CD8+ T cells induce medullary thymic epithelium and CD4+CD8+CD25+ TCRbeta- thymocytes in SCID mice

Pathogenesis of autoimmunity after xenogeneic thymus transplantation

Thymus transplantation is a promising strategy to induce xenotolerance, but may also induce an autoimmune syndrome (AIS). The pathogenesis of this AIS was explored using nude rats as recipients. Thymus grafts consisted of fetal hamster thymic tissue with or without mixing with fetal rat tissue such as thymus, thyroid, salivary gland, and heart. All hamster thymus recipients died of AIS within 2-3 mo. In most recipients of xenothymus mixed with rat tissues such as thymus, thyroid, and salivary gland, but not heart, AIS was prevented, indicating an insufficient presence of rat epithelial cell Ags within the xenothymus. AIS could be transferred to control nude rats by whole splenocytes or by splenocyte subpopulations such as CD3(+), CD3(-), and B lymphocytes, but not by non-T, non-B cells from AIS animals. This transfer could be suppressed by cotransferring either CD4(+) or CD8(+) lymphocytes from euthymic rats, but not by splenocytes from recipients of syngeneic or xenogeneic thymus mixed with rat tissue, indicating a defective generation of regulatory lymphocytes. As for CD4(+) regulatory cells this defect was probably qualitative, because the percentages of CD4(+)CD25(+) or CD4(+)CD45RC(low) populations were normal after xenothymus transplantation. As for the CD8(+) regulatory cells, the defect was quantitative, as CD8(+) cell levels always remained low. The latter was related to the nonvascularized nature of thymus grafts. In conclusion, AIS after xenothymus transplantation in nude rats is due to a combination of insufficient intrathymic presence of host-type epithelial cell Ags and a defective generation of regulatory T lymphocytes.

Long-term CD4+ and CD8+ memory T cells developed in severe combined immunodeficiency mice during homoeostasis exhibit differences in sensitivity to antigen

T cells transferred in small numbers to lymphopenic hosts proliferate spontaneously, and naïve T cells turn into memory cells without complete cellular reconstitution of the lymphoid compartment. In this study, neonatal severe combined immunodeficiency mice were treated with peripheral CD4+ or CD8+ T cells purified from the spleen of syngeneic C.B-17 mice. At 2 weeks and more pronounced at 10 weeks post treatment, a majority of the residing donor T cells showed memory phenotype, with high expression of CD44 and an early onset of proliferation and cytokine production upon stimulation. These memory type of donor cells were sustained in numbers for at least 1.5 years post treatment in a homoeostatic fashion, recognized by normal CD4/CD8 ratio and no bias towards type 1 or type 2 immune response. Furthermore, amongst the memory type of cells, there was a striking difference in their response, where the CD8+ donor cells had higher threshold for stimulation than the CD4+ donor cells.