KEY EPITOPES ON THE ESAT-6 ANTIGEN RECOGNIZED IN MICE DURING THE RECALL OF PROTECTIVE IMMUNITY TO MYCOBACTERIUM-TUBERCULOSIS

The recall of long-lived immunity in a mouse model of tuberculosis (TB ) is defined as an accelerated accumulation of reactive T cells in the target organs, We have recently identified Ag 85B and a 6-kilodalton early secretory antigenic target, designated ESAT-6, as key antigenic targets recognized by these cells, In the present study, preferential recognition of the ESAT-6 Ag during the recall of immunity was found t o be shared by five of six genetically different strains of mice, Over lapping peptides spanning the sequence of ESAT-6 were used to map two T cell epitopes on this molecule, One epitope recognized Fn the contex t of H-2(b,d) was located in the N-terminal part of the molecule, wher eas an epitope recognized in the context of H-2(a,k) covered amino aci ds 51 to 60, Shorter versions of the N-terminal epitope allowed the pr ecise definition of a 13-amino acid core sequence recognized in the co ntext of H-2(b), The peptide covering the N-terminal epitope was immun ogenic, and a T cell response with the same fine specificity as that i nduced during TB infection was generated by immunization with the pept ide in IFA, In the C57BL/6j strain, this single epitope was recognized by an exceedingly high frequency of splenic T cells (similar to 1:100 0), representing 25 to 35% of the total culture filtrate-reactive T ce lls recruited to the site of infection during the first phase of the r ecall response, These findings emphasize the relevance of this Ag in t he immune response to TB and suggest that immunologic recognition in t he first phase of infection is a highly restricted event dominated by a limited number of T cell clones.