Deficit of specific thymus-dependent lymphocytes in transplantation tolerance in the rat

Transplant Tolerance, Not Only Clonal Deletion

The quest to understand how allogeneic transplanted tissue is not rejected and how tolerance is induced led to fundamental concepts in immunology. First, we review the research that led to the Clonal Deletion theory in the late 1950s that has since dominated the field of immunology and transplantation. At that time many basic mechanisms of immune response were unknown, including the role of lymphocytes and T cells in rejection. These original observations are reassessed by considering T regulatory cells that are produced by thymus of neonates to prevent autoimmunity. Second, we review "operational tolerance" induced in adult rodents and larger animals such as pigs. This can occur spontaneously especially with liver allografts, but also can develop after short courses of a variety of rejection inhibiting therapies. Over time these animals develop alloantigen specific tolerance to the graft but retain the capacity to reject third-party grafts. These animals have a "split tolerance" as peripheral lymphocytes from these animals respond to donor alloantigen in graft versus host assays and in mixed lymphocyte cultures, indicating there is no clonal deletion. Investigation of this phenomenon excludes many mechanisms, including anti-donor antibody blocking rejection as well as anti-idiotypic responses mediated by antibody or T cells. This split tolerance is transferred to a second immune-depleted host by T cells that retain the capacity to effect rejection of third-party grafts by the same host. Third, we review research on alloantigen specific inhibitory T cells that led to the first identification of the CD4+CD25+T regulatory cell. The key role of T cell derived cytokines, other than IL-2, in promoting survival and expansion of antigen specific T regulatory cells that mediate transplant tolerance is reviewed. The precise methods for inducing and diagnosing operational tolerance remain to be defined, but antigen specific T regulatory cells are key mediators.

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

The intravenous injection of large doses of xenogeneic erythrocytes into rats completely abrogated the ability of thymocytes from such animals to provide help for antierythrocyte antibody responses in an adoptive transfer system. Thymocyte tolerance developed at a time when the thymocyte donors were producing antibody to the tolerizing antigen, and spleen helper T cells were not tolerized by the injection protocol. A minimum of two injections of 1 ml of packed erythrocytes, spaced 4 - 5 days apart, was required to induce loss of helper activity. Tolerance was fully developed within 16 h of the last injection of antigen. Helper activity recovered over a 24-h period starting 5 days after this injection but could be delayed by giving further injections of antigen. These results suggest that the intrathymic pool of cells with assayable helper activity turns over in about one day and that precursors of this pool are probably also tolerable. Suppressor cells could not be detected in tolerant thymocyte populations indicating that the loss of helper activity was either a consequence of clonal inactivation or that an antigen-specific mechanism existed for the recruitment of helper cells from the thymus. The significance of these findings to the acquisition of self-tolerance is discussed.

Recirculating, suppressor T cells in transplantation tolerance

An adoptive transfer system was used to examine the capacity of cellular inocula from rats fully tolerant of Ag-B antigens to transfer tolerance to irradiated recipients. Permanent tolerance in these irradiated recipients involved specific suppression of the regenerating immune response. Cells obtained from tissues rich in recirculating lymphocytes were the most effective suppressors. Highly purified inocula of T cells from tolerant donors were potent suppressors in irradiated hosts, but were not capable of direct suppression of peripheral antigen-sensitive T cells.. The role of the thymus in maintaining the complement of recirculating suppressor T cells in tolerant animals was examined after adult thymectomy. Thymectomized tolerant rats did not reject their tolerated grafts, and the longevity of the suppression in tolerant rats was confirmed by showing that adoptive transfer of cells from thymectomized tolerant donors was effective in suppressing irradiated recipients up to 180 days after thymectomy. Cellular inocula from these donors appeared to lose their suppressor function marginally faster than they lost effector function (as measured by their capacity to mediate rejection of third party control grafts). Thymectomy made tolerant rats more vulnerable to the termination of tolerance by challenge with normal cells. Transplantation tolerance is maintained in adult rats by long-lived rapidly recirculating suppressor T cells. The target for the suppressor action of these cells is probably the precursor of alloantigen-sensitive lymphocytes, and the effect of suppression may be deletion or inactivation of the relevant clone of these cells.

Persisting T cells in rats tolerant of human serum albumin. The significance of tolerant and nonimmune T cells which preferentially restrict high affinity antibody synthesis

The adoptive response of primed rat thoracic duct lymphocytes ('TDL) following specific antigen challenge (soluble human serum albumin, s-HSA) was restricted when cells were transferred into syngeneic, adult (AS2 X AS)F1 hybrid recipients in comparison with irradiated hosts. This adoptive memory response was also inhibited in irradiated recipients by transferring nonimmune TDL along with 'TDL. Recirculating B cells (B-TDL) did not inhibit the 'TDL response, indicating that the adoptive secondary response was regulated by T cells. Antibody synthesis was preferentially restricted in the high affinity memory cell precursor population, demonstrating a role for T cells in regulating the maturation of antibody affinity. The adoptive memory response was liberated from this T regulatory effect in adult recipients when hosts were challenged with the alum-precipitated adjuvant form (HSA-adj) rather than the soluble form of HSA. Since the adoptive memory response was sensitive to the presence or absence of T cells, this experimental model was used to determine whether or not T cells were eliminated from HSA-tolerant rats. Antibody synthesis by 'TDL was reduced approximately 10-fold compared with controls when transferred into tolerant recipients and challenged with either s-HSA or HSA-adj; a similar reduction was not observed by substituting bovine serum albumin (BSA) 'TDL and challenging with s-BSA. The tolerance-induced inhibition of HSA 'TDL was destroyed by irradiation and TDL from HSA-tolerant donors were more effective than normal nonimmune TDL in reducing the adoptive HSA 'TDL response. HSA-tolerant TDL did not inhibit the BSA 'TDL response significantly. The results indicate that T cells are not eliminated by tolerance induction in this model and after interaction with tolerogen may exert an active (or competitive) role in restricting antibody synthesis by high affinity B memory cell precursors. However, the fact that tolerant T cells are not able to prevent a primary response suggests that unresponsiveness to HSA in the T compartment represents a functional deficiency and not an active suppression at this level. Nevertheless, the presence of these tolerant cells probably accounts for the failure of antibody affinity to mature in partially tolerant rats.

Cellular basis of tolerance in neonatally induced mouse chimeras

In mice, thymectomized as young adults, neonatally induced tolerance persists in the putative absence of cell chimerism. The finding provides evidence that a selective deficiency of specific clones of lymphocytes exists in transplantation tolerance when induced under the conditions of these experiments.