Immunosenescence: Viral Reactivation in the Aging Immune System

June 22, 2026

Allergy/Immunology Class Act Geriatrics 7 min read

File:Altenpflege 07.jpgBy Madeline Kowalski

Peer Reviewed

A 91-year-old woman with no chronic illness,1 who until recently lived independently, developed rapidly progressive dementia and ataxia over several months. A paraneoplastic syndrome was suspected, yet an extensive workup revealed no malignancy. MRI revealed multifocal white-matter lesions, and cerebrospinal fluid PCR confirmed human polyoma virus 2 (also known as JC virus), establishing the diagnosis of progressive multifocal leukoencephalopathy (PML). Classically, PML only occurs in the profoundly immunocompromised, including those with HIV/AIDS, hematologic malignancies, or on aggressive immunosuppressive regimens. Her only “immunosuppressive condition” was advanced age. Her immune system had likely harbored this virus for decades. What changes in her immune system permitted its reactivation? More broadly, are there warning signs that can help us identify those older adults at risk for viral reactivation?

Older adults are less able to mount an immune response to new viruses, making them particularly susceptible to new variants of influenza or Covid. Yet an equally consequential phenomenon is the reactivation of viruses that the immune system has previously controlled for many years. Classic examples include varicella-zoster virus, which causes shingles; herpes simplex virus type 1, which causes recurrent cold sores; and Epstein-Barr virus, which may lead to lymphoma. JC virus reactivation is far less common yet devastating. This unusual occurrence has been reported in older adults who are otherwise immunocompetent.1-3 A fascinating question is: what biological changes in the aging immune system allow these dormant viruses to reawaken? The phenomenon is deemed “immunosenescence,” the age-related impairment of the immune response. While this is a complex topic, three major, age-related changes in antiviral immunity help explain why dormant viruses can re-emerge in older adults: reduced naïve T cell output due to thymic involution (degeneration), chronic immune stimulation from viruses such as cytomegalovirus (CMV), and the gradual erosion of T cell functional quality (“T cell exhaustion”) that limits the ability to control persistent viruses.

One major mechanism of immunosenescence is thymic involution. The thymus, the sole site of naïve T cell production, begins to involute as early as one year of age.4 During this process, which is conserved across all vertebrates with a thymus, organized epithelial architecture is gradually replaced by adipose tissue. This striking pattern has led researchers to hypothesize that there is an evolutionary advantage to an early period of thymocyte production. By generating a diverse T cell repertoire early in life, when pathogen exposure is high, energy can be redirected later, given that thymocyte production is energetically costly and most thymocytes fail selection.5 By age 50, thymic involution is largely complete, which is accompanied by a marked reduction in the output of naïve CD4? and CD8? T cells and a progressive narrowing of the T cell repertoire.6 After midlife, the immune system relies predominantly on the maintenance and expansion of existing T cell clones rather than on the generation of new ones, thereby reducing its flexibility to respond to novel antigens or reactivated viruses.

However, immunosenescence does not truly accelerate until decades after thymic involution is complete, indicating that additional forces shape antiviral immunity in aging. One potential contributor is persistent viral infections such as cytomegalovirus. CMV establishes lifelong latency, and intermittent antigen release drives continual oligoclonal expansion of CMV-specific memory T cells. This phenomenon is known as “memory inflation,” in which virus-specific clones progressively accumulate long after the acute infection has resolved. Although the precise triggers remain incompletely understood, subclinical, sporadic CMV reactivation is thought to provide the low-level antigenic stimulation needed to maintain this expanded pool.7 CMV seropositivity exceeds 90% in Americans over age 80,8 and up to one quarter of the entire CD8 T cell repertoire is dedicated to CMV alone.9 This disproportionate clonal expansion constricts the overall T cell receptor (TCR) repertoire and reshapes T cell homeostasis, most notably by lowering the CD4:CD8 ratio, a finding repeatedly observed in CMV-seropositive adults.10

The immune restructuring associated with CMV infection has important clinical consequences. Multiple cohort studies have linked higher CMV antibody titers with increased mortality in older adults and in hospitalized older patients.11-13 These associations remain controversial, in part because CMV seropositivity is thought to lead to systemic inflammation and ischemic heart disease, raising the possibility that CMV contributes to immunosenescence both directly, through clonal expansion and repertoire restriction, and indirectly, through chronic inflammation.14

A third pathway through which latent viruses can escape immune control is declining T cell function. Exhausted or senescent T cell clones tend to accumulate with age and exhibit dysfunctional responses to viral antigens. With repeated rounds of antigen exposure over decades, many CD8? T cells acquire features of replicative senescence: shortened telomeres, diminished proliferative capacity, and reduced cytotoxicity.15 These cells often lose expression of costimulatory molecules, such as CD28 and CD27, which normally aid in T cell activation, expansion, and survival.16 They may also upregulate inhibitory receptors, including PD-1 and Tim-3.17 Thus, T cells remain present in normal or even elevated numbers in older patients, but they are less capable of recognizing infected cells, producing antiviral cytokines, or mounting an effective antiviral response. This offers a compelling explanation for why older adults who appear immunocompetent by routine labs may nonetheless be vulnerable to viral reactivation. Furthermore, this may explain why patients can function relatively well despite limited naïve T cell production: immune dysfunction only becomes evident when these T cells are severely dysfunctional and novel T cell production stops. It is an important challenge to recognize when this perfect storm of aging-associated dysfunction occurs.

Current clinical tools are limited, but some readily available biomarkers may be useful. OCTO-immune, a longitudinal Swedish cohort study, demonstrated that declining CD4? T cell counts, combined with an expanded CD8? population, were associated with increased mortality.18-20 These findings suggest that relative changes in T cell subsets, rather than absolute lymphocyte counts alone, may signal clinically significant immunologic aging. Accordingly, the CD4:CD8 ratio has emerged as a pragmatic biomarker: both an inverted ratio (<1) and a high ratio correlate with frailty and increased susceptibility to infections in older adults.21 For the clinician, this metric may offer an early, actionable window into declining antiviral immunity. Flow cytometry is not performed routinely at this time, but improvements in technology and cost may make this a feasible screening procedure in older patients.

These markers only scratch the surface of the complex changes the immune system undergoes with age. As longevity improves and comorbidities such as cancer and cardiovascular disease become more survivable, viral reactivations may represent a growing source of morbidity and mortality in older adults. To meet this challenge, we will need more refined tools to detect early immune dysfunction, before clinical signs of viral reactivation occur.

Emerging technologies offer promising avenues for enhanced understanding and recognition of immune dysfunction. Advances in “omics” profiling allow researchers to characterize T cell populations at unprecedented resolution, including quantifying T cell subtypes, mapping TCR repertoires, and identifying clonal expansions.22,23 These approaches could reveal subtle signatures of immune dysregulation before they are apparent on routine laboratory testing. In our PML case, for example, the patient showed a reduced CD4:CD8 ratio via flow cytometry despite otherwise normal hematologic indices. Had deeper immune profiling been performed earlier, signs of T cell repertoire restriction or emerging clonal dominance might have prompted earlier diagnostic consideration of JC virus reactivation, potentially shortening time to diagnosis and earlier application of the limited therapeutic strategies. Increased immunologic profiling of patients using next-generation “omics” tools will shed light on how the immune system changes with age, aiding the development of potential therapies to ameliorate this decline.

Madeline Kowalski is a Class of 2028 medical student at NYU Grossman School of Medicine

Peer Reviewed by Michael Tanner, MD  Executive Editor, Clinical Correlations

Image Courtesy of Andreas Bohnenstengel, CC BY-SA 3.0 DE <https://creativecommons.org/licenses/by-sa/3.0/de/deed.en>, via Wikimedia Commons

References

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