Detection of hybrid (combinatorial) Ia antigens using parent anti-F₁ sera

Ia specificities 22 and 23 were initially identified by using conventional alloantisera and were mapped to the I-E subregion of k and d haplotypes on the basis of their reactivity with selected recombinants. Recently we found that Ia 22 and Ia 23 are hybrid determinants on the basis of their expression on selected F₁ cells, but absence from both parental cells. Initial attempts to detect hybrid Ia antigens by immunizing parents with F₁ cells were unsucessful. By utilizing lipopolysaccharide (LPS)-stimulated F₁ spleen lymphoblasts as immunogens, 1 of the parents as recipient and the other parent cells for absorption of antisera, specific anti-Ia.22 and 23 antibodies were produced. The specificity of these parent anti-F₁ sera was confirmed by cytotoxic and immunoprecipitation analyses.