Immunological studies of aging. Decreased production of and response to T cell growth factor by lymphocytes from aged humans

Human lymphocytes from elderly and young donors were cultured with phytohemagglutinin (PHA) or concanavalin A. Cultures from old donors produced less T cell growth factor (TCGF) and incorporated less tritiated thymidine (3H-Tdr) than did similar cultures from young donors in the presence of either mitogen. Furthermore, the response of lymphocytes from elderly donors to TCGF was impaired. Thus, PHA-activated T cells from aged donors showed no increase tritiated thymidine incorporation when incubated with exogenous human TCGF. In contrast, addition of exogenous human TCGF to PHA-activated peripheral blood leukocytes from younger individuals increased tritiated thymidine incorporation by 30-50%. The impaired response to TCGF was associated with decreased binding of TCGF by PHA-activated cells from old donors. TCGF production or responsiveness was not associated with the presence of "suppressor" activity in elderly T cell preparations. These studies suggest a possible molecular mechanism for the impaired proliferative response of elderly human T cells. These data lend support to the hypothesis that defects in the capacity to either produce or respond to TCGF may be a fundamental cause of immune deficiency.

Cytological localization of the genes for the four classes of ribosomal RNA (25S, 18S, 5.8S and 5S) in polytene chromosomes of Phaseolus coccineus

Homologous tritiated 25S, 18S and 5.8S rRNAs were used separately for in situ hybridization to the polytene chromosomes of the embryo suspensor cells of phaseolus coccineus. Hybridization occurred at the same chromosomal sites which were labeled in previous in situ hybridization experiments with 25 + 18S rRNAs in the same material (Avanzi et al., 1972), namely: nucleolus organizing system (satellite, nucleolar constriction and organizer) of chromosome pairs I (S1) and V (S2), proximal heterochromatic segment of the long arm of chromosome pair I, and terminal heterochromatic segment of chromosome pair II. Competition hybridization experiments confirmed for P. coccineus the high sequence homology between 25S and 18S rRNA already known for other plants. Homologous 125I-5S rRNA was found to hybridize to three sites in the polytene chromosomes of P. coccineus: the proximal heterochromatic segment in the long arm of chromosome pair I (which also bears the sequences complementary to 25S, 18S and 5.8S RNAs), most of the proximal heterochromatic segment plus a small portion of adjoining euchromatin in the long arm of chromosome pair VI and the large intercalary heterochromatic segment in the same chromosome pair. Simultaneous labeling of the two 5S RNA sites in chromosome VI was quite rare (3%), the rule being labelling of one site to the exclusion of the other, with a labeling frequency of 43.7% and 53.3% for sites no. 1 and no. 2 respectively. These results are interpreted as being due to differential hybridizability of chromosomal sites such as described in other materials.