European Society of Mycobacteriology - Instituto Nacional de Saúde ...

GL-7
A NOVEL DIAGNOSTIC TEST TO DIFFERENTIATE
LATENT TB INFECTION AND ACTIVE DISEASE
Lee W. Riley
MD, School of Public Health, University of California, Berkeley
It is well recognized that the treatment of latent TB infection (LTBI) is a highly effective TB prevention strategy, which is
still not widely practiced in most parts of the world. Most TB-endemic countries rely on BCG vaccine to prevent TB.
LTBI treatment requires contact investigation, which is not done in most “BCG countries”. One reason for this reluctance
to practice contact investigation is the lack of a reliable test that can distinguish LTBI from TB. Thus, a test that can
unequivocally distinguish LTBI from TB could alter the current national prevention programs in TB-endemic countries.
We have identified a set of M. tuberculosis cell wall proteins that are expressed when the bacilli replicate in vivo, but not
when they are in a nonreplicative state. Their continued expression is associated with disease progression in infected
mice, and mouse T cells are sensitized as these proteins are continually expressed in vivo. Exploiting this observation, we
developed a bioassay that is able to distinguish LTBI from active disease in a mouse model. The assay is based on IFNγ
induction by T cells exposed to a set of synthetic peptides based on the cell wall protein called Mcep1A. The Cornell
mouse model was used to study the response of spleen cells exposed ex vivo to these peptides. Cells from untreated
mice expressed 7-9-fold higher levels of IFNγ than those from treated mice at 24 and 32 weeks of infection, as measured
by ELISA. Blood cells from healthy tuberculin skin-test positive, QuantiFERON-negative (n=3) and TST-negative,
QuantiFERON-negative (n=3) human volunteers showed no response to the peptides. These peptides are currently
under evaluation in newly diagnosed TB patients. If the assay can show a response in these TB patients at levels similar
to those observed in diseased mice, this assay can be converted into an immunochromatographic (“dip stick”) format.
Such a test then can be used to readily differentiate those with LTBI and active disease, and could then be incorporated
as part of National TB Control Programs.
32 ESM 2009