A functional gamma gene formed from known gamma-gene segments is not necessary for antigen-specific responses of murine cytotoxic T lymphocytes

T gamma protein is expressed on murine fetal thymocytes as a disulphide-linked heterodimer

During the search for genes coding for the mouse alpha and beta subunits of the antigen-specific receptor of mouse T cells we encountered a third gene, subsequently designated gamma. This gene has many properties in common with the alpha and beta genes, somatic assembly from gene segments that resemble the gene segments for immunoglobulin variable (V), joining (J) and constant (C) regions; rearrangement and expression in T cells and not in B cells; low but distinct sequence homology to immunoglobulin V, J and C regions; other sequences that are reminiscent of the transmembrane and intracytoplasmic regions of integral membrane proteins; and a cysteine residue at the position expected for a disulphide bond linking two subunits of a dimeric membrane protein. Despite these similarities the gamma gene also shows some interesting unique features. These include a relatively limited repertoire of the germ-line gene segments, more pronounced expression at the RNA level in immature T cells such as fetal thymocytes and an apparent absence of in-frame RNA in some functional, alpha beta heterodimer-bearing T cells or cultured T clones and hybridomas. To understand the function of the putative gamma protein it is essential to define the cell population that expresses this protein. To this end we produced a fusion protein composed of Escherichia coli beta-galactosidase and the gamma-chain (hereafter referred to a beta-gal-gamma) using the phage expression vector lambda gt11 and raised rabbit antisera against the gamma determinants. Using the purified anti-gamma antibody we detected a polypeptide chain of relative molecular mass 35,000 (Mr 35K) on the surface of 16-day old fetal thymocytes. The gamma-chain is linked by a disulphide bridge to another component of 45K. No such heterodimer was detected on the surface of a cytotoxic T lymphocyte (CTL) clone 2C from which an in-phase gamma cDNA clone was originally isolated.

The T-cell antigen receptor gamma gene: rearrangement and cell lineages

Most adult T cells have a T-cell antigen receptor composed of αß heterodimers in a complex with the CD3 antigen. Immature thymocytes, however, can express CD3-associated heterodimers of a γ chain and another poorly characterized chain ('δ'). Here James Allison and Lewis Lanier suggest that thymocytes can develop along two mutually exclusive ontological pathways, giving rise to T cells that express either αß or γδ heterodimers, but not both or any other heterodimeric combination.

Repertoire of the human T cell gamma genes: high frequency of nonfunctional transcripts in thymus and mature T cells

To further understand the structural diversity and function of the human T cell-specific gamma gene, 20 human gamma gene cDNA from both thymocyte and peripheral blood T lymphocyte libraries were sequenced and analyzed. Evidence of greater germ-line multiplicity and more extensive use of junctional flexibility, N-region diversity and perhaps D gamma gene segment incorporation was observed. In spite of the larger gamma gene repertoire found in humans compared to mice, the potential of the human gamma gene message to encode a functional product appears to be severely limited. In addition to the extremely low levels of gamma gene expression observed, each of the 20 gamma gene cDNA exhibit some form of deleterious mutation defect. This is in sharp contrast to the finding that human and mouse T cell antigen receptor alpha and beta messages isolated from various sources have functional coding potential in most cases. Thus, the role of the human T cell gamma gene becomes even more uncertain than before.

Athymic mice express a high level of functional gamma-chain but greatly reduced levels of alpha- and beta-chain T-cell receptor messages

T lymphocytes differentiate and mature in the thymus. It is here that thymocytes with reactivities to self antigens are eliminated and those with specificities to 'altered' self-major histocompatibility complex (MHC) gene products are positively selected. The selections are presumably carried out on the basis of their T-cell antigen receptors (TcR). The genes of the alpha- and beta-chain T-cell antigen receptors have been cloned. A third T-cell specific gene capable of undergoing somatic rearrangement has also been identified; the role of this third gene is not known. An order of expression of gamma, beta, then alpha is found during T-cell ontogeny. But although alpha- and beta-chain messages are often functional, gamma transcripts are rarely functional in thymocytes or mature T cells. To define the sequential order of expression of these genes further and to continue the search for a possible role for the TcR gamma gene products, we investigated the expression of 'functional' alpha-, beta- and gamma-chain transcripts in young athymic mice. These mice express an undetectable amount (less than one in 8 X 10(5) spleen messages) of 'full-length' alpha- and beta-chain T-cell receptor transcripts, but an increased level of expression of 'full-length' gamma chain messages. Nucleotide sequence analysis of four gamma complementary DNAs show that all four gamma transcripts sequenced are functional. These findings suggest that gamma gene products may be important in a prethymic or extrathymic pathway and may represent a second type of T-cell recognition, possibly in a lineage in which alpha and beta genes are not used.

Transcripts of functionally rearranged gamma genes in primary T cells of adult immunocompetent mice

The T-cell specific, rearranging gamma-chain genes bear striking resemblance to T-cell receptor and immunoglobulin genes, but the role of gamma remains unknown. A central problem is to understand the conditions under which gamma RNA is expressed in cells. The transcription of gamma is abundant in T cells of fetal thymi, but is negligible in peripheral T cells of adults, suggesting that gamma is involved in development of the T-cell repertoire. However, gamma RNA was originally cloned from established lines of cytotoxic T cells (CTLs) derived from adult mice and this expression has been ascribed to non-physiological cell growth. Possibly consistent with this, most of the gamma RNA derives from genes rearranged abortively at the V gamma-J gamma junction of immunoglobulin genes, where V is the variable segment and J the joint segment. Here, we report the detailed analysis of gamma transcription in T cells of adult mice, and find that transcription may occur in T cells with a broad range of surface phenotypes; that it is predominantly of a single V gamma-C gamma unit (where C is the constant region); and that in cells freshly explanted from animals it can be of productively rearranged genes.

A phosphorylated, disulfide-linked membrane protein in murine cytotoxic T lymphocytes

The previously determined sequence of the murine T-cell gamma gene and its transcription in cloned T lymphocytes suggests that the polypeptide encoded by this gene is generally present in cytotoxic T cells as a 33-kDa monomer in a disulfide-bonded dimer. The gamma chain is also expected to be phosphorylated because a sequence in its cytoplasmic domain is homologous to an active site for serine phosphorylation in the regulatory subunit of cAMP-dependent protein kinase. We describe here a cytotoxic-T-cell-associated phosphorylated protein, many of whose properties suggest that it may be the product of the T-cell gamma gene. Its phosphorylation is greatly enhanced by interleukin 2 stimulation.

Diversity of murine gamma genes and expression in fetal and adult T lymphocytes

The search for the genes encoding the T-cell receptor alpha and chains revealed a third gene, T gamma (ref. 1), which shares with t T alpha (refs 2-7) and T beta (refs 8-15) genes a number of structure features, including somatic rearrangement during T-cell development. T gamma gene expression appears to be unnecessary in son mature T cells and is at its greatest in fetal thymocytes encouraging speculation that T gamma has a role in T-cell development and may be involved in the recognition of polymorphic major histocompatibility complex (MHC) products during thymic education. One argument against the participation of T gamma in such a process has been its apparently limited diversity, due to the small number of gene segments available for rearrangement. We here describe the identification of additional T gamma V-gene segments and demonstrate that they can be rearranged to previously identified J- and C-gene segments and are expressed in fetal thymocytes. In addition we describe a variety of patterns of T gamma mRNA processing which may be significant for T gamma gene regulation.

Serine esterase in cytolytic T lymphocytes

The mechanisms that enable cytotoxic T lymphocytes (Tc cells) to destroy target cells are only vaguely understood. However, recent studies have identified in Tc cells and natural killer cells cytoplasmic granules that contain perforin, a cytolytic protein that resembles the ninth component of complement (C9). Antigen-specific lysis of target cells, traditionally ascribed solely to Tc cells, has now also been demonstrated in some T-helper cell (Th cell) lines, referred to here as T helper-killer or Th/c cells. We recently found a novel serine esterase that is present at greatly elevated levels in cloned murine Tc cell lines and one Th/c cell line, but not in two non-cytolytic Th cell lines. These findings suggest that the serine esterase is involved in cytolytic activity and that a variety of effector cells share a common cytolytic mechanism. To explore the role of the serine esterase in this process, we have been studying additional properties of the enzyme in murine T cells. We show here that it is a membrane-associated, disulphide-linked dimer, it has trypsin-like properties but is not a general protease, in density gradient centrifugation it sediments with perforin, it is secreted by Tc cells during their cytolytic attack on target cells, and antiserum to Tc-cell serine esterase reacts with the enzyme in Th/c cells.