Investigating the role of circulating CXCR5-expressing CD8+ T-cells as a biomarker for bacterial infection in subjects with pneumonia.

Background: Recently, lymphoid follicle-confined and circulating CD8+ T-cells expressing the C-X-C chemokine receptor type 5 (CXCR5) were described, which was involved in anti-virus immune response. However, the dynamics and role of circulating CXCR5-expressing CD8+ T-cells during bacterial infection is unknown. So, we asked whether CXCR5+ CD8+ T cells were also generated during bacterial infections in lower respiratory tract.
Methods: The clinical data of 65 pneumonia patients were analyzed. The patients were divided into groups as tuberculosis, bronchiectasis and community or hospital acquired pneumonia (CAP, HAP). The sputum/bronchial secretion or bronchoalveolar lavage fluid (BALF) samples were taken for microbiological examination. The procalcitonin (PCT) was used to evaluate disease severity of these groups and compared among patients. We characterized the number and phenotype (PD-1 and CD103) of CXCR5 + CD8+ T cells in the peripheral circulation by flow cytometry in all individuals and analyzed their association with the serum PCT level and disease severity.
Results: Patients were mainly infected with Escherichia coli, Acinetobacter baumannii, Klebsiella pneumonia (K.p), Pseudomonas aeruginosa, and Staphylococcus aureus. Of note is the finding that PCT was weakly correlated with severity of respiratory infections. Furthermore, it was revealed an increase of CXCR5-expressing CD8+ T cells in peripheral blood of un-controlled CAP and progressive HAP compared controlled CAP and HAP, respectively (P < 0.05). Strikingly, the circulating CXCR5-expressing CD8+ T-cells in K.p-infected group was higher than that non-K.p-infected group (P < 0.05). Meanwhile, the ratio of CXCR5 + CD8+/CD8 was positively correlated with PCT level (P < 0.05). In clinic, the determination of CXCR5-expressing CD8+ T-cells showed better results compared to PCT and can be useful for the prediction of exacerbation of CAP or HAP. Phenotypically, CXCR5+ CD8 + T cell expressed comparable level of inhibitory molecules PD-1 and lower CD103 compared to their CXCR5- counterparts.
Conclusion: The circulating CXCR5-expressing CD8+ T-cell has diagnostic value for current pneumonia severity, and could act as a biomarker for identifying a bacteria-associated exacerbation. These cells may provide novel insight for the pathogenesis of pneumonia.