Equine class II MHC antigens: identification of two sets of epitopes using anti‐human monoclonal antibodies

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Abstract
Six mouse and 13 rat monoclonal antibodies (mAb) recognizing HLA‐DR, DQ and DP antigens were used for the detection of cell surface class II MHC antigens of equine lymphocytes. The monoclonal antibodies were tested against peripheral blood lymphocytes (PBL) from a panel of thoroughbred horses, using two‐color fluorescence flow cytometry. Seven of these mAbs reacted with both surface immunoglobulin positive (sIg+) and surface immunoglobulin negative (sIg‐) lymphocytes. sIg+ cells stained consistently brighter than sIg− cells. The fluorescence pattern did not vary from donor to donor for each of the mAbs tested, except for SFR1‐MI.2, which reacted with a variable intensity with cells from 47 of 53 horses tested. Immunoprecipitation with mAb SFR1‐MI.2 and analysis by two‐dimensional electrophoresis demonstrated the presence of light and heavy chains equivalent to HLA class II α and β chains. Antibody N297 (DQ specific), previously shown to react with an epitope expressed on human B cells but not on mitogen‐induced T cells, reacted only with sIg+ cells in 42 of 53 horses tested. The lack of staining of horse sIg− cells with N297 may be due to a low or lack of expression of this determinant on these cells or to a weak cross‐reactivity of this antibody with equine antigens.