Rv3615c is a highly immunodominant RD1 (Region of Difference 1)-dependent secreted antigen specific for Mycobacterium tuberculosis infection

Abstract

The 6-kDa early secretory antigenic target of Mycobacterium tuberculosis (ESAT-6) and the 10-kDa culture filtrate antigen (CFP-10), encoded in region of difference 1 (RD1) and secreted by the ESAT-6 system 1 (Esx-1) secretion system, are the most immunodominant and highly M. tuberculosis (MTB)-specific antigens. These attributes are responsible for their primary importance in tuberculosis (TB) immunodiagnosis and vaccine development. Rv3615c [Esx-1 substrate protein C (EspC)], encoded outside RD1, is similar in size and sequence homology to CFP-10 and ESAT-6, suggesting it might be a target of cellular immunity in TB. Using ex vivo enzyme-linked immunospot-and flow cytometry-based cytokine-secretion assay, we comprehensively assessed cellular immune responses to EspC in patients with active TB, latently infected persons, and uninfected bacillus CalmetteGuérin (BCG)-vaccinated controls. EspC was at least as immunodominant as ESAT-6 and CFP-10 in both active and latent TB infection. EspC contained broadly recognized CD4 + and CD8 + epitopes, inducing a predominantly CD4 + T-cell response that comprised functional T-cell subsets secreting both IFN-γ and IL-2 as well as functional T-cell subsets secreting only IFN-γ. Surprisingly, T-cell responses to EspC were as highly specific (93%) for MTB infection as responses to ESAT-6 and CFP-10, with only 2 of 27 BCG-vaccinated controls responding to each antigen. Using quantitative proteomics and metabolically labeled mutant and genetically complemented MTB strains, we identified the mechanism of the specificity of anti-EspC immunity as the Esx-1 dependence of EspC secretion. The high immunodominance of EspC, equivalent to that of ESAT-6 and CFP-10, makes it a TB vaccine candidate, and its high specificity confers strong potential for T-cell-based immunodiagnosis.

The risk of tuberculosis in transplant candidates and recipients: a TBNET consensus statement

“…New diagnostic approaches are needed that allow for a more targeted identification of patients at risk to develop TB. This may involve modifications of in vitro immunodiagnostic assays such as the use of novel stimulatory antigens [15,164,165], alternative biomarkers other than IFN-c [166][167][168][169][170][171][172], variations in incubation time [173,174], the readout system [9,12,13,175,176] or the clinical specimen instead of blood [177][178][179]. In addition, both for the detection of LTBI with a risk of reactivation and for suspected active TB, a novel approach based on a whole blood transcriptional signature could provide a biomarker system with high discriminative potential [180].…”

IP-10 release assays in the diagnosis of tuberculosis infection: current status and future directions

“…IP-10 has been shown to be specifically induced, not only in response to the TB-specific antigens used in the QFT-IT and QFT-G [78] tests, but also in response to purified protein derivative [25] and alternative TB antigens, including selected region of difference 1-specific peptides [21,50,51,56], peptides spanning the mycobacterial protein TB10.4 [49], whole M. tuberculosis sonicate [21], cyto megalovirus, HIV [84] and peptides specific for Mycobacterium leprae [85]. Alternative or additional antigens appear to have the potential for improving IGRA sensitivity [86,87] and enable discrimination between active and latent TB [88][89][90][91]; in this context, IP-10 may be a favorable bio marker for measuring responses to novel antigens with limited immunogenicity because of the high levels of biomarker produced [51].…”

MTBVAC from discovery to clinical trials in tuberculosis-endemic countries

“…Relevant to these observations, recent studies have shown that most of the experimentally confirmed human T cell epitopes of M. tuberculosis are concentrated in ESAT-6 and CFP-10, and up to 80% of humans and nonhuman primates show reactogenicity to both of them [83][84][85][86]. Moreover, these human T cell epitopes were shown to be evolutionary conserved in the related members of the MTBC [48].…”