Identification of immune correlates of protection in tuberculosis infection

Abstract

Immunity against tuberculosis depends on memory T cells following sensitisation to mycobacterium antigens. Clinically healthy people may be naive to Mycobacteriumtuberculosis antigens, but may also have prior vaccination with M. bovis bacille Calmette-Guerin, exposure to various environmental Mycobacterium species, or have latent tuberculosis infection. The latter is detectable by reactivity of peripheral blood mononuclear cells to Mtb-specific antigens ESAT-6, CFP-10 or PPE68. Purified protein derivative and Ag85A are antigens shared by most Mycobacterium species. Acr1 and Acr2 are Mtb latency-associated antigens as they are upregulated in dormant mycobacteria. To identify immune mechanisms due to differing immune experience of mycobacteria, a study group of healthy young adults in Singapore was characterised for their reactivity to these antigens, which was then matched with their cytokine profiles and regulatory T cells. In the LTBI group, defined by ESAT-6/CFP-10 reactivity, there was a balance of pro- and anti-inflammatory responses, the latter could be regulated by Tregs. Immunosuppressive cytokine IL10 and CD4+CD25+ Treg responses were associated with PPD-specific IL6 and TNFa responses, and IL12p35 was correlated with TGFb expression, thus homeostatic mechanisms may be in place to limit excessive inflammatory responses. Induction of IFNg responses was likely to be mediated by IL12 and not IL18 in this group. CD8+ Tregs could be a source of IL10 as their levels were correlated. Given the weak concordance between PPE68 and ESAT-6/CFP-10 reactivity, combining these antigens was required to increase LTBI detection sensitivity. This combined LTBI group, especially those recently exposed (defined byAcr2 reactivity), most strongly expressed pro-inflammatory cytokines IL12p35 and IFNg and their CD8 Tregs correlated with Foxp3 expression. This work is the first to demonstrate that clinically healthy subjects, often regarded as a homogenous cohort in TB immunity studies, exhibit a wide range of immune responses to Mtb antigens and these response patterns enable stratification of their anti-tuberculosis immunity levels.