Protein and Microbial Biomarkers in Sputum Discern Acute and Latent Tuberculosis in Investigation of Pastoral Ethiopian Cohort

Abstract

Differential diagnosis of tuberculosis (TB) and latent TB infection (LTBI) remains a public health priority in high TB burden countries. Pulmonary TB is diagnosed by sputum smear microscopy, chest X-rays, and PCR tests for distinct Mycobacterium tuberculosis (Mtb) genes. Clinical tests to diagnose LTBI rely on immune cell stimulation in blood plasma with TB-specific antigens followed by measurements of interferon-gamma concentrations. The latter is an important cytokine for cellular immune responses against Mtb in infected lung tissue. Sputum smear microscopy and chest X-rays are not sufficiently sensitive while both PCR and interferon-gamma release assays are expensive. Alternative biomarkers useful for developing diagnostic tests to discern TB disease states are desirable. This study's objective was to discover biomarkers in sputum, assessing the proteomes and microbiomes of 74 TB patients, 46 individuals with LTBI, and 51 negative community controls (NCC). Study participants were from the South Omo province, a pastoral region in southern Ethiopia. A total of 161 and 115 samples were used to determine the 16S rRNA sequence-based bacterial taxonomies and proteomic profiles, respectively. Sputum microbiota did not reveal statistically significant differences in alpha-diversity comparing the three groups. The genus Mycobacterium, representing Mtb, was only identified for the TB group. The latter featured reduced abundance of the genus Rothia in comparison to the LTBI and NCC groups. Rothia is a human respiratory tract commensal and may be sensitive to the inflammatory milieu caused by TB infection. Proteomic data strongly supported innate immune responses against Mtb in subjects with pulmonary TB. Ferritin, an iron storage protein released by damaged host cells, was markedly increased in abundance in TB sputum compared to the LTBI and NCC groups, along with alpha-1-acid glycoproteins ORM1 and ORM2. These proteins are acute phase reactants and inhibit excessive neutrophil activation. Proteomic data also supported effector roles of neutrophils in the anti-Mtb response which was not observed for LTBI cases. Less abundant in sputum of the LTBI group versus the NCC group were two immunomodulatory proteins, mitochondrial TSPO and the extracellular ribonuclease T2. If validated, these proteins are of interest as diagnostic biomarkers for LTBI.