Canine marrow transplantation: are serum blocking factors necessary to maintain the stable chimeric state?

Nine long-term canine radiation chimeras and their canine histocompatibility locus (DL-A) compatible, mixed leukocyte culture negative littermate marrow donors were studied between 545 and 1282 days after 1,200 R total body irradiation and marrow grafting. Before the time of testing, marrow donors were immunized against their chimeras by repeated skin grafts which they rejected. Skin fibroblasts from chimeras and their donors were tested for cell inhibition (CI) in the microcytotoxicity assay be exposure to lymphocytes from chimeras, donors and normal dogs and the effects of various sera on CI were evaluated. Lymphocytes from sensitized marrow donors consistently inhibited fibroblasts from the chimeras (eight of nine dogs); CI was abrogated by chimeric serum in only three of eight cases. Only two chimeras showed consistent CI of their "own" fibroblasts; CI was blocked by chimeric serum in one of the two. The remaining seven chimeras did not show consistent CI. Sequential studies in 16 additional recipients of DL-A compatible littermate marrow were carried out from 45 to 439 days after marrow grafting. Seven of the 16 did not show CI of chimeric fibroblasts by chimeric lymphocytes at any time. Nine showed CI on one or several occasions. Serum blocking factors were seen on one occasion in each of two chimeras. In conclusion, the CI assay is able to detect immunity against "minor" histocompatibility systems in dogs. Both long- and short-term chimeras occasionally demonstrated CI of chimeric fibroblasts but serum blocking factors did not appear to be necessary for maintaining stable graft-host "tolerance."