Torquetenovirus in bronchoalveolar lavage fluid for early detection of pulmonary infections in immunocompromised hosts

Immunocompromised hosts (ICH), particularly those undergoing organ transplants or receiving high-dose immunosuppressive therapies, are highly susceptible to various opportunistic infections. The early prediction and diagnosis of these infections pose significant clinical challenges, largely due to their subtle and atypical presentations [1–3]. Existing biomarkers and diagnostic tools often fall short of providing timely and accurate results, complicating the management of these vulnerable patients. However, recent studies have brought attention to Torquetenovirus (TTV) levels in peripheral blood as a promising biomarker for predicting infection risk in ICH [4,5]. Expanding this investigation, Zhu et al. focused on the utility of TTV in bronchoalveolar lavage fluid (BALF) in patients with suspected lung infections, aiming to provide a more localized and precise diagnostic tool [6]. Their findings mark a significant advancement in enhancing the predictive accuracy of opportunistic lung infections and improving the prognosis of ICH, where clinical margins for error are exceedingly narrow.

TTV is a small, circular, single-stranded DNA virus that is prevalent in nearly 90% of the human population [7,8]. While not directly pathogenic, TTV is closely linked to immune system dynamics, with its levels serving as a surrogate marker for immune suppression [7,8]. Recent studies have demonstrated that TTV viremia in solid organ transplant recipients and patients undergoing chimeric antigen receptor T-cell (CAR-T) therapy is a reliable predictor of infection risk [9–12]. The above studies indicate that elevated TTV levels are often detected before the onset of infection, and could serve as an early warning signal suggesting the presence of an impending infection in ICH. Conversely, a decrease in TTV levels may reflect improved immune function [12]. This strong correlation between TTV levels and immune status opens broader possibilities for utilizing TTV as a diagnostic and prognostic tool, enhancing clinical decision-making and guiding therapeutic strategies in high-risk ICH.

Recent advancements in regional lung immunity research underscore the importance of analyzing localized immune responses directly at the site of infection. BALF, a less invasive alternative to lung biopsy, provides direct access to immune cells, pathogens, and biomarkers specific to the lower respiratory tract [13,14]. Compared to peripheral blood biomarkers, BALF offers a precise snapshot of localized immune activity, enabling superior accuracy in respiratory diagnostics. Metagenomic next-generation sequencing (mNGS) has further revolutionized this field by enabling comprehensive detection of pathogens, including rare organisms, with unmatched sensitivity compared to conventional microbiological methods [13–15]. Its ability to detail the etiological spectrum and identify co-infections has been transformative in managing complex cases, particularly in ICH, who often present with atypical or subclinical infections and are especially vulnerable to opportunistic pathogens [13–15]. These advancements firmly establish BALF as an essential resource for studying localized immune suppression and diagnosing respiratory infections.

Building on these advancements, Zhu et al. explored the potential of TTV levels in BALF as a biomarker for assessing lung-specific immune suppression [6]. BALF directly represents immune dynamics within the lower respiratory tract – an environment that blood-based TTV levels may not adequately reflect. Their study demonstrated that TTV levels in BALF correlate with immune dysfunction, highlighting its potential as a novel biomarker for detecting pulmonary infections in ICH [6]. By focusing on the host’s immune response rather than solely on pathogen identification, this approach complements mNGS-based diagnostics. Integrating TTV measurements with advanced sequencing technologies offers a framework for achieving earlier and more targeted interventions for pulmonary infections.

The study presented compelling evidence supporting the diagnostic utility of TTV load in BALF among ICH but not in the non-ICH population [6]. The authors demonstrated its potential as a predictive biomarker by correlating TTV levels in BALF with opportunistic pathogens, specifically lung aspergillosis and Mycobacterium infections. The ICH cohort with detectable BALF-TTV showed higher clinical severity and worse prognosis compared to those without detectable TTV, findings that were not observed in the non-ICH cohort [6]. Moreover, these results highlight the role of BALF-TTV as an immune predictive biomarker, independent of traditional markers like CD4⁺ T cell counts, offering clinicians a valuable tool for improving prognosis and guiding therapeutic strategies in high-risk patients [6].

The clinical implications of TTV detection in BALF are significant. As a site-specific biomarker, BALF-TTV could significantly enhance early infection detection in ICH. Assessment of BALF-TTV levels during diagnostic evaluations may serve as an early warning signal, prompting timely interventions and adjustments to immunosuppressive therapies [6]. This targeted approach could also reduce unnecessary antimicrobial use, helping to mitigate the growing issue of antimicrobial resistance [16,17]. Overall, this novel diagnostic tool could allow clinicians to personalize patient management, adjust treatment plans according to TTV levels, and reduce the risks of both under-treatment and over-treatment, thereby optimizing outcomes for ICH.

The study by Zhu et al. presents several limitations [6]. As a single-center study with a relatively small sample size, its findings may lack generalizability, particularly considering variations in lung infection profiles among different ICH, such as those undergoing hematopoietic stem cell transplants or with advanced HIV. The study also did not specify the exact number of cases within specific immunocompromised states, limiting the ability to draw robust conclusions for these populations [6]. Larger, multicenter trials are essential to validate the predictive value of BALF-TTV levels across diverse ICH populations and establish standardized thresholds for reliable infection risk prediction. Additionally, understanding how TTV dynamics shift with different immunosuppressive therapies will be crucial for optimizing its clinical utility [4]. Given the dynamic nature of TTV replication, its rise in BALF may not always correlate directly with infection but could instead reflect fluctuating immune suppression [8,9]. This makes it difficult to distinguish between transient immune dysregulation and active infection – an essential distinction for clinical decision-making.

While mNGS was used, the study did not account for absolute pathogen load, reflecting only the presence or relative quantification of microorganisms, which may affect interpretation [6]. Procedural variability in BALF, including differences in the volume of saline instilled (dilution effect), the size of the bronchoscope, and the choice of fraction analyzed, further complicates reproducibility, as these factors significantly influence cell recovery and marker quantification [18,19]. Evidence suggests that the pellet fraction predominantly contains intact microbial DNA, while the supernatant reflects extracellular DNA from cell lysis, making fraction selection potentially critical for interpreting biomarker levels [19]. Quantification challenges, including variability in DNA extraction efficiency, assay sensitivity, and the presence of inhibitors, further complicate result interpretation [19]. Addressing these limitations through robust procedural and analytical standardization will be essential to fully establish the clinical utility of BALF-TTV as a biomarker. Furthermore, exploring its use alongside other markers of immune function, such as immune cell profiling, or infection, such as procalcitonin, could provide a more comprehensive assessment of infection risk in ICH [20].

The study by Zhu et al. highlights the potential of TTV levels in BALF as a valuable biomarker for detecting and predicting pulmonary infections in ICH [6]. While the findings demonstrate a promising advancement in diagnostic precision, particularly in localizing infection risk, several challenges must be addressed before BALF-TTV can be widely adopted in clinical practice. Standardizing assay techniques, establishing reliable TTV thresholds, and integrating TTV with other infection markers will be essential for fully realizing its clinical utility. With further research, BALF-TTV could significantly enhance early diagnosis and support personalized management of infections in high-risk ICH populations.

Funding Statement

The authors have no financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Disclosure statement

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.

Author contribution

¹AKP: conceptualization, drafted, and revised the manuscript

Ethical conduct of research

The authors state that they have obtained appropriate institutional review board approval () and/or have followed the principles outlined in the Declaration of Helsinki for all human or animal experimental investigations. In addition, for investigations involving human subjects, informed consent has been obtained from the participants involved.