A sensitive method to detect defined peptide among those eluted from murine MHC class II molecules

We developed a sensitive competitive inhibition radioimmunoassay able to trace pmoles of a defined peptide eluted from major histocompatibility complex (MHC) class II molecules that were subsequently fractionated by RP-HPLC. In this assay we used a model synthetic peptide corresponding to amino acid residues 110-120 from the hemagglutinin (HA) of PR8 influenza virus, and affinity purified rabbit antibodies specific for this peptide. The HA110-120 peptide binds to I-E^d class II molecules on the surface of APCs and is recognized by specific CD4⁺ T helper cells. 2PK3 B lymphoma cells (H-2^d) were pulsed with HA110-120 peptide or PR8 virus, lysed, the MHC class II molecules extracted, and bound peptides eluted. After separation by RP-HPLC, the fractions were tested for inhibition of the binding of rabbit anti-HA110-120 antibodies to peptide coated microtiter plates. A significant inhibitory activity was observed with one peak when the cells were pulsed with HA110-120 peptide and two peaks when pulsed with PR8 virus. The inhibitory activity was correlated with the presence of HA110-120 peptide as demonstrated by peptide sequencing. The assay is reproducible and sensitive to 1 pmol of antigenic peptide. This assay can be useful to identify microbial peptides with defined structure and antigenicity among the multiple peptides bound to class II molecules.