Requirement for positive selection of gamma delta receptor-bearing T cells

Human fetal liver gamma/delta T cells predominantly use unusual rearrangements of the T cell receptor delta and gamma loci expressed on both CD4+CD8- and CD4-CD8- gamma/delta T cells

Substantial numbers of both alpha/beta and gamma/delta T cells are present in human fetal liver, which suggests a role of the fetal liver in T cell development. The diversity of fetal liver T cell receptor (TCR) gamma and delta chain rearrangements was examined among both CD4+CD8- and CD4-CD8- gamma/delta T cell clones. In addition, TCR delta chain transcripts from three fetal livers were sequenced after polymerase chain reaction amplification of TCR delta chains with V delta 1 or V delta 2 rearrangements. Five of six fetal liver gamma/delta T cell clones had a V delta 2-D delta 3-J delta 3 gene rearrangement with limited junctional diversity; three of these clones had an unusual CD4+CD8- phenotype. V delta 2-D delta 3-J delta 3 gene rearrangements were also common among both in-frame and out-of-frame transcripts from three fetal livers, indicating that they are the result of an ordered rearrangement process. TCR gamma chain sequences of the fetal liver gamma/delta T cell clones revealed V gamma 1-J gamma 2.3, V gamma 2-J gamma 1.2, and V gamma 3-J gamma 1.1 rearrangements with minimal incorporation of template-independent N region nucleotides. TCR gamma chain rearrangements found in these fetal liver T cell clones were different from those that have been observed among early thymic gamma/delta T cell populations, while similar TCR delta chain rearrangements are found among gamma/delta T cells from both sites. These data demonstrate that the fetal liver harbors gamma/delta T cell populations distinct from those found in the fetal thymus, suggesting that the fetal liver is a site of gamma/delta T cell development in humans. These unusual T cell populations may serve a specific function in the fetal immune system.

MHC class I-deficient mice

A great deal has already been learned from the analysis of beta 2m-mutant mice, but it is clear that a great deal remains to be learned. A significant (though unanticipated) problem with this model system is that it is functionally leaky: residual functional class I expression can be detected in beta 2m- mice, and small numbers of functional CD8+ lymphocytes are present in the animals. In many cases, this has frustrated the initial attempts at obtaining immediate definitive resolution of important questions regarding the function of class I molecules. This has occurred primarily in instances in which the class I-deficient mice fail to express an expected phenotype--for example, in studies showing that beta 2m- mice make adequate protective immune responses against certain intracellular pathogens, and are able to reject some allogeneic tissues with a relatively normal pace. On the other hand, it appears that combining the use of beta 2m- mice with other methods (for example, antibody-mediated depletion of CD8+ T cells) is usually adequate to circumvent these difficulties. It remains to be seen whether other better class I deficiencies can be engineered--for example, large deletions of class I genes or mutations in transcription factors essential for class I gene expression. The extent of immunocompetence of beta 2m- mice was somewhat surprising. It was widely expected that class I-deficient mice would be exquisitely sensitive to many viral infections, though the results indicate that sensitivity varies dramatically with the virus and conditions of infection. However, it appears that in lieu of one major arm of the immune system, compensatory immune mechanisms are in many cases able to deal with infection. Similar conclusions are developing from the analysis of several other recently generated mutant mice. Nevertheless, the results indicate a very important role for class I-directed responses in clearing infections mediated by various viral and parasitic agents, particularly in the case of more severe conditions of infection. Although the class I-deficient mice were initially considered primarily a vehicle for analysis of the role of CD8+ T cells, evidence is accumulating that they manifest deficiencies in several other types of lymphocytes, including NK cells, TCR alpha beta+CD4-CD8- cells, and a subset of TCR gamma delta+ cells. This has been a boon for analysis of the development of these cells, but at the same time it has created difficulties in assigning a biological effect of the mutation to a specific lymphocyte deficiency.(ABSTRACT TRUNCATED AT 400 WORDS)

The extrathymic T-cell development pathway

In normal mice, not all T-lineage cells are generated and selected in the thymus; an alternative, extrathymic, development pathway exists. Extrathymic T cells are rare in the spleen and lymph nodes, but are abundant in some tissues, such as the gut. Here, Benedita Rocha, Pierre Vassalli and Delphine Guy-Grand discuss the rules of selection of extrathymic T cells, assess the possible role of these cells in the defence of epithelial integrity and their potential role in autoimmune disease.

Commitment to the T cell receptor-alpha beta or -gamma delta lineages can occur just prior to the onset of CD4 and CD8 expression among immature thymocytes

Two types of T lymphocytes, distinguishable by their surface expression of either the gamma delta or the alpha beta T cell receptor (TcR) for antigen, populate the periphery in the adult. In addition, immature precursors of both T cell types can be found in the thymus. While it is generally accepted that these two cell types represent distinct lineages, it is not known at which developmental stage these lineages diverge. The most mature thymocyte precursor population not yet expressing T lineage-specific surface markers (i.e. CD3, CD4, and CD8) is known to be capable of generating TcR-alpha beta T cells, and has been thought to be preprogrammed into the TcR-alpha beta lineage at an earlier developmental stage. We now show that this late-stage precursor is capable of giving rise to cells of both the TcR-alpha beta and -gamma delta lineages, both in vitro after intrathymic transplantation, and in vitro in simple culture medium or medium with cytokines. Thus it appears that the divergence of TcR-alpha beta and -gamma delta cells can occur at a relatively late stage of intrathymic development, just prior to the onset of CD4 and CD8 expression in most cells.

Selection of murine T cell receptor alpha beta and gamma delta cells in organ cultures established from 14-day embryos

The expression of minor lymphocyte stimulatory locus (Mls) determinants in combination with murine major histocompatibility complex (MHC) class II molecules, leads to the destruction of lymphocytes bearing specific V region-encoded T cell receptor (TcR) products. A much studied example is the elimination of V beta 6+ cells in IE+/Mls-1a mice, in which deletion can be detected 7-10 days after birth but is not fully operational earlier in embryonic life. Here we investigate this transitional period in development and show that selective deletion of V beta 6 occurs in vitro, approximately 1 week after organ cultures are established from 14 day embryos. These unmanipulated organ cultures receive no additional cell immigrants after day 14, suggesting that the cellular elements mediating negative selection (or their direct precursors), are already resident in the fetal thymus by day 14 of gestation. Hence, the developmental timing of the outset of rigorous negative selection of V beta 6 is not dictated by the postnatal entry of deleting elements into the thymus, but perhaps by the maturation of the pre-existing environment. Using a parallel organ-culture approach we have looked at the development of V delta 4 and V gamma 3, TcR gamma delta+ cells in a variety of mouse strains. These receptors have recently been reported to be subject of MHC and non-MHC linked selection, respectively. We find that after an initial period of expansion, the number of V gamma 3-expressing cells dramatically declines. However, this selective loss of V gamma 3 cells is not contingent on the C57BL/6 mouse strain (in contrast to a previous report). These findings are discussed in the context of current models of ontogeny and repertoire selection.

V(D)J recombination in mouse thymocytes: double-strand breaks near T cell receptor delta rearrangement signals

In the murine T cell receptor delta locus, V(D)J recombination events frequently involve the D2 and J1 elements. Here we report the presence of double-strand breaks at recombination signals flanking D2 in approximately 2% of thymus DNA. An excised linear species containing the sequences between D2 and J1 and a circular product of the joining of D2 and J1 recombination signals were also found. Although broken molecules with signal ends were detected, no species with coding ends could be identified. Observation of these broken molecules in thymus, but not in liver or spleen, provides the first direct evidence for an association between specific cleavage of chromosomal DNA and recombination in mammalian cells, and supports a breakage-reunion model of V(D)J recombination.

Development and selection of gamma delta T cells

The gamma delta T-cell population, a subpopulation of T cells formed through cell lineages that are independent of the alpha beta T-cell lineage, consists of multiple subsets with distinct receptor repertoires and homing properties. While the cell sublineage is a critical factor in the determination of homing specificity, both cell sublineage and receptor-dependent selection are instrumental in the determination of the functional repertoire.

Most gamma delta T cells develop normally in beta 2-microglobulin-deficient mice

The specificity of T cells bearing gamma delta T-cell receptors (gamma delta+ T cells) is poorly characterized. Earlier studies suggest that like alpha beta+CD8+ T cells, some gamma delta+ T cells may recognize antigens associated with class I major histocompatibility complex molecules. alpha beta+CD8+ T cells are nearly absent in class I-deficient mice (mutant for beta 2-microglobulin), reflecting a requirement for intrathymic "positive selection" of these cells by class I molecules. Here, we examine whether the development of gamma delta+ T cells is altered in the beta 2-microglobulin mutant mice. We show that the cellularity, marker expression, repertoire, and functional competence of gamma delta+ T cells are not detectably deficient in beta 2-microglobulin mutant mice. We conclude that class I expression is unnecessary for the development of most gamma delta+ T cells.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)