Biomarkers for tuberculosis: the case for lipoarabinomannan

Abstract

Tuberculosis (TB) is considered the most onerous of infectious diseases according to recent reports from the World Health Organization. Available tests for TB diagnosis present severe limitations, and a reliable point-of-care (POC) diagnostic test does not exist. Neither is there a test to discern between the different stages of TB, and in particular to predict which patients with Mycobacterium tuberculosis infection and no clinical signs are more at risk of advancing to overt disease. We here review the usefulness of mycobacterial lipoarabinomannan (LAM) as a diagnostic marker for active and latent TB and, also, aspects of the immune response to LAM relevant to such tests. There is a high potential for urinary LAM-based POC tests for the diagnosis of active TB. Some technical challenges to optimised sensitivity of the test will be detailed. A method to quantify LAM in urine or serum should be further explored as a test of treatment effect. Recent data on the immune response to LAM suggest that markers for host response to LAM should be investigated for a prognostic test to recognise individuals at the greatest risk of disease activation.

FIGURE 1

Schematic rendition of the structure of Mycobacterium spp. LAM. Only the major common structural features are shown (see various works for more structural detail [38, 39]). LAM: lipoarabinomannan.

FIGURE 2

Correlation between CD4⁺ T-cell counts and lipoarabinomannan (LAM) detection in urine. Compilation of results from a meta-analysis of 12 different studies of tuberculosis (TB)-LAM for TB diagnosis and screening. Plots of a) sensitivity and b) specificity of TB-LAM stratified by CD4⁺ T-cell counts. Circles represents the pooled estimates (median), with bars representing 95% credible intervals. Reproduced from [57] with permission from the publisher.

FIGURE 3

Signal intensity of Mycobacterium tuberculosis lipoarabinomannan in urine of children with tuberculosis (TB) in a high TB/HIV setting by ELISA at different time points after initiation of anti-TB treatment. Optical density (OD) results for HIV-positive participants are shown in red solid lines, results for HIV-negative participants as black dashed lines. The signal intensity became undetectable after 3 months of treatment in six participants. y-axis shows logarithmic scale for mean OD. Neg.: negative. Reproduced from [73].

FIGURE 4

Lipoarabinomannan (LAM) signal by ELISA in urine samples from children with suspected tuberculosis. Samples were collected at Machava General Hospital in Maputo, Mozambique, and stored at −80°C. The samples were transported frozen to the laboratory at Karolinska Institutet, Sweden. After thawing each urine was divided into four samples: 1) nonmanipulated; 2) spiked with 250 pg·mL⁻¹ LAM; 3) pH adjusted with 40 mM Tris final concentration; 4) pH adjusted and spiked with LAM. LAM signal was measured by ELISA with absorbance at 450 nm following the protocol described previously [97]. PBS spiked with 250 pg·mL⁻¹ LAM was used as a control. LAM was prepared in house as described previously [79].

FIGURE 5

Effect of storage of urine at 4°C on lipoarabinomannan (LAM) signal detected by ELISA. Urine was spiked with various concentrations of LAM and stored at 4°C. LAM signal was measured by ELISA with absorbance at 450 nm following the protocol described previously [97].