Interspecific chimerism--the characterization and immunological responsiveness of Bos taurus-Bos indicus haemopoietic chimeras produced by embryo transfer.

Bos taurus-Bos indicus twin calves were produced by embryo transfer and their degree of haemopoietic chimerism and immune responsiveness examined. All 10 pairs of Friesian-Brahman twins were chimeric compared with only 6 out of 12 pairs of Jersey-Brahman twins. In the Friesian-Brahman twins, Friesian lymphocytes predominated in all calves while in the Jersey-Brahman twins there was no uniform dominance of one cell type over another. All calves that had chimeric lymphocyte populations were also erythrocyte chimeras, although in two pairs of twins the genotype of the erythrocytes was different from the genotype of the lymphocytes. The lymphocytes of all chimeric calves failed to respond in mixed lymphocyte culture (MLC) tests to the lymphocytes of their co-twins but gave vigorous responses to cells from other calves. Primary and secondary immune responses to keyhole limpet haemocyanin (KLH) antigen were studied in the efferent prefemoral lymph in normal Jersey, Friesian and Brahman calves, in 1 set of Jersey-Brahman twins and in 5 sets of Friesian-Brahman twins. The immune responses of chimeric twins with genetically identical lymphocyte populations were quite dissimilar and were characteristic of the somatic genotype of the calf and not of the genotype of the lymphocyte population. The chimeric status of the cell population in the lymph did not change significantly during the response. These results suggest that either the genetic capacity of a population of lymphocytes to recognize and respond to antigens can be modified during ontogeny in an alien genetic environment or that a crucial determinant of the final outcome of an immune response is not the immunocompetent cell itself but the physiological and anatomical environment in which it exists.