The anatomy of an antigen molecule: functional subregions of L-tyrosine-p-azobenzenearsonate

The structural components of antigen molecules that interact with class II major histocompability complex (MHC) molecules on antigen-presenting cells (APCs) (agretopes) and with antigen receptors of T-lymphocytes (epitopes) in class II restricted T-cell responses have not been precisely defined. This issue was addressed here using murine T-cell clones specific for the simple immunogen L-tyrosine-p-azobenzenearsonate (ABA-tyr) and a series of analogs of the homologous antigen. Two experimental approaches were used. First, APCs were pulsed with analogs and used to stimulate T-cell proliferation. The patterns of stimulation segregated the clones into two specificity groups and indicated that the epitope recognized by the T-cell included the arsonate group and elements in the side chain of tyrosine. Furthermore, the clones manifest different sensitivities to antigen. Second, non-stimulatory analogs were used to block the presentation of ABA-tyr in an effort to define the agretope. Compounds containing the azophenyl group blocked presentation of ABA-tyr in a dose-dependent fashion, whereas p-arsanilic acid and L-tyrosine were ineffective. The blocking was specific inasmuch as the compounds had no effect on the antigen-induced proliferative responses of giant keyhole limpet hemocyanin (KLH) or hen egg white lysozyme (HEL)-reactive T-cell clones. The blocking pattern indicated that the feature required for productive association with the APC centered on the planar structure of the azo-linked aromatic rings, with little or no contribution from either the arsonate moiety or the tyrosyl side chain. We propose that this structure forms an agretope for this family of compounds.