Cross‐recognition by T cells of an epitope shared by two unrelated mycobacterial antigens

The molecular mimicry represented by cross‐recognition of determinants shared by unrelated antigens by antibodies or T cells is of broad immunological interest. In this study, we analyzed the cross‐recognition by CD4⁺ T cells of a peptide epitope shared by two mycobacterial proteins of diverse sequence, represented by the 19‐kDa antigen of Mycobacterium tuberculosis and the 28‐kDa antigen of Mycobacterium leprae. This epitope was immunodominant with respect to the 19‐kDa antigen, but cryptic in relation to the 28‐kDa antigen. The cross‐reactive epitope cores were identified by Pepscan window analysis and found to be eight residues long in both antigens (residues 69–76 and 127–134). Alignment of these octameric sequences revealed two identical and five conservatively related amino acids. Within the epitope core, two residues (⁷³Asn and ⁷⁶Ile) were identified as critical for recognition on the basis of inhibition of the cross‐reactive T cell proliferative response using singly substituted analog peptides. These results suggest that T cell cross‐reactive epitopes can exist in proteins with apparently not more than random levels of sequence homology. Their potential for unsuspected cross‐sensitization may play a role in the maintenance of T cell memory, in the pathogenesis of autoimmune diseases and possibly in a wide range of host immune responses to infectious pathogens.