Higher soluble CD8 correlates with poor prognosis in hospitalized pulmonary infection patients.

OBJECTIVES: Pulmonary infections pose significant challenges in accurate prognosis due to the lack of reliable biomarkers. This study aims to explore the prognostic potential of soluble CD4 (sCD4) and sCD8 for adverse outcomes in hospitalized pulmonary infection patients. METHODS: This study enrolled 112 hospitalized pulmonary infection patients and 62 healthy controls (HCs). Hospitalized patients were grouped into survivors (n = 87), who recovered and were discharged, and non-survivors (n = 25), who either died or discontinued treatment due to severe conditions. Plasma sCD4 and sCD8 levels were quantified by ELISA. T cell subsets were determined using flow cytometry. IL-6 and procalcitonin (PCT) levels were detected using electrochemiluminescence, and C-reactive protein (CRP) levels were measured using rate nephelometry. Inflammatory markers derived from complete blood count (CBC) were calculated accordingly. Receiver operating characteristic (ROC) analyses, Cox proportional hazard models and correlation analyses were conducted. RESULTS: Non-survivors were significantly older than survivors and HCs. Compared to survivors, non-survivors presented significantly higher levels of sCD8, PCT, IL-6, CRP and CBC-derived inflammatory markers, along with significantly reduced CD3+ T and CD8+ T cell counts. ROC analyses demonstrated that sCD8 had the highest area under the curve (AUC) value of 0.85 (0.78-0.92) with sensitivity and specificity of 100% and 66.67%, respectively. Other markers showed weak to moderate prognostic power with AUCs ranging from 0.64 to 0.75. Multivariate Cox hazard regression analysis indicated that higher sCD8, lower CD8+ T cell counts and the presence of mixed-type pathogenic infections remained independently associated with undesirable outcomes. sCD8 was positively correlated with all inflammatory markers but negatively correlated with T cell subset counts. CONCLUSIONS: Our data suggest that a plasma sCD8 level above the optimal cut-off value is independently associated with poor outcomes. Elevated sCD8 may be a novel prognostic biomarker in hospitalized pulmonary infection patients.