INDUCTION OF ALLOGRAFT NONRESPONSIVENESS AFTER INTRATHYMIC INOCULATION WITH DONOR CLASS-I ALLOPEPTIDES - II - EVIDENCE FOR PERSISTENT CHRONIC REJECTION DESPITE HIGH-LEVELS OF DONOR MICROCHIMERISM

We have recently demonstrated that three synthetic peptides correspond ing to the donor class I RT1.A(a) molecule induce long-term survival o f cardiac allografts in the PVG,R8-to-PVG,1U rat strain combination (d isparate for one isolated class I, RT1.A, molecule) when presented to the recipient immune system in the thymus, Long-term graft survivors h ad measurable levels of donor-reactive alloantibodies in their serum, In this study, we examined long-term allografts for the presence of ch ronic rejection and donor microchimerism to assess whether this regime n of immune modulation establishes true tolerance and whether this tol erance is dependent upon the presence of donor-recipient microchimeris m. Histological examination of long-term heart grafts (>100 days) demo nstrated chronic rejection, including a mild degree of myocardial infi ltration by mononuclear cells, mild to moderate myocardial fibrosis, a nd various vascular changes ranging from focal intimal thickening to t otal vascular lumen blockade due to smooth muscle cell proliferation, In contrast, long-term syngeneic hearts transplanted under similar exp erimental conditions lacked these pathological manifestations, Donor m icrochimerism was analyzed using the polymerase chain reaction with a pair of oligonucleotides specific for the donor class I RT1,A(a) gene and genomic DNA harvested from various tissues from graft recipients, We detected high levels of donor microchimerism in the heart, kidney, liver, skin, bone marrow, thymus, and lymph nodes of long-term graft r ecipients, Donor microchimerism was also detected in unmanipulated con trol graft recipients at rejection (7 days) and in intrathymically man ipulated recipients that rejected allografts in a delayed fashion (12- 82 days), These data clearly demonstrate that intrathymic inoculation of donor class I allopeptides induces long-term graft survival but doe s not prevent chronic rejection, Allograft rejection occurred despite high levels of donor microchimerism, providing direct evidence that do nor-recipient microchimerism is not sufficient for the prevention of a cute or chronic rejection in this model.