A DR17-RESTRICTED T-CELL EPITOPE FROM A SECRETED MYCOBACTERIUM-TUBERCULOSIS ANTIGEN ONLY BINDS TO DR17 MOLECULES AT NEUTRAL PH

The assembly of peptide-major histocompatability class II complexes in vitro is accelerated at low pH, comparable to that found in the intra cellular compartments of metabolically active antigen-presenting cells (APC). Mycobacteria such as Mycobacterium tuberculosis reside in phag osomes with only mildly acidic pH. Therefore, we investigated the pH d ependency of peptide-HLA-DR binding for several T cell epitopes of myc obacterial proteins, focussing particularly on well-defined, immunodom inant HLA-DR17(3)-restricted T cell epitopes: peptide (p) 3-13 from th e cytoplasmic 65-kDa heat shock protein of M. tuberculosis/M. leprae, and peptide 56-65 from the secreted 30/31-kDa protein from M. tubercul osis/M. leprae. p3-13 bound to purified, cell-free DR17 under both aci dic and neutral conditions. Four other, unrelated DRl7-binding peptide s showed the same pi-I-dependent binding characteristics as p3-13. p56 -65, however, only bound to purified DR17 at pH 7 but not at all at pH 4.5. These DR17 peptide binding data were confirmed in cell-bound DR1 7, in T cell stimulation assays in which fixed APC were peptide-pulsed at acidic or neutral pH before addition of peptide-specific DR17-rest ricted T cells. As far as we are aware, p56-65 is the only human T cel l epitope binding to HLA exclusively at neutral pH. The binding charac teristics of p56-65 may reflect dominant processing in alternative, le ss acidic vacuolar compartments specifically related to the generation of epitopes from (secreted) mycobacterial proteins. The observation t hat p56-65 is an immunodominant epitope for anti-mycobacterial T cells suggests the relevance of such novel processing compartments in T cel l-mediated immunity.