Gain-of-function p53 mutations enhance alteration of the T-cell receptor following X-irradiation, independently of the cell cycle and cell survival.

Missense mutations are by the far the most common types of mutations found in p53 of human tumors, suggesting that mutant p53 proteins function either by abrogating wild-type function or by gaining new oncogenic functions. To distinguish between the dominant-negative effect and gain of new function of p53 missense mutants, we measured the ability of transfected missense mutant p53s in p53-null Jurkat cells to alter T-cell receptor (TCR) surface expression. The TCR is a key signal transduction moiety common to T lymphocytes and is one of the major sites for aberrations in T-cell leukemias/lymphomas. Three p53 mutants (248trp, 249ser, and 273his) enhanced the frequency of TCR mutants after graded doses of X-radiation compared to null p53 parent- and wild-type p53-possessing normal lymphocytes; the parent Jurkat and normal lymphocyte showed no difference. These enhancements were not the results of a change in radiosensitivity or in G1 checkpoint arrest characteristics. Therefore, the creation of this mutator phenotype by missense-type p53 mutations implies that a more direct mechanism, apart from changes of cell cycle kinetics or cell death, may be responsible for the selection of certain p53 point mutations, which eventually result in the tumorigenesis of the cell.

UVB radiation preferentially induces recruitment of memory CD4+ T cells in normal human skin: long-term effect after a single exposure.

Acute, low-doses of ultraviolet (UV)-B radiation affect the immune competent cells of the skin immune system. In this study, we examined the time-dependent changes of the cutaneous T cell population in normal human volunteers following a single local exposure to UV. Solar-simulated UV radiation caused an initial decrease in intraepidermal T cell numbers, even leading to T cell depletion at day 4, whereupon a considerable infiltration of T cells in the epidermis occurred that peaked at day 14. In the dermis the number of T cells was markedly increased at days 2 (peak) and 4 after irradiation, and subsequently declined to the nonirradiated control values at day 10. Double-staining with several T cell markers showed that the T cells, infiltrating the (epi)dermis upon UV exposure, were almost exclusively CD4+ CD45RO+ T cells, expressing an alpha/beta type T cell receptor, but lacking the activation markers HLA-DR, VLA-1, and IL-2R. Application of UVB radiation resulted in similar dynamics of T cells, indicating that the UVB wavelengths within the solar-simulated UV radiation were responsible for the selective influx of CD4+ T cells. In conjunction with UVB-induced alterations in the type and function of antigen-presenting cells (i.e., Langerhans cells and macrophages), the changes of the cutaneous T cell population may also contribute to UVB-induced immunosuppression at skin level in man.

Increased frequency of CD4-8-T cells bearing T-cell receptor alpha beta chains in peripheral blood of atomic bomb survivors exposed to high doses.

A rare T-cell subpopulation, CD4-8- alpha beta T cells, may be differentiated through a pathway (or pathways) different from the pathway(s) of conventional CD4+ or CD8+ T cells. In the present study, the frequencies of CD4-8-T cells in peripheral-blood alpha beta T cells in 409 atomic bomb survivors (160 estimated to have been exposed to 1.5 Gy or more and 249 controls) were determined to investigate late effects of radiation on the composition of human T-cell subpopulations. The frequency of CD4-8- alpha beta T-cell decreased significantly with the subject's age and was higher in females than males. A significant increase in the frequency was found in the survivors exposed to more than 1.5 Gy, suggesting that the previous radiation exposure altered differentiation and development of T cells.

Long-lasting changes in the T-cell receptor V beta repertoires of CD4 memory T-cell populations in the peripheral blood of radiation-exposed people.

To study the long-term effects of radiation-induced T-cell depletion on the T-cell receptor (TCR) Vbeta repertoires of human peripheral CD4 T-cell populations, we measured the percentages of CD4 T cells representing each of the full range of possible TCR Vbeta families in a cohort of atomic bomb survivors. We then estimated the extent to which the expression levels for individual TCR Vbeta families differed from the average expression level for that particular TCR Vbeta family across the entire cohort. We found no evidence of a systematic change in the TCR Vbeta repertoires of the naïve CD4 T-cell populations, but memory CD4 T-cell TCR Vbeta family expression levels diverged significantly from the population average for counterpart families, especially in individuals who had been exposed to higher doses and were at least 20 years of age at the time of the bombing. Comparisons of the TCR Vbeta family expression profiles in the naïve and memory CD4 T-cell pools of the same group of adult survivors revealed that differences in the TCR Vbeta repertoires of these two types of CD4 T-cell pool were larger in more heavily exposed survivors than in unexposed controls. These findings suggest that the memory CD4 T-cell pools of individuals who received significant radiation doses in adulthood may well have become (and could still be) dependent upon a much less diverse complement of TCR Vbeta families than would otherwise have been the case.

Thymocyte differentiation in gamma-irradiated severe-combined immunodeficient mice: characterization of intermediates and products of V(D)J recombination at the T cell receptor alpha locus.

Treatment with DNA-damaging agents promotes rescue of V(D)J recombination, limited thymocyte differentiation, and development of thymic lymphomas in severe-combined immunodeficient (SCID) mice. One intriguing aspect of this system is that irradiation rescues rearrangements at the T cell receptor (TCR) beta, gamma and delta loci, but not at the TCR alpha locus. Current models posit that only those loci that are recombinationally active at the time of irradiation can be rescued. Here, we employ sensitive, semiquantitative ligation-mediated polymerase chain reaction assays to detect a specific class of recombination intermediates, hairpin coding ends, at the TCR alpha locus. We found that J alpha-coding ends are undetectable in unirradiated SCID thymocytes, but accumulate after irradiation at times coincident with the emergence of a CD4+ CD8+ thymocyte population. Coding joints produced by joining of these ends, however, are extremely rare. To test whether the presence of hairpin coding ends at TCR alpha is sufficient for irradiation-mediated rescue of coding joint formation, we administered a second dose of gamma-irradiation after abundant CD4+ CD8+ thymocytes and hairpin TCR alpha coding ends had accumulated. This treatment failed to stimulate rescue of TCR alpha coding joints. Thus, the presence of hairpin coding ends at the time of irradiation, while perhaps necessary, is not sufficient for rescue of V(D)J rearrangements. These results support a refined model for irradiation-mediated rescue of TCR rearrangements in SCID mice.

Frequent occurrence of in vivo clonal expansion of CD4- CD8- T cells bearing T cell receptor alpha beta chains in adult humans.

We have previously reported 2 cases of healthy men showing in vivo monoclonal expansion of mature CD4- CD8- alpha beta T cells. In the present study, an additional 3 adults were found to exhibit such an expansion, among a total 464 adult donors studied. These 5 individuals were otherwise physiologically normal, with no history of severe illness and autoimmune disease at the time of examination. To investigate the mechanisms of the clonal expansion, further characterization of the clonal cells was attempted. No apparent preference for usage of the T cell receptor beta chain variable region was observed in the clonal T cells. These clonal T cells showed lectin-dependent or redirected antibody-dependent cell-mediated cytotoxicities, whereas they could not lyse autologous lymphoblastoid cell lines. Failure of Fas antigen expression was not observed for any of these clones. These results suggest that clonal expansion of CD4- CD8- alpha beta T cells frequently occurs in the periphery without any T cell abnormalities.