Processing of lysozyme by macrophages: identification of the determinant recognized by two T-cell hybridomas.

The purpose of this study was to identify the fragment of the hen egg-white lysozyme (HEL) molecule presented by macrophages to helper T cells. This was investigated by using T-cell hybridomas and macrophages prefixed in paraformaldehyde. We previously had shown that such prefixed macrophages could present a tryptic digest of HEL. The tryptic peptides were separated by HPLC and tested for their ability to stimulate the T-cell hybridomas. Only one tryptic peptide was found to be immunogenic. This immunogenic peptide was identified as the tryptic peptide T-8, containing amino acids 46-61. The precise determinant on the peptide T-8 being recognized was further defined by testing the response of the two T-cell hybridomas to human lysozyme. Neither clone responded to human lysozyme. From the amino acid sequence of human lysozyme, the determinant was localized to the four amino-terminal residues. Cleavage of the immunogenic peptide with either chymotrypsin or protease V-8 completely abolished the immunogenicity. This suggested that the T-cell determinant is located in the hydrophilic amino-terminal residues and that it must be associated with a hydrophobic stretch of amino acids, which allows the peptide to associate with the macrophage plasma membrane.