Luk Van Parijs

Induction of peripheral T cell tolerance in vivo requires CTLA-4 engagement

Studies of T cell anergy in vitro have led to the widely accepted view that anergy is induced by T cell antigen recognition without costimulation. We show that the induction of T cell anergy in vivo is due to an abortive T cell response that requires recognition of B7 molecules, since blocking B7 maintains T cells in an unactivated but functionally competent state. Furthermore, the induction of anergy is prevented by blocking CTLA-4, the inhibitory T cell receptor for B7 molecules. Thus, in vivo T cell anergy may be induced not because of a lack of costimulation, but as a result of specific recognition of B7 molecules by CTLA-4. In contrast, blocking CD28 on T cells prevents priming but not the induction of tolerance. Therefore, the outcome of antigen recognition by T cells is determined by the interaction of CD28 or CTLA-4 on the T cells with B7 molecules.

The Roles of Costimulation and Fas in T Cell Apoptosis and Peripheral Tolerance

Using cells from TCR transgenic mice that do or do not express Fas, we show that there are two mechanistically distinct forms of apoptosis in CD4+ T cells. Naive T cells undergo apoptosis if cultured in the absence of antigen or costimulation. This form of programmed cell death (PCD) is not dependent on Fas, and is prevented by CD28-mediated signals, which lead to the secretion of growth factors and the expression of survival genes, such as bcl-xL. Recently activated T cells undergo apoptotic death upon repeated stimulation. This activation-induced cell death (AICD) is mediated by Fas, but is independent of costimulation and is not prevented by IL-2 or bcl-xL. Finally, we show that peripheral tolerance may be induced in vivo independent of Fas-mediated cell death.

Role of Fas-mediated cell death in the regulation of immune responses

Interaction of the cell-surface molecule Fas (CD95 APO-1) with its specific ligand triggers apoptotic death of T and B lymphocytes. This pathway is important for eliminating self-reactive lymphocytes and thus preventing autoimmunity. Fas is also involved in controlling injurious lymphocyte reactions in immunologically privileged tissues, and may provide a strategy for reducing graft rejection.

THE FAS/FAS LIGAND PATHWAY AND BCL-2 REGULATE T CELL RESPONSES TO MODEL SELF AND FOREIGN ANTIGENS

We have examined the role of Fas and Bcl-2 in T cell survival and responses to antigen in vivo using T cells that express a transgenic antigen receptor specific for hen egg lysozyme (HEL) and that either lack functional Fas or Fas ligand (FasL) or overexpress Bcl-2 as a transgene. HEL-specific, Bcl-2-transgenic T cells showed prolonged responses to immunization with cognate peptide but were eliminated rapidly when exposed to HEL expressed systemically as a self antigen. In contrast, Fas- and FasL-defective T cells did not display exaggerated responses to immunization with HEL peptide, but did show increased expansion and survival in response to systemic self antigen and were able to activate anti-HEL (self) antibody-forming cells. Thus, Bcl-2 and Fas play different roles in the regulation of T cell responses to antigen in vivo and in self tolerance.

Combined Deficiency of p50 and cRel in CD4+ T Cells Reveals an Essential Requirement for Nuclear Factor κB in Regulating Mature T Cell Survival and In Vivo Function

Signaling pathways involved in regulating T cell proliferation and survival are not well understood. Here we have investigated a possible role of the nuclear factor (NF)-κB pathway in regulating mature T cell function by using CD4+ T cells from p50−/− cRel−/− mice, which exhibit virtually no inducible κB site binding activity. Studies with these mice indicate an essential role of T cell receptor (TCR)-induced NF-κB in regulating interleukin (IL)-2 expression, cell cycle entry, and survival of T cells. Our results further indicate that NF-κB regulates TCR-induced expression of antiapoptotic Bcl-2 family members. Strikingly, retroviral transduction of CD4+ T cells with the NF-κB–inducing IκB kinase β showed that NF-κB activation is not only necessary but also sufficient for T cell survival. In contrast, our results indicate a lack of involvement of NF-κB in both IL-2 and Akt-induced survival pathways. In vivo, p50−/− cRel−/− mice showed impaired superantigen-induced T cell responses as well as decreased numbers of effector/memory and regulatory CD4+ T cells. These findings provide the first demonstration of a role for NF-κB proteins in regulating T cell function in vivo and establish a critically important function of NF-κB in TCR-induced regulation of survival.

Role of interleukin 12 and costimulators in T cell anergy in vivo (Journal of Experimental Medicine (1997) 186, 7, (1119-1128))

The induction of T cell anergy in vivo is thought to result from antigen recognition in the absence of co-stimulation and inflammation, and is associated with a block in T cell proliferation and Th1 differentiation. Here we have examined the role of interleukin (IL)-12, a potent inducer of Th1 responses, in regulating this process. T cell tolerance was induced by the administration of protein antigen without adjuvant in normal mice, and in recipients of adoptively transferred T cells from T cell receptor transgenic mice. The administration of IL-12 at the time of tolerance induction stimulates Th1 differentiation, but does not promote antigen-specific T cell proliferation. Conversely, inhibiting CTLA-4 engagement during anergy induction reverses the block in T cell proliferation, but does not promote full Th1 differentiation. T cells exposed to tolerogenic antigen in the presence of both IL-12 and anti–CTLA-4 antibody are not anergized, and behave identically to T cells which have encountered immunogenic antigen. These results suggest that two processes contribute to the induction of anergy in vivo; CTLA-4 engagement, which leads to a block in the ability of T cells to proliferate to antigen, and the absence of a prototypic inflammatory cytokine, IL-12, which prevents the differentiation of T cells into Th1 effector cells. The combination of IL-12 and anti–CTLA-4 antibody is sufficient to convert a normally tolerogenic stimulus to an immunogenic one.

Role of T Cell Death and Cytokines in Autoimmunity

The immune system is effective at fighting a vast diversity of microbes and eliminating toxic substances. In contrast to most other organ systems, the total number of immune cells can increase is often attributalbe to a 1000- to 50,000-fold expansion of the lymphocytes specific for the antigens of the microbe. At the same time, the immune system continuously generates new lymphocytes, each specific for a different antigen, in the hope that one or more of these will prove useful against future infections. In the face of this massive production of cells, the immune system has developed mechanisms to eliminate lymphocytes, so that it can maintain an optimal size and a balanced representation of cells. These mechanisms rely on an event called programmed cell death, or apoptosis.

The Regulatory Functions of Co-Stimulators Revealed in Transgenic Mice

A general principle of lymphocyte activation is that functional responses of mature T and B lymphocytes require at least two signals. The first signal is provided by antigen recognition, and is responsible for ensuring that the resultant immune response is specific for the eliciting antigen. For T lymphocytes, the second signal is provided by proteins, called co-stimulators, that are expressed on the surfaces of antigen-presenting cells (APCs). Only professional APCs, such as dendritic cells, macrophages, and B lymphocytes, express co-stimulators, and this expression is enhanced by inflammatory reactions that often accompany infections or the administration of antigens with adjuvants. Therefore, the requirement for co-stimulators ensures that T lymphocytes are activated at the correct place—where foreign antigens are presented by professional APCs—and the correct time—when the introduction of antigen elicits the warning sign of local inflammation. T lymphocytes that encounter the first signal, antigen, in the absence of co- stimulators either fail to respond or are rendered anergic, i.e., unresponsive to subsequent interactions with the antigen. It is postulated that because of this, antigens that are presented on nonprofessional APCs or antigens, including self antigens, that do not normally elicit local inflammation fail to stimulate any specific T cells that may be present. Thus, the absence of co- stimulators on normal, “resting” tissue APCs may induce and maintain T-cell tolerance to self antigens that may be presented by these APCs. Conversely, the aberrant expression of co-stimulators on tissue APCs may contribute to the development of autoimmune reactions.