HUMAN ‘Ia’ ANTIGEN POPULATIONS DEFINED BY MONOCLONAL ANTIBODIES

The relationships between the antigens recognized by four monoclonal anti‐human ‘Ia’‐like antibodies were investigated using sequential immunoprecipitation and capping techniques. Two of the antibodies were ‘monomorphic’ and have previously been shown to recognize epitopes in which carbohydrate residues are involved, whereas the two ‘polymorphic’ antibodies recognized protein‐defined epitopes—one of these epitopes being present on MB⁺DR^‐ molecules. In the absence of an indisputable anti‐DR monoclonal antibody, it was not possible to conclusively verify which ‘Ia’‐encoded antigens were detected by the anti‐‘Ia’‐like monoclonal antibodies. Nevertheless, several firm conclusions could be drawn: (a) so‐called ‘monomorphic’ antibodies do not necessarily react with all ‘Ia’ molecules encoded by a single locus—from the results using the two monomorphic antibodies, B5.1 and 3F1.1, described herein, two populations of antigens being B5.1⁺3F1.1⁺ and B5.1⁺3F1.1^‐ were identified; (b) cross‐reactivity of a polymorphic determinant expressed on antigenically‐separable ‘Ia’ molecules was noted—using the two polymorphic antibodies, 26.1 and F5C9, molecules which were 26.1⁺F5C9⁺ and 26.1^‐F5C9⁺ were identified; and (c) the data clearly point to the existence of at least two loci coding for ‘Ia’‐like antigens (one of which may or may not be the HLA‐DR locus). Given that polymorphisms can now include protein‐ and carbohydrate‐defined epitopes, that cross‐reactions occur and that the definition of DR itself by monoclonal antibodies is not clear, the complexity of the human ‘Ia’ antgens is apparent.