Mechanisms of tolerance to parental parathyroid tissue when combined with human allogeneic thymus transplantation

Abstract

Background: The induction of tolerance toward third-party solid organ grafts with allogeneic thymus tissue transplantation has not been previously demonstrated in human subjects.

Objective: Infants with complete DiGeorge anomaly (having neither thymus nor parathyroid function) were studied for conditions and mechanisms required for the development of tolerance to third-party solid organ tissues.

Methods: Four infants who met the criteria received parental parathyroid with allogeneic thymus transplantation and were studied.

Results: Two of 3 survivors showed function of both grafts but subsequently lost parathyroid function. They demonstrated alloreactivity against the parathyroid donor in mixed lymphocyte cultures. For these 2 recipients, parathyroid donor HLA class II alleles were mismatched with the recipient and thymus. MHC class II tetramers confirmed the presence of recipient CD4(+) T cells with specificity toward a mismatched parathyroid donor class II allele. The third survivor has persistent graft function and lacks alloreactivity toward the parathyroid donor. All parathyroid donor class II alleles were shared with either the recipient or the thymus graft, with minor differences between the parathyroid (HLA-DRB1∗1104) and thymus (HLA-DRB1∗1101). Tetramer analyses detected recipient T cells specific for the parathyroid HLA-DRB1∗1104 allele. Alloreactivity toward the parathyroid donor was restored with low doses of IL-2.

Conclusion: Tolerance toward parathyroid grafts in combined parental parathyroid and allogeneic thymus transplantation requires matching of thymus tissue to parathyroid HLA class II alleles to promote negative selection and suppression of recipient T cells that have alloreactivity toward the parathyroid grafts. This matching strategy may be applied toward tolerance induction in future combined thymus and solid organ transplantation efforts.

Figure 1

Parathyroid graft function in Subject 4 after combined parental parathyroid with allogeneic thymus transplantation. Calcium supplementation was weaned off by 15 days after transplantation. The subject remains off calcium supplementation. Intact PTH levels are shown vs. time after parathyroid transplantation. The lower limit of normal (15 pg/ml) is demarcated.

Figure 2

Mixed lymphocyte cultures. Non-irradiated responder PBMCs were co-cultured with irradiated autologous or parathyroid (PT) donor PBMCs. Results represent cpm of ³H-thymidine incorporation. For Subject 4, healthy adult control PBMC responses to irradiated PBMCs from the PT donor for Subject 4 and responses by Subject 4 and the control to pooled allogeneic PBMCs are shown for comparison. **p < 0.005.

Figure 3

Tetramer analyses. Subjects 1 and 4 were tested for CD4⁺ T cells with allorecognition toward parathyroid donor oligopeptides (from the HLA-DRB1 allele not shared with the recipient) presented within recipient-type MHC class II tetramers (“positive”). “Negative” tetramers contained an antigenically irrelevant peptide. Percentages of tetramer-positive cells are shown, gated on CD4⁺ T cells.

Figure 4

Assessments for anergic alloreactive T cells. Subject 4 and control PBMCs were cultured with irradiated PBMCs (autologous, parathyroid (PT) donor, or allogeneic) or in medium without irradiated cells. In parallel, IL-2 was added to Subject 4 PBMCs in all four conditions. Results (incorporated ³H-thymidine) are shown at 3.3 years after transplantation. NS, p > 0.05; *p < 0.05; **p < 0.005.