The thymus contains a high frequency of cells that prevent autoimmune diabetes on transfer into prediabetic recipients

Abstract

Rats of the PVG.RT1u strain develop autoimmune diabetes when thymectomized at 6 wk of age and are rendered relatively lymphopenic by a cumulative dose of 1,000 rads 137Cs gamma-irradiation given in four split doses. Previous studies have shown that the disease is prevented by the intravenous injection of 5 x 10(6) CD4+ CD45RC-TCR alpha beta+ RT6+ peripheral T cells from normal syngeneic donors. These cells have a memory phenotype and are presumably primed to some extrathymic antigen. However, we now report that the CD4+ CD8- population of mature thymocytes is a very potent source of cells, with the capacity to prevent diabetes in our lymphopenic animals. As few as 6 x 10(5) of these cells protect approximately 50% of recipients and the level of protection increases with cell dose. It appears that one characteristic of the intrathymic selection of the T cell repertoire is the generation of cells that regulate the autoimmune potential of peripheral T cells that have been neither clonally deleted intrathymically nor rendered irreversibly anergic in the periphery.

Figure 1

CD4⁺CD8⁻ thymocytes are more potent than peripheral CD4⁺CD45RC⁻ T cells in protecting from diabetes. T cells subsets were negatively selected from TDLs or thymus from normal PVG.RT1^u rats by rosette depletion as described in Material and Methods. Three different doses of CD4⁺CD45RC⁻ peripheral T cells and five doses of CD4⁺ CD8⁻ thymocytes were injected i.v. into recipient groups of prediabetic rats shortly after their last irradiation. Results are expressed as percentage of animals that are protected from development of diabetes and are a compilation of 10 individual experiments involving, with controls, a total of 136 animals. Numbers in parenthesis represent the aggregate number of animals in each group. The purity of isolated cells was >95%.

Figure 2

Naive CD4⁺CD8⁻ thymocytes are less potent in providing in vivo B cell help than primed peripheral CD4⁺ T cells. Primed CD4⁺ T cells and primed B cells were negatively selected from TDLs from PVG.RT1^c rats primed with 10⁸ SRBC 4–9 wk before cannulation. Naive CD4⁺CD8⁻ thymocytes were negatively selected from thymus of normal PVG.RT1^c rats by rosette depletion as described in Materials and Methods. Syngeneic recipients were given 800 rads and injected i.v. 1 d later with 10⁸ SRBC, 3 × 10⁷ SRBC-primed B cells and either three different doses of SRBC-primed CD4⁺ peripheral T cells or three different doses of naive CD4⁺CD8⁻ thymocytes. The controls were given 10⁸ SRBC and 3 × 10⁷ SRBC-primed B cells. The recipients were bled 7 d later and the anti-SRBC antibody titers determined by radioimmunoassay. The results are expressed in arbitrary units made by comparison with a pool of sera from rats immunized with SRBC.