IMMUNOCHEMICAL EVIDENCE FOR MULTIPLE ? UNITS OF THE CLASS II MOLECULE IN THE RAT

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Abstract
The RTI‐B/D region‐associated antigens which were serologically defined in the previous study (Ohhashi et al., 1981), were partially purified from membranes of a rat B cell leukaemia, KNL‐14. Sequential immunoprecipitation test, with the partially purified 125I‐B/Dak preparation using four different rat alloantisera, including a monoclonal antibody, disclosed three distinctive populations of β units of the class II molecules. Highly purified β units of three discriminable class II molecules were shown to have different structural properties in terms of molecular weights and of electrophoretic profiles on the isoelectric focusing. The β units shifted to a position of higher molecular weight on SDS‐PAGE under reducing condition, thus suggesting to carry intradisulfide bonds. Furthermore, the highly purified β units crossreacted with murine anti‐Ia sera. The rebinding test revealed that at least two discriminable species of β units cross‐react with anti‐I‐Ak monoclonal antibody, whereas β units purified by binding with the IE4 monoclonal antibody cross‐reacted with anti‐I‐Ab and/or anti‐I‐Ad antiserum. On the basis of structural and antigenic properties, we have postulated that the rat class II region can be divided into at least three subregions, each containing a locus which encodes a distinctive β unit of the class II molecule.