Restricted usage of T-cell receptor V alpha sequence and variable-joining pairs after normal T-cell development and bone marrow transplantation

A model for TCR gene segment use

The TCR alpha-chain is assembled by somatic recombination of variable (V) and joining (J) gene segments at the CD4+ CD8+ stage of development. In this study, we present the first analytical model for deletional rearrangement and show that it is consistent with almost all available data on Valpha Jalpha use in mice and humans. A key feature of the model is that both "local" and "express service" models of rearrangement can be obtained by varying a single parameter that describes the number of gene segments accessible at a time. We find that the window is much larger for Valpha segments than Jalpha segments, which reconciles seemingly conflicting data for the former. Implications for the properties of the repertoire as a whole and experiments that seek to probe them are discussed. Special considerations for allelic inclusion are treated in the Appendices.

Isoforms of terminal deoxynucleotidyltransferase: developmental aspects and function

The immune system develops in a series of programmed developmental stages. Although recombination-activating gene (RAG) and nonhomologous end-joining (NHEJ) proteins are indispensable in the generation of immunoglobulins and T-cell receptors (TCRs), most CDR3 diversity is contributed by nontemplated addition of nucleotides catalyzed by the nuclear enzyme terminal deoxynucleotidyltransferase (TdT) and most nucleotide deletion is performed by exonucleases at V(D)J joins. Increasing TdT expression continuing into adult life results in N region addition and diversification of the T and B cell repertoires. In several species including mice and humans, there are multiple isoforms of TdT resulting from alternative mRNA splicing. The short form (TdTS) produces N additions during TCR and B-cell receptor (BCR) gene rearrangements. Other long isoforms, TdTL1 and TdTL2, have 3' --> 5' exonuclease activity. The two forms of TdT therefore have distinct and opposite functions in lymphocyte development. The enzymatic activities of the splice variants of TdT play an essential role in the diversification of lymphocyte repertoires by modifying the composition and length of the gene segments involved in the production of antibodies and T-cell receptors.

Distinct and opposite activities of human terminal deoxynucleotidyltransferase splice variants

Evidence for potential human TdT (hTdT) isoforms derived from hTdT genomic sequences led us to identify the short isoform (hTdTS), as well as mature long transcripts containing exon XII (hTdTL1) and another including exon VII (hTdTL2) in lymphoid cells. Normal B and T lymphocytes express exclusively hTdTS and hTdTL2, whereas hTdTL1 expression appears to be restricted to transformed lymphoid cell lines. In in vitro recombination and primer assays, both long isoforms were shown to have 3'-->5' exonuclease activity. Overexpression of hTdTS or hTdTL2 greatly reduced the efficiency of recombination, which was reverted to normal levels by the simultaneous expression of both enzymes. Therefore, alternative splicing may prevent the adverse effects of unchecked elongation or diminution of coding ends during V(D)J recombination, thus affecting the survival of a B or T cell precursor during receptor gene rearrangements. Finally, the newly discovered hTdT isoforms should be considered in future screening of human leukemias.

T-cell clonal change after allo-kidney transplantation in humans

Whether T cells circulating peripherally express changes at a clonal level after renal transplantation is uncertain. To clarify this issue, we analyzed T-cell clonality of peripheral blood lymphocytes (PBLs) in 12 renal transplant recipients by a novel polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) method that can discriminate T-cell clones with different T-cell receptor (TCR) Vbeta motifs. The PCR-SSCP study showed that after transplantation, only a few distinct T-cell clonotypes accumulated in the absence of clinical episodes, irrespective of the compatibility of HLA antigens. In contrast, various T-cell clones appeared in cases of acute rejection (AR) and infection. These subsided immediately after the AR was resolved; however, they remained long after the resolution of the infection. In a case of AR followed by an infectious episode, distinct T-cell clones appeared concomitantly with each episode. Several of them disappeared or remained thereafter. In one case, significant numbers of accumulating bands were observed by in-vitro stimulation by mixed lymphocyte reaction (MLR); several were identical to those found in vivo. However, some of those that did not appear in vitro were apparent in vivo. In conclusion, the appearance of T-cell clonotypes at a peripheral level indicates the existence of immunologically activated T-cell clones, which were significantly affected by immunosuppressive therapy. It was also determined that the T-cell immune system is much more complicated in vivo than in vitro.

T cell receptor repertoire of CD4+ and CD8+ T cell subsets in the allogeneic bone marrow transplant recipient

Allogeneic bone marrow transplantation (BMT) has become a therapy of choice for the treatment of certain malignancies and hematopoietic disorders. However, immunodeficiencies following BMT continue to cause significant morbidity and mortality. We have compared the T cell receptor (TCR) repertoire of BMT patients and healthy control individuals by staining peripheral blood mononuclear cells with fluorochrome-labeled TCR-specific antibodies. Several patients exhibited a biased pattern of TCR expression atypical of the healthy controls, yet no particular TCR bias characterized all patients. For example, we found that 2%-8% of T cells from healthy individuals expressed the V beta 19 TCR. One BMT patient exhibited V beta 19 expression on more than 60% of peripheral T cells, while additional patients expressed V beta 19 on less than 1% of T cells. The patients with the most extreme skewing of TCR types suffered from graft-versus-host disease. The causes of skewed TCR V beta expression patterns in BMT patients are not fully understood, yet some researchers have suggested that an oligoclonal expansion of CD8+ T cell populations may be largely responsible. To test this hypothesis, we examined the TCR V beta repertoire of CD4+ and CD8+ T cell populations. We found that biased V beta expression characterized both CD4+ and CD8+ T cell populations, sometimes within a single individual. Thus, therapies directed toward CD8+ T cells alone may not fully correct repertoire abnormalities following BMT.