The alloantibody response to semiallogeneic pregnancy in the rat. II. Antibodies in serum directed to multiple epitopes on conventional RT1A antigens.

Evidence for a unique class I MHC antigen (termed Pa), which is believed to be expressed on rat trophoblast during pregnancy and which stimulates alloantibody formation with unusual interstrain cross-reactivity, has been examined in inbred rats. The previously reported pattern of crossreactivity was confirmed but was not unique to antisera produced by pregnancy. Antibody blocking studies using biotinylated rat monoclonal antibodies to distinct epitopes on RT1Aa antigens suggested that antibodies present in pregnancy sera, especially from multiparous rats, reacted with several epitopes on these molecules. Moreover, a rat monoclonal antibody, 381- 1E10, directed against the putative Pa epitope was shown by synergistic lysis and cold antibody competition to be directed to the immunodominant S epitope on RT1Aa. These data argue against the existence of a distinct Pa antigen or epitope detected by pregnancy sera.