Harald von Boehmer

Early αβ T cell development in the thymus of normal and genetically altered mice

The vast majority of T lymphocytes, with the exception of gut-associated, intraepithelial lymphocytes, differentiate and mature inside the thymus. Early T cell development is characterized by expansion and differentiation of thymocytes which do not yet express mature TCRs on their cell surface. Important events in early thymocyte development are controlled by a pre-TCR complex consisting of a conventional TCR beta chain and a novel transmembrane protein termed pre-TCR alpha (p T alpha chain) which are noncovalently associated with components of CD3. Recent studies of pre-TCR function have led to a better understanding of the molecular events in early thymocyte development.

Pleiotropic changes controlled by the pre-T-cell receptor.

The construction of various gene-deficient mice has facilitated the understanding of the role of various receptors and signaling pathways that control the generation of alphabeta lineage cells. A predominant role is occupied by the pre-TCR, which not only generates large numbers of alphabeta lineage cells but also controls TCRbeta allelic exclusion as well as commitment to the gammadelta lineage versus the alphabeta lineage.

The αβ T Cell Receptor Can Replace the γδ Receptor in the Development of γδ Lineage Cells

Abstract In peripheral lymphoid tissues of TCR transgenic mice that express the nominal antigen (HY peptide plus H-2D b MHC) recognized by the transgenic TCR, there exist unusual CD4 − CD8 − and CD4 − CD8 low cells bearing the transgenic TCR. Here we show that, unlike TCRαβ T cells that are generated in the absence of nominal antigen, these unusual cells do not express endogenous TCRα genes, have maintained the TCRδ locus on both chromosomes, and can coexpress TCRαβ and TCRγδ chains on the cell surface. The latter is also true for CD4 − CD8 − , HSA + TCRαβ + thymocytes in male and female TCR transgenic mice. The number of TCRαβ and TCRγδ coexpressing cells is increased in pre-TCR-deficient mice. The data indicate that the TCRαβ can replace the TCRγδ in the development of γδ lineage cells and that the pre-TCR interferes with the generation of γδ-expressing cells.

Anergy and suppression regulate CD4 + T cell responses to a self peptide

To examine the role of cognate peptide in establishing CD4+ T cell tolerance, we have mated transgenic mice that express the major I‐Ed‐restricted determinant (S1) from the influenza virus PR8 hemagglutinin (HA28 mice) with mice expressing a S1‐specific T cell receptor (TS1 mice). Surprisingly, S1‐specific CD4+ T cells were not substantially deleted in TS1xHA28 mice; indeed, lymph node cells expressing the S1‐specific TCR were as abundant in TS1xHA28 mice as in TS1 mice. The S1‐specific T cells in TS1xHA28 mice were, however, impaired in their ability to respond to S1 peptide both in vitro and in vivo, and contained two distinct populations. Approximately half expressed a unique cell surface phenotype (CD25hiu2009/u2009CD45RBint) and had been anergized by the neo‐self S1 peptide. The remainder responded normally to the S1 peptide if purified away from the anergic T cells, but their proliferation was suppressed when the anergic T cells were also present in unfractionated lymphnode cells or in mixed cultures. These findings establish that anergy and suppression are coordinated mechanisms by which autoreactive CD4+ T cells are regulated and that anergicu2009/u2009suppressor CD4+ T cells can develop in response to self peptides.

Allelic Inclusion of T Cell Receptor α Genes Poses an Autoimmune Hazard Due to Low-Level Expression of Autospecific Receptors

Organ-specific autoimmune disease can be caused by alphabeta T cells that have escaped self-tolerance induction. Here we show that one of the causes of escape from self-tolerance is the coexpression of two different T cell receptors by the same cell, which can occur in up to 30% of all T cells in normal mice and can lead to low-level surface expression of an autospecific TCR. We found that double receptor-expressing T cells can escape tolerance even to ubiquitously expressed antigens but can nevertheless induce autoimmune diabetes when the relevant protein is expressed in pancreatic tissue. Such diabetogenic T cells are absent, however, among T cells expressing the autospecific TCR as the sole receptor.

Restoration of Thymopoiesis in pTα−/− Mice by Anti-CD3ε Antibody Treatment or with Transgenes Encoding Activated Lck or Tailless pTα

Mice deficient for the pre-TCR alpha (pT alpha) chain cannot form a pre-T cell receptor (TCR) and exhibit a severe defect in early T cell development, characterized by lack of beta selection and impaired generation of double-positive (DP) thymocytes. Here, we demonstrate that intraperitoneal injection of CD3epsilon-specific antibodies into pT alpha-/- x RAG-/- mice or introduction of an activated p56(lck) transgene in pT alpha-/- mice fully restores the number of DP thymocytes, and that expression of a transgenic pT alpha chain lacking its cytoplasmic portion can overcome all developmental defects associated with pT alpha deficiency. These results allow a better definition of the role of pT alpha in pre-TCR signal transduction and provide conclusive evidence that the cytoplasmic tail of pT alpha is not essential for pre-TCR signaling.

Crucial function of the pre‐T‐cell receptor (TCR) in TCRP selection, TCRβ allelic exclusion and αβ versus γδ lineage commitment

Summary: The analysis of T‐cell receptor (TCR) βselection, TCRβ allelic exclusion and TCRβ rearrangement in γδ T cells from normal and pre‐TCR‐deficient mice has shown that the pre‐TCR has a crucial role in T‐lyinpbocyte development:

On the Role of the Pre–T Cell Receptor in αβ versus γδ T Lineage Commitment

Abstract The role of the pre–T cell receptor (TCR) in lineage commitment to the γδ versus αβ lineage of T cells was addressed by analyzing TCRβ chain rearrangements in γδ T cells from wild-type and pre-TCR-deficient mice by single cell polymerase chain reaction. Results show that the pre-TCR selects against γδ T cells containing rearranged Vβ genes and that γδ T cell precursors but not γδ T cells express the pre-TCRα protein. Furthermore, pre-TCR-induced proliferation could not be detected in γδ T cells. We propose that the pre-TCR commits developing T cells to the αβ lineage by an instructive mechanism that has largely replaced an evolutionary more ancient stochastic mechanism of lineage commitment.

Autoimmune insulitis and diabetes in the absence of antigen‐specific contact between T cells and β‐islet cells

Autoimmune diabetes develops following recognition of organ‐specific antigens by T cells. The disease begins with peri‐islet infiltration by mononuclear cells, proceeds with insulitis and becomes manifest with destruction of insulin‐producing islet β‐cells. T cells are necessary to induce insulitis and diabetes, but it is not clear by what mechanisms they can do so, i.u2009u2009e. whether the T cells need to make antigen‐specific contact with the β‐cell or whether other interactions are sufficient to induce β‐cell death. In the present study we have constructed chimeric mice in which the bone marrow‐derived antigen‐presenting cells, but not the islet β‐cells, are capable of presenting antigen to monospecific T cells. We show that both insulitis as well as β‐cell destruction can proceed in the absence of islet β‐cell surface antigen recognition by T cells. Our results support the notion that diabetes can be caused by distinct effector mechanisms.

Genomic structure of the human pre-T cell receptor α chain and expression of two mRNA isoforms

The pre‐TCR, which is minimally composed of the TCRβ chain, the pre‐Tα chain, and the CD3 complex, regulates early T cell development. The pre‐Tα chain is a 33‐kDa type I transmembrane glycoprotein with an extracellular part similar to the constant domain of the immunoglobulin supergene family. We have sequenced (11 kb) the human pTα gene, which like the murine pTα gene consists of four exons: exon 1 encodes the 5u2009′ untranslated region, the leader peptide and the first three amino acids of the mature protein, exon 2 the extracellular immunoglobulin (Ig)‐like domain, exon 3 a 15‐amino acid peptide including a cysteine required for heterodimerization with TCRβ, exon 4 the transmembrane region, the cytoplasmic tail and the 3u2009′ untranslated sequence. The human pTα gene is located on chromosome 6p21.3, close to the HLA‐A locus. Reverse transcription‐PCR studies with human thymus and leukemic cells showed that alternative splicing produces a shorter pTα isoform, which lacks the Ig‐like domain but contains the transmembrane elements and the extracytoplasmic cystein and which could thus permit pairing with TCRβ chain and association with CD3 molecules. The conserved splice sites suggest a yet ill‐defined biological function of the short pTα protein.