ESAT-6 and CFP-10 in tuberculosis: bridging virulence to diagnostic applications

Tuberculosis (TB) remains a major global public health challenge. Accurate detection of Mycobacterium tuberculosis ( Mtb ) infection, particularly latent tuberculosis infection (LTBI), is essential for targeted preventive treatment and progress toward TB elimination. The traditional tuberculin skin test (TST) uses the purified protein derivative (PPD), a complex antigenic mixture that exhibits substantial cross-reactivity in individuals vaccinated with bacille Calmette-Guérin (BCG) vaccine and those exposed to nontuberculous mycobacteria (NTM), thereby limiting its specificity, particularly in BCG-vaccinated populations. Comparative genomic analyses of Mtb led to the identification of early secreted antigenic target 6 kDa (ESAT-6) and culture filtrate protein 10 (CFP-10), two immunodominant proteins encoded within region of difference 1 (RD1). The genes encoding these antigens are absent from BCG vaccine strains and most clinically relevant NTM species, providing the molecular basis for improved discrimination between Mtb infection and vaccine-induced sensitization, although cross-reactive homologs occur in some NTM species. As key substrates of the ESX-1 secretion system, ESAT-6 and CFP-10 are secreted early during infection, elicit strong Th1-biased cellular immune responses, and induce measurable interferon-gamma (IFN-γ) release, thereby enabling the development and widespread implementation of IFN-γ release assays (IGRAs). Although IGRAs generally provide greater specificity than TST, they measure host immune memory rather than ongoing bacillary activity; consequently, they cannot reliably distinguish LTBI from active tuberculosis (aTB), and their sensitivity may be reduced in immunocompromised individuals. This review examines how these virulence-associated proteins were repurposed as diagnostic antigens and discusses the current limitations of, and future directions for, TB infection diagnostics.