238 Suppressor T Cells in Delayed

There is increasing evidence for cell-to-cell interactions in immune response (1, 2). While the interaction between B and T lymphocytes has been studied by many investigators both in vivo and in vitro, there is a paucity in the literature concerning subpopulations of T cells and their interactions. The nature of T-Tcell interactions is considered an important key in resolving the regulatiry mechanisms of cell-mediated immune responses (CMI). 1 Cantor et al. (3, 4) first reported the cooperation of two T-cell subpopulations, termed precursor of effector cells and amplifier cells, in the development of graft-vs.-host reactions (GVH) in mice. Suppressive actions o f t cells were also demonstrated in GVH by Gershon et al. (5, 6) and by Hardin et al. (7). T-T-cell cooperations and suppression in vitro have been observed in mixed lymphocyte reactions (MLR) (8-11) and in cytotoxic allograft responses (12-15). Delayed hypersensitivity (DH) is an important phenomenon manifested by T-cell activity, yet few papers have discussed such activity. A decline in immunocompetence with respect to both cell-mediated and humoral immunity is considered an age-related phenomenon in mammalians and this decline appears to be linked to involution of the thymus (16). In the present study using mice, we found with age, an increasing reactiveness in DH to methylated human serum albumin (MHSA). After reaching peak responses at around 10-15 mo of age, the animals showed a gradual reduction in the degree of the response. In a preceding p a p e r , 2 w e reported that foot pad reaction (FPR) as a manifestation of DH to MHSA is a phenomenon with high T-cell function dependence, and that the grade of foot pad swelling was limited by the sensitized T-cell population. The discrepancy observed in involution of the thymus and