Transplantation analysis of B cell destruction in (NOD x CBA)F1 mouse bone marrow chimeras

Abstract

F1 hybrids produced by outcross of non-obese diabetic (NOD) mice with diabetes resistant strains are also diabetes resistant. This resistance is abrogated if F1 hybrids are lethally irradiated and then haematopoietically reconstituted with NOD bone marrow. This model was employed to determine whether T lymphocyte recognition and elimination of pancreatic B cells in NOD mice is restricted by the MHC haplotype of the target B cell. Diabetes resistant (NOD/Lt x CBA/J)F1 hybrids were lethally irradiated and reconstituted with NOD/Lt bone marrow. Following haematopoietic reconstitution, donor matched NOD/Lt or CBA/J pancreatic islet and anterior pituitary grafts were grafted under a renal capsule to determine whether effector cells derived from NOD/Lt marrow progenitors would reject islet grafts in a MHC restricted fashion. The H-2^k haplotype expressed by CBA/J mice differs from all known loci of the unique H-2 haplotype of NOD; therefore, if NOD/Lt T lymphocytes eliminate pancreatic B cells in a MHC restricted fashion, NOD islet grafts would be eliminated in these chimeras while CBA islet grafts would be retained. Overt diabetes developed in 80% of the female and 40% of the male F1 hybrids following reconstitution with NOD/Lt marrow, while no hybrids reconstituted with CBA/J marrow became diabetic through a year of age. The retention of CBA/J skin and pituitary grafts in NOD/Lt marrow reconstituted F1 hybrids confirmed that the F1 thymic environment imparted tolerance to CBA/J alloantigens. Nonetheless, responses to a T cell dependent model antigen were restricted to the unique MHC haplotype of NOD. This was associated in the hyperglycaemic chimeras with rejection (8–21 days post-implantation) of both CBA/J and NOD/Lt islet grafts. This indicated generation of islet specific effectors that were not restricted by the MHC haplotype of the target B cell. Islet specific effectors were less active in those chimeras remaining normoglycaemic following NOD/Lt marrow reconstitution, since both NOD/Lt and CBA/J islet grafts remained intact 21 days post-implantation. Islet and pituitary grafts of each genotype remained free of leucocytic involvement in CBA/J marrow reconstituted F1 hybrids. The generation of B cell specific, but not genotype specific, effectors elicited from NOD/Lt marrow was age-dependent. Following priming with porcine insulin in vivo, T lymphocytes from 8-weekold NOD male mice failed to respond to porcine insulin in vitro, while primed T lymphocytes isolated from 16-weekold mice proliferated vigorously. This suggested that as pancreatic insulitis progressed in aging NOD/Lt mice, T lymphocytes capable of responding to B cells in a tissue specific, but not genotype specific fashion, were activated. Thus, we propose that immunological effectors derived from NOD/Lt, but not CBA/J marrow, are capable of eliminating islet grafts in non-MHC restricted fashion. However, this population could be activated as a consequence of B cell erosion initated in the pancreas by another class of effectors which may be restricted by the MHC haplotype of the target B cell.