Analysis of early fetal T-cell receptor delta chain in humans

T-cell receptor V delta-J alpha rearrangements in human thymocytes: the role of V delta-J alpha rearrangements in T-cell receptor-delta gene deletion

The differentiation mechanisms that force thymocytes into the T-cell receptor (TCR)-alpha beta or TCR-gamma delta lineage are poorly understood, but rearrangement processes in the TCR-alpha/delta locus are likely to play an important role. It is assumed that the TCR-delta gene is deleted prior to V alpha-J alpha rearrangements by rearrangement of the so-called TCR-delta-deleting elements delta Rec and psi J alpha. However, the TCR-delta gene can also be deleted via V delta-J alpha rearrangements. We studied the different TCR-delta-deleting rearrangements of V delta 1, delta Rec, V delta 2 and V delta 3 to J alpha gene segments in human thymocytes and peripheral blood using polymerase chain reaction analysis. The V delta 1 gene segment is the most upstream V delta gene segment tested and appears to rearrange to almost all J alpha gene segments. In contrast, the delta Rec and V delta 2 gene segments only rearrange to the 5'-located J alpha gene segments, thereby preserving an extensive TCR-alpha combinatorial diversity, because most J alpha gene segments are kept available for subsequent V alpha-J alpha rearrangements. Based on our combined data we hypothesize that the different V delta gene segments and the delta Rec gene segment play different roles in T-cell development with regard to TCR-delta deletion.

Comparison of human and mouse T-cell receptor variable gene segment subfamilies

Like the immunoglobulin Igh-V and Igk-V gene families, the human or mouse TCRV gene families may be grouped into subfamilies displaying > 75% nucleic acid sequence similarity among their members. Systematic interspecies sequence comparisons reveal that most mouse Tcr-V subfamilies exhibit clear homology to human TCRV subfamilies (> 60% amino acid sequence similarity). Homologous pairs of TCRV genes in mice and humans show higher sequence similarity than TCRV genes from different subfamilies within either species, indicating transspecies evolution of TCRV genes. Mouse and human homologues show conservation of their relative map order, particularly in the 3' region and a similar sequential and developmentally programmed expression. When the V regions from both species were analyzed together, local length differences and conserved residues in the loop regions were revealed, characteristic of each of the four TCRV families.

Usage of TCRAV and TCRBV gene families in human fetal and adult TCR rearrangements

We have investigated fetal and adult T-cell receptor (TCR) A and B V-gene repertoires both by fluorescence-activated cell sorter (FACS) analysis with the available TCR V region-specific mAbs and by the polymerase chain reaction (PCR) with TCR V gene family-specific oligonucleotides. Among the low number of CD3+ T cells, most of the TCR V regions tested for could be detected by FACS analysis in liver, bone marrow, and spleen derived from a 14-week-old fetus and two 15-week-old fetuses. Similarly, the PCR analysis showed that the majority of the TCRAV and TCRBV families were expressed in the peripheral organs of the 13-week-old fetus, although an apparent absence of particular TCR V families was found in liver and bone marrow. This was most probably the consequence of the low number of CD3+ T cells in these organs. In 17-week-old fetal thymi the level of expression of some TCRAV and TCRBV gene families, in particular those that contain a single member, was lower compared to post-partum thymi and adult peripheral blood mononuclear cells. The combined data of FACS and PCR analysis demonstrate that TCR V genes belonging to the majority of TCR V gene families can be used in TCR alpha and beta chain rearrangements during early human fetal life. Our data also suggest that the expression levels of some of the single member TCR V gene families may be influenced by the developmental stage.

Phenotypic characteristics of lymphoid populations of middle gestation human fetal liver, spleen and thymus

Mononuclear cells isolated from liver, spleen and thymus of fetuses between 18 and 24 weeks gestational age were stained for a number of lymphoid cell markers by indirect immunofluorescence and analyzed by flow cytometry. Studies were carried out on freshly isolated mononuclear cell preparations and on cultured cells after selective expansion in interleukin 2 (IL2). Many mononuclear cells in fresh isolates of liver and spleen could not be identified with antibodies to mature T- and B-cell markers. An average of 3% of isolated liver cells and 34% of isolated spleen cells stained positively for CD3, and 19% of liver cells and 37% of spleen cells stained positively for CD20. Lymphoid cells of the fetal thymus were an average 67% CD3+, 76% CD4+, 84% CD8+, and showed greater CD45RO staining (93%) than mononuclear cells of other tissues. Propagation of liver and spleen cell populations in culture favored CD3 phenotypes and CD8 phenotypes. Propagated T cell populations of liver and spleen were primarily TCR alpha/beta+ (81% in liver, 85% in spleen), suggesting a selective advantage in IL2 expansion of alpha/beta T cells over gamma/delta T cells. Propagated gamma/delta T cells of liver and spleen were predominantly TCR gamma/delta 2+. Whereas propagated cells of liver and spleen consisted of approximately 10% gamma/delta+ cells, thymus-derived cells expanded in culture were only an average of 2% TCR gamma/delta+, demonstrating a rarity of IL2-responsive gamma/delta T cells in middle gestation fetal thymus.

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.

Functionally distinct subsets of human gamma/delta T cells

To determine if effector subsets exist among human gamma/delta T cells, we examined the cytokine production and cytotoxic activity of gamma/delta T cell clones with different accessory molecule phenotypes, V delta and V gamma gene expression, and J gamma rearrangements. T cell clones bearing gamma/delta T cell receptor produce an array of cytokines like alpha/beta T cell clones. Individual gamma/delta T cell clones produced a characteristic array of cytokines without correlation with V delta or V gamma gene expression. However, when phenotypic subsets were considered, CD4+ gamma/delta clones produced significantly higher levels of interleukin 2 and granulocyte-monocyte colony-stimulating factor compared with CD4-CD8- and CD8+ gamma/delta clones. Similarly, when cytotoxic potential was assessed, CD4+ gamma/delta clones exhibited minimal activity when compared with CD4-CD8- and CD8+ adult peripheral blood gamma/delta clones. We conclude that functionally distinct gamma/delta T cell subsets exist and suggest that these subsets may correlate with expression of the CD4 accessory molecule.