Diagnosis and biomarkers for ocular tuberculosis: From the present into the future

Abstract

Tuberculosis is an airborne disease caused by Mycobacterium tuberculosis (Mtb) and can manifest both pulmonary and extrapulmonary disease, including ocular tuberculosis (OTB). Accurate diagnosis and swift optimal treatment initiation for OTB is faced by many challenges combined with the lack of standardized treatment regimens this results in uncertain OTB outcomes. The purpose of this study is to summarize existing diagnostic approaches and recently discovered biomarkers that may contribute to establishing OTB diagnosis, choice of anti-tubercular therapy (ATT) regimen, and treatment monitoring. The keywords ocular tuberculosis, tuberculosis, Mycobacterium, biomarkers, molecular diagnosis, multi-omics, proteomics, genomics, transcriptomics, metabolomics, T-lymphocytes profiling were searched on PubMed and MEDLINE databases. Articles and books published with at least one of the keywords were included and screened for relevance. There was no time limit for study inclusion. More emphasis was placed on recent publications that contributed new information about the pathogenesis, diagnosis, or treatment of OTB. We excluded abstracts and articles that were not written in the English language. References cited within the identified articles were used to further supplement the search. We found 10 studies evaluating the sensitivity and specificity of interferon-gamma release assay (IGRA), and 6 studies evaluating that of tuberculin skin test (TST) in OTB patients. IGRA (Sp = 71-100%, Se = 36-100%) achieves overall better sensitivity and specificity than TST (Sp = 51.1-85.7%; Se = 70.9-98.5%). For nuclear acid amplification tests (NAAT), we found 7 studies on uniplex polymerase chain reaction (PCR) with different Mtb targets, 7 studies on DNA-based multiplex PCR, 1 study on mRNA-based multiplex PCR, 4 studies on loop-mediated isothermal amplification (LAMP) assay with different Mtb targets, 3 studies on GeneXpert assay, 1 study on GeneXpert Ultra assay and 1 study for MTBDRplus assay for OTB. Specificity is overall improved but sensitivity is highly variable for NAATs (excluding uniplex PCR, Sp = 50-100%; Se = 10.5-98%) as compared to IGRA. We also found 3 transcriptomic studies, 6 proteomic studies, 2 studies on stimulation assays, 1 study on intraocular protein analysis and 1 study on T-lymphocyte profiling in OTB patients. All except 1 study evaluated novel, previously undiscovered biomarkers. Only 1 study has been externally validated by a large independent cohort. Future theranostic marker discovery by a multi-omics approach is essential to deepen pathophysiological understanding of OTB. Combined these might result in swift, optimal and personalized treatment regimens to modulate the heterogeneous mechanisms of OTB. Eventually, these studies could improve the current cumbersome diagnosis and management of OTB.

Keywords: Ocular tuberculosis; biomarkers; molecular diagnostic techniques; multi-omic; precision medicine.

Figure 1

Shows the molecular diagnosis by various approaches of the ocular samples obtained from OTBpatients and identification of novel biomarkers for better treatment of ocular tuberculosis. (A-D) Diverse clinical approaches and diagnostic tools that are used to detect OTB from the patient's samples by using mass spectrometry based-proteomics/peptidomics and others for discovery of novel biomarkers (BM) of potential therapeutic targets and better treatment options of OTB.

Figure 2

Flow chart showing the clinical approaches and diagnostic tools for detecting OTB from patient's perspective.

Figure 3

Uveitis is a form of eye inflammation that affects the middle layer of tissue in the eye wall (uvea). It presents with eye redness, pain, blurred vision and deterioration of visual acuity occurs quickly. (A-B) Inflammatory cytokine Interleukin 1 (IL-1) is an essential mediator of innate immunity and promotes inflammatory tissue damage in ocular uveitis (OU). The inflammatory constituents include Th1 and Th17 lymphocytes, which produce pro-inflammatory cytokines such as IL-1β, IL-2, IL-17, IL-18, IL-23, iNOS, COX2, TNF-α and INF-gamma that recruit leukocytes from circulation to result in tissue damage of the eye. The IFN-gamma-driven antimicrobial properties of phagocytes are augmented by IL-18 and IL-1β, and inflammatory cytokines processed by caspase-1, which are recruited to the inflammasomes. Inflammasomes are multimeric protein complexes that serve as a platform for caspase-1, the enzyme responsible for proteolytic cleavage of IL-1β and IL-18 precursors. However, this inflammasome activation triggers the multifaceted action of pro-inflammatory cytokines, a prerequisite for developing an effective inflammatory response against Mtb. The NLRP3 and AIM2 inflammasomes play an important role in innate immunity against Mtb.