Lymphocyte alloantigens of the horse. III. ELY-2.1: a lymphocyte alloantigen not coded for by the MHC.

A new polymorphic locus of the horse which has several unusual properties is described. The suggested name for the locus is ELY-2. The gene product of one allele at this locus, designated ELY-2.1, has been identified with antisera raised as a result of pregnancy. Antibody to ELY-2.1 was first detected on day 55 after conception in the serum of a mare in first pregnancy. This early onset of antibody is similar to that seen for antibody to ELA antigens, and suggests that the source of the antigenic stimulus may be the tissue of the equine endometrial cups. The antisera identifying ELY-2.1 are cytotoxic and kill all peripheral blood lymphocytes from horses carrying the antigen. ELY-2.1 is a cell surface molecule expressed on lymphocytes, erythrocytes, and platelets. Other cell types have not been investigated. The overall phenotypic frequency of ELY-2.1 in several horse breeds was 16%. The ELY-2.1 antigen is controlled by an autosomal, dominant gene which is not coded by the ELA region (the major histocompatibility complex of the horse), nor is it identical to the ELY-1 locus, which codes for another cell surface alloantigen of equine lymphocytes. Stimulator cells carrying ELY-2.1 did not induce proliferation of ELY-2.1 negative responder cells in mixed cultures of horse lymphocytes. Attempts to raise alloantisera to other alleles of the ELY-2 locus through immunization with lymphocytes were unsuccessful. It is possible that the alternate allele(s) does not code for a gene product which is expressed. The function and biochemical nature of the ELY-2.1 molecule are unknown.