Elevated antibody responses to
Mycobacterium tuberculosis antigens in individuals with
latent infection (LTBI) have previously been linked to an increased risk for progression to active disease. Studies in the field focussed mainly on
IgG antibodies. In the present study,
IgA and/or
IgG responses to the mycobacterial
protein antigens AlaDH, NarL, 19 kDa, PstS3, and MPT83 were determined in a blinded fashion in sera from 53 LTBI controls, 14 healthy controls, and 42 active TB subjects. Among controls, we found that elevated
IgA levels against all investigated
antigens were not randomly distributed but concentrated on a subgroup of <30%-with particular high levels in a small subgroup of ~5% comprising one progressor to active TB. Based on a specificity of 100%, anti-NarL
IgA antibodies achieved with 78.6% sensitivity the highest accuracy for the detection of active TB compared to healthy controls. In conclusion, the consistently elevated
IgA levels in a subgroup of controls suggest higher mycobacterial load, a risk factor for progression to active TB, and together with high
IgG levels may have prognostic potential and should be investigated in future large scale studies. The novel
antigen NarL may also be promising for the antibody-based diagnosis of active TB cases.