The toleration of rat thymocytes to xenogeneic erythrocytes: kinetics of induction and recovery

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

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.

CD4+CD8- thymocytes that express L-selectin protect rats from diabetes upon adoptive transfer

Previous studies have shown that insulin-dependent diabetes can be induced in normal PVG.RT1u rats by a protocol of adult thymectomy and irradiation. The injection of CD4+ T cells from non-irradiated syngeneic donors prevents the onset of disease in approximately 50% of pre-diabetic recipients but all rats are protected if a particular subset of CD4+ cells is transferred. These protective cells express TCR alpha beta and have a memory phenotype, being CD45RClow RT6+. Further studies have demonstrated that the transfer of CD4+CD8- thymocytes, like that of unfractionated CD4+ peripheral T cells, also protects approximately half of recipients from diabetes suggesting that, as with the peripheral T cells, a functional heterogeneity may exist amongst CD4+CD8- thymocytes. In this study, we show that L-selectin is expressed by 50-60% of all CD4+CD8- thymocytes from 6-week-old rats. Adoptive transfer of these populations into thymectomized and irradiated rats revealed that the protection from diabetes observed by CD4+CD8- thymocytes was mediated almost entirely by the L-selectin+ subset. Cells with this phenotype were also able to mediate both humoral and cell mediated responses, providing primed B cells with help for secondary antibody responses and mediating local graft-versus-host reactions. L-selectin- CD4+CD8- thymocytes failed to mediate these responses. These data indicate that CD4+CD8- thymocytes must mature to the stage of L-selectin expression, before they can mediate normal T cell function. The implications of these results are discussed with respect to the possible role of murine NK1.1+ thymocytes in the control of autoimmunity.

An intermediate cell in thymocyte differentiation that expresses CD8 but not CD4 antigen

Thymocytes of CD4-,CD8+,OX-44- phenotype have been shown to be an intermediate of thymopoiesis that give rise to cells of CD4+, CD8+, OX-44- normal cortical thymocyte phenotype both in vitro and in vivo during thymic regeneration.

The MRC OX-44 antigen marks a functionally relevant subset among rat thymocytes

A monoclonal antibody called MRC OX-44 is described that labels all myeloid cells and peripheral lymphoid cells but only 12% of thymocytes. The OX-44+ thymic cells include most if not all cells found in the medulla but only a small fraction of the cortical cells. Together with CD4 and CD8 antigens, seven subsets of thymic cell were defined and it was notable that most CD4- CD8- cells were OX-44+ whereas almost all CD4+ CD8+ cells were OX-44-. In functional tests, the OX-44+ cells accounted for all proliferation by thymocytes when stimulated by allogeneic spleen cells or concanavalin A plus growth factors and OX-44- cells were completely negative in these assays. Also, in tests for thymopoiesis after intra-thymic injection of cells, all activity was OX-44+. It seems possible that the OX-44+ set may include all functionally relevant cells in the rat thymus.

Contribution of thymus lymphocytes to the peripheral lymphoid tissues and the effect of antigen on the rate of cell exit from the thymus

Some of the important questions concerning the development of T cells in the thymus can be answered by a study of the different thymocyte subpopulations and a comparison of their properties with those of the cells exported to the peripheral lymphoid tissues. What is the relationship between cortical and medullary thymocytes? Why do most cortical cells die? Which subpopulation gives rise to thymus migrants? How many cells are exported from the thymus? Are the exported cells fully mature? Are any of these functions affected by antigen stimulation or other peripheral events? In this paper we review the background to some of these questions and focus on the effect of peripherally administered antigen on the export of cells from the thymus. Experimental data are presented which suggest that the overall rate of emigration is not grossly affected by large doses of intravenous protein antigens. Nor is there any obvious qualitative change, at least in terms of the size of the cells released. The possibility of changes in the specificity of the exported cells is discussed, but as yet there are no data which throw light on this point.

MRC OX-22, a monoclonal antibody that labels a new subset of T lymphocytes and reacts with the high molecular weight form of the leukocyte-common antigen

A mouse monoclonal antibody (MRC OX-22) is described that labels rat T cells which mediate graft-versus-host reactions and those responsible for the suppression of antibody synthesis in hosts undergoing these reactions. In contrast, most of the T cells that provide help for B cells are MRC OX-22 negative. These results, taken together with those published previously, demonstrate that the rat contains at least three phenotypically and functionally distinct subsets of T cells. The MRC OX-22 antibody also labels all B cells, 50% of bone marrow cells, but only 2% of thymocytes. Of these latter cells about half are found at the edge of the medulla and the remainder are randomly distributed throughout the cortex and medulla. These findings lend support to the view that mature thymocytes leave the thymus at the cortico-medullary junction, and also suggest that both cortex and medulla may be sites where thymocytes mature. Biochemical studies showed that the MRC OX-22 antibody reacts with the high molecular weight form of the leukocyte-common antigen (L-CA). Comparison with data on human L-CA suggests that the molecular and antigenic heterogeneity of this set of glycoproteins has been conserved between rat and man.

Functional clonal deletion of cytotoxic T-lymphocyte precursors in chimeric thymus produced in vitro from embryonic Anlagen

Chimeric thymus, formed by fusing the prelymphoid third pharyngeal pouches of fetal mice with fetal liver, have been allowed to develop entirely in vitro. Syngeneic and allogeneic chimeras were prepared and both types of thymus were shown to contain substantial numbers of functional cytotoxic T lymphocyte precursors reactive against "third party" alloantigens. However, alloreactivity specific for H-2 antigens present on either the third pharyngeal pouch or the fetal liver was minimal. In three different allogeneic chimeric thymuses, the frequencies of cytotoxic T lymphocyte precursors reactive to H-2 antigens present on the third pharyngeal pouches were reduced to 1%, 4%, and 0% of control values, whereas, in the one allogeneic chimera tested for alloreactivity to H-2 antigens present on the fetal liver, the cytotoxic T lymphocyte precursor frequency was reduced to less than 1% of control values. The phenotype of the H-2 tolerance is shown to be one of functional clonal deletion of the cytotoxic T lymphocyte precursor.

A spleen-thymus interaction is involved in the tolerization of thymocytes to xenogeneic erythrocytes

Previous work has shown that thymocytes of rats receiving two i.v. injections, 4 days apart, of large numbers of xenogeneic erythrocytes become tolerant of the injected cells whereas peripheral T cells in the same recipients make a vigorous response to the injected antigen. Results are herein presented of experiments on the effect of splenectomy on the induction of thymocyte tolerance produced in this way. Splenectomy performed either immediately before the first injection of erythrocytes, or between the two injections, completely prevented tolerance induction and tolerance was still incomplete when splenectomy was performed as late as a few hours after the second injection of tolerogen. Sham splenectomy had no effect on the erythrocyte-induced thymocyte tolerance. The results demonstrate an essential role for the spleen in the tolerization of thymocytes in this system. Possible mechanisms and the implications for self tolerance are discussed.

Mouse monoclonal antibodies against rat major histocompatibility antigens. Two Ia antigens and expression of Ia and class I antigens in rat thymus

A rat Ia (RT-1B) antigen (called Ia-A) equivalent to the mouse I-A product has been defined with mouse monoclonal antibodies (W. R. McMaster and A. F. Williams, Eur. J. Immunol. 1979. 9: 426). To identify other Ia antigens mouse monoclonal antibodies were raised against rat spleen glycoproteins depleted of the Ia-A antigen. An IgG antibody (called MRC OX17) was obtained and used to purify a molecule which had a similar structure to the Ia-A antigen and reacted with anti-Ia alloantibodies. There was no cross-reaction between the two Ia glycoproteins in assays with mouse monoclonal antibodies, alloantibodies or rabbit antibodies. In one alloantiserum almost all the detectable anti-Ia antibodies reacted with a mixture of the two Ia glycoproteins. The MRC OX17 antibody did not bind to mouse cells, but rabbit antibodies to the pure rat glycoprotein cross-reacted and recognized determinants mapping to the mouse I-E region. In the thymus the rat Ia-E antigen was on cortical epithelial and medullary reticular cells. An IgG monoclonal antibody (MRC OX18) to isotypic determinants of rat histocompatibility RT-1A antigens was also produced and used to analyze these antigens on thymus cells. The heavily labeled thymocytes were those with characteristics of mature T lymphocytes. Cortical epithelial cells and medullary dendritic-like cells were also RT-1A positive.