EsxB

EsxB, also known as CFP-10 (Culture Filtrate Protein 10), is a small secreted protein of Mycobacterium tuberculosis and related species. It belongs to the WXG100 protein family and is typically about 100 amino acids in length, with a characteristic Trp-X-Gly (WXG) motif near the middle of the sequence. EsxB forms a functional heterodimer with EsxA (ESAT-6), and these two proteins are co-secreted via the ESX-1 type VII secretion system.

Genetic and secretion context: EsxB is encoded in the ESX-1 gene cluster, adjacent to esxA, and is co-regulated with it. The EsxA-EsxB heterodimer is essential for efficient secretion of virulence factors and for the disruption of phagosomal membranes, which facilitates cytosolic access of the bacterium.

Function and virulence: The EsxA/EsxB pair contributes to virulence in animal models and modulates host immune responses. Because RD1, the genomic region encoding this cluster, is deleted in most BCG vaccine strains, EsxA and EsxB are key virulence determinants distinguishing pathogenic Mycobacterium tuberculosis from BCG.

Clinical relevance: EsxB-derived antigens, together with EsxA, are major targets of the host immune response and are used in diagnostic assays, such as interferon-gamma release assays, to distinguish TB infection from BCG vaccination or non-tuberculous mycobacterial infections.