients. Sera from acutely infected patients had higher EBV copy number than convalescent sera. Our results suggest that serology should be performed as the initial diagnostic test for acute EBV. The role for polymerase chain reaction in immunocompromised patients with impaired antibody responses or as a 2nd-line diagnostic test when serologic results are equivocal deserves further study.

Fatal reactivation of postnatal cytomegalovirus infection with rapid emergence of ganciclovir resistance in an infant after allogeneic stem cell transplantation

Human cytomegalovirus (HCMV) can cause serious problems after hematopoietic stem cell transplantation. The death of a pediatric transplant recipient after reactivation of a postnatal HCMV infection with bilateral retinitis and pneumonitis is described. Sequencing of the HCMV UL97 region revealed a compartment-specific mutation (H520Q) in urine conferring ganciclovir resistance.

[Value of the polymerase chain reaction in the diagnosis of herpes infections of the nervous system]

OBJECTIVE

To assess the performance of a polymerase chain reaction (PCR) method in cerebrospinal fluid (CSF) for the diagnosis of nervous system infections caused by herpesvirus, and to estimate the incidence of encephalitis due to herpes simplex virus type 1 in the adult population of the island of Gran Canaria.

METHODS

We studied 330 CSF specimens from 312 patients (281 HIV-negative and 31 HIV-positive) remitted to investigate clinically suspected encephalitis or meningitis, or to study neuropathy or demyelinating disease. A multiplex PCR technique was used to detect herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), human cytomegalovirus (CMV), Epstein-Barr virus and human herpesvirus type 6. The patients' clinical records were reviewed to establish the definite diagnosis.

RESULTS

Nine samples from eight patients (2.6%) showed positive results (9.7% of patients with pathological CSF and none with normal CSF). The eight patients had clinical and analytic findings of herpesvirus nervous system infection: HSV-1 DNA in four patients with encephalitis, HSV-2 DNA in one patient with meningitis, VZV DNA in two patients with meningitis and CMV DNA in one HIV-positive patient with encephalitis. Herpesvirus was the cause of 50% of encephalitis cases and 10% of meningitis cases. The incidence of HSV-1 encephalitis was five cases per million inhabitants per year.

CONCLUSIONS

Diagnosis of herpesvirus nervous system infections by PCR in CSF is not appropriate when CSF parameters are normal. We found a higher incidence of herpesvirus encephalitis than has been reported in other studies.

Subclinical activation of latent cytomegalovirus (CMV) infection and anti-CMV immune response in patients with atopic dermatitis

BACKGROUND

Microbiological infections are considered to be of pathophysiological importance in atopic dermatitis (AD). As yet, no information is available regarding cytomegalovirus (CMV) infection in this disease. This, however, is of interest because of the high prevalence of latent infections in the general population, the frequent reactivation in inflammatory diseases, and the immunomodulating capacity of CMV.

OBJECTIVES

To investigate the prevalence of latent CMV infection, the frequency of active CMV infection, and the immune response to CMV in patients with moderate to severe AD. Methods To detect active infection we analysed CMV antigen expression by peripheral blood mononuclear cells (PBMC) from 27 patients with moderate to severe AD in comparison with 53 healthy volunteers. We used three monoclonal antibodies recognizing different CMV-encoded antigens and immunocytological staining (alkaline phosphatase-antialkaline phosphatase technique).

RESULTS

Patients with AD had a higher mean frequency of CMV-positive PBMC: 2.25 per 10 000 vs. 0.74 per 10 000 in controls (P = 0.001) as well as a higher incidence of CMV antigenaemia: 29.6% vs. 7.5% (P < 0.01). Seropositivity for anti-CMV IgG antibodies indicated subclinical activation of latent infection. Remarkably, a clearance of CMV antigenaemia was observed during anti-eczematous treatment. Significantly higher plasma levels of tumour necrosis factor-alpha, which is involved in CMV reactivation, and interleukin-12, which is crucial for an antiviral cellular immune response, were observed in AD patients in comparison with healthy volunteers. Furthermore, a significantly enhanced frequency of circulating activated HLA-DR+ T cells especially in CMV-seropositive AD patients (19.3% vs. 13.5% in seronegative AD patients vs. 10.2% in controls) suggested that the active CMV infection triggers a cellular immune response. This was also supported by a high frequency of CMV-specific interferon-gamma-producing T cells in CMV-seropositive patients with AD.

CONCLUSIONS

Our data suggest that active, subclinical CMV infection is more frequent in patients with moderate to severe AD and may have immunopathophysiological relevance.

CNS manifestations of cytomegalovirus infections: diagnosis and treatment

Cytomegalovirus (CMV) infection of the CNS occurs most commonly in patients with severe immunosuppression such as those with advanced HIV infection (i.e. AIDS) or those who have undergone bone marrow or solid organ transplantation. Immunocompetent patients are affected very rarely. The infection of the CNS may affect the brain (diffuse encephalitis, ventriculoencephalitis, cerebral mass lesions) or the spinal cord (transverse myelitis, polyradiculomyelitis). Diagnosis is very difficult and should be based on clinical presentation, results of imaging and virological markers. The most specific diagnostic tool is the detection of CMV DNA by polymerase chain reaction in the CSF. Treatment should be initiated promptly if CMV infection is suspected. Antiviral therapy consists of intravenous ganciclovir, intravenous foscarnet or a combination of both. Cidofovir is the treatment of second choice. Patients who experience clinical improvement or stabilisation during induction therapy should be given maintenance therapy. After immune reconstitution (in HIV-positive patients) or discontinuation of immunosuppressive therapy (in transplant recipients), maintenance therapy may be stopped. Despite therapy, the prognosis for long-term survival is very poor, especially in patients with AIDS.

Detection of herpesvirus genomes by polymerase chain reaction in cerebrospinal fluid and clinical findings

BACKGROUND

The viruses of the Herpesviridae family, in particular herpes simplex virus types 1 (HSV-1) and 2 (HSV-2), cytomegalovirus (CMV), Epstein-Barr virus (EBV), varicella zoster virus (VZV), and human herpesvirus 6 (HHV-6), are responsible for numerous infections of the central nervous system (CNS). These infections manifest as diverse clinical signs, many of which are not specific. The diagnosis of these infections is necessary to make it possible to adapt treatment appropriately, as treatment is specific for the particular virus concerned.

OBJECTIVES

To apply a polymerase chain reaction (PCR) technique for the diagnosis in a single reaction of the six herpesviruses most frequently detected in the cerebrospinal fluid (CSF) and to analyse clinical events in patients presenting positive results in PCR for herpesviruses.

STUDY DESIGN

We studied 141 patients, from whom 180 CSF samples were collected. The clinical files of the patients were consulted retrospectively, and a list of clinical signs was recorded. After testing by targeted PCR, at the clinician's demand, we tested these samples by herpes consensus PCR, which detects six herpesviruses (HSV-1, HSV-2, CMV, EBV, VZV, HHV-6), in a single PCR.

RESULTS

Targeted PCR tests identified 25 CSF samples (13.9%), corresponding to 18 patients (12%), as positive. The herpes consensus PCR test detected 49 samples (27.2%) as positive, resulting in the identification of 54 individual viruses (four samples displayed co-infection) from 39 patients (27%). 130 CSF samples, from 101 patients, tested negative by both techniques. 23 HIV-positive patients (30.6%), three HIV-negative immunocompromised patients (27%), and 14 immunocompetent patients (25%) were CSF PCR-positive. In HIV-positive patients, CMV was the virus most frequently identified (13%), followed by EBV (10.6%), VZV (5.3%) and finally HSV-1 and HSV-2 (both 1.3%). We did not detect HHV-6 in any of these samples. We detected only HSV-2, EBV and VZV in the 11 HIV-negative immunocompromised patients. CSF samples of immunocompetent patients contained mostly VZV (9%) and HSV-1 (7.3%).

CONCLUSIONS

The herpes consensus PCR for a given virus was more sensitive than the standard, targeted PCR used in our laboratory. The clinical signs presented by patients infected with HSV-1, HSV-2 and CMV were similar to those reported in previous studies. For VZV, we report the possibility of mild, transient cerebral viral reactivation. Our data on the detection of EBV by PCR suggest that the PCR test is of predictive value for cerebral lymphoma in immunocompromised patients. The possible role of HHV-6 in a subacute neurological disorder merits further investigation.

High variability between results of different in-house tests for cytomegalovirus (CMV) monitoring and a standardized quantitative plasma CMV PCR assay

A total of 2,718 blood samples were analyzed in five virological laboratories for the presence of cytomegalovirus (CMV) by in-house tests and one standardized plasma PCR assay. Results from in-house tests showed remarkable variability. Detection of CMV pp65 antigen or DNA from cells was more sensitive than that by plasma CMV PCR assay.

[Cytomegalovirus encephalitis in an immunocompetent patient: clinical, neuropathological and ultrastructural analysis]

Cytomegalovirus encephalitis is rarely diagnosed in immunocompetent patients. It is more frequent among immunocompromised hosts. We present a case of a young male patient without evidence of immunosupression who had a cytomegalovirus encephalitis. The diagnosis was made by detection of viral DNA in cerebral tissue obtained by biopsy. Neuropathologic and ultrastructural changes are described and compared with the descriptions in immunocompromised patients.

Human cytomegalovirus infections in nonimmunosuppressed critically ill patients

OBJECTIVE

To assess the occurrence of active human cytomegalovirus (HCMV) infection and HCMV disease and to evaluate potential risk factors in immunocompetent intensive care patients after major surgery or trauma.

DESIGN

A prospective clinical study.

SETTING

An anesthesiological intensive care unit (ICU) in a university hospital.

PATIENTS

Fifty-six anti-HCMV immunoglobulin G (IgG) seropositive patients without manifest immunodeficiency whose simplified acute physiology score (SAPS II) value rose to >or=41 points during their ICU stay.

INTERVENTIONS

Once a week, the patients were examined for active HCMV infection by polymerase chain reaction and by viral cultures from blood and lower respiratory tract secretions. Three times a week, detailed clinical examination for signs of HCMV disease was carried out.

MEASUREMENTS AND MAIN RESULTS

Twenty of the 56 ICU patients (35.6%) who met the study criteria of a SAPS II score >40 points and anti-HCMV IgG seropositivity developed an active HCMV infection as diagnosed by the detection of HCMV DNA in leukocytes, plasma, or respiratory tract secretions. In seven patients, the virus was isolated in the respiratory tract secretions. Severe HCMV disease appeared in two patients with pneumonia or encephalitis respectively. In patients with active HCMV infection, the mortality tended to be higher (55%) than in those without (36%); the duration of intensive care treatment of the survivors was significantly longer in the patients with active HCMV infection (median 30 vs. 23 days; p = .0375). Univariate testing for factors associated with active HCMV infection showed the importance of sepsis at admission (p = .011) and prolonged pretreatment on the ward or in an external ICU (p = .002); the relevance of underlying malignant disease was borderline (p = .059). Multiple regression analysis identified only sepsis to be independently associated with active HCMV infection (p = .02; odds ratio, 4.62).

CONCLUSIONS

Even in a group of ICU patients without manifest immunodeficit who were anti-HCMV IgG seropositive and had reached a SAPS II score of >or=41 points, active HCMV infection occurred frequently (35.6%). Septic patients were affected twice as often as the total study population. In 2 of the 20 cases, active HCMV infection progressed to severe HCMV disease. Proper diagnosis demands special clinical attention combined with extended virological examinations. Further studies in a larger patient group should evaluate the influence of HCMV on ICU mortality.

A novel link between stress and human cytomegalovirus (HCMV) infection: sympathetic hyperactivity stimulates HCMV activation

Recently, inflammatory mediators such as TNFalpha were identified as triggering active human cytomegalovirus (HCMV) infection. Here, we demonstrate that a highly stressful event in the absence of systemic inflammation, as observed in patients with acute myocardial infarction, leads to the development of an active HCMV infection in latently infected patients. Elucidating the molecular mechanism of virus activation, we could show that catecholamines directly stimulate the HCMV immediate-early (IE) enhancer/promoter in monocytic cells via beta-2 adrenergic receptors. Subsequent activation of the cAMP/PK-A-signaling pathway results in enhanced synthesis and binding of the transcription factor CREB-1/ATF-1 to the cAMP-responsive elements within the IE enhancer. Epinephrine also enhanced HCMV gene expression in infected THP-1 cells by about 50% in three of four experiments. These data suggest that HCMV, like HSV-1 and VZV, can be (re)activated under stress conditions.