Asthma is a heterogeneous chronic airway disease in which type 2 (T2) inflammation is prominent, yet additional endotypes linked to systemic inflammation, metabolic comorbidity, and variable severity complicate stratified care [1]. While Th2 cells are canonical T2 effector cells, CD8+ T cells can extend beyond cytotoxicity and adopt helper-like, cytokine-secreting programs relevant to airway inflammation [2,3]. Interleukin-6 (IL-6) is implicated in asthma severity and systemic inflammation [4,5], and surface IL-6Rα (CD126) provides a practical receptor-level readout of IL-6 classic signaling in human samples [6]. CD126 is expressed on a subset of CD8+ T cells, and together with SLAMF7 can define CD8 memory subsets spanning cytotoxic versus helper-like inflammatory states [7]. Clinically, circulating CD126high effector memory (EM) CD8+ T cells are increased in asthma and associated with lung function [8]. However, direct quantitative comparisons of CD126-defined CD8+ subsets across circulation and airway remain limited, and their associations with atopy, disease severity, and symptom control, remain incompletely characterized. To address this gap, we conducted a cross-sectional study to quantify CD126+ CD8+ T-cell subsets in peripheral blood and induced sputum from adults with asthma and healthy controls (HC) and to assess their clinical relevance.

Adults with asthma and age-matched HC were recruited from the First Affiliated Hospital of Guangzhou Medical University between December 2022 and May 2024. Asthma was diagnosed and graded according to Global Initiative for Asthma (GINA) criteria [9]. Atopy was defined by serum specific IgE sensitization (≥0.35kUA/L) [10]. Participants with acute respiratory infection, recent biologic therapy, or alternative causes of respiratory symptoms were excluded. All participants underwent standardized clinical assessments, including lung function testing, fractional exhaled nitric oxide (FeNO) measurement, blood and sputum inflammatory profiling, and symptom control assessment using the Asthma Control Questionnaire-5 (ACQ-5). CD126 expression on CD8+ T cells was quantified by flow cytometry in two independent cohorts (PBMCs and induced sputum). CD8+ memory subsets were defined by CCR7 and CD45RA as central memory (CM; CD45RA−CCR7+), effector memory (EM; CD45RA−CCR7−), and terminally differentiated effector re-expressing CD45RA (EMRA; CD45RA+CCR7−). Dimensionality reduction (t-SNE) was applied to sputum CD3+ CD8+ events. Group comparisons and Spearman correlations were performed using appropriate parametric or non-parametric tests (two-sided p<0.05). Exploratory adjusted analyses were performed controlling for age, sex and BMI, including linear regression for severity and partial Spearman correlation for ACQ-5. A detailed description of the methodology is provided in the Supplementary Material (Appendix Detailed Methods). The study was approved by the local ethics committee, and written informed consent was obtained from all participants.

Sixty-six asthma patients and thirty-six HC were included in the PBMC cohort, baseline characteristics are shown in Appendix Table S1. Flow cytometric analysis demonstrated a higher frequency of CD126+ cells within total CD8+ T cells in asthma compared with controls (2.52±0.34 vs. 1.50±0.32, p<0.05), with a similar enrichment observed within EM CD8+ T cells (1.89±0.26 vs. 0.97±0.21, p<0.01) (Fig. 1a, b). Within the asthma group, non-atopic patients exhibited a higher frequency of CD126+ EM CD8+ T cells than atopic patients (2.80±0.60 vs. 1.43±0.23, p<0.05) (Fig. 1c). In contrast, stratification by type-2 inflammatory biomarkers, including FeNO, blood eosinophil counts, or sputum eosinophil percentages, revealed no significant differences in CD126+ EM CD8+ T-cell frequencies (all p>0.05; data not shown). Overall, circulating CD126+ EM CD8+ T cells were enriched in asthma, particularly in non-atopic disease.

Fig. 1.

Peripheral blood CD126+ CD8+ T cell frequencies in asthma versus healthy controls. (a) Frequency of CD126+ within total CD8+ T cells (%). (b) Frequency of CD126+ within effector memory (EM) CD8+ T cells (%). (c) Within asthma, frequency of CD126+ within EM CD8+ T cells (%) stratified by atopy. Peripheral blood mononuclear cells were purified and stained with CD45, CD3, CD4, CD8, CD45RA, CCR7 and CD126, then analyzed by multiparameter flow cytometry with gating detailed in Methods. Data are shown as mean±SEM with individual values. Mann–Whitney U test was used for statistical evaluation of differences between study groups. *p<0.05, **p≤0.01. HC, healthy controls; AS, asthma; AA, atopic asthma; NAA, non-atopic asthma.

Induced sputum was analyzed from severe asthma (SA, n=15), non-severe asthma (NSA, n=16) and HC (n=19); baseline characteristics are shown in Appendix Table S2. t-SNE visualization of sputum CD3+CD8+ events showed that CD126 expression localized predominantly to CM CD8+ T cells, with minimal expression in EM or EMRA subsets (Fig. 2a). In groupwise analysis, asthma exhibited a higher frequency of sputum CD126+ within total CD8+ T cells than HC (3.75±0.57 vs. 0.42±0.14; p<0.0001; Fig. 2b). CD126+ CM CD8+ T cells (expressed as a fraction of total sputum CD8+ T cells) were increased in asthma compared with HC (Fig. 2c). Among asthma samples with CM CD8+ event counts>5 (n=20), SA showed a greater frequency of sputum CD126+ within CM CD8+ T cells than NSA (50.83±7.04 vs. 24.06±6.59; p<0.05; Fig. 2d). Finally, the frequency of sputum CD126+ within CM CD8+ T correlated positively with ACQ-5 (Spearman's r=0.463, p=0.0398), linking CD126+ CM CD8+ T cells enrichment to worse symptom control (Fig. 2e). In exploratory adjusted analyses controlling for age, sex and BMI, the association with severity remained significant (adjusted β=25, 95% CI 2.61–47.3, p=0.0309), whereas the association with ACQ-5 was attenuated (partial Spearman's r=0.399, p=0.081), although the direction remained positive.

Fig. 2.

CD126 expression across airway CD8+ T cell compartments from induced sputum. (a) t-SNE analysis of flow cytometry data using CD8+ T cells from asthma patients, depicting either central-memory (CM; CD45RA−CCR7+) T cells, effector-memory (EM; CD45RA−CCR7−) T cells, terminally differentiated effector memory T cells re-expressing CD45RA (EMRA; CD45RA+CCR7−), CD126+ cells in red. (b) Frequency of CD126+ within total CD8+ T cells (%). (c) Frequency of CD126+ CM CD8+ T cells among total sputum CD8+ T cells. (d) Within AS, frequency of CD126+ within CM CD8+ T cells (%) stratified by severity. (e) Correlation between ACQ-5 score and CD126+ within CM CD8+ T cells (%). For panels d) and e), within-CM percentages were analyzed only in asthma samples with CM CD8+ event counts>5 (n=20). Data are shown as mean±SEM with individual values. Between-group differences were assessed by Mann–Whitney U test and correlations by Spearman's r. *p<0.05, **p≤0.01, ***p≤0.001, ****p≤0.0001. HC, healthy controls; AS, asthma; SA, severe asthma; NSA, non-severe asthma; ACQ-5, Asthma Control Questionnaire-5.

Overall, CD126 identifies distinct CD8+ memory phenotypes in peripheral blood versus induced sputum in asthma, with CD126+ EM CD8+ T cells enriched in blood and CD126+ CM CD8+ T cells enriched in the airway. Airway enrichment of CD126+ CM CD8+ T cells, and its association with ACQ-5 and severity, supports a clinically relevant IL-6-linked immune signature beyond canonical T2 markers. The findings in PBMC align with prior work identifying a CD126high EM CD8+ T cell subset in human blood that is elevated in asthma compared with HC [8]. In our cohort, this enrichment was most evident in non-atopic asthma and was not associated with eosinophils or FeNO, suggesting an IL-6-linked immune program that can be partly uncoupled from eosinophilic T2 inflammation [4,5,11]. Beyond Tc2 outputs, IL-6R+ CD8 memory also encompasses helper-like subsets such as Tc17 and Tc22, indicating broader cytokine-secreting inflammatory states within the IL-6R+ CD8+ T cells rather than a purely Th2-restricted program [7]. Prior studies suggest that IL-6 signaling supports Th17 programs and can antagonize Treg stability [12,13]. In experimental allergen-challenge models, IL-6 axis activity, including trans-signaling, has been implicated in mixed type-2/17 airway inflammation [14,15], providing context for an IL-6R-linked, helper-like CD8 signature that may emerge independently of eosinophilic T2 biomarkers. Collectively, our data suggest that blood CD126+ EM CD8+ T cell enrichment may reflect a greater contribution of IL-6-linked pathways in non-atopic or mixed-inflammation endotypes that can be partly uncoupled from eosinophilic T2 inflammation, consistent with reports linking circulating IL-6 to higher disease burden beyond classic T2-high asthma.

Prior work shows that membrane IL-6R is enriched on naïve and CM subsets [16]. Consistent with this receptor distribution, our airway t-SNE analysis localized CD126 expression predominantly to CM CD8+ T cells (Fig. 2a). Lee et al. reported that the frequency of circulating IL-6Rαhigh EM CD8+ T cells declines with poorer lung function, and they proposed preferential homing to the lung as a contributing mechanism [8]. Our airway data are broadly consistent with this preferential lung-homing hypothesis. In induced sputum, CD126+ within CM CD8+ T cells increased with severity and correlated with ACQ-5, although the ACQ-5 association was attenuated in exploratory analyses after adjustment for age, sex and BMI, suggesting that IL-6R-competent CD8 memory cells are more represented at the site of active disease as severity rises.

Several limitations should be acknowledged. This study is cross-sectional, so temporal dynamics of CD126 expression during exacerbation and remission are not defined. The PBMC and induced sputum cohorts were independent rather than paired, precluding within-subject blood-airway comparisons. Treatment intensity (including ICS dose and recent systemic corticosteroid exposure) may influence IL-6R/CD126 expression, and residual treatment confounding cannot be excluded. In spite of these limitations, the findings are clinically informative. CD126-based flow cytometric phenotyping of CD8 memory subsets is feasible in routine laboratories and may complement standard T2 biomarkers. Validation of prespecified thresholds for CD126+ EM CD8+ T cells in blood and CD126+ CM CD8+ T cells in airway could support patient selection and monitoring in studies that target the IL-6 axis.

In summary, we characterize a receptor-defined pattern of CD126 on CD8+ T cells that differs between peripheral blood and the local airway and relates to clinical status. CD126+ within EM CD8+ T cells is increased in blood, with greater enrichment in non-atopic asthma, while CD126+ within CM CD8+ T cells is increased in the airway, rises further in severe disease, and correlates with ACQ-5. These findings indicate that CD126-based phenotyping may capture IL-6-linked biology that is not fully reflected by eosinophilic T2 markers and may complement current tools for patient stratification and monitoring.