Changes in human T lymphocytes after thymectomy and during senescence

Peripheral lymphocytes from individuals who had been thymectomized in adult life for myasthenia gravis (MG) or for other, nonimmunological reasons showed a moderate decrease in proliferative response capacity to several T-cell mitogens as compared to lymphocytes from normal individuals. The decrease of the response to mitogens and allogeneic lymphocytes was 20–30% within 5 years after thymectomy and about 50% more than 15 years after thymectomy. A comparable decrease in lymphocyte proliferative response capacity was found in healthy aged humans (68–97 years old). Analysis of T lymphocytes from both aged and thymectomized individuals with monoclonal (OKT) antibodies showed a similar pattern: the proportion of T lymphocytes binding OKT3 was reduced, and the OKT4/OKT8 ratio was increased. Hardly any T lymphocytes binding OKT6, OKT10, or OKT1 were found. A biochemical parameter for human T-cell differentiation, the lactate dehydrogenase (LDH) isoenzyme pattern, showed a significantly lower H/M ratio in the group of elderly people compared to young individuals. Furthermore, among patients thymectomized for MG, a significant correlation was observed between the LDH isoenzyme pattern of the T lymphocytes and the proliferative response to mitogens of these cells. In contrast, in healthy thymectomized individuals the LDH isoenzyme pattern appeared to be normal. These findings indicate that, after thymectomy or involution of the thymus, at least part of the peripheral blood T lymphocytes have properties different from those of the cells of young individuals. These cells might represent immature and/or not fully differentiated lymphocytes.