The genes of human cytomegalovirus (CMV) chronically exists in the host myeloid lineage including CD34+ hematopoietic progenitor cells as extrachromosomal plasmid after asymptomatic or self-limited primary lytic infection that occurs during childhood and adolescence [1,2,3]. Unlike other viruses of Herpesviridae family, latently established CMV can express a handful of transcripts such as latency-associated unidentified nuclear antigen (LUNA), UL81-82 antisense transcript, and latency-associated homolog of interleukin-10 (LAcmvIL-10) together with periodic active replication events during the life-long [2,4,5,6,7]. The outstanding feature of sleepless latency leads to the extraordinary expansion of CMV-specific resting effector memory CD8+ T-cell subpopulation and then chronic inflammatory condition as well as dysregulation of host immune mechanisms [7,8,9,10,11,12]. This long-term pathophysiologic mechanisms of CMV acquisition may be the direct cause of chronic inflammatory cardiovascular diseases (CVDs) or autoimmune diseases including systemic lupus erythematosus and systemic sclerosis [13,14].

Although the pathogenesis of type 2 diabetes mellitus (T2DM, non-insulin-dependent diabetes mellitus [DM]) is quite complex and involves various cross-linked molecules including adipocytokines, receptors, and genetic pathways as well as immune system, T2DM seemed to be a fundamentally chronic low-grade inflammatory metabolic disease with clinical detrimental effects through various micro- and macrovascular complications [15,16,17,18,19,20,21]. Lohr and Oldstone [22] detected the CMV immediate-early and late gene products using reverse transcription polymerase chain reaction (PCR) and in situ hybridization in pancreatic tissues of T2DM patients. Recent studies suggested that viperin (endoplasmic reticulum-associated, interferon-inducible virus inhibitory protein), which is directly induced by CMV, may play a role in lipid and glucose metabolism through interaction with the CMV mitochondrial inhibitor of apoptosis (vMIA) protein [23,24].

Considering these molecular biological experiments, we hypothesized that CMV diseases may contribute to the development of T2DM. Although only a few clinical studies evaluating the association between CMV and T2DM were performed nearly 2 decades ago, these studies evaluated CMV seroepidemiology measured by anti-CMV immunoglobulin G (IgG) antibody test or titer, to determine previous CMV exposure and latent status by humoral immunity, with age being a variable influence [25,26,27,28,29]. The anti-CMV IgG is not useful for evaluating CMV-specific cell-mediated immunity (CMV-CMI), which plays a major role in immunosenescence and immune exhaustion caused by CMV [30]. The active replication of whole CMV genes can be delicately categorized into CMV infection and diseases, defined as detection of DNA (DNAemia) or competitive virions (viremia) in the peripheral blood and cytopathic inflammatory end-organ tissue-invasive disease, respectively [30]. The CMV infection or diseases, in relation to more powerful immune boosting of CMV-CMI than anti-CMV IgG serostatus, will better reflect the chronic inflammatory dysregulation phenomenon by CMV-related indirect effect [30].

The impact of CMV infection or diseases on new-onset T2DM had been primarily evaluated as posttransplant DM in adult solid organ transplant recipients [31,32]. However, little is known about the causal connection of CMV diseases to T2DM development in the entire population including both transplant and non-transplant patients. Therefore, we performed the general population-based matched case-control cohort study in both immunocompetent and immunocompromised patients to explore whether the CMV diseases contributes to the development of T2DM.

This study showed that the occurrence of CMV diseases increases the incidence of new-onset T2DM, with a relative risk of ≥200%, which was higher than the CMV-attributable risk of CVD evaluated by meta-analyses (22% and 67%) [14,36]. This result was identical with additional analysis from the matched cohort of 3:1 ratio (1,488 of control group and 496 of case group) with age, sex, cohort entry year, lower income status, hypertension, and dyslipidemia as 2.01 of OR (Supplementary Table 3). Interestingly, patients aged <60 years had a higher risk of T2DM after CMV diseases. However, the existence of chronic inflammatory diseases such as hypertension, dyslipidemia, COPD, IHD, cerebral infarction, HF, and ESRD did not influence the incidence of post-CMV T2DM. These findings were consistently derived from different regression models adjusted by clinical factors of usual risks for T2DM [37] and confounding variables showing higher frequencies in patients with CMV diseases (Table 1). The Kaplan-Meier curve indicated that most post-CMV T2DM cases developed shortly after CMV diseases, in particular, within 1 year.

Majority of epidemiological studies evaluating the causal relationship between CMV and metabolic diseases have focused on CVD, especially coronary heart disease (CHD), type I DM, and autoimmune diabetes [13,14,36,38]. The meta-analysis conducted by Ji et al. [14] revealed that CMV infection was associated with high risk of CHD. However, only seven (12.7%) among the total 55 studies including the meta-analysis reported the detection of CMV DNAemia by PCR [14]. Interestingly, the OR reported in seven studies was higher than that in studies using CMV seroprevalence (8.12 vs. 1.56, respectively) [14]. A few studies measuring anti-CMV IgG showed that T2DM patients had higher frequency of CMV seropositivity and titer (OR, 2 to 12) than healthy subjects [26,27]. In addition, CMV seropositive status was associated with higher glucose levels [26]. However, these studies may have considerable limitations and are underpowered due to the small sample size and rare CMV events and because the specific subjects do not reflect the universal population.

The pathophysiological mechanisms of post-CMV T2DM remain uncertain. Although Lohr and Oldstone [22] revealed that CMV genes existed in the pancreatic islets of Langerhans cells without cytopathic inflammation and Numazaki et al. [39] investigated that CMV was found to replicate in in vitro human fetal pancreatic islet cells in the early 1990s, further studies had not been reported. Yoneda et al. [38] recently reported that the immunologic indirect effect of CMV infection was associated with direct pancreatic β-cell injury, showing T lymphocyte infiltration. The reliability of the experiments conducted in human samples was confirmed although the absence of CMV in normal controls [22,38]. The discrete impact of CMV infection on the failure of islet allografts to support the active CMV replication may be an attributing factor to the development of T2DM through direct and/or indirect pancreatic damages from CMV [40]. On the contrary, we may presume that the substances used in the envelop composition such as viperin, which is essential to extracellular budding and shedding of complete CMV virion, might alter the lipid and glucose metabolism pathways [24].

Our new findings of post-CMV T2DM discrepancy by age group and early development after CMV diseases could provide novel insights for the causative effects of CMV replication in chronic inflammatory diseases including T2DM. Another unique analysis for the refractory nature of CMV diseases, which may suggest prolonged active CMV replication, had earlier development of and more strong relationship with T2DM. The complete lytic reactivation of CMV resulting in end-organ cytopathic disease or DNAemia, especially in the younger population who have more prominent immunologic response than the elderly with profound immunosenescence or subjects who have uncontrolled CMV replication, could lead to deeper deterioration of immune homeostasis. The comprehensive measurement of CMV-specific immunologic profiles using mass cytometry (cytometry by time of flight [CyTOF]) and genetic evaluation for alteration of glucose metabolism by CMV are warranted. If information regarding causative mechanisms in CMV replication and T2DM would be accumulated along with further epidemiologic studies in various ethnic populations, the development of CMV vaccine could be targeted to prevent several chronic inflammatory diseases including T2DM in this era of aging population.

This study has some limitations, same as those of large-scale studies using coding data processing. Therefore, the data collection method could cause the selected disease to be underestimated. However, our analyses can overcome this drawback through a matched case-control design. In addition, the frequencies of CMV diseases incidence according to ICD-10 code showed that our database from the entire population had the majority of B25.8 and B25.9 code (1,025 of 1,188, 86.3%), which may indicate the confirmatory CMV diagnosis driven by pathology in gastrointestinal tract such as esophagus, stomach, duodenum, and colon. The absence of rare cytomegaloviral pancreatitis as well as the minority of cytomegaloviral pneumonitis (75 of 1,188, 6.3%), which could be presumptive or over-diagnosed with chest computed tomography scan or quantitative nucleic acid amplification test in bronchoalveolar lavage without standardization and universal cut-off level for proper diagnosis, will guarantee the validity of our database for the confirmatory CMV tissue-invasive end-organ disease (Supplementary Table 4) [30]. Secondly, our subjects were restricted by South Korean nationality. A previous study showed that the effect of CMV infection on CHD might be different among ethnic groups, with increased risk among Asian populations [14]. In addition, the refractory CMV diseases were identified simply by repeated administration of anti-CMV drugs. The huge database could not allow a more precise definition of clinical information-based refractory cases in each CMV patient. Finally, our cohort includes some proportion of solid organ transplantation recipients of about 20% among total individuals with CMV diseases (Supplementary Table 5). However, the patients with new-onset T2DM had higher frequencies of several chronic inflammatory diseases, including hypertension, dyslipidemia, IHD, cerebral infarction, and ESRD, in the well-known relation to T2DM (Supplementary Table 1) [37]. These basic characteristics will ensure the good quality in this cohort. In addition, the low IR of CMV end-organ disease in general population may make our population-based study worthy to evaluate the relationship of CMV and T2DM.

In spite of these limitations, our study has several strengths and outstanding features: (1) first assessment of the T2DM incidence after CMV diseases including tissue-invasive end-organ damage, but not the CMV serostatus in T2DM patients, (2) long wash-out period for ≥4 years that can discriminate the casual relationship between CMV diseases and T2DM as well as can analyze the post-CMV T2DM, and (3) largest sample size among studies reporting about the relation between T2DM and CMV

In conclusion, our data indicate that CMV diseases increase the development of T2DM. These results suggest that active CMV replication may play an important role in T2DM pathogenesis.