title: 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 Calmette-Guérin (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.
authors.affiliation: Tuberculosis Research Unit, National Heart and Lung Institute, Imperial College London, London W2 1PG, United Kingdom.
authors.forename: Kerry A
authors.initials: KA
authors.lastname: Millington
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Fortune
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Low
authors.forename: Alejandra
authors.initials: A
authors.lastname: Garces
authors.forename: Suzanne M
authors.initials: SM
authors.lastname: Hingley-Wilson
authors.forename: Melissa
authors.initials: M
authors.lastname: Wickremasinghe
authors.forename: Onn M
authors.initials: OM
authors.lastname: Kon
authors.forename: Ajit
authors.initials: A
authors.lastname: Lalvani
mesh_terms: D000942:Antigens, Bacterial / Q000276:immunology*; D001500:BCG Vaccine; D001426:Bacterial Proteins / Q000276:immunology; D058501:Enzyme-Linked Immunospot Assay; D005434:Flow Cytometry; D006801:Humans; D016056:Immunodominant Epitopes / Q000276:immunology; D009169:Mycobacterium tuberculosis / Q000276:immunology*; D040901:Proteomics; D016176:T-Lymphocyte Subsets / Q000276:immunology*; D014376:Tuberculosis / Q000175:diagnosis / Q000276:immunology*
chemical_list: D000942:Antigens, Bacterial; D001500:BCG Vaccine; D001426:Bacterial Proteins; C117940:CFP-10 protein, Mycobacterium tuberculosis; C093276:ESAT-6 protein, Mycobacterium tuberculosis; D016056:Immunodominant Epitopes