A DR17‐restricted T cell epitope from a secreted Mycobacterium tuberculosis antigen only binds to DR17 molecules at neutral pH

Abstract The assembly of peptide‐major histocompatability class II complexes in vitro is accelerated at low pH, comparable to that found in the intracellular compartments of metabolically active antigen‐presenting cells (APC). Mycobacteria such as Mycobacterium tuberculosis reside in phagosomes with only mildly acidic pH. Therefore, we investigated the pH dependency of peptide‐HLA‐DR binding for several T cell epitopes of mycobacterial proteins, focussing particularly on well‐defined, immunodominant HLA‐DR17(3)‐restricted T cell epitopes: peptide (p) 3–13 from the cytoplasmic 65‐kDa heat shock protein of M. tuberculosis/M. leprae , and peptide 56–65 from the secreted 30/31‐kDa protein from M. tuberculosis/M. leprae. p3–13 bound to purified, cell‐free DR17 under both acidic and neutral conditions. Four other, unrelated DR17‐binding peptides showed the same pH‐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 DR17, in T cell stimulation assays in which fixed APC were peptide‐pulsed at acidic or neutral pH before addition of peptide‐specific DR17‐restricted T cells. As far as we are aware, p56–65 is the only human T cell epitope binding to HLA exclusively at neutral pH. The binding characteristics of p56–65 may reflect dominant processing in alternative, less acidic vacuolar compartments specifically related to the generation of epitopes from (secreted) mycobacterial proteins. The observation that p56–65 is an immunodominant epitope for anti‐mycobacterial T cells suggests the relevance of such novel processing compartments in T cell‐mediated immunity.