A role for CD4⁺CD25⁺ T cells in regulation of the immune response during human tuberculosis

Summary

Active tuberculosis (TB) is associated with prolonged suppression of Mycobacterium tuberculosis (MTB)-specific immune responses, but mechanisms involved are understood incompletely. We investigated a potential role for CD4⁺CD25⁺ regulatory T cells in depressed anti-MTB immunity by evaluating serially CD4 cell phenotype and interferon (IFN)-γ production by mononuclear cells from patients with TB. At diagnosis, frequencies of CD4⁺CD25⁺ T cells were increased in blood from TB patients compared to healthy purified protein derivative (PPD)-positive controls (with a history of prior TB exposure), and remained elevated at completion of therapy (6 months). By contrast, expression of another activation marker, CD69, by CD4 T cells was increased at diagnosis, but declined rapidly to control levels with treatment. Among CD4⁺CD25⁺ T cells from TB patients at diagnosis those expressing high levels of CD25, probably representing regulatory T cells, were increased 2·9-fold when compared to control subjects, while MTB-stimulated IFN-γ levels in whole blood supernatants were depressed. A role for CD4⁺CD25⁺ T cells in depressed IFN-γ production during TB was substantiated in depletion experiments, where CD25⁺-depleted CD4 T cells produced increased amounts of IFN-γ upon MTB stimulation compared to unseparated T cells. At follow-up, IFN-γ production improved most significantly in blood from TB patients with high baseline frequencies of CD4⁺CD25⁺ T cells (more than threefold higher than controls for both total and CD25^hi+ CD4 T cells), who also had a significant drop in frequencies of both total and ‘regulatory’ CD4⁺CD25⁺ T cells in response to treatment. Expansion of CD4⁺CD25⁺ regulatory T cells during active TB may play a role in depressed T cell IFN-γ production.