Irradiated and thymectomized CBA mice are markedly depressed in several immunological parameters (skin homograft rejection, graft-vs.-host activity and hemolytic plaque-forming cells of the spleen, hemolysin and hemagglutinin formation, and peripheral lymphocyte counts). In the present experiments the ability of homografts of neonatal thymus placed beneath the kidney capsule to restore immunological capacity of such animals was studied.

Thymus homografts which share the same H-2 locus with the CBA mouse were permanently tolerated and immunological restoration was complete. Skin from the thymus donor was specifically retained, but third party skin with even minor (non-H-2) incompatibility was normally rejected and hemolytic plaque-forming cells of the spleen were restored.

Thymus homografts which differ at the H-2 locus were promptly rejected and led to accelerated rejection of skin subsequently grafted from the thymus donor. With such H-2 incompatible thymus grafts, third party skin with minor histo-incompatibility was retained while there was slight to moderate restoration of rejection of skin with major (H-2) incompatibility. Graft-vs.-host activity was restored, but there was no return of plaque-forming spleen cells, hemolysins, hemagglutinins, or peripheral lymphocyte counts.

In view of the cross-reactivity at the H-2 locus in CBA mice between thymus and third party skin donors, it was felt that restoration of skin rejection and graft-vs.-host activity could be adequately explained on the basis of immunization by the thymus graft and did not require the postulation of true immune restoration or a thymus hormone.

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