This study aimed at determining the association between active CMV infection and lung fibroproliferation in adult patients with ARDS. Among adult patients with ARDS, the incidence of active CMV infection was 16.1%. In addition, clinical characteristics, including laboratory findings (lower level of platelet and NK cells), blood transfusion, and septic shock, were related to active CMV infection. Most importantly, further correlation statistical analysis revealed that active CMV infection was associated with pulmonary fibrosis based on chest HRCT and NT-PCP-III. Additionally, active CMV infection was associated with several adverse prognoses.
Active CMV infection is not an uncommon phenomenon in critically ill patients. Our research team’s systematic review and meta-analysis have shown that the incidence of reactivation in immunocompetent patients is up to 31% in critical care settings . In a prospective, multicenter epidemiological study that enrolled 399 patients with ARDS, 271 (68%) cases were CMV seropositive, and reactivation occurred in 74 (27%) of them; so the incidence of active CMV infection was 18.6% . An additional study, which enrolled 123 ARDS patients receiving extracorporeal membrane oxygenation (ECMO), revealed that CMV DNAemia occurred at any level in 22.0% of the patients . Due to the different inclusion and exclusion criteria, diagnostic methods, and specimen detection, the above incidence was slightly different from the results of this study. In particular, the detection methods (time points and monitoring periods) of CMV in different studies may have an effect on the incidence of active CMV infection. The present study was retrospective and could not detect each time point clearly; so, the incidence of active CMV infection may be lower than the real situation. It is also essential to note that our study used blood DNAemia for evaluating active CMV infection, which might underestimate the active CMV infection, combined with a more comprehensive detection of lower respiratory tract specimens.
Previous studies have verified that low-level platelet and NK cells, blood transfusion, and septic shock are associated with active CMV infection [3, 20, 21], consistent with this study's results. The reason behind these associations is related to the direct pathological damage and indirect damage.
The inhibition of platelet and NK cells may be related to the damage caused by CMV infection to the hematopoiesis and immune system [21,22,23]. Specifically, for bone marrow transplantation patients, CMV infection causes delayed platelet recovery . CMV infection can inhibit NK cells through driving adaptive epigenetic diversification with altered signaling and effector function [21, 24]. Moreover, active CMV infection through blood transfusion is a risk in treating patients with a critical illness. A study has indicated that donations from new CMV-IgG-positive donors bear the highest risk for transmitting CMV infections because they contain elevated CMV-DNA levels, a risk factor for active CMV infection . Several studies have found that sepsis is associated with active CMV infection [10, 19, 26, 27]. An in vivo model has shown that inflammatory factors such as lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF-α), and interleukin-1beta (IL-1β) are associated with sepsis and could activate CMV immediate early genes and promote viral replication . From clinical studies, it has also been speculated that CMV reactivation is associated with multiple inflammatory factors (IL-6 and TNF-α) [29, 30]. There was an interaction between septic shock and active CMV infection. However, there were cases of prolonged mechanical ventilation causeing ventilator-acquired pneumonia clinically, which might also lead to sepsis-related active CMV infection, though no statistically significant difference was reached between these pathogens and active CMV infection. Therefore, the limited sample size and unmeasured confounders were the inadequacies of this study.
Furthermore, our results showed that septic shock and platelet were independent risk factors for active CMV infection and were independent of each other in the study subjects. It was further found that platelet levels had a moderate value for predicting active CMV infection. The mechanisms through which septic shock causes active CMV infection have been described above, and this result was consistent with most of the current studies. Nonetheless, no relevant study is consistent with our results, and the association between CMV and platelet has never been shown before. Platelets are small masses of cytoplasm released from the cytoplasm of mature megakaryocytes in the bone marrow and are extremely important for the body’s hemostatic function. However, the latest view considers that platelets can be an important mediator of immunity and inflammation . In allogeneic bone marrow transplantation recipients, platelet recovery was significantly slower in CMV-positive patients than in CMV-negative patients, the condition may be related to CMV-associated myelosuppression . Evidence for the association between CMV and myelosuppression has been provided from in vitro/vivo studies and clinical studies, those results indicated the direct infection of hematopoietic progenitors and that the supportive microenvironment is infected, thereby compromising its supportive function . Importantly, more patients need to be included for further confirmation of the role of platelets. Meanwhile, it needs to be emphasized that the relatively small sample size in this study opens the possibility for a type II error.
Current treatment protocols for ARDS continue to evolve and be refined; nevertheless, its incidence and case fatality remain high [5, 6]. Pulmonary fibrosis is a common complication of ARDS, and its severity significantly influences clinical outcomes and long-term quality of life in ARDS patients . Multiple factors have been associated with ARDS-associated fibroproliferation, in which viral infection plays an important role, but the role of CMV infection in this is not definite. At present, in vitro results have shown that CMV from latent infection in the lung could undergo reactivation under LPS intervention, and active CMV infection could lead to the development of fibrosis in the lungs of mice . Besides, a clinical study showed that active CMV infection could influence the oxygenation function (PaO2/FiO2) of critically ill patients with sepsis . In summary, we hypothesized that active CMV infection was associated with pulmonary fibrosis leading to low oxygenation levels in ARDS patients; so, we further designed and carried out this study. There is no relevant clinical research consistent with our results, and the association between active CMV infection and lung fibroproliferation in adult patients with ARDS has never been studied before. We found that active CMV infection was associated with ARDS-associated fibroproliferation based on chest HRCT and NT-PCP-III. The active CMV infection group had a lower oxygenation level than the non-active CMV infection group, although statistical differences have not been observed. This revealed that CMV could damage the respiratory barrier, leading to pulmonary fibrosis and subsequently to decreased oxygenation. There is not well-established method to assess CT image of ARDS associated the lung fibrosis, thus in the study, we adapted the scoring system of CT image which is commonly used in evaluation of pulmonary fibrosis which is characterized by honeycombing. In this study, ARDS patients do not show typical honeycombing on HRCT scans. Meanwhile, pulmonary fibrosis once present in HRCT was largely refractory to disappear, so excluded patients with pulmonary fibrosis at the first HRCT scan who had excluded pre-existing pulmonary fibrosis. Moreover, the level of NT-PCP-III (Day 28) was high in cases with active CMV infection than in cases with non-active CMV infection, although statistical significance is not reached. Only 38 patients (6 in active CMV infection group and 32 in non-active group) still stayed in ICU on day 28 and NT-PCP-III were evaluated in these patients. The relatively small sample may be the potential reason for negative correlation between active CMV infection and NT-PCP-III at day 28. Furthermore, antiviral therapy was used in the active CMV infection group which effectively in suppressing the virus and reduced pulmonary fibroproliferation.
In addition, our assessment of pulmonary fibrosis using chest HRCT and NT-PCP-III is clinically feasible; both of these are reliable diagnostic methods or markers for ARDS-associated lung fibroproliferation [12, 13, 34], with the advantages of non-invasiveness and high specificity. Our results also found that chest HRCT score and NT-PCP-III had a strong correlation. Although lung histopathology is the gold standard, performing open lung biopsies in ARDS patients is complicated. Besides, the testing time points for chest HRCT were not consistent, and the endpoints were 28 days after admission to the ICU, and testing samples for NT-PCP-III were sera, which are slightly less sensitive relative to lower respiratory specimens (endotracheal aspirate or bronchoalveolar lavage fluid). Thus, the incidence of pulmonary fibrosis may have been underestimated. Furthermore, the source of fibrosis is also a matter of concern. Studies have shown that CMV infection could lead to epithelial–mesenchymal transition (EMT) in various cells, and continuous EMT is closely related to organ fibrosis [35,36,37]. EMT refers to the biological process in which epithelial cells lose epithelial characteristics and obtain mesenchymal characteristics under some physiological or pathological conditions . EMT is transient and reversible, depending on its biological and functional environment, so preventive or preemptive anti-CMV treatment may benefit ARDS patients. In addition, the current studies have revealed that some patients occurred pulmonary fibrosis after COVID-19 pulmonary infection owing to SARS-CoV-2 infection promoted the occurrence of lung fibroproliferation [39, 40]. Due to the outbreak control requirements of the Chinese government, all patients with confirmed COVID-19 diseases were sent to the specialized infectious disease hospital or department for isolation and treatment. Therefore, no patients with COVID-19 pulmonary infection were included during this study period. Hemopexin (hx) level of pulmonary disease was associated with lung fibroproliferation [41, 42], but the study lacked detection of hx.
Most studies have shown that active CMV infection was associated with several poor clinical outcomes, which is consistent with our results, including prolonged duration of mechanical ventilation, hospitalization and ECMO, and increased complications and mortality [2,3,4, 7, 8, 10, 20, 23, 26,27,28,29,30]. The mechanisms include direct CMV pathogenicity or indirect CMV effects such as CMV-mediated immunosuppression and CMV-mediated lung injury [29, 43]. Moreover, the cause of death in both groups was multiple organ dysfunction syndrome in this study. Furthermore, the main cause of death in the active CMV infection group was multiorgan failure. As a matter of fact, it is very hard to tell the exact attribution to death when the patient is being in the end-stage of ARDS in clinical practice. In most cases, we believe that above conditions may contribute to the death of ARDS patients, the attribution of pulmonary fibroproliferation to death of ARDS patients need to be evaluated in a prospective well controlled study. In addition, the notable role of antiviral therapy in active CMV infection is well established. However, the preemptive and prophylactic application of anti-CMV therapy remains controversial for non-immunosuppressed patients [30, 44, 45], and the role of preemptive and prophylactic anti-CMV therapy in pulmonary fibrosis, which is not clear, should be further investigated in future studies.
This study has several limitations. First, it was a single-center retrospective study, so the time points at which CMV detections and chest HRCT scans were made were difficult to be determined. Therefore, the timing of when CMV was detected relative to admission and relative to HRCT scans might affect assessment of the association between active CMV infection and lung fibroproliferation. Second, only active infection, and not reactivation, could be assessed because of the lack of detection of CMV IgG. Third, the number of patients included was relatively insufficient to comprehensively evaluate the epidemiological characteristics and lung fibroproliferation of CMV for adult patients with ARDS. Fourth, due to the lack of CMV and NT-PCP-III detection in the lower respiratory tract, we could not further assess the association between CMV and NT-PCP-III among ARDS patients. Therefore, a prospective, multicenter study is needed in the future, and more study participants with ARDS should be included. Moreover, the effect of prophylactic and preemptive therapy using anti-CMV agents for pulmonary fibrosis should be evaluated in ARDS patients. Eventually, further assessment of the effect of CMV in the airways of ARDS-associated fibroproliferation markers is urgently needed.