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A panel of crossreactive anti‐human, ‐mouse and ‐rat MHC class II monoclonal antibodies (Mabs) was used to examine MHC class II antigen expression on canine T lymphocytes by cytofluorometry. The presence of MHC class II antigens was demonstrated on activated T lymphoblasts as well as on non‐stimulated peripheral blood T lymphocytes. A number of anti‐MHC class II Mabs reacted only with activated T lymphoblasts. Immunoprecipitation studies confirmed the Ia‐like or MHC class II molecular character of the antigens on canine T cells. The expression of MHC class II antigens on peripheral blood T lymphocytes appeared to be not induced by stimulation of the T cells, as purified T lymphocytes of specific pathogen free dogs reacted with anti‐MHC class II Mabs. Moreover, the study indicates that MHC class II antigen expression is present in the neonatal thymus. Lectin stimulated and allogeneically stimulated T lymphoblasts showed a stronger expression of MHC class II antigens in comparison with non‐stimulated T cells.

The Nethe-lands MHC-class I1 antigens on canine lymphoid cells were detected with monoclonal antibodies (Mabs). On the basis of reactivity with resting or activated T lymphocytes two groups of Mabs were reported earlier. Non-activated T lymphocytes expressed MHC-class I1 antigens recognized by the first group whereas the second group recognized only MHC-class I1 antigens on activated T lymphocytes. In this study we analysed the reactivity pattern of both groups of Mabs with purified canine B lymphocytes. One-and two-colour immunoflowcytometric analysis, immunoprecipitation, immunohistology and MLC inhibition were performed. The Mabs which only reacted with activated T lymphocytes appeared to stain also purified sIg' lymph node cells. Two-colour fluorescence and immunohistology confirmed the reactivity of these Mabs with B lymphocytes. Immunoprecipitation showed the bimolecular structure of these antigens. It was concluded that two subsets of MHCclass I1 antigens can be detected on canine lymphoid cells. One subset showed an aberrant distribution being expressed on both nonactivated and activated T and on B lymphocytes. The second subset of MHC-class I1 antigens could only be detected on B lymphocytes and activated T lymphocytes, a distribution pattern similar to that found in most other species, for instance man.

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