Spleen cells but not the thymus or the bursa cells of chicken embryos suppressed the in vitro mitogenesis of spleen cells of adult syngeneic or allogeneic chickens. The natural suppressor cell activity of embryo spleen was present at embryonation day 16, reached peak levels at embryonation day 18 and disappeared at hatch. The embryo spleen cells did not by themselves respond to phytohemagglutinin stimulation in vitro. The suppressive effect of embryonic spleen cells on adult spleen cells was present when the embryonic cells were added at the time of or after initiation of the adult spleen mitogenic cultures. When the embryonic cells were added to the cultures of adult spleen cells after the blastogenic response of the adult cells had peaked, the embryonic cells inhibited the incorporation of the label into adult spleen cell blasts. The suppressive activity of the embryonic spleen cells was mediated by soluble suppressor product(s) secreted by these cells, and direct cell-to-cell contact between embryonic and adult spleen cells was not necessary for suppression to occur. Infection of embryos with turkey herpesvirus and Marek's disease virus reduced the suppressor cell activity of embryonic spleen, although substantial residual suppressor cell activity remained in virus-infected embryos. Several pathogenic or non-pathogenic isolates of infectious bursal disease virus did not appreciably alter the suppressor cell activity of embryonic spleen cells.
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